Susceptibility Testing And Their Interpretation See Chapter

There are currently three susceptibility testing methods for anaerobic bacteria that provide reproducible results that correlate with a reference standard: the agar dilution, the broth micro dilution, and the the E-test (AB Biodisk, soloans, Sweden) (4,8,14). At present, there are no automated methods. The CLSI currently recommends that for surveillance purposes to monitor for resistance trends, the agar dilution method is used to test at least 100 anaerobic isolates per year at individual hospitals. The agar dilution method is reproducible, labor-intensive, and allows for batch testing of up to 30 anaerobic isolates at one time against a single antibiotic.

The broth micro dilution panel is a convenient, user-friendly method that determines susceptibilities of a single anaerobic isolate to several antibiotics at the same time. This panel provides results that correlate well with those of the agar dilution standard for anaerobes that grow well in broth supplemented with CLSI-recommended Brucella blood agar. Both methods are equivalent for determining antimicrobial susceptibilities of B. fragilis group isolates, but the CLSI does not recommend the broth micro dilution for non-bacteroides anaerobes unless the laboratory validates their results against the agar dilution standard.

The E-test (AB Biodisk, soloans, Sweden) is a simple user-friendly, gradient method that delivers accurate results. However, the limitation of this test is that each test strip is used only for a single isolate. Both the broth micro dilution and the the E-test methods are adequate for testing individual isolates and can provide guidance in selection of therapy on the basis of positive culture results.

The susceptibility results are considered when antibiotic treatment is chosen. The results are expressed as the minimal inhibitory concentration (MIC) or by providing degrees of susceptibility as sensitive (S), intermediate (I), and resistant (R). These sensitivity breakpoints are established by the CLSI and the U.S. Food and Drug Administration (FDA). The MIC value obtained by any of the methods does not represent an absolute number because the accurate MIC is actually between the obtained MIC and the next-lower or -higher test concentration. Also a two-fold difference on successive testing is allowed in all dilution-based susceptibility methods (8). The phenotypic interpretation of the results of the MIC tests as sensitive, intermediate, and resistant are based on the MIC distribution of the bacterial population, the antibiotic pharmacokinetics and pharmacodynamics, and the verification of antibiotic efficacy in clinical studies. The dosages of antibiotics administered for infections caused by anaerobic organisms whose MICs are at or near the S or I breakpoints, should be maximal to overcome their lower penetration and instability at the site of most infections (8).

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