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TB, Etest, Myla, VITEK are used, pending and/or registered the blue logo, Empowering Clinical Decisions, API, A / RCS Lyon B 398 160 242.
The information in this booklet is given as a guideline only and is not intended to be exhaustive.
It in no way binds bioMérieux S.A. to the diagnosis established or the treatment prescribed by the physician.
bioMérieux S.A.
B/A / This document is not legally binding. bioMérieux reserves the right to modify specifications without notice / BIOMERIEUX, 69280 Marcy l'Etoile Tel. : 33 (0)4 78 87 20 00Fax : 33 (0)4 78 87 20 90www.biomerieux.com 01-12 / 93025 trademarks belonging to bioMérieux S.A. or one of its subsidiaries / bioMérieux S.A. RCS Lyon 673 620 399 / Printed in France / ANSWERSon Antimicrobial Of all the laboratory examinations performed daily by clinical microbiologists, in vitro susceptibility testing is of particular clinical importance as an aid for selecting the most appropriate antimicrobial Testing of Bacteria therapy for individual patients, monitoring the evolution of microbial resistance, and updating empirical therapeutic strategies.
The methodology for in vitro antibacterial testing and the criteria for interpretation are well established. Antibacterial susceptibility testing is routinely performed in microbiology laboratories worldwide. Methods for in vitro antifungal testing and criteria for interpretation have been This brochure is intended to provide developed more recently and are similar in concept to antibacterial succinct answers to common testing. Susceptibility testing of certain antiviral agents (e.g. anti-HIV agents) is also established but the methods and concepts are quite questions about the performance of different. The scope of this brochure will be limited to a discussion of in vitro antimicrobial susceptibility antibacterial and antifungal susceptibility testing.
testing of bacteria and fungi and the This brochure explains basic facts concerning the relevance and value of the results in guiding procedures of in vitro susceptibility testing. It provides information on the essential elements required to perform and utilize susceptibility antimicrobial therapy. testing as a tool for optimizing anti-infectious therapy. This brochure was compiled with the Prof. John TURNIDGE help of John Turnidge and Clinical Director of Microbiology and Infectious Diseases Jan Bell, SA Pathology, Adelaide, South Australia.
Jan BELLUnit Head, Antimicrobials and Multi-Resistant OrganismsSA Pathology, Women's and Children's HospitalAdelaide, South Australia 1. What is in vitro susceptibility
the empirical therapy and/or indicate appropriate alternative agents for treatment. Alternative agents may be required when resistance is detected or the patient experiences an adverse reaction to the empirical agent. Often, it is possible to identify appropriate agents Susceptibility testing measures the level at which a particular for oral step-down therapy or narrower spectrum agents likely to antimicrobial inhibits the growth of a specific microbial strain. be as effective as the broader empirical therapy.
A variety of laboratory methods can be used to measure the A second important purpose of routine in vitro susceptibility testing is in vitro susceptibility of microbial pathogens to antimicrobial to monitor the evolution of bacterial and fungal resistance. This agents. Methods should be standardized based on international role requires periodic statistical analysis of the accumulated levels of standards such as EUCAST, CLSI and ISO 20774 for antibacterials resistance per species, type of specimen, and patient, in order to (antifungal standard under development).
guide the initial empiric choice of antimicrobial therapy while awaiting "Sensitivity" is a widely used term and is essentially synonymous laboratory test results. The pattern of antimicrobial resistance by with susceptibility in the context of susceptibility testing.
ward, healthcare establishment, region or country guides empiric antibiotic therapy choices and antibiotic formulary decisions. Detailed statistical analysis enables the detection of outbreaks, especially in the hospital or long-term care setting, caused by multi- 2. Why perform
resistant bacteria, which justify investigation and appropriate infection control intervention. The detection of a new resistance pattern or in vitro susceptibility testing? a large number of patients infected with multi-resistant bacterial strains at one time and in the same place may indicate the need for implementation or change of infection control practices.
The goal of in vitro antimicrobial susceptibility testing is to assess the activity of an antimicrobial agent on a bacterial or fungal Data from routine antimicrobial susceptibility testing performed in strain in order to predict the likelihood of in vivo efficacy of clinical microbiology laboratories therefore influences the therapeutic antimicrobial therapy when the antimicrobial is administered to decisions for current and future patients.
