Bacteriology of moderate-to-severe diabetic foot infections and in vitro activity of antimicrobial agents

As part of a United States-based multicenter clinical trial, conducted from 2001 to 2004, that compared ertapenern to piperacillin-tazobactam for the treatment of moderate-to-severe diabetic foot infections (DFIs), we obtained 454 pretreatment specimens from 433 patients. After debridement, the investigators collected wound specimens, mostly by curettage or biopsy, and sent them to the R. M. Alden Research Laboratory for aerobic and anaerobic culture. Among the 427 positive cultures, 83.8% were polymicrobial, 48% grew only aerobes, 43.7% had both aerobes and anaerobes, and 1.3% had only anaerobes. Cultures yielded a total of 1,145 aerobic strains and 462 anaerobic strains, with an average of 2.7 organisms per culture (range, I to 8) for aerobes and 2.3 organisms per culture (range, I to 9) for anaerobes. The predominant aerobic organisms were oxacillin-susceptible Staphylococcus aureus (14.3%), oxacillin-resistant Staphylococcus aureus (4.4%), coagulase-negative Staphylococcus species (15.3%), Streptococcus species (15.5%), Enterococcus species (13.5%), Corynebacterium species (10.1%), members of the family Enterobacteriaceae (12.8%), and Pseudonumas aeruginosa (3.5%). The predominant anaerobes were gram-positive cocci (45.2%), Prevotella species (13.6%), Porphyromonas species (11.3%), and the Bacteroides fragilis group (10.2%). Pure cultures were noted for 20% of oxacillin-resistant Staphylococcus aureus cultures, 9.2% of Staphylococcus epidermidis cultures, and 2.5% of P. aeruginosa cultures. Two or more species of Staphylococcus were present in 13.1% of the patients. Ertapenem and piperacillin-tazobactam were each active against > 98% of the enteric gram-negative rods, methicillin-sensitive S. aureus, and anaerobes. Among the fluoroquinolones, 24% of anaerobes, especially the gram-positive cocci, were resistant to moxifloxacin; 27% of the gram-positive aerobes but only 6% of the members of the family Enterobacteriaceae were resistant to levofloxacin. Moderate-to-severe DFIs are typically polymicrobial, and almost half include anaerobes. Our antibiotic susceptibility results can help to inform therapeutic choices.