Bacteriology of Infected, Subcutaneous Tissue in Diffuse Cellulitis and Foot Ulcer Infections in Patients with Diabetes Mellitus using Cultures and Molecular Identification Techniques

Background: The microbiology of diabetic foot infections remains poorly understood, and polymicrobial cultures result in unnecessarily broad antibiotic coverage.  This study compares the microbiology of cellulitis and infected foot ulcers in diabetics via bacterial culture and molecular methods at varying sample levels to attempt to delineate which organisms are likely causative.

Methods: Single-center prospective case series from 2010 - 2011, involving 30 diabetic patients ≥18 years old, 9 with non-ulcer lower extremity cellulitis and 21 with an infected foot ulcer. Patients with cellulitis had a skin aspirate done for aerobic/anaerobic culture, pyrosequencing, and real-time polymerase chain reaction (RT-PCR). Patients with infected foot ulcers had superficial and deep ulcer tissue samples, and a skin biopsy of the adjacent cellulitis performed for aerobic/anaerobic culture and pyrosequencing.

Results: Cultures of subcutaneous aspirates of cellulitis showed no growth. RT-PCR yielded pathogenic bacteria in 89.0% of subcutaneous aspirates, identified as Streptococcus anginosus, MSSA, and Streptococcus agalactiae in decreasing frequency. For ulcer-related foot infections, cultures revealed fewer bacteria at deeper sample levels, with the predominant pathogens being MSSA, Streptococcus agalactiae, Streptococcus anginosus and Streptococcus viridans in all levels. Enterococcus spp. and anaerobes were also isolated in all levels, and deemed to be likely commensal organisms. Of note, Enterobacteriaceae were present in superficial ulcer cultures but infrequently isolated in skin biopsies. Pyrosequencing identified significantly more bacteria at all tissue levels in both patient populations, most of which are non-pathogenic, without improving the identification for pathogenic bacteria.

Conclusion: In diabetics with infected ulcers and non-ulcer cellulitis, this study identified MSSA and Streptococcus spp. most frequently in all levels of soft tissue. Pyrosequencing yielded more commensal bacteria and is thus of limited utility. Gram-negative bacteria were isolated in superficial ulcer but decreased in frequency at deeper levels, with rare isolation in subcutaneous skin biopsy cultures, suggesting its role as colonizers.