No abstract

Loss of culturability of Salmonella enterica subsp. enterica serovar Typhimurium has been observed in mixed cultures with anaerobic fecal bacteria under conditions that allow local interaction between cells, such as cell contact. A reduction of a population of culturable S. Typhimurium on the order of ϳ10 4 to 10 5 CFU/ml was observed in batch anaerobic mixed cultures with fecal samples from different human donors. Culturability was not affected either in supernatants collected at several times from fecal cultures, when separated from fecal bacteria by a membrane of 0.45-m pore size, or when in contact with inactivated fecal bacterial cells. Loss of culturability kinetics was characterized by a sharp reduction of several logarithmic units followed by a pronounced tail. A mathematical model was developed to describe the rate of loss of culturability as a function of the frequency of encounters between populations and the probability of inactivation after encounter. The model term F(S · F) 1/2 quantifies the effect of the concentration of both populations, fecal bacteria (F) and S. Typhimurium (S), on the loss of culturability of S. Typhimurium by cell contact with fecal bacteria. When the value of F(S · F) 1/2 decreased below ca. 10 15 (CFU/ml) 2 , the frequency of encounters sharply decreased, leading to the deceleration of the inactivation rate and to the tailing off of the S. Typhimurium population. The probability of inactivation after encounter, P, was constant, with an estimated value of ϳ10 ؊5 for all data sets. P might be characteristic of the mechanism of growth inhibition after a cell encounter. Bacteria in nature display social activities such as cooperation and/or competition with other species. The adult human gut houses a bacterial community containing thousands of specieslevel phylogenetic types typically dominated by two bacterial phyla (divisions), the Firmicutes and the Bacteroidetes (1). These two main phylogenetic types interact with each other and with the host to ensure the stability of the gut ecosystem (1).The gut microbiota is a major luminal barrier against pathogenic bacterial colonization (2). It profoundly influences the gut barrier function, host immunity, mucin biosynthesis, epithelial proliferation, and bacterial pathogenesis (2-4).Most of the identified pathogenic bacteria exclusion mechanisms associated with the gut microbiota are mediated by production of extracellular components, especially short-chain fatty acids, produced by the microbiota as end metabolites (3, 4). However, secretion and detection of extracellular molecules in the surrounding environment is not the only form of bacterial interaction. Recently, cell-cell contact between bacteria has been demonstrated to be required for some bacterial interactions (5-10).Salmonella enterica subsp. enterica serovar Typhimurium has been reported to be effectively outcompeted by gut bacteria in healthy mice (11). The aim of this work was to investigate if, similarly, human gut bacteria could have an effect on the population dynam...

No abstract