Comparison of the invasion strategies used by Salmonella cholerae-suis, Shigella flexneri and Yersinia enterocolitica to enter cultured animal cells: endosome acidification is not required for bacterial invasion or intracellular replication

Multiple beta 1 chain integrins are receptors for invasin, a protein that promotes bacterial penetration into mammalian cells

Mammalian cell receptors that promote entry of intracellular bacteria into nonphagocytic cells have not been identified. We show here that multiple members of the integrin superfamily of cell adhesion receptors bind the Y. pseudotuberculosis invasin protein prior to bacterial penetration into mammalian cells. Affinity chromatography of crude detergent extracts demonstrated that integrins containing the subunit structures alpha 3 beta 1, alpha 5 beta 1, and alpha 6 beta 1 bound to immobilized invasin. Furthermore, phospholipid vesicles containing isolated integrin proteins were able to attach to invasin. Specificity for invasin binding to the identified integrin receptors was also demonstrated, as immunoprobing and phospholipid reconstitution studies showed that the alpha 2 beta 1 integrin, beta 2 chain integrins, and vitronectin receptor (alpha v beta 3) were not involved in cellular attachment to invasin.

Invasion and replication of Salmonella typhimurium in animal cells

A total of 81 avirulent Tn10 insertion mutants of Salmonella typhimurium have previously been described. These mutants were selected for the inability to survive in murine macrophages. We have characterized the abilities of the most avirulent of these mutants to adhere to, invade, and replicate in both macrophages and nonphagocytic epithelial cells. The results suggest that most mutants contain a defect that is specific to survival within professional phagocytes. These mutants invaded and replicated normally within nonphagocytic human colon adenocarcinoma cells (Caco-2) but did not survive in the macrophage cell line J774. One mutant invaded both macrophages and epithelial cells much less efficiently than the parental strain. The defect associated with this mutant appears to be a result of decreased adherence to animal cells.

The role of IFN-gamma in murine Salmonella typhimurium infection

Influence of both recombinant interferon gamma (r-IFN-gamma) and monoclonal anti-IFN-gamma on murine Salmonella typhimurium infection was studied in vivo. As a challenge we used either a virulent or an avirulent strain of S. typhimurium. The avirulent strain is unable to replicate in the mouse spleen. The effect of IFN-gamma or anti-IFN-gamma treatment on infection was assayed by counting the number of bacteria in the spleens 1, 2, 4 or 7 days after infection. Exogenously supplied IFN-gamma was found to decrease the number of both the virulent and the avirulent bacteria in the spleens 1 day after challenge but thereafter had no detectable effect. Anti-IFN-gamma-treated mice were unable to clear a sub-lethal dose of the virulent bacteria. The enhanced growth of the bacteria in the spleen was seen after 2 days of infection. In the spleens of anti-IFN-gamma-treated mice and control mice the fate of the non-replicating bacteria was similar. We conclude that the mice produce IFN-gamma during the early phase of Salmonella infection and that this causes a reduction in the apparent growth rate of the virulent bacteria. Because the avirulent bacteria, which do not multiply, were not affected by anti-IFN-gamma treatment, the effect of IFN-gamma is primarily bacteriostatic rather than bactericidal. After challenge, the production of IFN-gamma seems to start with a lag of approximately 2 days.

Salmonella as an intracellular parasite

Salmonella species are facultative intracellular parasites, capable of penetrating (invading), surviving, and often multiplying within diverse eukaryotic cell types, including epithelial and phagocytic cells. These processes are essential for virulence, and involve both bacterial and host cell products. The use of cultured eukaryotic cells and other model systems has facilitated the study of bacterial-host cell interactions, and has led to a better understanding of the genetic and molecular basis of Salmonella pathogenicity.

Common themes in microbial pathogenicity

A bacterial pathogen is a highly adapted microorganism which has the capacity to cause disease. The mechanisms used by pathogenic bacteria to cause infection and disease usually include an interactive group of virulence determinants, sometimes coregulated, which are suited for the interaction of a particular microorganism with a specific host. Because pathogens must overcome similar host barriers, common themes in microbial pathogenesis have evolved. However, these mechanisms are diverse between species and not necessarily conserved; instead, convergent evolution has developed several different mechanisms to overcome host barriers. The success of a bacterial pathogen can be measured by the degree with which it replicates after entering the host and reaching its specific niche. Successful microbial infection reflects persistence within a host and avoidance or neutralization of the specific and nonspecific defense mechanisms of the host. The degree of success of a pathogen is dependent upon the status of the host. As pathogens pass through a host, they are exposed to new environments. Highly adapted pathogenic organisms have developed biochemical sensors exquisitely designed to measure and respond to such environmental stimuli and accordingly to regulate a cascade of virulence determinants essential for life within the host. The pathogenic state is the product of dynamic selective pressures on microbial populations.

Epithelial cell surfaces induce Salmonella proteins required for bacterial adherence and invasion

Salmonella bacteria are capable of entering (invading) and multiplying within eukaryotic cells. Stable adherence to and invasion of epithelial cells by S. choleraesuis and S. typhimurium were found to require de novo synthesis of several new bacterial proteins. This inducible event appears to be a coordinately regulated system dependent on trypsin- and neuraminidase-sensitive structures present on the epithelial cell surface. Mutants of S. choleraesuis and S. typhimurium were unable to synthesize these proteins and did not stably adhere to nor invade eukaryotic cells. Two such S. typhimurium mutants were avirulent in mice, an indication that these proteins are required for Salmonella virulence.

Identification and characterization of TnphoA mutants of Salmonella that are unable to pass through a polarized MDCK epithelial cell monolayer

Surface protein mutants of the invasive Salmonella species, S. choleraesuis, were generated using the transposon TnphoA. 626 alkaline phosphatase (PhoA+) fusion mutants were identified and screened for their ability to pass through (transcytose) polarized epithelial monolayers of Madin Darby canine kidney (MDCK) cells grown on membrane filters. Forty two mutants were unable to pass through this barrier. All of these transcytosis mutants were unable to adhere to or invade MDCK monolayers, yet these mutations were not in the genes encoding type 1 pili or mannose-resistant haemagglutination (MRHA). These transcytosis mutants could be grouped into six classes. Class 1 mutants had altered lipopolysaccharide (LPS) O side-chain structures while Class 2 mutants had defects in their LPS core. Mutants belonging to Classes 5 and 6 did not decrease the transepithelial electrical resistance of polarized MDCK cell monolayers, in contrast to the parental strain and the other mutants (Classes 1, 2, 3 and 4). Mutants belonging to Class 1 were less virulent in mice, while Class 2 (defective core) and Classes 4 and 5 (normal LPS) mutant strains were avirulent in mice. Mutants from Classes 3 and 6 were as virulent in mice as S. choleraesuis. These results suggest that the ability to pass through epithelial barriers may be an important virulence characteristic of Salmonella. These data indicate that bacterial adherence, internalization and monolayer transcytosis are closely linked events. It was also demonstrated that a mutant with decreased rates of intracellular replication still passed through the monolayer at rates similar to wild-type S. choleraesuis.