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Clinical and Laboratory Standards Institute (CLSI) MIC Breakpoints For Susceptibility Testing Of Anaerobes

Anaerobic bacteria predominate in normal skin and the bacterial flora of mucous membranes and are therefore a common cause of endogenous infection. Infections caused by anaerobic bacteria are common and may be serious or life threatening. Their recovery requires proper methods of collection, transportation, and cultivation. Diagnosis and management of anaerobic bacterial infections are complicated by their slow growth, polymicrobial nature, and resistance to antimicrobial agents.
Susceptibility testing of anaerobe should be performed to monitor local, national, or international susceptibility patterns periodically and to determine activity of new agents. Testing of an individual patient's isolates may be required for serious or chronic infection, such as bacteremia, osteomyelitis, or brain abscess, or, or when serious infection is due to an organism with unpredictable antibiotic susceptibility
Problems in antimicrobial susceptibility testing of anaerobic bacteria include lack of standardization of techniques, fastidiousness of bacterial growth, appropriate choice of antimicrobial susceptibility breakpoints, lengthy test period, and limited clinical correlation of in vitro observations. Treatment usually is based on statistical knowledge of suspected or proven pathogens and antimicrobial susceptibility patterns as tested in batches in reference laboratories.
Till recently there were no standard testing method, because of the lack of reproducibility and inability to grow all clinically significant anaerobe on a standard medium. However Clinical and Laboratory Standards Institute (CLSI) 2011 has given MIC breakpoints for both agar dilution and broth dilution; disc diffusion and broth disc elution are inappropriate. According to this recent document, members of the Bacteroides fragilis group are presumed to be resistant to penicillins. However other gram negative rods may be screened for beta-lactamase activity with a chromogenic cephalosporin (Nitrocefin disc). In addition, the routine primary testing panel includes beta-lactam/beta-lactamase inhibitor combination, carbapenems, clindamycin and metronidazole. Hence for the proper management of anaerobic infection, ancillary therapy such as drainage and debridement along with antibiotic therapy are of great importance.
The recommended agar / broth for testing anaerobe are Brucella agar/broth supplemented with hemin, Vitamin K1 and lysed horse blood. The quality control strains recommended are Bacteroides fragilis ATCC 25285, Bacteroides thetaiotaomicron ATCC 29741, Clostridium difficile ATCC 700057 and Eubacterium lentum ATCC 43055. It is recommended to test any 2 of these strains for agar dilution; test 1 for a single broth dilution test.

Table1: Breakpoints for antibiotics that are routinely tested and reported for anaerobes


MIC Interpretive Standard




Amoxicillin-clavulanic acid

≤ 4/2


≥ 16/8


≤ 8/4


≥ 32/16




≥ 128/4


 ≤ 4


≥ 16


≤ 8


≥ 32


≤ 2


≥ 8













Dr Sadia Omer







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