Bacteroides Fragilis Vertebral Osteomyelitis and Discitis: “Back” to Susceptibility Testing

Additionally, none of the isolates carried the nim or bft toxin genes. Although the nim gene is associated with metronidazole resistance,its presence does not invariably result in resistant strains of B fragilis; in fact, metronidazole resistance is relatively uncommon, with the majority of B fragilis showing < 1% resistance, based on CLSI breakpoints (≥ 32 mg/L).^13,20,21 However, one recent epidemiologic study on anaerobic wound isolates from Iraq and Afghanistan casualties found that 12% (2/17) of B fragilis isolates were resistant to metronidazole.¹⁵ Given the improvement of the patient’s symptoms while on metronidazole, it is likely that the B fragilis was susceptible. Nevertheless, susceptibility testing with minimum inhibitory concentrations is necessary to verify this result. Also, although enterotoxigenic strains of B fragilis have been associated with bloodstream infections, our patient’s isolates lacked the 3 subtypes of B fragilis enterotoxin gene.²²

Conclusions

We report a case of B fragilis bacteremia and vertebral osteomyelitis complicated by challenges in anaerobic identification and sensitivities that led to brief use of a possibly inactive antimicrobial and the subsequent use of carbapenem therapy, which may have been avoided if susceptibility testing were more readily available. This case led to changes in our hospital’s processing of anaerobic isolates to include susceptibility testing on request.

Acknowledgments

We thank Keith Thompson, MD (staff pathologist, Naval Medical Center Portsmouth Virginia), for providing the pathology images from the initial vertebral biopsy, and Dr. Kate Hinkle (director, Multidrug-Resistant Organism Repository and Surveillance Network, Silver Spring, Maryland ) for providing the whole genome sequencing results from the B fragilis isolates.