The purpose of our work was to determine the ability of selected L. innocua and L. monocytogenes strains to adapt to benzalkonium chloride (BC). The minimum inhibitory concentration (MICa) of BC was determined with broth microdilution method, and with growth curve. From the latter we determined the lowest concentration of BC that had an inhibitory effect on growth of each listeria strain. This indicated sufficiently low concentrations at which the adaptation assay was started. We found that lower concentrations of BC, between 0.125x - 0.5x MICa, have an inhibitory effect on growth of selected strains of L. monocytogenes and L. innocua. Therefore, the adaptation assay started at 0.25x MICb BC, as determined by the growth curve, using 5 strains of L. innocua and 5 strains of L. monocytogenes. BC concentration was increased stepwise for all strains, up to a concentration of 16x MICb. 40 % of L. monocytogenes and 60 % of L. innocua strains were able to adapt to the higher BC concentrations than MICb. For those strains that were adapted to the higher BC concentrations, broth microdilution method was used to determine whether MICa of adapted strains remained raised. If MICa remained constant for seven days, we concluded that a BC resistant strain had developed. Among five adapted Listeria strains, MICa remained unchanged in one of L. monocytogenes strain which is a significant proportion for laboratory conditions. These results provide a great basis for further research, which might provide a more detailed insight into mechanisms responsible for adaptation and the development of BC resistance in Listeria.
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