Co-existence of Citrobacter freundii exacerbated Pseudomonas aeruginosa infection in vivo.

The presence of bacterial species other than the pathogen at infection site can affect the progression of a bacterial infection. Based on the fact that Citrobacter freundii can coexist during Pseudomonas aeruginosa infection, this study aims to investigate the impact of the co-existing C. freundii on the pathogenesis of P. aeruginosa infection. A murine peritonitis model was used to compare the mortality rates and histopathology of P. aeruginosaPAO1 infection in the presence and absence of a C. freundii clinical isolate C9. We also investigated the intercellular interaction between PAO1 and C9 by examining pyocyanin production and comparing gene expression levels. The results demonstrate that co-infection with C9 significantly increased the mortality rate and tissue damages in PAO1 infected mice. At an inoculum of 106 CFU, no mortality was observed in the C9 infected group at three days post-infection, whereas the mortality rate in the PAO1-C9 co-infection group was 64%, compared with 24% in the PAO1 infected group. Pyocyanin production in P. aeruginosa PAO1 increased 8 folds approximately in the presence of C. freundii C9, and operons associated with phenazine synthesis, phzA1 and phzA2, were also upregulated. Disruption of the phzA1 and phzA2 eliminated the exacerbated pathogenicity in the co-infection group, indicating that the elevated pyocyanin production was the main contributing factor. The results suggest that co-existing C. freundii during P. aeruginosa infection can exacerbate the pathogenicity, which may have significant implications in patients infected with these bacteria.