Discussion
In the era of growing antibiotic resistance patterns, antimicrobial stewardship programs recommend interventions to improve antimicrobial use through targeted narrow- spectrum antibiotics.12 The Clinical and Laboratory Standards Institute (CLSI) maintains guidelines on the major indications for anaerobic antimicrobial susceptibility testing (AST) to help direct narrow-targeted antimicrobial therapy. However, in a 2008 practice survey Goldstein and colleagues reported that less than half of US hospitals performed anaerobic AST, and only 21% of these facilities did it in-house, while the remainder sent out their isolates for testing.11-14 The CLSI major indications for AST include situations in which the selection of agents is important because of the (1) known resistance of a particular species; (2) confirmation of appropriate therapy for severe infections or for those that may require long-term therapy; (3) persistence of infection despite adequate treatment with an appropriate therapeutic regimen; and (4) difficulty in making empirical decisions based on precedent.14 Additionally, isolates from brain abscess, endocarditis, osteomyelitis, joint infection, infection of prosthetic devices or vascular grafts, bacteremia, and normally sterile body sites (unless contamination suspected) should be tested.14
Because of the lack of anaerobic AST, health care providers must base empiric treatment on reported sensitivities from the medical literature. Empiric selection of antimicrobials for anaerobic infections is made even more challenging by the increased rates of resistance reported in the literature, leading to recommendations to increase susceptibility testing to guide therapy.13,15,16 Empiric therapy of deep-seated anaerobic infections may lead to use of inactive agents or overly broad-spectrum antibiotics. Current antimicrobial stewardship initiatives recognize the importance of narrow-spectrum antibiotics to minimize risk of adverse events and selective pressure for antimicrobial resistance.
Although we attempted to confirm the identification of the anaerobic isolates via commercially available methods, it was not until we performed genetic testing that we were able verify the isolates as B fragilis. Furthermore, earlier susceptibility testing would have allowed for more narrow-targeted antimicrobial therapy and could have potentially prevented our patient’s readmission and use of ertapenem, despite its > 98% susceptibility rates against B fragilis.13,17
All of the B fragilis isolates carried the cepA gene, which is a cephalosporinase that encodes for resistance to cephalosporins and aminopenicillins but not to ß-lactam ß-lactamase inhibitor combinations.13 Although not a substitution for susceptibility analysis, genetic testing showed that all of the isolates carried a nonsynonymous mutation from serine to a phenylalanine at amino acid position 82 (S82F) in the gyrA gene. The S82F mutation has been implicated in fluoroquinolone resistance, via inhibition of substrate–target recognition and binding between fluoroquinolones and the target topoisomerase protein,18 and may potentially explain why our patient clinically worsened while on moxifloxacin monotherapy. Although moxifloxacin susceptibility was not performed, susceptibility rates remain highly variable, ranging from 50% to 70% for B fragilis.13,15,16
It is important to note that the metronidazole the patient received during his first hospital admission could have sterilized the vertebral body without completely eradicating the microbe; thus could explain his clinical worsening while on moxifloxacin monotherapy despite no growth from the repeat biopsy culture. Our rationale for initially continuing moxifloxacin was based on its excellent bioavailability and bone penetration properties. Additionally, of the fluoroquinolones it has the most reliable anaerobic activity and is the only one recommended as monotherapy for complicated intraabdominal infections.19 However, guidelines recommend avoiding its use in patients who have received a fluoroquinolone in the past 90 days or at institutions with high rates of resistance. At our institution Escherichia coli has a > 90% susceptibility rate to fluoroquinolones. Given this rate and our concern that the patient had a polymicrobial infection, we felt that moxifloxacin would provide appropriate anaerobic and aerobic coverage, especially since he had no previous fluoroquinolone exposure.