In this episode, I’ll discuss double anaerobic coverage.

Anaerobic pathogens

Anaerobic pathogens are found in the mouth and the lower gastrointestinal tract. Oral anaerobes are the gram positives Peptococcus and Peptostreptococcus. Intestinal anaerobes are Fusobacterium, Prevotella melaninogenica, and more commonly Bacteroides fragilis.

Oral anaerobes are easily covered with penicillins or clindamycin – think “Anything that a dentist would prescribe.”

Intestinal anaerobes are not as easily covered. Antibiotic coverage against Bacteroides is excellent for metronidazole, piperacillin-tazobactam, and carbapenems. Resistance against these agents is generally less than 1%.

Coverage against Bacteroides is not as good for ampicillin-sulbactam, moxifloxacin, and clindamycin with resistance rates exceeding 35% for the latter 2 of these agents.

Double anaerobic coverage is the combination of 2 or more of the following agents:

Ampicillin/sulbactam

Carbapenems (Doripenem, Ertapenem, Imipenem, Meropenem)

Cefotetan

Cefoxitin

Clindamycin

Metronidazole

Moxifloxacin

Piperacillin/tazobactam

Ticarcillin/clavulanate

Tigecycline

A common antibiotic stewardship intervention is the elimination of double anaerobic coverage. I approach the question of double anaerobic coverage this way:

If an agent with less than 1% chance of resistance vs Bacteroides is already being used, there is no need to add a 2nd anaerobic agent.

If an agent with a high chance of resistance vs Bacteroides is being used, it makes more sense to switch to an agent with low resistance rather than to add a 2nd anaerobic agent.

Examples

For the combination of clindamycin + piperacillin/tazobactam, I would stop the clindamycin since it has a higher resistance rate.

For the combination of metronidazole + a carbapenem, I would stop the metronidazole since the carbapenem covers aerobic pathogens in addition to anaerobic ones.

Guideline confusion

Anyone reading the IDSA guidelines for intra-abdominal infections might be led to believe that double anaerobic coverage is indicated. Take a look at the recommendation:

Reading this recommendation, it is not clear whether the guideline is suggesting metronidazole be added to the other anaerobic agents or only to ceftazidime or cefepime.

However, in Table 3 of the guidelines it is clear that metronidazole is only to be combined with ceftazidime or cefepime, and not with carbapenems or piperacillin-tazobactam.

Does double anaerobic coverage ever make sense?

There are two scenarios where double anaerobic coverage is appropriate:

1. When a patient has an infection that requires anaerobic coverage AND they require metronidazole for Clostridium difficile infection.

2. When a patient has necrotizing fasciitis and clindamycin is added to reduce bacterial toxin production.

If you like this post, check out my book – A Pharmacist’s Guide to Inpatient Medical Emergencies: How to respond to code blue, rapid response calls, and other medical emergencies.

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