The main purpose of routine in vitro susceptibility testing in the clinical microbiology laboratory is to guide physicians in selecting antimicrobial therapy for treatment of individual patients. The DUAL PURPOSE OF SUSCEPTIBILITY TESTING: susceptibility testing is performed on bacterial and fungal strains isolated from individual patients and presumed to be the etiology Individual (to guide the selection of their infection. The physician utilizes the susceptibility test result and modification of antimicrobial therapy) along with other available clinical information (e.g. site of infection, severity of infection, immune status of patient, co-morbidities, etc.) to select the optimal therapeutic agent for that particular patient's infection. Usually the susceptibility test results become available after initiation of empirical antimicrobial therapy. When this occurs, the susceptibility test results serve to confirm the appropriateness of 3. When should a susceptibility
test be performed? In general, assuming that standardized testing methodologies have Sometimes microbiologists cannot definitely determine if been developed, susceptibility testing is indicated for microorganisms susceptibility testing is required, without obtaining the clinical causing infections warranting antimicrobial therapy when the information that only a clinician can provide.
susceptibility cannot be reliably predicted based on the known characteristics of the organism.
For example, a commensal bacterium (e.g. Staphylococcus epidermidis) is occasionally isolated from sterile site cultures In vitro susceptibility testing methodology is well established for (e.g. blood, joint fluid, cerebrospinal fluid) due to inadequate bacteria and is considered a routine part of the culture process decontamination of the skin during specimen collection. (Clinical Laboratory Standards Institute (CLSI), 2009 and European Committee for Antimicrobial Susceptibility Testing (EUCAST), 2000, Susceptibility testing should not be performed on probable 2003). In vitro susceptibility testing is usually performed each time contaminants. However, the same S. epidermidis can cause a true bacteria considered to be responsible for a patient's infection are bloodstream infection in an immuno-compromised patient or an isolated from a clinical specimen. infection at a specific body site (e. g. prosthetic joint, cerebrospinal fluid shunt) in which case, susceptibility testing should be Susceptibility testing for yeast species is less commonly performed and is not available for all species. There are published reference methods (CLSI and EUCAST) and some commercial products are Clinical symptoms can also be a determining factor when deciding available. Each laboratory will determine the need for routine testing whether to perform susceptibility tests (e.g. diagnosis of urinary of yeast isolates from clinical specimens based on the need of the tract infection with a low bacterial count).
patient population. Reference methodology for in vitro testing of mould species is under development and currently only available from specialised mycology laboratories.
Establishing the need for susceptibility testing requires a close working relationship When the same species is isolated from specimens taken from different body sites (e.g. urine and blood) or from multiple specimens between microbiologists and clinicians.
from the same body site (e.g. multiple blood culture bottles are positive), the laboratory may elect not to perform susceptibility testing on all of the isolates from the patient. Susceptibility testing should not be routinely performed on organisms that are part of the normal bacterial flora and usually not considered pathogenic.
4. Can susceptibility and/or
resistance of bacteria to an antibiotic be predicted? Each antibiotic is characterized by a natural spectrum of antibacterial Acquired resistance is a characteristic specific to some strains, activity. This spectrum is the list of bacterial species which, in their within a naturally susceptible bacterial species, in which the naïve (wild-type) state, have their growth inhibited at a concentra- genotype has been modified by gene mutation or gene tion known to predict efficacy in vivo. These bacterial species are acquisition. Contrary to natural resistance, acquired said to be naturally susceptible to this antibiotic. Bacterial species resistances are evolutionary and their frequency is often which are not included in this spectrum are said to be naturally dependent on the amount of exposure to antibiotics. Given the evolution of acquired resistances, the natural activity spectrum is no longer sufficient to guide the choice of antibiotic therapy for numerous species. Acquired resistance results from a Natural resistance is a stable characteristic of all strains of mutation in the microbial chromosome or the acquisition of the same bacterial species. It occurs as a result of the extra-chromosomal DNA. In bacteria, the spread of resistance microorganism's genetic composition. This intrinsic resistance mechanisms occurs through vertical transmission (parent to means that the antimicrobial agent is unlikely to ever be useful daughter cells) of inherited mutations from previous genera- to treat an infection due to strains of this species. Knowledge tions as well as horizontal spread of mobile genetic elements of naturally resistant species enables prediction of the likely such as plasmids (moving between cells and often between inactivity of a molecule in relation to the identified or probable dif erent species of bacteria). bacterial pathogen. It sometimes constitutes an identification aid as some species can be characterized by their natural Acquisition of antimicrobial resistance mechanisms can render previously useful antimicrobial agents useless for treating most strains of the species (e.g. penicillin and Staphylococcus aureus). Therefore, susceptibility testing becomes essential for the • Natural resistance of Klebsiella pneumoniae to aminopenicillins (ampicillin, detection of acquired resistance. amoxicillin) due to a β-lactamase (mostly SHV-1).
• Natural resistance of Proteus mirabilis to tetracyclines and colistin (due to natural targets with reduced binding ability).
NATURAL RESISTANCE: permanent characteristic of the species, which is known and predictable.
