a b s t r a c tObjectives: The prevalence of resistance genes in two important anaerobic genera, Bacteroides and Prevotella, was assessed by applying PCR specifically directed to genes of interest. Methods: A total of 101 Bacteroides spp. and 99 Prevotella spp. human clinical isolates were identified using MALDI-TOF MS. The presence of the resistance genes cfxA, cepA, cfiA, tetQ, ermF and nim, was assessed. Prevalence of resistance genes was compared with the phenotypic resistance against amoxicillin, clindamycin, meropenem and metronidazole. Results: Even though the majority of the Bacteroides isolates (95.0%) showed resistance towards amoxicillin, only 52/101 of the isolates harboured one of the resistance genes, accounting for this resistance. Within the genus Prevotella the presence of cfxA (50/99) almost perfectly matched the amoxicillin resistance (48/99). No difference in prevalence of the ermF gene (16/101 and 9/99) and clindamycin resistance (16/101 and 10/99) was observed within Bacteroides and Prevotella, respectively. Two isolates of Prevotella were resistant to metronidazole. One harboured the nim gene. One metronidazolesusceptible isolate of Bacteroides harboured a nim gene. Within the Bacteroides and Prevotella genera, 6/101 strains and 5/99 isolates harboured three different resistance genes, respectively, among them tetQ. TetQ is often located on a conjugative transposon, increasing the chance of horizontal gene transfer between isolates.Conclusions: An unknown mechanism in Bacteroides non-fragilis isolates causes resistance to b-lactam antibiotics. The fact that the prevalence of the tetQ gene among Prevotella is increasing and the existence of isolates harbouring three resistance genes are worrisome developments. A.C.M. Veloo, Clin Microbiol Infect 2019;25:1156

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