Variations in susceptibility to latamoxef(moxalactam) and cefoxitin within the Bacteroides fragilis group

Conventional agar dilution MIC titrations, anaerobic turbidimetric monitoring and viable counting were used to investigate the response to latamoxef (moxalactam) and cefoxitin of four well characterized species within the Bacteroides fragilis group. MIC results indicated that Bact. fragilis and Bact. vulgatus were more susceptible to both antibiotics than were Bact. ovatus and Bact. thetaiotaomicron and that latamoxef was more active than cefoxitin. Continuous turbidimetric monitoring with parallel viable counting confirmed the greater susceptibility of Bact. fragilis and Bact. vulgatus, but also suggested that the differential in activity between latamoxef and cefoxitin was less than MIC titrations had indicated. Moreover, against the more resistant Bact. ovatus and Bact. thetaiotaomicron strains, cefoxitin actually appeared more active than latamoxef in bactericidal and bacteriolytic terms. The interpretation of these results is discussed in the light of the morphological response of the bacteria to the two antibiotics. Because species within the Bact. fragilis group appear to differ considerably in their beta-lactam antibiotic susceptibility there is a need for laboratories to test the antibiotic susceptibility of these organisms or to identify them presumptively to the species level if treatment with beta-lactam agents is contemplated.