There has been quite a tradition for including Gram negative anaerobic cover for patients suspected to have aspiration pneumonia. There are virtually no randomised trials that directly address the issue with nearly all such trials including agents that have Gram negative anaerobic cover across all treatment arms.   One exception was a small trial from 1997 in a paediatric population that compared penicillin with clindamycin (42 patients, no significant outcome difference).

Jason Kwong et al unpacked the debate well in a 2011 perspective piece in the Medical Journal of Australia. They failed to find strong support from either microbiological or clinical studies.  Their summary recommendations are shown below .

In the current Australian Therapeutic Guidelines: Antibiotic (edition 15, 2014), an entire chapter is devoted to management of aspiration pneumonia in patients presenting from the community or early (within 72 hrs)  of admission. For mild or moderate aspiration pneumonia (community or nursing home-associated), amoxycillin or IV benzylpenicillin is recommended with addition of metronidazole reserved for moderate disease with risk factors (including alcoholism or severe periodontal disease*) or severe disease. They rightly emphasise the need to consider Staphylococcus aureus (including MRSA) as a cause in some patients (e.g. severe disease with toxicity, cavitatory disease and/or sputum with profuse Gram positive cocci in clusters).

In terms of local practice, there has been a tendency to turn, perhaps without sufficient thought, to use of piperacillin+tazobactam for patients where aspiration pneumonia is suspected, even in community cases. This is an approach that we discourage – simple benzylpenicillin +/- metronidazole as above will suffice in nearly all cases.

* It is accepted that patients with periodontal disease there is a shift in oral flora from a predominant Gram positive picture to higher prevalence of Gram negatives, including Bacteroides, Prevotella and similar anaerobes.  Gram negative anaerobes usually produce penicillinases , causing resistance to ampicillin and benzylpenicillin. These betalactamases are inhibited thought by clavulanate (in amoxycillin+clav) or tazobactam (in piperacillin+tazo). 

Image from Medscape