Chemical treatments for biofilms

Chlorine-based sanitisers are the most widely used in the food industry, but resistance to chlorine treatments has arisen in some microbes. For example, in enterica, chlorine resistance was correlated to its cellulose production phenotype. This phenotype depended on the environmental stress conditions found in the food processing plants. Aqueous ClO₂is the most widely used sanitiser in the food industry, although gaseous ClO₂ has been demonstrated to be more effective against B. cereus endospores present in biofilms on food steel surfaces. In the case of E. coli O157:H7 biofilms, aqueous ClO₂ was more effective than NaOCl (sodium hypochlorite, commonly used at 50–1,000 ppm), especially when a drying step followed the surface treatment at the food factory. Furthermore, this treatment with ClO₂ increased the sensitivity of E. coli cells to other stressors, such as drying.

Staphylococcus aureus and enterica are common pathogens in raw milk microbiota and can easily form biofilms in dairy factories. NaOCl is an effective chemical for eradicating these biofilms on stainless steel and polypropylene surfaces. However, this disinfectant was unable to eradicate the pathogen Cronobacter sakazakii biofilms in the same environments.

H₂O₂is a potent oxidising disinfectant, widely use in food industry. It generates free radicals in contact with the biofilm structures, destroying them at concentrations of 0.08–5% without toxic side effects. Its combination with acetic acid generates peracetic acid, a strong oxidant with pH 2.8, which is used, for example, in water pipes treatment at 0.5%, with high success against monocytogenes and S. aureus populations.

Ozone is a toxic gas with a potent oxidising activity as well. It destroys different kinds of microorganisms, even biofilms, viruses and protozoans, by breaking down the cellular envelopes. Its use in dairy industry prevents mould overgrowth on stainless steel structures, powdered formulas and cheeses, for example.

Quaternary ammonium compounds (as Metaquats) are widely used as sanitisers in food industry, including biofilms removal. These positively charged water soluble compounds disrupt the bacterial cell membrane, causing bacterial lysis. However, some monocytogenesstrains isolated from food environments harbour genes involved in resistance to quaternary ammonium sanitisers (qacH and bcrABC), which act as pumps for secretion of these compounds. This characteristic can allow them to persist after sanitisation procedures. These genes provide growth advantages for bacteria in food manufacturing plants and, therefore, another type of sanitiser or higher concentrations must be used. In this cases, a multi-faceted approach using a combination of different treatments could improve the removal of biofilms formed by these resistant bacteria. For example, a combination of NaOCl, H₂O₂, iodophor and benzalkonium chloride with steam heating was able to eliminate biofilms formed by E. coli O157:H7, S. enterica and L. monocytogenes, decreasing both sanitiser concentrations and treatment times.

Other less common chemical treatments for biofilms, such as salicylate-based polyanhydride esters, interfere with the biofilm formation in entericaat the air-liquid interface. This implies that biofilm formation by this pathogen can be prevented in the initial stages. The synthetic brominated furanone F202, also an uncommon sanitiser, inhibited S. enterica and E. coli O103:H2 growth at temperatures used in the food industry, preventing the formation of these biofilms on abiotic surfaces and also targeting the flagellar function of both bacteria. This fact accentuates the potential of furanones as a treatment for eradicating biofilms in the food industry. Finally, an experimental short-chain fatty acid formulation resulted in a promising sanitiser against E. coli O157:H7 biofilm formation on fresh vegetables.

It is interesting to note that decades of sanitisers use in the food industry may be one of the main driving forces with respect to development of antibiotic resistance in bacteria. Therefore, development of alternative chemical treatments for biofilms could be a first step toward reduction of this important health problem worldwide.