ACQUIRED RESISTANCE: characteristic of some bacterial strains, which is evolutionary, unpredictable and justifies the need for susceptibility testing.

Examples of natural resistance (adapted from Livermore DM et al., 2001) + beta-lactamase inhibitor Cephalosporins: C1G = Natural resistance C1G : 1st generation cephalosporins C2G : 2nd generation cephalosporinsC3G : 3rd generation cephalosporins 5. What is an antibiotic clinical
6. How is the susceptibility to an
antimicrobial measured? In order to take into account the evolution of acquired resistances Most susceptibility testing is growth based. It involves exposing a and therefore provide clinicians with useful information when pure culture of a microorganism to a range of concentrations of choosing empiric antibiotic therapy, the concept of clinical spectrum an antimicrobial agent and observing the presence or absence of complements that of the natural spectrum. growth after a period of incubation. The results can vary widely The clinical spectrum is defined for each antibiotic and in some depending on the conditions of testing. It is therefore imperative jurisdictions is included in the technical package insert which is to use standardized methods.
approved during the registration of antibiotics. This spectrum is When performing in vitro susceptibility testing, technical factors initially defined by clinical breakpoints, which are devised by must be controlled by rigorous standardization of all the analysis integrating microbiological data (Minimum Inhibitory Concentra- stages (purity and density of the bacterial inoculum, medium tions – MICs, and wild-type MIC distributions), pharmacokinetics/ composition, reagents, incubation conditions, reading method and pharmacodynamics, and clinical outcome data. Regulators may biological and clinical criteria for interpretation of these results). also define susceptible species as only those species for which the Detailed and continuously updated international recommendations clinical activity of the product has been demonstrated. Strains other are available, such as those compiled by the CLSI and EUCAST. than those defined by the regulator may still be susceptible to an Quality control procedures for evaluating analytical accuracy and antimicrobial agent at the same clinical breakpoints. The treating precision must also be applied regularly in order to guarantee the clinician then takes responsibility when using that agent to treat quality of the susceptibility test. infections caused by such strains. Broth dilution was one of the earliest antimicrobial susceptibility The clinical spectrum is regularly revised to take into account the testing (AST) methods. Originally performed in test tubes (macro- evolution of acquired resistances. The prevalence of resistance may broth dilution), it has been miniaturized into microtiter plates (micro- vary geographically and with time for selected species and local broth dilution). Two-fold dilutions of the antimicrobial are made in a information on resistance is desirable, particularly when treating nutrient broth and then each well is inoculated with a standardized severe infections.
number of microorganisms. As defined by standardized methodol-ogy, the plates are incubated at a defined temperature for a defined period of time. The lowest concentration of antimicrobial with no visible growth is the minimum inhibitory concentration (MIC). CLINICAL SPECTRUM OF ACTIVITY: Modern technology has allowed miniaturization and automation of • useful to guide empiric antibiotic therapy broth dilution methodology which has reduced the time to results. In general, for routine bacterial susceptibility testing, results are • depends on the clinical breakpoints available in several hours to a day. and frequency of resistance and the in vivo activity of the antibiotic Automation increases precision, minimizes operator error, and provides traceability to AST methods.
Antimicrobial gradient diffusion is another form of AST in which a Routine Laboratory methods for MIC determination concentration gradient is established in an agar medium onto which a standardized suspension of microorganism is inoculated. This method has the capability to generate a MIC value across an extensive range of dilutions for a wide range of organism/antimicrobial Bacterial growth The disk diffusion method involves placing antimicrobial impreg- nated disks onto an agar surface that has been inoculated with a standardized suspension of microorganism. After a defined period Gradient diffusion of incubation, the zone of no growth around each disk is measured and interpreted based on published interpretive criteria, which have been developed by comparison with MIC methods.
Bacterial growth 7. What is a MIC and how
The basic measurement of the susceptibility of a microorganism to an antimicrobial agent is based on the determination of the minimum inhibitory concentration (MIC). The MIC is defined as the lowest concentration of a range of Bacterial growth antibiotic dilutions that inhibits visible growth of bacteria within according to one 4 hrs
a defined period of time.
The MIC is a measure of antimicrobial potency. It is the fundamental reference value that enables a range of antimicro- bial activity to be established for different microbial species, and to which all other testing methods are compared. Each microbial species will have a unique MIC distribution in the naïve or wild-type state (EUCAST website), i.e. not all members of a species will have exactly the same wild-type MIC.
Various laboratory techniques enable the MIC value to be measured or estimated semi-quantitatively in routine use (see opposite). Using the MIC, a tested strain can then be categorized according to its susceptibility for the antimicrobial being tested. This strain is SEMI-AUTOMATED OR AUTOMATED METHODS said to be Susceptible (S), Intermediate (I) or Resistant (R) to the antibacterial or antifungal agent.
