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 minimum inhibitory concentration (MIC) obtained during antimicrobial susceptibility testing is helpful in indicating the concentration of antimicrobial agent required at the site of infection necessary to inhibit the infecting organism. The MICs are accompanied by interpretive categories (ie, susceptible, intermediate, or resistant) when applicable.
Determining the in vitro susceptibility of anaerobic bacteria involved in human infections
A "susceptible" category result and a low minimum inhibitory concentration value indicate in vitro susceptibility of the organism to the antimicrobial tested.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
In vitro susceptibility does not guarantee clinical response. Therefore, the decision to treat with a particular agent should not be based solely on the antimicrobial susceptibility testing result.
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.
Results are reported as minimum inhibitory concentration (MIC) in mcg/mL and as susceptible, intermediate, or resistant according to the Clinical and Laboratory Standards Institute (CLSI) guidelines.
In some instances (vancomycin, ciprofloxacin, and minocycline) an interpretive category cannot be provided based on available data and the following comment will be included: "There are no established interpretive guidelines for agents reported without interpretations."
The "susceptible" category implies that isolates are inhibited by the usually achievable concentrations of antimicrobial agent when the dosage recommended to treat the site of infection is used, resulting in likely clinical efficacy.
The "intermediate" category includes isolates with antimicrobial agent minimum inhibitory concentrations (MICs) that approach usually attainable blood and tissue levels, and for which response rates may be lower than for susceptible isolates.
Note: The intermediate category implies clinical efficacy in body sites where the drugs are physiologically concentrated or when a higher than normal dosage of a drug can be used. This category also includes a buffer zone, which should prevent small, uncontrolled, technical factors from causing major discrepancies in interpretations, especially for drugs with narrow pharmacotoxicity margins.
The "resistant" category implies that the isolates are not inhibited by the usually achievable concentrations of the agent with normal dosage schedules and/or that demonstrate MIC that fall in the range where specific microbial resistance mechanisms are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.
(Clinical and Laboratory Standards Institute: Performance Standards for Antimicrobial Susceptibility Testing. 26th Informational Supplement. CLSI document M100S. Wayne, PA, 2016)
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Rosenblatt JE, Brook I: Clinical relevance of susceptibility testing of anaerobic bacteria. Clin Infect Dis 1993;16(Suppl 4):S446-S448
2. Clinical and Laboratory Standards Institute: Performance Standards for Antimicrobial Susceptibility Testing. 26th Informational Supplement. CLSI document M100S. Wayne, PA, 2016
3. Jenkins SG, Schuetz AN: Current concepts in laboratory testing to guide antimicrobial therapy. Mayo Clin Proc 2012 Mar;87(3):290-308