Tn6188 - A Novel Transposon in Listeria monocytogenes Responsible for Tolerance to Benzalkonium Chloride

Controlling the food-borne pathogen Listeria (L.) monocytogenes is of great importance from a food safety perspective, and thus for human health. The consequences of failures in this regard have been exemplified by recent large listeriosis outbreaks in the USA and Europe. It is thus particularly notable that tolerance to quaternary ammonium compounds such as benzalkonium chloride (BC) has been observed in many L. monocytogenes strains. However, the molecular determinants and mechanisms of BC tolerance of L. monocytogenes are still largely unknown. Here we describe Tn 6188 , a novel transposon in L. monocytogenes conferring tolerance to BC. Tn 6188 is related to Tn 554 from Staphylococcus (S.) aureus and other Tn 554 -like transposons such as Tn 558 , Tn 559 and Tn 5406 found in various Firmicutes . Tn 6188 comprises 5117 bp, is integrated chromosomally within the radC gene and consists of three transposase genes ( tnpABC ) as well as genes encoding a putative transcriptional regulator and QacH, a small multidrug resistance protein family (SMR) transporter putatively associated with export of BC that shows high amino acid identity to Smr/QacC from S. aureus and to EmrE from Escherichia coli . We screened 91 L. monocytogenes strains for the presence of Tn 6188 by PCR and found Tn 6188 in 10 of the analyzed strains. These isolates were from food and food processing environments and predominantly from serovar 1/2a. L. monocytogenes strains harboring Tn 6188 had significantly higher BC minimum inhibitory concentrations (MICs) (28.5 ± 4.7 mg/l) than strains without Tn 6188 (14 ± 3.2 mg/l). Using quantitative reverse transcriptase PCR we could show a significant increase in qacH expression in the presence of BC. QacH deletion mutants were generated in two L. monocytogenes strains and growth analysis revealed that Δ qacH strains had lower BC MICs than wildtype strains. In conclusion, our results provide evidence that Tn 6188 is responsible for BC tolerance in various L. monocytogenes strains.