Interpretive Handbook

Test 56031 :
Antimicrobial Susceptibility, Anaerobic Bacteria, MIC

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Anaerobic bacteria make up a large part of the human body's normal flora and generally do not cause infection. When usual skin and mucosal barriers are penetrated and in an anaerobic environment, these bacteria 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 nonspore-forming 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 Suggests clinical disorders or settings where the test may be helpful

Directing antimicrobial therapy for anaerobic infections

Interpretation Provides information to assist in interpretation of the test results

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.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Susceptibility test results are only one of many factors that influence the outcome of antimicrobial therapy of infections.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

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 Provides recommendations for further in-depth reading of a clinical nature

Rosenblatt JE, Brook I: Clinical relevance of susceptibility testing of anaerobic bacteria. Clin Infect Dis 1993;16(Suppl 4):S446-S448