Abstract
The genus Salmonella includes many pathogens of great medical and veterinary importance. Bacteria belonging to this genus are very closely related to those belonging to the genus Escherichia. lacZYA operon and lacI are present in Escherichia coli, but not in Salmonella enterica. It has been proposed that Salmonella has lost lacZYA operon and lacI during evolution. In this study, we have investigated the physiological and evolutionary significance of the absence of lacI in Salmonella enterica. Using murine model of typhoid fever, we show that the expression of LacI causes a remarkable reduction in the virulence of Salmonella enterica. LacI also suppresses the ability of Salmonella enterica to proliferate inside murine macrophages. Microarray analysis revealed that LacI interferes with the expression of virulence genes of Salmonella pathogenicity island 2. This effect was confirmed by RT-PCR and Western blot analysis. Interestingly, we found that SBG0326 of Salmonella bongori is homologous to lacI of Escherichia coli. Salmonella bongori is the only other species of the genus Salmonella and it lacks the virulence genes of Salmonella pathogenicity island 2. Overall, our results demonstrate that LacI is an antivirulence factor of Salmonella enterica and suggest that absence of lacI has facilitated the acquisition of virulence genes of Salmonella pathogenicity island 2 in Salmonella enterica making it a successful systemic pathogen.
Highlights
The genus Salmonella includes Gram-negative facultative intracellular pathogens that infect reptiles, birds and mammals
Our results demonstrate that LacI is an antivirulence factor of Salmonella enterica and suggest that absence of lacI has facilitated the acquisition of virulence genes of Salmonella pathogenicity island 2 in Salmonella enterica making it a successful systemic pathogen
CadA and ompT are absent in Shigella, but are present in nonpathogenic E. coli
Summary
The genus Salmonella includes Gram-negative facultative intracellular pathogens that infect reptiles, birds and mammals. This genus includes two species, viz. Salmonella bongori (S. bongori) and Salmonella enterica (S. enterica). Salmonella pathogenicity island 2 (SPI-2) imparts S. enterica an ability to cause systemic infection. One part is 25 kb in size and encodes a type three secretion system (TTSS) This part is essential for the systemic virulence and is present in S. enterica, but absent in S. bongori. The other part is 15 kb in size and is not essential for the systemic virulence of S. enterica This region encodes the tetrathionate reductase involved in anaerobic respiration and is present in both S. bongori and S. enterica [3,4,5]. Evolutionary significance of S. bongori not having the 25 kb part and S. enterica having both parts of SPI-2 is not clear
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