Plasmid transmitted small multidrug resistant (SMR) efflux pumps differ in gene regulation and enhance tolerance to quaternary ammonium compounds (QAC) when grown as biofilms

Small multidrug resistance (SMR) efflux pump genes are commonly identified from integrons carried by multidrug-resistant (MDR) plasmids. SMR pumps are annotated as qac for their ability to confer resistance to quaternary ammonium compounds (QACs) but few qac are characterized to date. Hence, we have examined SMR sequence diversity, antimicrobial susceptibility, and gene expression from >500 sequenced proteobacterial plasmids. SMR sequence diversity from plasmid database surveys identified 20 unique SMR sequences annotated as qacE/E{Delta}1/F/G/H/I/L, or sugE. Phylogenetic analysis shows Qac sequences are homologous to archetypical SMR member EmrE, and share a single sequence origin. In contrast, SugE sequences are homologous to archetypical member Gdx/SugE and likely originate from different species. SMR genes, qacE, qacE{Delta}1, qacF, qacG, qacH, and sugE(p), were over-expressed in Escherichia coli to determine their QAC antimicrobial susceptibility as planktonic, colony, and biofilms. SMRs (except qacE{Delta}1/sugE) expressed in biofilms significantly increased its QAC tolerance as compared to planktonic and colony growth. Analysis of upstream SMR nucleotide regions indicate sugE(p) genes are regulated by type II guanidinium riboswitches, whereas qacE and qacE{Delta}1 have a conserved class I integron Pq promoter, and qacF/G/H are regulated by integron Pc promoter in variable cassettes region. Beta-galactosidase assays were used to characterize growth conditions regulating Pq and Pc promoters and revealed that Pq and Pc have different expression profiles during heat, peroxide, and QAC exposure. Altogether, this study reveals that biofilm growth methods are optimal for SMR-mediated QAC susceptibility testing and suggests SMR gene regulation on plasmids is similar to chromosomally inherited SMR members.