Background The indigenous intestinal microflora may generate beta-lactamase activity in response to beta-lactam antibiotic selective pressure. We hypothesized that expansion of subpopulations of Bacteroides fragilis group organisms that hyperproduce cephalosporinases could prevent disruption of the indigenous microflora in the face of ceftriaxone selective pressure.
Methods In vitro mixing studies were performed to determine the ability of Bacteroides fragilis TAL3636 (BF3636), a hyperproducer of a broad-spectrum beta-lactamase, to raise the minimum bactericidal concentration (MBC) of susceptible organisms to ceftriaxone. Continuous-flow (CF) cultures of human colonic microflora were established with or without the addition of BF3636 and increasing concentrations of ceftriaxone were added to the inflowing media. Beta-lactamase activity, ceftriaxone concentrations, and densities of bacterial populations were monitored. A mouse model was also utilized to evaluate the ability of BF 3636 to preserve colonization resistance in vivo. Concomitant with injections of ceftriaxone, mice received orogastric inoculation of CF culture containing BF3636, BF3636 culture alone, or normal saline (control group). Mice subsequently received a culture of vancomycin-resistant enterococcus (VRE) by orogastric gavage. Stool samples were collected on succeeding days and plated for the presence of VRE and resistant Bacteroides species.
Results In mixing studies, BF3636 raised the MBC of a ceftriaxone-susceptible Escherichia coli strain from < 0.25 to 64, whereas organisms that did not produce beta-lactamase did not (p < .001). The CF culture containing intestinal microflora was not affected when 8 μg/mL of ceftriaxone was added to the inflowing media, but 100 μg/mL resulted in disruption of the microflora and detection of ceftriaxone in the culture. In the CF culture containing BF3636, the population of Bacteroides species with high-level resistance to cephalosporins expanded over time and beta-lactamase activity increased; infusion of 100 μg/mL of ceftriaxone did not disrupt the indigenous microflora and ceftriaxone levels remained undetectable. In vivo, ceftriaxone-treated mice that had been orally inoculated with CF culture containing BF3636 did not develop intestinal overgrowth with VRE despite exposure; this was in contrast to ceftriaxone-treated mice that had received an oral inoculation with saline alone. Conclusion: Expansion of subpopulations of Bacteroides species that hyperproduce cephalosporinases can inactivate high concentrations of ceftriaxone and preserve the indigenous intestinal microflora. CF culture containing beta-lactamase-producing Bacteroides species was able to preserve colonization resistance of ceftriaxone-treated mice against VRE.