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MACCL
89218
and generally do not cause infection. When usual skin and mucosal barriers are penetrated and in an
anaerobic environment, they can behave as pathogens. Anaerobes grow aggressively in the body under
anaerobic conditions and may possess a variety of virulence factors including capsules and extracellular
enzymes. They also can develop resistance to antimicrobials by producing beta-lactamase and other
modifying enzymes and by alterations in membrane permeability and structure of penicillin-binding
proteins. Because anaerobic bacteria are a significant cause of human infection and they are often resistant
to commonly used antimicrobials, susceptibility testing results are useful to clinicians. The following
antimicrobials are routinely tested for the various groups of anaerobes. Anaerobic gram-negative bacilli
Beta-lactamase positive Piperacillin/tazobactam, ertapenem, metronidazole, clindamycin Beta-lactamase
negative Penicillin, clindamycin, metronidazole Anaerobic cocci and nonsporeforming gram-positive
bacilli, Clostridium perfringens Penicillin, clindamycin, metronidazole Clostridium species other than
Clostridium perfringens Penicillin, clindamycin, metronidazole, ertapenem, piperacillin/tazobactam
Propionibacterium species Penicillin, moxifloxacin, minocycline Actinomyces species Penicillin,
clindamycin Additionally, the following antimicrobials may be individually requested or added to the
above panels: -Amoxicillin/clavulanic acid, ceftriaxone, meropenem, ciprofloxacin, and vancomycin
Useful For: Directing antimicrobial therapy for anaerobic infections
Interpretation: Minimal inhibitory concentration (MIC) and interpretive category (susceptible,
intermediate, or resistant) results are reported. Note that vancomycin, ciprofloxacin, and minocycline
will have no interpretation attached to the MIC. Resistant results indicate an organism is resistant to the
antimicrobial tested and that the agent should not be used for treatment. Intermediate results indicate
that treatment with high doses might well be successful. Susceptible results indicate that therapy with
usual doses is appropriate.
Reference Values:
Minimum inhibitory concentration (MIC) results are reported in mcg/mL with an interpretation of
S-susceptible, R-resistant, or I-intermediate. Vancomycin, ciprofloxacin, and minocycline are reported
as MIC values only (without interpretation).
Clinical References: Rosenblatt JE, Brook I: Clinical relevance of susceptibility testing of
anaerobic bacteria. Clin Infect Dis 1993;16(Suppl 4):S446-S448
Antimicrobial Susceptibility, Mycobacterium avium Complex,
Clarithromycin
Clinical Information: Mycobacterium avium complex (MAC) organisms are the most common
environmental mycobacteria causing human disease. Clinical presentation can include pulmonary
disease, lymphadenitis, and disseminated disease. The Clinical Laboratory Standards Institute (CLSI),
the American Thoracic Society (ATS), and the Infectious Diseases Society of America (IDSA)
recommend that MAC isolated from previously untreated patients should be tested for susceptibility to
clarithromycin because this drug acts as the class agent for the newer macrolides and shares
cross-resistance with azithromycin. Clarithromycin susceptibility testing should also be performed for
MAC isolates from patients who fail macrolide therapy or prophylaxis. Testing of other drugs for MAC
is not recommended by the CLSI or ATS/IDSA because no correlation has been established between in
vitro susceptibility testing and clinical response to agents other than macrolides.
Useful For: Determining resistance of Mycobacterium avium complex to clarithromycin
Interpretation: Using broth microdilution at pH 7.3 to 7.4, a clarithromycin minimum inhibitory
concentration (MIC) of < or =8 mcg/mL is susceptible, a MIC=16 mcg/mL is intermediate, and a MIC
> or =32 mcg/mL is resistant.
Reference Values:
Results are reported as MIC in mcg/mL and interpreted as susceptible, intermediate, or resistant
according to Clinical Laboratory Standards Institute (CLSI) guidelines.
Clinical References: 1. Griffith DE, Brown-Elliot BA, Langsjoen B, et al: Clinical and molecular
analysis of macrolide resistance in Mycobacterium avium complex lung disease. Am J Respir Crit Care
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