Bacteroides fragilis BmeABC efflux systems additively confer intrinsic antimicrobial resistance.

OBJECTIVES To determine the prevalence of expression and function(s) of Bacteroides fragilis RND family efflux transport systems (bmeABC1-16). METHODS The mRNA transcripts of bmeB efflux pump genes were detected in a wild-type strain ADB77 by RT-PCR and expression in different strains was quantified by comparative quantitative real-time RT-PCR. In order to determine independent or additive functions, BmeB 1, 3, 12 and 15 (the first efflux pumps identified) were deleted as singles, doubles, triples or quadruples by the double cross-over technique with pADB242 and antimicrobial susceptibility was assayed by the spiral gradient endpoint technique. RESULTS All efflux pumps except bmeB9 were expressed in the wild-type parental strain. Susceptibility to beta-lactams, fluoroquinolones, ethidium bromide, SDS and triclosan was increased in ADB77DeltabmeB3 (up to 3-fold) and ADB77DeltabmeB1DeltabmeB3DeltabmeB12 (up to 5-fold). Expression of bmeB9 was increased and that of bmeB11 repressed in the latter deletant. A quadruple deletant (ADB77DeltabmeB1DeltabmeB3DeltabmeB12DeltabmeB15) had similar changes as well as a 2-fold increase in expression of bmeB16 and norfloxacin resistance. Expression of bmeB3 was increased in two triple deletants ADB77DeltabmeB1DeltabmeB12DeltabmeB15-type I (2-fold) and ADB77DeltabmeB1DeltabmeB12DeltabmeB15-type II (5.8-fold). Antimicrobial MICs were also increased in the latter deletant; ampicillin (2.6-fold), cefoperazone (3.4-fold), cefoxitin (1.8-fold), tetracycline (36.4-fold), SDS (1.7-fold) and triclosan (2-fold). CONCLUSIONS These data demonstrate that constitutive bmeB expression is prevalent in B. fragilis. At least seven BmeB efflux pumps are functional in transporting antimicrobials and have overlapping substrate profiles, and at least four confer intrinsic resistance.