8. What are clinical breakpoints?
the zone diameters generated by the standardized disk diffusion method on large numbers of relevant strains, using sophisticated In general, two antimicrobial concentrations, known as «break- points» or interpretive criteria determine three categories: Susceptible (S), Intermediate (I), and Resistant (R). Below (or equal to) one concentration, the clinical isolate is categorized as S, above (or equal to) the second concentration, it is categorized as R, and 9. What do categories S, I, R mean?
between these two concentrations, it may be categorized as I. For some newer antimicrobials, where resistance is very rare or un- For a given antimicrobial, a bacterial or yeast strain is classified reported, the categories may be Susceptible and Non-susceptible. according to the following criteria. Although the term "breakpoint" has been used in a wide range of contexts, it should be reserved for the values determined by the Susceptible means that the infection caused by that strain is highly methods described below. likely to respond to treatment, at the site from which the strain Breakpoints are developed through a detailed examination of MIC was isolated, with the usual antimicrobial regimen for that type data and distributions, resistance data and mechanisms, pharmaco- of infection.
kinetic and pharmacodynamic properties of the antimicrobial and available data on clinical outcome (by MIC of the infecting Intermediate means that the infection is likely to respond to higher strain if possible). They are regularly reviewed and revised as dosing regimens (where possible) or because the antimicrobial is appropriate when new information becomes available in one or concentrated at the site of infection. It is also a buffer category to more of these data sources.
ensure day-to-day test variation does not result in a resistant isolate Breakpoints are used by clinical microbiology laboratories to being categorized as susceptible or vice versa.
categorize and report clinical isolates as S, I or R, which will then assist physicians in selection of antimicrobial therapy.
Resistant means that the infection caused by that strain is unlikely AST interpretive category classifications are based on the in vitro to respond to treatment with any regimen of the antimicrobial agent.
response of an organism to an antimicrobial agent at levels Susceptible-dose dependent (SDD) correlated to blood or tissue levels attainable with the usually The "susceptible-dose dependent" category implies clinical efficacy prescribed doses of that agent. The intention is to correlate clinical when a higher than normal dosage of a drug is used and maximal breakpoints with the likely performance of the drug when used blood level achieved. This is currently reserved for certain antifungal to treat an infected patient.
drugs (e.g. fluconazole) Breakpoints are of two types: Nonsusceptible (NS) • MIC breakpoints, applied to broth or agar-based methods, This category is used for organisms that currently have only a including the automated methods such as VITEK 2. susceptible interpretive category, but not an intermediate or • zone diameter breakpoints, used to categorize test results for resistant interpretive category. It is often given to new antimicrobial the disk diffusion method. Disk diffusion breakpoints compare agents for which few or no resistant clinical isolates have yet been the MICs determined using a reference MIC testing method with 10. What criteria are used to select
Often laboratories report only a selection of the antimicrobials the antibiotics to be tested? tested, those that are most appropriate or commonly used. This selected reporting is often preferred because it assists in "antimicrobial stewardship" by guiding the prescriber to the most The selection of the antimicrobial agents to be tested must be carefully appropriate agents for the treatment of the infection and determined depending on the microbial species and their natural withholding results for agents which may be effective but are resistance, local epidemiology of acquired resistances, the site of unnecessarily broad or are only used as reserve agents. infection and local therapeutic options (formulary).
For example, the laboratory may choose "cascade reporting", e.g. Each laboratory selects which antimicrobials are appropriate to test to withhold the results of a third-generation cephalosporin and a and report for each organism isolated.
carbapenem for a strain of Escherichia coli which is susceptible to • The antibiotics tested are those of therapeutic value for the type earlier generations of cephalosporins and beta-lactamase inhibitor of infection and the body site from which the specimen has been combinations, or to modify the report to include additional agents when certain resistances are detected.
• Due to the importance of acquired resistance, it is sometimes necessary to test antibiotics which serve as resistance «markers», i.e. which are useful to detect resistance mechanisms.
EXAMPLE: Ertapenem is an excellent carbapenemase resistance marker for Entero- 12. What is antibiotic equivalence?
Equivalence is the prediction of in vivo activity for one antimicrobial based on results obtained by testing another, related antimicrobial The choice of antibiotics to be tested is made agent. In this case, only a category result (S, I, R) can be reported.
in relation to their therapeutic value and their usefulness to detect resistance mechanisms.
EXAMPLE: Equivalence between erythromycin which is tested and other macrolides (e.g. azythromycin and clarithromicin) which are not tested. The category (S, I, or R) result for the other antimicrobials can be predicted from that obtained for erythromycin. It is possible to test a restricted number 11. How are susceptibility results
of antibiotics without limiting therapeutic Susceptibility testing results are usually reported as S, I, or R for each antimicrobial tested for that isolate. Most laboratories will also include the actual MIC value, as it provides critical information that assists clinicians in making therapeutic decisions.

