Deletion of the lon gene augments expression of Salmonella Pathogenicity Island (SPI)-1 and metal ion uptake genes leading to the accumulation of bactericidal hydroxyl radicals and host pro-inflammatory cytokine-mediated rapid intracellular clearance

Abstract

In the present study, we characterized the involvement of Lon protease in bacterial virulence and intracellular survival in Salmonella under abiotic stress conditions resembling the conditions of a natural infection. Wild type (JOL401) and the lon mutant (JOL909) Salmonella Typhimurium were exposed to low temperature, pH, osmotic, and oxidative stress conditions and changes in gene expression profiles related to virulence and metal ion uptake were investigated. Expression of candidate genes invF and hilC of Salmonella Pathogenicity Island (SPI)-1 and sifA and sseJ of SPI-2 revealed that Lon protease controls SPI-1 genes and not SPI-2 genes under all stress conditions tested. The lon mutant exhibited increased accumulation of hydroxyl (OH•) ions that lead to cell damage due to oxidative stress. This oxidative damage can also be linked to an unregulated influx of iron due to the upregulation of ion channel genes such as fepA in the lon mutant. The deletion of lon from the Salmonella genome causes oxidative damage and increased expression of virulence genes. It also prompts the secretion of host pro-inflammatory cytokines leading to early clearance of the bacteria from host cells. We conclude that poor bacterial recovery from mice infected with the lon mutant is a result of disrupted bacterial intracellular equilibrium and rapid activation of cytokine expression leading to bacterial lysis.

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“…Animals were monitored for Salmonella-induced clinical signs such as ruffled fur, weight loss, and mortality. 49 Samples such as sera, splenocytes, lung homogenate, and intestinal lavage were collected from mice, whereas only sera samples were obtained from hamsters. The humoral and cellular immune responses were evaluated 3 weeks after the final inoculation.…”

Coordinated interaction between Lon protease and catalase-peroxidase regulates virulence and oxidative stress management during Salmonellosis

“…When the two proteins are intact, they coordinately regulate the response to intracellular oxidative stress and the expression of virulence genes, ensuring the systemic establishment of an infection. 7 Here, we establish that Lon protease and KatG cooperate to regulate the virulence factors and are required for ST pathogenicity. The ability of ST to adhere, invade and proliferate inside the macrophages is associated with virulence and colonization of the host.…”

“…Earlier we have shown that the deletion of lon causes an increased accumulation of hydroxyl radicals in the Salmonella . 7 In support, the expression levels of katE, katN , and tsaA were downregulated in lon mutant. It should be noted that the primary hydrogen peroxide scavenging role of tsaA ( ahpC ) over katE, katN, katG , and ahpF has been reported earlier.…”

“…Although previous reports from our lab and others have shown that the deletion of lon increases the ST susceptibility to H 2 O 2 , 7 , 9 the role of Lon protease in regulating the levels of crucial enzymes associated with H 2 O 2 scavenging is yet to be elucidated. To study the effect of Lon protease on the whole Salmonella proteome with special emphasis on the H 2 O 2 degrading enzymes, we conducted a proteome comparison of wild-type (WT) and lon mutant Salmonella strains intending to recognize differentially expressed proteins using 2D gel electrophoresis.…”

“… 5 It does not seem to have a crucial impact on SPI-2 genes, which are regulated during later phases of infection. 7 Lon protease is shown to potentiate bacteria evolution and antimicrobial resistance. 8 The dysregulation of Lon protease, a negative regulator of SPI-1 genes, leads to increased expression and orchestration of early virulence genes.…”