Virulence of Listeria Monocytogenes in Mice is Enhanced by Deletion of Pathogenicity Island 4

Listeria monocytogenesis ( L. monocytogenes)a facultative anaerobic zoonotic intracellular pathogen. Pathogenicity island 4 (LIPI-4) is a newly discovered virulence gene cluster involved in the central nervous system (CNS) infection of L. monocytogenes. To explore the role of LIPI-4 in the virulence of L. monocytogenes, a frozen chicken isolate LM928 LIPI-4 gene deletion strain (ΔLIPI-4) and complement strain (CΔLIPI-4) were constructed to infect human brain microvascular endothelial cells (HCMECs). The effect of LIPI-4 on L. monocytogenes virulence was determined through bacterial adhesion, cellular invasion, and intracellular proliferation evaluation by noting the median lethal dose (LD50) in mice, the number of bacteria in the tissue, and the expression of virulence factors in vivo and in vitro by RT-qPCR. The results showed that LIPI-4 deletion decreased cellular adhesion, cellular invasion, and intracellular proliferation of L. monocytogenes to HCMECs cells. The LD50 of ΔLIPI-4 infected mice was 1.0 and 0.7 orders of magnitude lower than that of LM928 and CΔLIPI-4, respectively. The tissue load of ΔLIPI-4 was significantly higher (P<0.01) than that of LM928 and CΔLIPI-4. In BHI culture, the expression of important virulence genes was significantly down-regulated (P<0.01) in the ΔLIPI-4 strains. However, transcription levels of actA, inlA, inlB, and inlC were significantly up-regulated (P<0.01) while hly, prfA, plcA, and plcB were significantly down-regulated (P<0.01) in ΔLIPI-4 infected HCMECs.This data suggests that LIPI-4 acts as a virulence factor involved in L. monocytogenes infection. Its deletion may contribute to decreasing the virulence of L. monocytogenes in mice.