Answer:

Infections caused by Pasteurella multocida are usually treated empirically with penicillin, ampicillin, or amoxicillin. Because rare isolates from humans produce beta-lactamase, susceptibility testing using an MIC method, such as broth microdilution, could be helpful in identifying resistant strains. Since NCCLS has not defined a method for testing or reporting results for P. multocida, only MIC results without interpretation should be reported. It is not clear which, if any, of the current interpretive criteria for ampicillin or penicillin would be suitable for identifying emerging resistance to penicillin in P. multocida. According to recent data, the MIC90 for penicillin for P. multocida is 0.25 µg/ml (meaning 90% of isolates encountered would have penicillin MICs at or below 0.25 µg/ml). Based on interpretive criteria for staphylococci, pneumococci, or enterococci, a result of 0.25 µg/ml would indicate a resistant, intermediate, or susceptible result, respectively. Thus, MIC results alone, perhaps with a recommendation for an Infectious Disease consult, are best. Secondary agents that might be prescribed include fluoroquinolones, extended-spectrum cephalosporins, or chloramphenicol.

The following articles provide additional information:

1. Goldstein, E. J., D. M. Citron, C. V. Merriam, Y. Warren, and K. Tyrrell. 2000. Comparative in vitro activities of ABT-773 against aerobic and anaerobic pathogens isolated from skin and soft-tissue animal and human bite wound infections. Antimicrob Agents Chemother. 44:2525-9.
    
2. Naas, T., F. Benaoudia, L. Lebrun, and P. Nordmann. 2001. Molecular identification of TEM-1 beta-lactamase in a Pasteurella multocida isolate of human origin. Eur J Clin Microbiol Infect Dis. 20:210-3.
    
3. Gilbert, D.N., R.C. Moellering, and M.A. Sande (eds). 2001. The Sanford Guide to Antimicrobial Therapy, 2001. 31st ed. Antimicrobial Therapy, Inc., Hyde Park, VT.