Factors of the Bacterial Pathogen Listeria monocytogenes which Influence Niche Acquisition within Hosts

Invasive cardiac infections are a recognized yet poorly understood facet of Listeria monocytogenes pathogenesis. It had been previously reported that cardiotropic strains of L. monocytogenes shared highly related alleles of an operon whose gene products are known to contribute to host cell invasion. The genes, inlA and inlB have been well-studied in multiple systems and have been shown to influence tissue tropism in infected animals. To investigate whether inlA and/or inlB contributed to cardiotropism, the inlA and inlB alleles from the laboratory non-cardiotropic strain, 10403s, as well as the highly cardiotropic strain, 07PF0776, were inserted into plasmid vectors under the control of IPTG-inducible promoters and introduced into mutant strains of 10403s lacking inlAB. Increased levels of either 10403S or 07PF0776 InlA reduced L. monocytogenes invasion of H9c2 heart cells. In contrast, increasing InlB expression enhanced bacterial invasion of heart cells, such that the 07PF0776 allele of inlB enhanced 10403s invasion to the level of 07PF0776, whereas the 10403s allele of inlB did not show similar enhancement. Infection of female Swiss Webster mice at sub-lethal doses indicated that mice infected with strains over-expressing InlB from 07PF0776 were more likely to exhibit bacterial colonization of the heart than those expressing InlB from 10403s. Mice infected with 10403S expressing 07PF0776 InlB also exhibited higher colony burdens in the heart, whereas liver and spleen colonization was similar between the strains. In pregnant animals, strains over-expressing InlB were enhanced for their ability to colonize the placentas and fetuses during blood-stream infections, independently of allelic variation. Additionally, the relative expression of inlA to inlB was found to perturb cellular invasion, suggesting a dynamic invasion complex between the two gene products. Using in vitro assays of cellular invasion, it was found that variants of InlB responded differently to host proteoglycans. Given the variability of host proteoglycans between different tissues, it is likely that the variation in InlB heparan utilization ultimately affected the ability of the bacterium to invade the cells across different tissues, leading to different manifestations of disease.