Bacterial-microplastic interaction can affect disinfection efficiency of methods routinely used in the food industry

High abundance of microplastics in the environment and its potential dangers have been an increasingly emerging topic in the last decade. Thus far, research on microplastics has focused mainly on water systems, sewage, and soil, where they have been described as additional surfaces and vectors for microorganism growth and transmission. In our study we hypothesized that microplastics play a role in aiding the survival of bacteria in clinical or food production environments by influencing effectiveness of routine disinfection. To test this hypothesis, two typical foodborne pathogens, Listeria monocytogenes and Escherichia coli, were exposed to different disinfection methods, physical (heat and ultraviolet (UV) radiation) and chemical (lactic acid and benzalkonium chloride), in presence of microplastics. Two experimental groups were tested and subsequently compared with the control group: bacteria incubated overnight in presence of microplastics, and bacteria mixed with microplastics shortly before exposure to disinfectant methods. Our study included two different types and sizes of microplastics: 1-5 μm thermoset amino-formaldehyde (TAFP) polymers and 425-500 μm polyethylene (PE). Disinfection efficiency was determined from the log₁₀ reduction in bacterial counts after 10, 30 or 60 min of exposure to physical and chemical disinfectants, and after 60 min of exposure to UV. Our results indicate that, while microplastics do not shield the bacteria from heat-shock, they can support their survival in presence of benzalkonium chloride and under the exposure of UV light. More specifically, TAFP appears to cancel out the effect of benzalkonium chloride by absorbing or reacting with it. On the other hand, both types of microplastics seem to create a physical barrier protecting bacteria against UV light. Our research points out the necessity in taking microplastics into account when assessing disinfection methods efficiency.