Listeria monocytogenes Strains Selected on Ciprofloxacin or the Disinfectant Benzalkonium Chloride Exhibit Reduced Susceptibility to Ciprofloxacin, Gentamicin, Benzalkonium Chloride, and Other Toxic Compounds

Abstract

Listeria monocytogenes is a leading agent for severe food-borne illness and death in the United States and other nations. Even though drug resistance has not yet threatened therapeutic interventions for listeriosis, selective pressure associated with exposure to antibiotics and disinfectants may result in reduced susceptibility to these agents. In this study, selection of several L. monocytogenes strains on either ciprofloxacin (2 g/ml) or the quaternary ammonium disinfectant benzalkonium chloride (BC; 10 g/ml) led to derivatives with increased MICs not only to these agents but also to several other toxic compounds, including gentamicin, the dye ethidium bromide, and the chemotherapeutic drug tetraphenylphosphonium chloride. The spectrum of compounds to which these derivatives exhibited reduced susceptibility was the same regardless of whether they were selected on ciprofloxacin or on BC. Inclusion of strains harboring the large plasmid pLM80 revealed that MICs to ciprofloxacin and gentamicin did not differ between the parental and plasmid-cured strains. However, ciprofloxacin-selected derivatives of pLM80-harboring strains had higher MICs than those derived from the plasmid-cured strains. Susceptibility to the antimicrobials was partially restored in the presence of the potent efflux inhibitor reserpine. Taken together, these data suggest that mutations in efflux systems are responsible for the multidrug resistance phenotype of strains selected on ciprofloxacin or BC.

The Connection between Persistent, Disinfectant-Resistant Listeria monocytogenes Strains from Two Geographically Separate Iberian Pork Processing Plants: Evidence from Comparative Genome Analysis

“…MICs were determined in at least two separate assays, and each strain was assayed in duplicate. To determine efflux pump activity, the inhibitor reserpine (final concentration, 10 mg/liter) was added to each Mueller-Hinton agar plate containing BAC (26,27). All compounds were purchased from Sigma-Aldrich (St. Louis, MO, USA), and stock solutions were prepared and stored according to the manufacturer's recommendations.…”

“…The efflux systems encoded by all of these genes belong to the small multidrug resistance (SMR) pro-tein family (25). Other efflux pumps, such as MdrL (multidrug resistance Listeria) and Lde (Listeria drug efflux) (26,27), which belong to the major facilitator superfamily (MFS) (25), are also associated with L. monocytogenes adaptation and resistance to BAC (26,27).…”

Identification of the mechanisms of resistance and potential cross resistance of Listeria monocytogenes upon adaptation to different antimicrobials

“…Different studies are suggesting variable incidence of resistance to the Selective pressure has been hypothesized to contribute to the increase in L monocytogenes resistance as BC has been extensively used in food processing and health care environments in the US (124,146,151,190). Furthermore, a number of studies have suggested cross-resistance between BC and antibiotics (182,187,216).…”

“…Different studies are suggesting variable incidence of resistance to the quaternary ammonium compound bezalkonium chloride ( Selective pressure has been hypothesized to contribute to the increase in L monocytogenes resistance as BC has been extensively used in food processing and health care environments in the US (124,146,151,190). Furthermore, a number of studies have suggested cross-resistance between BC and antibiotics (182,187,216). and L. welshimeri have been found to be more common than L. monocytogenes in food processing environments and in foods, and to grow faster than L. monocytogenes in foods and other media (8,47,104,140,168,197,201,221).…”

“…Additionally, we employed deletion mutants of wap and genomic region 18, along with corresponding parental strains. Lastly, mutants of L. monocytogenes adapted to antibiotic ciprofloxacin described in the previous study (182) were also tested.…”