Predicting Virulence in Diabetic Foot

BY MICHELE G. SULLIVAN

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Researchers have discovered five bacterial genes that predict virulent infections in diabetic foot ulcers caused by Staphylococcus aureus.

"Our study suggests that testing for the presence of the five genes may not only help clinicians to distinguish grade 1 from grades 2-4 ulcers, but also predict wound outcome," Dr. Albert Sotto and his colleagues wrote in the December issue of Diabetes Care.

Running a genome screen on bacterial samples could be a fast, easy, and inexpensive way to predict outcome in such infections. The one-step assays give results in 2 hours and only cost about $5 each, wrote Dr. Sotto of the University of Montpellier, Nimes, France (Diab. Care 2008;31:2318-24).

The prospective study included 118 patients with diabetic foot ulcers infected by S. aureus; none of the patients had been on antimicrobial therapy for the previous 6 months. Initially, 20% of the ulcers were classified as grade 1; over the 6-month follow-up period, 9 of these healed and 15 worsened.

A total of 132 S. aureus strains were obtained during the initial culture and over the 6-month follow-up period; 51 were methicillin-resistant S. aureus (MRSA). All the samples were genotyped. A logistic regression analysis identified five genes significantly associated with ulcers of grades 2-4 (sea, sei, lukE, hlgv, and cap8). The presence of these genes had a sensitivity of 98% and a specificity of 87% for differentiating grade 1 ulcers from ulcers of grades 2-4.

The researchers then assessed the pathogenicity of different strains by feeding them to Caenorhabditis elegans nematodes. Normally, the worms live for about 2 weeks, but their life span can be significantly shortened if they are fed pathogenic bacteria. The investigators randomly selected one MRSA strain and two methicillin-susceptible (MSSA) strains from each category of ulcer, as well as strains from slow- and fast-healing grade 1 ulcers. Highly virulent strains killed half of the worms in less than 2 days, while less virulent strains killed half in more than 3 days.

All but one of the strains isolated from grades 2-4 ulcers were highly virulent; all but one isolated from grade 1 ulcers were less virulent. "Interestingly," the authors wrote, "no significant differences in the killing potentials of MRSA and MSSA were observed within ulcers of the same grade."

Strains from the healing ulcers had significantly longer killing times than those from non-healing ulcers, indicating lower virulence.

"The use of the C. elegans model demonstrated that S. aureus virulence was not dependent on methicillin resistance as suggested previously," the authors noted. "We can thus speculate that within the same bacterial species there are pathogens with different virulence potential against the host."

The study was sponsored by the French government and the University of Montpellier. The authors reported no conflicts of interest.

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