Keys to unlocking the biofilm phenotype of virulent environmental isolate of Salmonella

Abstract

The aim of this research was to elucidate the phenotypic adaptation of an environmental isolate of Salmonella enterica grown in a single species biofilm using transcriptomic analysis. This environmental isolate was obtained from an outbreak in Gideon, MO, and was classified as Salmonella enterica serotype Missouri. Gene expression profiles obtained from this environmental isolate were compared with profiles of the ATCC type strain Salmonella enterica serotype Typhimurium LT2 grown under the same conditions. It was shown that there were distinct transcriptional differences in both of the strains between the biofilm and planktonic phenotypes. Both strains exhibited the strong up-regulation of several gene pathways that were unique to the biofilm phenotype. These included genes responsible for the cobalamin-dependent anaerobic utilization of 1,2-propanediol (cob-cbi-pdu), type III secretion system apparatus and effector proteins located on Salmonella Pathogenicity Island 2 (SPI-2) and the well characterized csg operon largely responsible for biofilm formation in Salmonella. A significant proportion of the genes present on the virulence plasmid PSLT were shown to be exclusively up-regulated in the biofilm phenotype of Salmonella Typhimurium LT2, illustrating the tendency of this pathogen to exhibit a promiscuous lifestyle whilst in the non-host environment. It was further demonstrated that the environmental isolate exhibited a more tenacious biofilm-forming tendency and overall greater survivability than the type strain in a low nutrient, non-host environment. It appeared from the transcriptional profile of Salmonella Typhimurium LT2 during planktonic growth that the organism struggled to adapt and survive under low nutrient conditions as evidenced by the increased expression of ribosomal subunit operons rps and rpl and several stress-related genes including dnaK and htp. The conclusion may be drawn that Salmonella Missouri has developed several key systems differentiating the biofilm and planktonic phenotypes and affording it a competitive advantage. While some of these traits have previously been studied exclusively in the context of host pathogenicity, this research indicates that perhaps these so-called virulence strategies may afford the pathogen enhanced survival in non-host environments as well. Therefore, these findings suggest that the use of excessively sub-cultured laboratory strains may be inappropriate surrogates for studying the behavior of real-world pathogens.