13. Why do some patients with
susceptible isolates fail therapy? A MIC is an in vitro measurement (a laboratory assay) that When multiple reports of the correlation of therapeutic outcome provides an estimate of antimicrobial potency. It does not take with in vitro susceptibility are examined (Rex J and Pfaller MA 2002), into account host factors, especially the kinetics of the agent in an a pattern referred to as the "90-60 rule", or natural response rate, individual host, which are just as important as susceptibility test emerges. This 90-60 rule observes that infections due to susceptible results in determining the outcome of treatment. Therefore, the isolates respond to appropriate therapy approximately 90% of the susceptibility of a microorganism in vitro does not always assure time, whereas infections due to resistant isolates (or infections successful therapy. As a corollary, resistance defi ned in vitro often, treated with inappropriate antibiotics) respond less than 60% of but not always, predicts therapeutic failure.
the time. Although there are some important exceptions, this rule is relatively robust and holds whether the outcome measurement is clinical response, bacteriological response, or mortality. Antimicrobial Drugs: why do they sometimes fail? Metabolism and Elimination Tissue penetration Adverse event profi le Underlying diseases Virulence factors SITE OF INFECTION Need for surgical 14. How do bacteria acquire
Modification of the antimicrobial target resulting in reduced binding.
The genetic mechanism of acquired resistance can be: EXAMPLE: Modification of Penicillin-Binding Proteins (PBPs) of: • The mutation of a gene involved in the mode of action of - oxacillin-resistant Staphylococcus the antimicrobial that results in an alteration in the molecule aureus (known as MRSA*).
that is the target of the antimicrobial. Most often, this type of - penicillin-resistant S. pneumoniae.
resistance mutation results in reduced antimicrobial binding to * methicillin-resistant Staphylococcus aureus the target. This mechanism is the most commonly observed for the following antibiotics: quinolones, rifampin, fusidic acid, fosfomycin, antituberculosis drugs, and sometimes cephalosporins.
EXAMPLE: Resistance to quinolones by modification of DNA gyrase in Enterobacteriaceae.
• Acquisition of resistance genes transferred from a strain Impermeability of the belonging to an identical or different species, usually on a bacterial outer membrane mobile genetic element such as a plasmid. Some antibiotics by alteration or quantitative are particularly affected by this mechanism: ß-lactams, decrease of porins.
aminoglycosides, tetracyclines, chloramphenicol, sulfonamides.
EXAMPLE: Imipenem-resistant EXAMPLE: Resistance to ampicillin in E. coli and Proteus mirabilis.
The biochemical mechanism of resistance can be due to: Production by the bacteria of Efflux mechanism: expulsion enzymes inactivating the of the molecule by active EXAMPLE: Penicillinase in staphylococci, EXAMPLE: Tetracycline-resistant extended spectrum ß-lactamase (ESBL) 15. Can antimicrobials «induce»
17. What is cross-resistance or
associated resistance? Antimicrobials do not cause resistance, but may allow resistant Cross-resistance is a resistance mechanism that affects an entire mutants to proliferate by eliminating susceptible microorganisms.
class or subclass of antibiotics.
This is known as selection pressure.
EXAMPLES:• For Streptococci, resistance to 14- and 15-membered macrolides can be The increase in the frequency of resistant strains is often linked to predicted by testing erythromycin.
increased use of a specific antimicrobial.
• Resistance to oxacillin in Staphylococci confers in vivo resistance to almost all ß-lactams.
In certain cases, a resistance mechanism can affect antibiotics from different classes.
EXAMPLE: Resistance due to impermeability to tetracyclines also affects chloramphenicol and trimethoprim.
16. Which methods enable
Resistances are said to be associated when several resistant resistance mechanisms to be mechanisms involving different antibiotic classes frequently occur together. Associated resistance is often plasmid-mediated, demonstrated in vitro? and in Gram-negative bacteria can often be encoded by genes strung together inside an integron.
For the moment, only certain techniques enable the direct detection EXAMPLE: Resistance to oxacillin in staphylococci is often associated with of biochemical mechanisms (example: detection of ß-lactamase by resistance to quinolones, aminoglycosides, macrolides and tetracyclines.
hydrolysis of the indictor ß-lactam nitrocefin) or genetic determinants of resistance (example: detection of the mecA gene responsible for staphylococcal resistance to oxacillin). Susceptibility test results can suggest the presence of a resistance mechanism.
