THE c-di-GMP-SPECIFIC PHOSPHODIESTERASE YhjH IS REQUIRED FOR SALMONELLA ENTERICA MOTILITY AND INVASION INTO NONPHAGOCYTIC HOST CELLS

Salmonella enterica is a major food-borne pathogen and the causative agent of salmonellosis in both developed and developing countries. Non-typhoid Salmonella serovars such as Salmonella enterica serovar Typhimurium (S. Typhimurium) induce in immunocompetent individuals a localized acute gastrointestinal disease, while typhoidal S. enterica serovars such as S. Typhi and S. Paratyphi A lead to systemic life-threatening disease known as enteric fever.

Active invasion into nonphagocytic host cells is central to Salmonella enterica pathogenicity, and depends on environmental cues regulating multiple genes within Salmonella pathogenicity island 1 (SPI-1) and elsewhere.

The yhjH gene belongs to EAL domain superfamily with cyclic dimeric GMP-specific phosphodiesterase activity, which is involved in regulation of cell motility and biofilm formation in Enterobacteria. Here we show that yhjH deletion (ΔyhjH) drastically reduced the typhoidal serovar S. Paratyphi A invasion into human epithelial cell-line following growth under aerobic conditions. A similar, though milder, effect was observed following microaerobic growth, and in the non-typhoidal serovar S. Typhimurium under aerobic and microaerobic growth conditions. Intriguingly, bacterial motility on soft agar plates was inhibited in the ΔyhjH S. Typhimurium strain, but was unaffected in S. Paratyphi A ΔyhjH.

SPI-1 genes expression was consistent with these phenotypic observations. In the S. Typhimurium ΔyhjH strain the transcription of multiple flagellar and chemotaxis genes, as well as the SPI-1 regulators HilA, HilC, HilD and other genes driving “trigger” mechanism invasion, was reduced. In the S. Paratyphi A background, yhjH deletion was associated primarily with reduced transcription of the “zipper” invasion mechanism genes ompX and pagN.

In summary, we show for the first time that YhjH is involved in host cell invasion by both typhoidal and non-typhoidal Salmonella strains through regulation of SPI-1 genes expression.