Adaptive and cross-adaptive responses of persistent and non-persistent Listeria monocytogenes strains to disinfectants

Persistent and non-persistent Listeria monocytogenes strains were tested for initial resistance and adaptive and cross-adaptive responses towards two quaternary ammonium compounds, alkyl-benzyl-dimethyl ammonium chloride and n-alkyldimethyl ethylbenzyl ammonium chloride, one tertiary alkylamine, 1,3-propanediamine-N-(3-aminopropyl)N-dodecyl, sodium hypochlorite and potassium persulphate. The initial resistance of two persistent and two non-persistent L. monocytogenes strains was observed to differ. Both types of strains adapted after a 2-h sublethal exposure to the quaternary ammonium compounds and the tertiary alkylamine, the highest increase in the minimum inhibitory concentration (MIC) being 3-fold. Progressively increasing disinfecting concentrations at 10 and 37 °C resulted in adaptation of L. monocytogenes to all disinfectants except potassium sulphate. The highest observed increase in MIC was over 15-fold, from 0.63 to 10 μg/ml of n-alkyldimethyl ethylbenzyl ammonium chloride. All strains reached approximately similar MICs. Stability of the increased resistance was tested by measuring MICs every seventh day for 28 days. The increased resistance to sodium hypochlorite disappeared in 1 week, but the quaternary ammonium compounds and the tertiary alkylamine showed increased resistance for 28 days. These results suggest that cellular changes due to adaptive responses continue to have an effect on the resistance some time after the exposure. All disinfectants were shown to cause cross-adaptation of L. monocytogenes, the highest increase in MIC being almost 8-fold. The only agent that L. monocytogenes could not be shown to cross-adapt to was potassium persulphate which did, however, cause cross-adaptation to the other disinfectants. The mechanism behind these adaptive responses seemed to be non-specific as cross-adaptation was observed not only between related but also unrelated disinfectants. These findings suggest that sustaining high disinfectant effectiveness may be unsuccessful by rotation, even when using agents with different mechanisms of action.