18. Why is it necessary to interpret
EXAMPLES: • Strains of Staphylococcus aureus that test Resistant to methicillin or oxacillin susceptibility test results? (MRSA) may test Susceptible in vitro to other beta-lactams, especially cepha-losporins. However, in vivo data have demonstrated a high level of treatment failures of MRSA infections with beta-lactam therapy. Therefore the interpretation The rapid evolution of acquired resistance mechanisms by clinically is changed to "Resistant" for all beta-lactams regardless of the actual MIC value.
signifi cant bacteria and the sometimes weak expression of these • A strain of S. aureus resistant to erythromycin can test in vitro as Susceptible to resistance characteristics may require the use of tests in addition clindamycin and as positive by an Inducible Clindamycin test. Treatment with to those routinely performed. The goal of the additional tests is clindamycin can result in the selection of resistant mutants that cause inactivation to avoid categorizing bacteria as susceptible when they express of the antibiotic and treatment failure. For strains of S.aureus testing positive with the Inducible Clindamycin test, the Susceptible result for clindamycin must only low-level resistance in vitro but are likely to cause therapeutic be modifi ed to Resistant or a standard comment must be attached to the report failure in vivo. alerting the prescriber of this possibility.
To avoid this, susceptibility testing must be interpreted to discern "Interpretive reading" can also be enhanced by the use of expert even a weakly expressed resistance mechanism (by comparing systems that are capable of examining resistance profi les and making results for each antibiotic). Therefore, with appropriate interpretation, predictions about other agents not tested or their likely therapeutic a strain initially testing as susceptible will be categorized as I or R.
effi cacy (see Question 19).
Through the judicious choice of antibiotics tested, the interpretation of susceptibility test results can help detect resistance weakly expressed in vitro.
Interpretive procedure Resistance phenotype observed in vitro tests if required 19. What is the role of an Expert
Validation of a susceptibility test result requires a comprehensive indicating a rare phenotype in a given context knowledge of resistance mechanisms and antibiotic activity. An Expert The regular update of information constituting the knowledge- System is a software package designed to help integrate this knowledge base is essential. The genetic make-up of the predominant and automatically interpret susceptibility tests, check results and strains varies over time and from one geographical location suggest the necessary corrections. The Expert System contributes to the reliability of the result by: • MRSA strains have been resistant to gentamicin for a very long time, but ensuring consistency between the susceptibility test result this association is less true today due to the emergence of community- and bacterial identifi cation associated strains, e.g. USA300, which are gentamicin susceptible.
• Vancomycin-resistant enterococci are frequent in the USA but are uncommon EXAMPLE: Klebsiella pneumoniae - ampicillin S = improbable phenotype.
identifying improbable or impossible resistance phenotypes screening for important resistance mechanisms EXAMPLE: E. coli - cefazolin S - cefotaxime R = impossible phenotype.
EXAMPLE: Detection of carbapenemases in Gram-negative bacteria or detecting insuffi ciently expressed resistances heterogeneous vancomycin resistance in Staphylococcus aureus.
EXAMPLE: Detection of a third-generation susceptible Enterobacter cloacae and correction of S results to R, or appending a standard comment to a report for third-generation cephalosporins.
The Expert System: a tool for interpretation of susceptibility test results Resistance phenotype probable resistance observed in vitro 20. Some current resistance issues
What is the probability of infection by Enterobacteriaceae with extended spectrum β-lactamase (ESBL) strains?In patients from the community, the frequency of this type Are bacteria responsible for community-acquired urinary of multi-resistant bacteria (i.e. with acquired resistance to tract infections affected by antibiotic resistance? numerous antibiotics) used to be linked to a previous hospital stay. These bacteria were mainly found in hospitals, where Although bacterial resistance is more frequent in hospitals their multi-resistance gives them a selective advantage. than in the community, bacteria that are most often found in Generally transmitted from one patient to another in the community-acquired pathologies, such as E. coli, can acquire same healthcare unit (hospital, clinic, nursing home, etc.), they were and still are responsible for nosocomial infections.
For example, in 2005 the level of acquired resistance of E.coli to antibiotics frequently used for the treatment of urinary tract infections were: In recent years, however, ESBL-producing E. coli have begun to emerge in community strains worldwide (Oteo J et al., 2010). These strains carry ß-lactamases of the CTX-M type, rather Fluoroquinolones than the more common TEM or SHV type found in hospital- associated strains. One clone in particular, O25:H4-ST131, Source: TRUST 2007 encoding CTX-M-15, seems to have spread widely across the world (Nicolas-Chanoine M-H et al, 2008). Not unexpectedly, they are most frequently isolated from urine. Many strains are What is Community-associated MRSA? resistant to multiple anti-microbial agents and are challenging The first reported cases of CA-MRSA began to appear in the for outpatient management. They are adding to the overall mid-1980s in Australia and New Zealand, and in the mid-1990s burden of ESBLs in hospitals because they require the same in the United States, the United Kingdom, Europe and Canada. infection control interventions.
