Polymicrobial Foot Infection Patterns Are Common and Associated With Treatment Failure.

Background: That foot infections are predominately polymicrobial has long been recognized, but it is not clear if the various species co-occur randomly or in patterns. We sought nonrandom species co-occurrence patterns that might help better predict prognosis or guide antimicrobial selection.

Methods: We analyzed tissue (bone, skin, and other soft tissue), fluid, and swab specimens collected from initial foot infection episodes during a 10-year period using a hospital registry. Nonrandom co-occurrence of microbial species was identified using simple pairwise co-occurrence rates adjusted for multiple comparisons, Markov and conditional random fields, and factor analysis. A historical cohort was used to validate pattern occurrence and identify clinical significance.

Results: In total, 156 unique species were identified among the 727 specimens obtained from initial foot infection episodes in 694 patients. Multiple analyses suggested that is negatively associated with other staphylococci. Another pattern noted was the co-occurrence of alpha-hemolytic , , , , , or , and absence of both and . Patients in a historical cohort with this latter pattern had significantly higher risk-adjusted rates of treatment failure.

Conclusions: Several nonrandom microbial co-occurrence patterns are frequently seen in foot infection specimens. One particular pattern with many species may denote a higher risk for treatment failure. rarely co-occurs with other staphylococci.