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.

感染部位除病原体外还存在其他细菌物种，可能会影响细菌感染的进程。基于这样的事实，弗氏柠檬酸杆菌可以在共存绿脓杆菌感染，本研究旨在探讨共存的影响弗氏柠檬酸杆菌的发病机制绿脓杆菌感染。使用鼠腹膜炎模型比较存在和不存在弗氏梭菌的情况下铜绿假单胞菌PAO1感染的死亡率和组织病理学临床分离株C9。我们还研究了花青素的产生并比较了基因表达水平，从而研究了PAO1和C9之间的细胞间相互作用。结果表明，与C9共同感染可显着提高PAO1感染小鼠的死亡率和组织损伤。在接种量为10 ⁶ CFU的情况下，C9感染组在感染后三天未观察到死亡率，而PAO1-C9联合感染组的死亡率为64％，而PAO1感染组为24％ 。大约在弗氏梭菌C9和与吩嗪合成有关的操纵子，phzA1和phzA2的存在下，铜绿假单胞菌PAO1中的氰化蛋白产量增加了8倍，也被上调。phzA1和phzA2的破坏消除了共感染组的致病性加剧，表明黄体生成素的增加是主要的致病因素。结果表明，在铜绿假单胞菌感染期间共存的弗氏梭状芽胞杆菌可加剧致病性，这可能会对感染这些细菌的患者产生重大影响。