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We currently do not perform anaerobe susceptibilities routinely but we call the physician when we isolate an anaerobe from a blood or critical source. The phone call seems to prompt a "yes" answer regardless of what the anaerobe is. Are there some anaerobes where, even when isolated from a critical source, susceptibilities need not be performed?
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This is a very timely question. Similar to what we are seeing with aerobic bacteria, anaerobes are also developing resistance to antimicrobial agents.
Resistance to clindamycin has increased to 20-40% in the Bacteroides fragilis group and to 15-20% in anaerobic gram-positive cocci and Prevotella species.
Resistance to cephalosporins is common among all groups.
Although infrequent, resistance to the most potent drugs such as imipenem, metronidazole and piperacillin/ tazobactam also occurs in the B. fragilis group.
Even some strains of Fusobacterium nucleatum may produce beta-lactamase resulting in high-level resistance to penicillin. Beta-lactamase production is also common in Prevotella and even some strains of Porphyromonas.
While Clostridium perfringens is still susceptible to most antimicrobial agents, resistance to other classes of antimicrobials is common among other species isolated from blood and other serious infections.
Thus susceptibilities among anaerobes are not predictable and for patients with serious anaerobic infections, it is understandable that the clinicial would want the empiric choice of therapy validated by the lab.
The CLSI (M11-A7)recommends that susceptibility testing be performed on isolates from sterile sites.
We hope that we were able to answer your question.
Please visit us again at the American Society for Microbiology.
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