Chitinase A, a tightly regulated virulence factor of Salmonella enterica serovar Typhimurium, is actively secreted by a Type 10 Secretion System

As a facultative intracellular pathogen, Salmonella enterica serovar Typhimurium is one of the leading causes of food-borne diseases in humans. With the ingestion of fecal contaminated food or water, S. Typhimurium reaches the intestine. Here, the pathogen efficiently invades intestinal epithelial cells of the mucosal epithelium by the use of multiple virulence factors. Recently, chitinases have been described as emerging virulence factors of S. Typhimurium that contribute to the attachment and invasion of the intestinal epithelium, prevent immune activation, and modulate the host glycome. Here we find that the deletion of chiA leads to diminished adhesion and invasion of polarized intestinal epithelial cells (IEC) compared to wild-type S. Typhimurium. Interestingly, no apparent impact on interaction was detected when using non-polarized IEC or HeLa epithelial cells. In concordance, we demonstrate that chiA gene and ChiA protein expression was solely induced when bacteria gain contact with polarized IEC. The induction of chiA transcripts needs the specific activity of transcriptional regulator ChiR, which is co-localized with chiA in the chitinase operon. Moreover, we established that after chiA is induced, a major portion of the bacterial population expresses chiA, analyzed by flow cytometry. Once expressed, we found ChiA in the bacterial supernatants using Western blot analyses. ChiA secretion was completely abolished when accessory genes within the chitinase operon encoding for a holin and a peptidoglycan hydrolase were deleted. Holins, peptidoglycan hydrolases, and large extracellular enzymes in close proximity have been described as components of the bacterial holin/peptidoglycan hydrolase-dependent protein secretion system or Type 10 Secretion System. Overall, our results confirm that chitinase A is an important virulence factor, tightly regulated by ChiR, that promotes adhesion and invasion upon contact with polarized IEC and is likely secreted by a Type 10 Secretion System (T10SS). Author summaryBeing considered as one of the key global causes of diarrheal diseases, Salmonella can cause diseases from mild salmonellosis to life-threatening diseases depending on host factors and the serotypes of Salmonella. Within the species Salmonella enterica, serovar Typhimurium is the most important human food-borne bacterial pathogen contracted through the consumption of contaminated food or drinking water. Highly adapted, S. Typhimurium owns a plethora of tightly regulated virulence factors, which when needed, are secreted by specific secretion systems to facilitate bacterial infection. Besides classical virulence factors such as toxins, S. Typhimurium also takes advantage of metabolic enzymes degrading various substrates important for colonization. Here, we demonstrate that chitinase A produced by S. Typhimurium is specifically induced and promotes adhesion and invasion of polarized intestinal epithelial cells. The gene encoding for chitinase A is organized within a defined operon that includes three other important genes. One encodes for the transcriptional regulator ChiR, which we show is essential for the induction of chitinase A expression upon contact with polarized intestinal epithelial cells. The other two encode for a holin and a peptidoglycan hydrolase, components of a Type 10 Secretion System, which is responsible for the active secretion of chitinase A in S. Typhimurium.

Automated summary

Salmonella enterica serovar Typhimurium is a leading cause of food-borne diseases in humans and is able to invade intestinal epithelial cells by using multiple virulence factors. Chitinases have recently emerged as important virulence factors of S. Typhimurium that promote attachment and invasion of the intestinal epithelium, modulate the host glycome, and prevent immune activation. Chitinase A is specifically induced upon contact with polarized intestinal epithelial cells and is likely secreted by a Type 10 Secretion System (T10SS), making it an important virulence factor in S. Typhimurium infection.