These cases were notable because they involved people who had not been exposed to a healthcare setting.
Why is it essential to check the susceptibility of This increase in the incidence of MRSA infection has been S. pneumoniae to antibiotics and notably to penicillin G? associated with the recognition of new MRSA clones known For over 3 decades, pneumococcal non-susceptibility to as community-associated MRSA (CA-MRSA). CA-MRSA strains penicillin G has continuously increased in many countries infect a different group of patients, cause different clinical (less than 1% in 1985, 10-50% in the mid 2000's (Linares J syndromes, and differ in antimicrobial susceptibility patterns et al., 2010). This resistance is often associated with resist- compared to healthcare-associated MRSA (HA-MRSA) strains, ance to other antibiotics (e.g. tetracyclines, macrolides). CA-MRSA strains can spread rapidly among healthy people This emerging resistance impacts empirical antibiotic therapy in the community and are now a frequent cause of infections for acute otitis media, sinusitis and bronchopulmonary in healthcare environments as well. The clinical spectrum of infections, as well as meningitis which are often caused by infectious syndromes associated with CA-MRSA strains ranges S. pneumoniae. In some cases, resistance of S. pneumoniae from a commensal state to severe, overwhelming infections, to penicillin G may indicate resistance to other ß-lactam anti- especially skin and pulmonary. (David MZ and Daum RS 2010).
biotics, making determination of susceptibility necessary.
Is there any problem with resistance to antifungal agents? Patient characteristics, antifungal prophylaxis, and other factors appear to have contributed to a change in the spectrum of n Clinical and Laboratory Standards Institute: Performance Standards for invasive fungal pathogens. Infections with more resistant Antimicrobial Disk Susceptibility Tests - Tenth Edition; M2-A10, 2009, CLSI, Wayne, species of Candida (e. g. C. glabrata, C. krusei), and Aspergillus PA. n Clinical and Laboratory Standards Institute: Methods for Dilution Antimicrobial (e.g. A. terreus) as well as non-Aspergillus moulds appear to Susceptibility Tests for Bacteria that Grow Aerobically. Approved Standard– be on the rise, at least among certain populations. (Pfaller MA Eighth Edition; M7-A7, 2009, CLSI, Wayne, PA. n Clinical and Laboratory Standards Institute: Reference Method for Broth Dilution Antifungal Susceptibility et al., 2006). These species are resistant or less susceptible to Testing of Yeasts. Approved Standard–Third Edition; M27-A3, 2008, CLSI, Wayne, some commonly used antifungal agents. With this change in PA. n Clinical and Laboratory Standards Institute: Reference Method for Broth epidemiology as well as an increased choice in available Dilution Antifungal Susceptibility Testing of Yeasts. Third Informational Supplement; antifungal agents from which to choose, antifungal M27-S3, 2008, CLSI, Wayne, PA. n Clinical and Laboratory Standards Institute: susceptibility testing is becoming more of a necessity, Reference Method for Broth Dilution Antifungal Susceptibility Testing of Filamentous especially for the commonly isolated yeasts.
Fungi. Approved Standard–Second Edition; M38-A2, 2008, CLSI, Wayne, PA. n Clinical and Laboratory Standards Institute: Analysis and Presentation of Cumulative Antimicrobial Susceptibility Test Dta; Approved Guideline–Second Edition; M39-A2, 2005, CLSI, Wayne, PA. n Clinical and Laboratory Standards Institute: Performance Standards for Antimicrobial Disk Testing. Twentieth Informational Supplement; M100-A7, 2010, CLSI, Wayne, PA. n David MZ and Daum RS. Community-Associated Methicillin-Resistant Staphylococcus aureus: Epidemiology and Clinical Consequences of an Emerging Epidemic. Clin Microbiol Rev, 2010; 23:616-687. n European Committee for Antimicrobial Susceptibility Testing (EUCAST) of the European Society of Clinical Microbiology and Infectious Diseases (ESCMID). Determination of minimum inhibitory concentrations (MICs) of antibac-terial agents by agar dilution. Clin Microbiol Infect, 2000; 6:509-15. n European Antibiotic susceptibility testing, at the interface between the Committee for Antimicrobial Susceptibility Testing (EUCAST) of the European Society clinical diagnosis and the therapeutic decision, is a key element of Clinical Microbiology and Infectious Diseases (ESCMID). Determination of essential for guiding both microbiologically-documented and minimum inhibitory concentrations (MICs) of antibacterial agents by broth dilution. empiric antibiotic therapy. Not only is the susceptibility test result Clin Microbiol Infect, 2003; 9:1-7. n EUCAST definitions of clinical breakpoints of immediate interest for the clinician to guide selection of and epidemiological cut-off values. 2010 At: http://www.srga.org/Eucastwt/ antimicrobial therapy, but it also plays a role as an epidemiological eucastdefinitions.htm. n EUCAST disk diffusion test. 2010. At: http://www. eucast.org/eucast_disk_ diffusion_test/ n Liñares J, Ardanuy C, Pallares R, Fenoll A. surveillance tool for local bacterial and fungal resistance patterns.
