Postadaptational resistance to benzalkonium chloride and subsequent physicochemical modifications of Listeria monocytogenes

Abstract

Many studies have demonstrated that bacteria, including Listeria monocytogenes, are capable of adapting to disinfectants used in industrial settings after prolonged exposure to sublethal concentrations. However, the consequent alterations of the cell surface due to sanitizer adaptation of this pathogen are not fully understood. Two resistant and four sensitive L. monocytogenes strains from different sources were progressively subcultured with increasing sublethal concentrations of a surfactant, benzalkonium chloride (BC). To evaluate the effects of acquired tolerance to BC, parent and adapted strains were compared by using several morphological and physiological tests. Sensitive strains showed at least a fivefold increase in the MIC, while the MIC doubled for resistant strains after the adaptation period. The hydrophobicities of cells of parent and adapted strains were similar. Serological testing indicated that antigen types 1 and 4 were both present on the cell surface of adapted cells. The data suggest that efflux pumps are the major mechanism of adaptation in sensitive strains and are less important in originally resistant isolates. A different, unknown mechanism was responsible for the original tolerance of resistant isolates. In an originally resistant strain, there was a slight shift in the fatty acid profile after adaptation, whereas sensitive strains had similar profiles. Electron micrographs revealed morphological differences after adaptation. The changes in cell surface antigens, efflux pump utilization, and fatty acid profiles suggest that different mechanisms are used by resistant and sensitive strains for adaptation to BC.

FIG. 1.

Electron micrographs of thin sections of L. monocyotgenes. (a) Resistant parent strain H7962P grown in TSB. Scale bar = 100 nm. (b) Adapted resistant strain H7962A grown in the presence of 8 μg of BC per ml. Scale bar = 200 nm. (c) Sensitive parent strain LJH381P grown in TSB. Scale bar = 100 nm. (d) Adapted sensitive strain LJH381A grown in the presence of 5 μg of BC per ml. Scale bar = 100 nm. Compared with the parent strains, adapted strains were elongated and slimmer. Waviness and shedding of unknown materials were apparent in adapted strains.