Listeria monocytogenes: clinical and experimental update

Experimental validation of low virulence in field strains of Listeria monocytogenes

Several reports have described Listeria monocytogenes strains which were nonpathogenic or weakly pathogenic, but little is known about these low-virulence strains. We found that 9 field L. monocytogenes strains were hypovirulent and 17 were avirulent, based on the number of mice contaminated and the colonization of their spleens after subcutaneous inoculation. All these strains possessed the known virulence genes. We have now assessed the low virulence of these strains in other assays before determining how they differ from virulent strains. We have shown that the low-virulence strains exhibited a phenotypic stability and were not a mixture of virulent and avirulent bacteria. They did not recover virulence after many passages in mice and colonized the spleens of mice more poorly than virulent strains after i.v. inoculation. Their lethal capacities, determined by 50% lethal dose (LD(50)), were lower than those of virulent strains. Like Listeria innocua, 14 of 17 avirulent strains had no LD(50) and were eliminated by the lymph nodes after subcutaneous inoculation. The virulent, hypovirulent, and avirulent strains were always significantly different, whatever the tests of virulence used, confirming the importance of these low-virulence field strains in identifying the proteins involved in virulence.

The lymphotoxin beta receptor is critically involved in controlling infections with the intracellular pathogens Mycobacterium tuberculosis and Listeria monocytogenes

Containment of intracellularly viable microorganisms requires an intricate cooperation between macrophages and T cells, the most potent mediators known to date being IFN-gamma and TNF. To identify novel mechanisms involved in combating intracellular infections, experiments were performed in mice with selective defects in the lymphotoxin (LT)/LT beta R pathway. When mice deficient in LT alpha or LT beta were challenged intranasally with Mycobacterium tuberculosis, they showed a significant increase in bacterial loads in lungs and livers compared with wild-type mice, suggesting a role for LT alpha beta heterotrimers in resistance to infection. Indeed, mice deficient in the receptor for LT alpha(1)beta(2) heterotrimers (LT beta R-knockout (KO) mice) also had significantly higher numbers of M. tuberculosis in infected lungs and exhibited widespread pulmonary necrosis already by day 35 after intranasal infection. Furthermore, LT beta R-KO mice were dramatically more susceptible than wild-type mice to i.p. infection with Listeria monocytogenes. Compared with wild-type mice, LT beta R-KO mice had similar transcript levels of TNF and IFN-gamma and recruited similar numbers of CD3(+) T cells inside granulomatous lesions in M. tuberculosis-infected lungs. Flow cytometry revealed that the LT beta R is expressed on pulmonary macrophages obtained after digestion of M. tuberculosis-infected lungs. LT beta R-KO mice showed delayed expression of inducible NO synthase protein in granuloma macrophages, implicating deficient macrophage activation as the most likely cause for enhanced susceptibility of these mice to intracellular infections. Since LIGHT-KO mice proved to be equally resistant to M. tuberculosis infection as wild-type mice, these data demonstrate that signaling of LT alpha(1)beta(2) heterotrimers via the LT beta R is an essential prerequisite for containment of intracellular pathogens.

Severe impairment in early host defense against Listeria monocytogenes in mice deficient in acid sphingomyelinase

The phagolysosomal compartment is crucial for the defense against infection with intracellular pathogens. Within this compartment, the TNF- and IFN-gamma-responsive acid sphingomyelinase (ASMase) generates the signaling molecule ceramide, resulting in the activation of proteases like cathepsin D. To investigate the possible role of ASMase as a mediator of the antibacterial effects of TNF and IFN-gamma, ASMase(-/-) mice were infected with Listeria monocytogenes. ASMase(-/-) mice showed a dramatically increased susceptibility to L. monocytogenes (LD(50) approximately 100 CFU) when compared with syngeneic wild-type mice (LD(50) approximately 10,000 CFU). In L. monocytogenes-challenged ASMase(-/-) mice, IFN-gamma serum levels as well as IL-1 beta and IL-6 secretion by macrophages were similar to those observed in wild-type C57BL/6 mice. Although macrophages and granulocytes from ASMase(-/-) mice showed intact production of reactive nitrogen intermediates and oxidative burst, ASMase(-/-) macrophages proved completely incapable of restricting the growth of L. monocytogenes in vitro. The results of this study suggest that ASMase is crucially required for the intracellular control of L. monocytogenes in macrophages and granulocytes by nonoxidative mechanisms.

