Abstract
Yersinia enterocolitica is one of three pathogenic Yersinia species that share a tropism for lymphoid tissues. However, infection of an immunocompromised host is likely to result in a systemic infection, which is often fatal. Little is known about the bacterial proteins needed to establish such an infection. The genes that encode these virulence factors are likely to be active only during systemic infection. A library of random cat fusions was used to inoculate BALB/c mice. Fusions expressed during a systemic infection were enriched by the administration of chloramphenicol-succinate. Y. enterocolitica isolates recovered from the mice were tested for chloramphenicol resistance in vitro. Fusions that were inactive in vitro were analyzed further and found to represent 31 allelic groups. Each was given a sif (for systemic infection factor) designation. Based on homology to known proteins, the sif genes are likely to encode proteins important for general physiology, transcription regulation, and other functions. During systemic infections, 13 of the sif-cat fusions were able to outcompete the wild type in the presence of chloramphenicol-succinate, confirming that the fusions were active. The in vitro expression of several sif genes was determined, showing modest changes in response to various growth conditions. A mutation in sif15, which encodes a putative outer membrane protein, caused attenuation during systemic infection but not during colonization of the Peyer's patches. Comparisons between the Y. enterocolitica sif genes and the previously identified hre genes imply that very different groups of genes are active during a systemic infection and during colonization of the Peyer's patches.