Integrated combined effects of temperature, pH and sodium chloride concentration on biofilm formation by Salmonella enterica ser. Enteritidis and Typhimurium under low nutrient food-related conditions

Abstract

Salmonella enterica is a major foodborne bacterial pathogen. This forms biofilms on surfaces and persists, depending on the strain and the environment. The integrative interaction of temperature (T; 13-39 °C), pH (5-8) and sodium chloride (NaCl) concentration (0.5-8.5%) on biofilm formation by two S. enterica strains (ser. Enteritidis and Typhimurium) was here evaluated under low nutrient conditions. This was achieved using response surface methodology to model the combined effect of each factor on the response, through mathematical quadratic fitting of the outcomes of a sequence of designed experiments. These last were executed by incubating stainless steel coupons carrying sessile bacteria, for 24 h, in 1:10 diluted tryptone soya broth, under 15 different combinations of three independent factors (T, pH and NaCl). For each strain, a second order polynomial model, describing the relationship between biofilm formation (log CFU/cm2 ) and the factors (T, pH and NaCl), was developed using least square regression analysis. Both derived models predicted the combined influences of these factors on biofilm formation, with agreement between predictions and experimental observations (R2 ≥ 0.96, P ≤ 0.0001). For both strains, the increase of NaCl content restricted their sessile growth, while under low salinity conditions (NaCl < 4%) biofilm formation was favored as pH increased, regardless of T. Interestingly, under low salt content, and depending on the strain, biofilm formation was either favored or hindered by increasing T. Thus, 34.5 and 13 °C were the T predicted to maximize biofilm formation by strains Enteritidis and Typhimurium, respectively, something which was also experimentally verified. To sum, these models can predict the interactive influences of crucial food-related factors on biofilm growth of a significant foodborne pathogen towards the efforts to limit its persistence in food industry.