IL-12-assisted immunization generates CD4+ T cell-mediated immunity to Listeria monocytogenes

Mice infected with virulent Listeria monocytogenes develop long-lived acquired immunity. We previously reported that acquired immunity to Listeria could also be elicited by immunizing mice with non-viable Listeria or listerial proteins/peptides in combination with IL-12. Here we show that this IL-12-assisted immunization strategy was effective in class I but not in class II MHC-deficient mice, suggesting that antigen-specific CD4(+) T cells are selectively generated using this adjuvant system. We have also evaluated the importance of endogenous production of IFN-gamma and IL-12 for the efficacy of IL-12-assisted immunization. IFN-gamma-deficient mice immunized with HKLM and IL-12 failed to produce effective Listeria-specific responses. In contrast, IL-12-deficient mice were able to generate protective antigen-specific T cell responses in response to immunization with HKLM and IL-12, indicating that exogenous IL-12 is sufficient to initiate a cytokine cascade that results in a potent T(H)1 response. IL-12-assisted immunization provides a model in which both the generation and effector mechanisms of anti-bacterial antigen-specific CD4(+) effector cells can be analyzed.

Listeria monocytogenes infection as a complication of treatment with tumor necrosis factor alpha-neutralizing agents

OBJECTIVE

Tumor necrosis factor alpha (TNFalpha) has been implicated in the pathogenesis of certain inflammatory diseases. Two TNFalpha-neutralizing agents are licensed in the US. Infliximab is licensed for the treatment of Crohn's disease (CD) and, when used with methotrexate, for the treatment of rheumatoid arthritis (RA). Etanercept is licensed for the treatment of RA, including juvenile RA, and, more recently, was licensed for the treatment of psoriatic arthritis. Because of the potential for decreased host resistance to infectious agents due to treatment with anti-TNFalpha agents, we sought to evaluate postlicensure cases of opportunistic infection, including Listeria monocytogenes, in patients treated with these products.

METHODS

The FDA Adverse Event Reporting System, a passive monitoring system, was reviewed to identify all reports of adverse events (through December 2001) associated with L monocytogenes infection in patients treated with infliximab or etanercept.

RESULTS

Fifteen cases of L monocytogenes infection associated with infliximab or etanercept treatment were identified. In 14 of these cases, patients had received infliximab. The median age of all patients was 69.5 years (range 17-80 years); 53% were female. Six deaths were reported. Among patients for whom an indication for use was reported, there were 9 patients (64%) with RA and 5 patients (36%) with CD (information was not reported for 1 patient). All patients for whom information was reported were receiving concurrent immunosuppressant drugs.

CONCLUSION

Postlicensure surveillance suggests that L monocytogenes infection may be a serious complication of treatment with TNFalpha-neutralizing agents, particularly infliximab.

Bacterial infections: small intestine and colon

Bacterial infections of the small intestine and colon represent a major health problem for developing and developed nations. Recent technological progress has helped research groups to obtain important information on bacterial structure, identify evolutionary relationship between bacterial species, and learn details of the mechanisms involved in the interplay between host and microbes that culminate in disease expression. It is hoped that accumulated knowledge from in vitro experiments and animal models will translate into clinical benefit by means of developing new therapeutic strategies and effective vaccines.

Foodborne enteric infections

Foodborne infections are estimated to affect one in four Americans each year. Most these (67%) are caused by the Norwalk-like viruses, but Campylobacter and nontyphoidal Salmonellae together account for about one fourth of cases of illness in which a pathogen can be detected. Less common bacterial infections, such as with Listeria monocytogenes and the Shiga toxin-producing Escherichia coli, cause fewer infections but are important because of their severe complications or high mortality rate, or both. This review describes the recent development of a national surveillance system for foodborne illness, newer methods for molecular characterization of organisms for epidemiologic studies, and individual etiologic agents in the order of frequency of occurrence. Methods for decreasing the disease burden are discussed, including education of health care professionals and the public, modification of food-handling behaviors, the use of food irradiation, and the application of probiotics to foods.

Listeria monocytogenes bile salt hydrolase is a PrfA-regulated virulence factor involved in the intestinal and hepatic phases of listeriosis

Listeria monocytogenes is a bacterial pathogen causing severe food-borne infections in humans and animals. It can sense and adapt to a variety of harsh microenvironments outside as well as inside the host. Once ingested by a mammalian host, the bacterial pathogen reaches the intestinal lumen, where it encounters bile salts which, in addition to their role in digestion, have antimicrobial activity. Comparison of the L. monocytogenes and Listeria innocua genomes has revealed the presence of an L. monocytogenes-specific putative gene encoding a bile salt hydrolase (BSH). Here, we show that the bsh gene encodes a functional intracellular enzyme in all pathogenic Listeria species. The bsh gene is positively regulated by PrfA, the transcriptional activator of known L. monocytogenes virulence genes. Moreover, BSH activity increases at low oxygen concentration. Deletion of bsh results in decreased resistance to bile in vitro, reduced bacterial faecal carriage after oral infection of the guinea-pigs, reduced virulence and liver colonization after intravenous inoculation of mice. Taken together, these results demonstrate that BSH is a novel PrfA-regulated L. monocytogenes virulence factor involved in the intestinal and hepatic phases of listeriosis.