Impact of Acanthamoeba cysts on stress resistance of Salmonella enterica serovar Typhimurium, Yersinia enterocolitica 4/O:3, Listeria monocytogenes 1/2a, and Escherichia coli O:26

Abstract

The formation of robust resting cysts enables Acanthamoeba to resist harsh environmental conditions. This study investigated to what extent these cysts are resistant to physical and chemical stresses as applied in food industry cleaning and disinfection procedures. Moreover, it was assessed whether certain intracystic meat-borne bacterial pathogens are more stress resistant than free-living bacterial monocultures and if intracystic passage and subsequent association with trophozoites induces cross-tolerance toward other stressors. Several physical and chemical stressors (NaCl, H2O2, benzalkonium chloride, 55°C, heating until boiling, ethanol, dishwashing detergent, and sodium hypochlorite) frequently used in domestic and industrial food-related environments were tested against (i) Acanthamoeba castellanii cysts, (ii) single strains of bacterial monocultures, (iii) intracystic bacteria, and (iv) bacteria after intracystic passage (cyst-primed bacteria). Only heating until boiling and hypochlorite treatment were cysticidal. After boiling, no viable trophozoites could be recovered from the cysts, and hypochlorite treatment caused a 1.34- to 4.72-log10 cells/ml reduction in cyst viability. All treatments were effective in reducing or even eliminating the tested bacterial monocultures, whereas bacteria residing inside cysts were more tolerant toward these stressors. All cyst-primed bacteria exhibited an increased tolerance toward subsequent H2O2 (>92% decrease in median log10 CFU/ml reduction) and 70% ethanol (>99% decrease) treatments. Moreover, intracystic passage significantly increased the survival of Yersinia enterocolitica (74% decrease in median log10 reduction), Escherichia coli (58%), and Salmonella enterica (48%) after NaCl treatment and of E. coli (96%), S. enterica (99%), and Listeria monocytogenes (99%) after sodium hypochlorite treatment compared with that of nonprimed bacteria.