Disinfectant resistance and resistance stability of bacteria isolated from Estonian dairies: Master's thesis

Disinfectant resistances of bacteria from three Estonian dairies (isolated in spring 2004), Finnish dairies and reference strains were investigated. Resistance of bacteria were determined to disinfectants that were used in Estonian dairies, in addition some disinfectants with similar mechanisms of action were tested. Resistance stability of four Listeria spp. were further examined to disinfectants that showed higher resistance than other bacteria. Survival of bacteria after disinfection represents a potential spoilage or food safety problem for the food industry and consumer. MICs for resistance and resistance stability were determined with the microdilution broth method at the optimum temperature of bacteria for Listeria spp., mycobacteria and Bacillus cereus. The MIC method has been used extensively to determine the resistance to antibiotics and disinfectants. This method gave variable results and may be therefore not so reliable. Listeria monocytogenes was the most adapted strain investigated, while Bacillus cereus and mycobacteria were more sensitive to in-use concentrations of disinfectants. Only six antimicrobial agents were efficient in inhibiting all microbes tested, while seven agents were not so effective and had at least one strain that was not inhibited by the recommended concentration. Disinfectants based on sodium hypochlorite were the most ineffective, while other chlorine-based disinfectants, QACs and cationic tensides were efficient to every bacteria tested. Cleaning agents did not have microbicidal properties. Stability of resistance was not constant in some cases. Instead of decreasing it was surprisingly increasing. Only two Listeria monocytogenes strains showed smaller resistance after l0th subcultivation than initial resistance to antimicrobial agent that contained sodium hypochlorite. Because results varied, they are not so reliable. In conclusion dairies have to start using immediately bigger concentrations of ineffective agents or choose more effective disinfectant, because using sublethal concentrations of disinfectants leads to resistance formation. Before disinfection should be carried out effective cleaning procedure that removes biofilms and organic material and help disinfectants to reach their targets.