Changes in antimicrobial resistance, serotypes and genotypes in Streptococcus pneu- The evolution of resistance, as well as the development of new moniae over a 30-year period. Clin Microbiol Infect 2010; 16:402-10. n Livermore antibiotics and laboratory techniques make a close working DM, Winstanley TG, Shannon KP. Interpretive reading: recognizing the unusual and relationship between the microbiologist and the clinician more inferring resistance mechanisms from resistance phenotypes. JAC 2001;48:87-102 n Nicolas-Chanoine M-H, Blanco J, Leflon-Guibout V, et al. Intercontinental necessary now than ever before.
emergence of Escherichia coli clone O25:H4-ST131 producing CTX-M-15. J Antimicrob Chemother 2008; 61:273-81. n Oteo J, Pérez-Vásquz M, Campos J. Extended-spectrum beta-lactamase producing Escherichia coli : changing epidemiology and clinical impact. Curr Opin Infect Dis 2010; 23:320-6. n Pfaller MA, Pappas PG, Wingard JR. Invasive Fungal Pathogens: Current Epidemiological Trends. CID 2006; 43:S3–14. n Rex J and Pfaller MA. Has Antifungal Susceptibility Testing Come of Age? Clin Infect Dis 2002; 35:982–9.
With over 45 years of experience working in close partnership with microbiologists, bioMérieux is acknowledged as a world leader in the field of microbial identification (ID) and antimicrobial susceptibility testing (AST).
Microbial Identification bioMérieux's expertise and extensive and Antimicrobial experience are recognized by microbiology Susceptibility Testing laboratories worldwide. We provide a large range of innovative diagnostic solutions to help ensure patients receive timely, appropriate antimicrobial therapy.
VITEK® 2 Technology Automated, rapid ID/AST system Agar gradient method Recognized as the world's leading ID/AST system, VITEK® 2 offers reliable same-day ID/AST results for the majority of organisms encountered Provides MIC values and tests a broad range of antimicrobial concentrations.
With over 45 years of experience working in the routine clinical lab.
Etest is recognized as a leader in MIC testing, and as the AST platform of Advanced Expert System™ (AES) choice for supplementing and/or confi rming primary AST methods in serious in close partnership with microbiologists, As part of the VITEK® 2 Technology, AES software uses MIC values to infections or when additional information is needed: review possible resistance patterns. The AES automatically reviews and • diffi cult organisms (e.g. fastidious, anaerobic, etc.), bioMérieux is acknowledged as a world leader interprets each susceptibility result to check for known, uncommon, and • critical infections/specimens, low-level resistance mechanisms.
• detection/confi rmation of antimicrobial in the fi eld of microbial identifi cation (ID) and resistance mechanisms, etc.
antimicrobial susceptibility testing (AST).
bioMérieux's expertise and extensive experience are recognized by microbiology Myla™ is an innovative middleware solution laboratories worldwide. We provide a large Identifi cation strips that is changing the way lab microbiology information is managed. Myla plays a pivotal role range of innovative diagnostic solutions Considered the reference to which all other ID methods are compared, API in bioMérieux's Full Microbiology Lab Automation strips cover almost all groups of commonly occurring bacteria and yeasts to help ensure patients receive timely, to deliver faster results to clinicians isolated from clinical specimens. Identifi cation products that provide an for improved patient care option when additional information is needed on identifi cation.
appropriate antimicrobial therapy.
The apiweb™ internet platform provides interpretive reading for each identifi cation strip.
Mass spectrometry system for rapid ID Manual / automated ID/AST strips VITEK® MS identifi es microorganisms in minutes instead of ID 32 and rapid ID 32 strips are automated versions of API strips providing hours, using cutting-edge mass spectrometry technology. microbial identifi cation in 18-24 hours or 4 hours respectively. They are With state of the art, user-friendly software and barcode associated with extensive databases for expert analysis. readers, sample preparation is fast. Automated ATB and rapid ATB susceptibility tests meet CLSI/NCCLS and • Review your results remotely, at your convenience, EUCAST standards.
• Fully integrated with the AST result from VITEK® 2

Source: http://www.biomerieux.com.br/sites/subsidiary_uk/files/ast_booklet_gb_final.pdf