Klebsiella pneumoniae type VI secretion system contributes to bacterial competition, cell invasion, type-1 fimbriae expression, and in vivo colonization.

Background: We previously isolated a Klebsiella pneumoniae strain, NTUH-K2044, from a community-acquired pyogenic liver abscess (PLA) patient. Analysis of the NTUH-K2044 genome revealed that this strain harbors two putative type VI secretion system (T6SS)-encoding gene clusters.

Methods: The distribution of T6SS genes in the PLA and intestinal-colonizing K. pneumoniae clinical isolates was examined. icmF1-, icmF2-, icmF1/icmF2- and hcp-deficient K pneumoniae strains were constructed using an unmarked deletion method. The roles of T6SSs in antibacterial activity, type-1 fimbriae expression, cell adhesion and invasion and intestinal colonization were determined.

Results: The prevalence of T6SSs is higher in the PLA strains than in the intestinal-colonizing strains (37/42 vs. 54/130). Deletion of icmF1/icmF2 and hcp genes significantly reduced interbacterial and intrabacterial killing. Strain deleted for icmF1 and icmF2 exhibited decreased transcriptional expression of type-1 fimbriae and reduced adherence to and invasion of human colorectal epithelial cells, and was attenuated for in vivo competition to enable colonization of the host gut. Lastly, Hcp expression in K. pneumoniae was silenced by the histone-like nucleoid structuring protein via direct binding.

Conclusions: These results provide new insights into T6SS-mediated bacterial competition and attachment in K. pneumoniae, and could facilitate the prevention of K. pneumoniae infection.