P. mosselii was formally described as a novel species in 2002 through a polyphasic taxonomic approach including 16SrDNA phylogeny, numerical analysis, DNA–DNA hybridization, thermal stability of DNA–DNA hybrids and siderophore-typing methodology . The several strains of P. mosselii described to date were isolated in hospital and some have been suggested as emerging human pathogens [19–21]. Our study aimed at investigating the virulence potential of two of these strains, namely ATCC BAA-99 and MFY161, belonging to the same cluster strongly related to the hospital-isolated P. putida on the basis of both oprD or oprF-linked phylogenies . Although P. putida species is mostly known for its huge capacity in degradation of numerous carbon sources , some clinical strains have emerged, causing infections in immunosuppressed hosts and patients with invasive medical devices. More recently, P. putida has been involved in war wound infection, and should be considered as a potential human pathogen, for a review see Carpenter et al. .
In the present study, we further investigated the cytotoxicity of P. mosselii ATCC BAA-99 and MFY161 strains, and show that they provoked the lysis of the intestinal epithelial cells Caco-2/TC7, with a major damage obtained after infection with P. mosselii MFY161. The cytotoxic levels were lower compared to the well-known opportunistic pathogen P. aeruginosa PAO1 but almost similar to those observed for P. mosselii strains on rat glial cells , and for the clinical strain P. fluorescens MFN1032 on Caco-2/TC7 cells . The gentamicin exclusion test showed that P. mosselii ATCC BAA-99 and MFY161 can enter Caco-2/TC7 cells. The invasion capacity of the two P. mosselii strains studied was similar and lower than that of the pathogen P. aeruginosa PAO1.
The bacterial proinflammatory effect of P. mosselii ATCC BAA-99 and MFY161 was then assessed by measuring the secretion of IL-6 and IL-8 cytokines in Caco-2/TC7 after 24 h of infection. The results showed that the two strains did not induce the production of these proinflammatory cytokines. We hypothesize that this may serve as a strategy for P. mosselii to escape the immune system. However, P. mosselii ATCC BAA-99 and MFY161were found to strongly increase the secretion of HBD-2. Human beta-defensins are known to play a key role in host defense. In fact, in addition to their potent antimicrobial properties against commensal and pathogenic bacteria , beta-defensins were demonstrated to function as multieffector molecules capable of enhancing host defense by recruiting various innate as well as adaptive immune cells to the site of infection. Nevertheless, some pathogens can be resistant to HBD-2  and surprisingly can induce and divert HBD-2 secretion in intestinal epithelial cells to enhance its capacity of virulence .
The effect of P. mosselii ATCC BAA-99 and MFY161 on epithelial permeability was then evaluated by measuring the TER across differentiated Caco-2/TC7 monolayers. The F-actin cytoskeleton was stained with Alexa-488 phalloïdin and examined using a confocal laser scanning microscope. We observed that the TER of the monolayers exposed to the bacteria was significantly decreased and that the F-actin cytoskeleton was completely broken. Similar results of TER decrease and F-actin disruption were previously observed with many pathogens including Salmonella typhimurium, P. aeruginosa and Escherichia coli[28–30].
Infections caused by multidrug-resistant (MDR) Gram-negative bacilli have become a growing challenge in hospital . In a recent study, Giani et al.  suggested that unusual human opportunistic pathogen like P. mosselii may probably play a role as shuttles for acquired metallo-β-lactamases resistance thus an antibiogram was made for P. mosselii ATCC BAA-99 and MFY161 (see Additional file 1: Table S1). We found that the two strains were resistant towards 6 of the 16 antibiotics tested including the ticarcillin beta-lactam, which could support the above hypothesis.