Evaluation of antimicrobial activity of probiotic bacteria against Salmonella enterica subsp. enterica serovar typhimurium 1344 in a common medium under different environmental conditions

The importance of probiotics in human nutrition has been gaining recognition in recent years. These organisms have been shown to promote human health by enhancing immunological and digestive functions and fighting respiratory tract infections. We propose an improved in vitro model for the study of probiotic antimicrobial activity against enteropathogens, by attempting to re-create, in a common culture medium, environmental growth conditions comparable to those present in the small intestine. A preliminary experiment was carried out in order to find a culture medium able to support both probiotics and pathogens. This was done with the aim of obtaining correct assessment of the interaction under shared growth conditions. BHI medium was selected as the common culture medium and was therefore used in antimicrobial activity assays. The interactions between Salmonella 1344 and Lactobacillus rhamnosus and Lactobacillus reuteri were then assessed at different pH and oxygen availability conditions mimicking the small intestinal environment. L. rhamnosus GG ATCC 53103 (LGG) had the strongest antimicrobial effect, in particular under anaerobic conditions and at lower pH levels. Its antagonistic activity involved both lactic acid and secreted non-lactic acid molecules. Our findings suggest that each probiotic strain has an optimum range of action and should therefore be thoroughly investigated to optimize its use.