A high-throughput genetic system for assessing the inhibition of proteins: identification of antibiotic resistance and virulence targets and their cognate inhibitors in Salmonella

Exposure to rumen protozoa leads to enhancement of pathogenicity of and invasion by multiple-antibiotic-resistant Salmonella enterica bearing SGI1

Multiple-antibiotic-resistant Salmonella enterica serotype Typhimurium is a food-borne pathogen that has been purported to be more virulent than antibiotic-sensitive counterparts. The paradigm for this multiresistant/hyperpathogenic phenotype is Salmonella enterica serotype Typhimurium phage type DT104 (DT104). The basis for the multiresistance in DT104 is related to an integron structure designated SGI1, but factors underlying hyperpathogenicity have not been completely identified. Since protozoa have been implicated in the alteration of virulence in Legionella and Mycobacterium spp., we attempted to assess the possibility that protozoa may contribute to the putative hypervirulence of DT104. Our study reveals that DT104 can be more invasive, as determined by a tissue culture invasion assay, after surviving within protozoa originating from the bovine rumen. The enhancement of invasion was correlated with hypervirulence in a bovine infection model in which we observed a more rapid progression of disease and a greater recovery rate for the pathogen. Fewer DT104 cells were recovered from tissues of infected animals when protozoa were lysed by preinfection chemical defaunation of the bovine or ovine rumen. The protozoan-mediated hypervirulence phenotype was observed only in DT104 and other Salmonella strains, including serovars Agona and Infantis, possessing SGI1.

SlyA regulates the collagenase-mediated cytopathic phenotype in multiresistant Salmonella

Salmonella enterica serotype Typhimurium phagetype DT104 (DT104) is a foodborne pathogen with a multiresistant phenotype conferred by a genomic-based integron structure designated as SGI1. Recently, a novel cytopathic phenotype was ascribed to several isolates of DT104 recovered from veal calves. This phenotype is dependent upon clg, a gene encoding a collagenase in Salmonella. Using a novel transposon system and an RT-PCR assay for detection of clg expression, we identified SlyA as a regulator of the collagenase-mediated phenotype. The function of SlyA, in regards to clg expression, is to repress the synthesis of Clg. Derepression ensued in the absence of SlyA or in the presence of a truncated version of SlyA with the latter being relevant for maintenance of another virulence aspect mediated by SlyA, i.e. survival within macrophages. The SlyA-mediated effect on clg expression was restricted to DT104 and other Salmonella phagetypes and serotypes possessing SGI1 thus suggesting co-regulation by an SGI1-specific component.