Differences in invasion and adherence of Listeria monocytogenes with mammalian gut cells

A comparison of selected methods for measuring the virulence properties of Listeria spp

The comparative ability of different methods to assess virulence of Listeria species was investigated in ten Listeria strains. All strains were initially subjected to pulsed-field gel electrophoresis analysis to determine their relatedness. Virulence characteristics were subsequently tested for by (i) determining the presence of six virulence genes by polymerase chain reaction; (ii) testing for the production of listeriolysin O, phosphatidylcholine phospholipase C, and phosphatidylinositol-specific phospholipase C; (iii) investigating the hydrophobicity of the strains; (iv) determining the strains ability to attach to, enter, and replicate within the Caco-2 cells. Variations in most of the virulence characteristics were obvious across the strains for the range of tests performed. A wide range of anomalous results among methods were apparent. In particular, the presence of virulence genes was found to be unrelated to the production of virulence-associated proteins in vitro, while virulence protein production and hydrophobicity in Listeria monocytogenes were found to be unrelated or marginally related, respectively, to the ability to invade the Caco-2 cell line. It was concluded that the methods investigated were unable to consistently and unequivocally measure the differences in the virulence properties of the strains.

Achieving continuous improvement in reductions in foodborne listeriosis--a risk-based approach

Listeria monocytogenes is a foodborne pathogen that can cause listeriosis, a severe disease that can lead to septicemia, meningitis, and spontaneous abortion. Ongoing efforts are needed to further reduce the incidence of listeriosis, due to its high mortality rate. The focus of this report is the use of a risk-based approach to identify strategies that will have the greatest impact on reducing foodborne listeriosis. A continuum of risk for listeriosis is observed in the human population, ranging from exquisitely sensitive groups, who are highly immunocompromised and at very high risk of listeriosis, through the normal healthy population younger than 65 years of age, who appear to have a minimal risk for listeriosis. In addition, unique subpopulations may exist; for example, pregnant Latina women appear to have a higher risk of listeriosis than pregnant women of other ethnic groups, most likely due to consumption of contaminated soft cheeses such as queso fresco and queso blanco. The International Life Sciences Institute Risk Science Institute Expert Panel concluded that certain foods pose a high risk for causing listeriosis. High-risk foods have all of the following properties: (1) have the potential for contamination with L. monocytogenes; (2) support the growth of L. monocytogenes to high numbers; (3) are ready to eat; (4) require refrigeration; and (5) are stored for an extended period of time. Control strategies are needed in the food chain from preharvest through consumption to minimize the likelihood that food will become contaminated by L. monocytogenes and to prevent the growth of the organism to high numbers. The Expert Panel identified three main strategies for ensuring continuous improvement in reducing foodborne listeriosis: (1) preventing contamination of foods with L. monocytogenes; (2) preventing growth of L. monocytogenes to high numbers in foods; and (3) science-based education messages targeted to susceptible populations and their caregivers. Of these strategies, the Expert Panel concluded that preventing growth of L. monocytogenes to high numbers would have the greatest impact in reducing cases of listeriosis. Dose-response models predict that the risk of listeriosis increases as the number of organisms in a food increases and can be used as a scientific basis for a target level below which the organism should be reduced to minimize the likelihood of listeriosis in high-risk populations. This requires implementation of effective food safety control measures and ensuring that these control strategies are consistently met. Most effective strategies to control L. monocytogenes in high-risk foods include (1) good manufacturing practices, sanitation standard operating procedures, and hazard analysis critical control point programs to minimize environmental L. monocytogenes contamination and to prevent cross-contamination in processing plants and at retail; (2) an intensive environmental sampling program in plants processing high-risk foods and an effective corrective action plan to reduce the likelihood of contamination of high-risk foods; (3) time and temperature controls throughout the entire distribution and storage period, including establishing acceptable storage times of foods that support growth of L. monocytogenes to high numbers; (4) reformulating foods to prevent or retard the growth of L. monocytogenes; and (5) using postpackaging treatments to destroy L. monocytogenes on products. Science-based education and risk communication strategies aimed at susceptible populations and focused on high-risk foods should be delivered through health care providers or other credible sources of information. Exquisitely sensitive consumers may become ill when exposed to low numbers of L. monocytogenes or other opportunistic pathogens, so reducing the risk to this population could be achieved by maintaining them on restricted low-microbe diets during those periods when they are most severely immunocompromised. High-risk individuals (i.e., the elderly, pregnant women, and most immunocompromised individuals) should be provided with guidance on healthy eating, including specific information on high-risk foods that they should avoid, and strategies to reduce their risk, such as thorough cooking, avoidance of cross-contamination, and short-term refrigerated storage of cooked perishable foods. Those at low risk for listeriosis should receive information on safe food handling practices, preferably starting at a preschool age.

Adhesion, invasion, and translocation characteristics of Listeria monocytogenes serotypes in Caco-2 cell and mouse models

Adhesion is a crucial first step in Listeria monocytogenes pathogenesis. In this study, we examined how the adhesion properties of serotypes correlate with their invasion efficiencies in a cell culture model (Caco-2) and in a mouse model. Adhesion characteristics of all 13 serotypes of L. monocytogenes (25 strains) were analyzed, which yielded three distinct groups (P < 0.05) with high-, medium-, and low-level-adhesion profiles. The efficiency of these strains in invading the Caco-2 cell line was analyzed, which produced two groups; however, the overall correlation (R(2)) was only 0.1236. In the mouse bioassay, all selected strains, irrespective of their adhesion profiles, translocated to the liver and the spleen with almost equal frequencies that did not show any clear relationship with adhesion profiles. However, the serotypes with increased adhesion showed a slightly increased translocation to the brain (R(2) = 0.3371). Collectively, these results indicate that an in vitro adhesion profile might not be an accurate assessment of a strain's ability to invade a cultured cell line or organs or tissues in a mouse model.

Influence of temperature and growth phase on expression of a 104-kilodalton Listeria adhesion protein in Listeria monocytogenes

Interaction of Listeria monocytogenes with mammalian intestinal cells is believed to be an important first step in Listeria pathogenesis. Transposon (Tn916) mutagenesis provided strong evidence that a 104-kDa surface protein, designated the Listeria adhesion protein (LAP), was involved in adherence of L. monocytogenes to a human enterocyte-like Caco-2 cell line (V. Pandiripally, D. Westbrook, G. Sunki, and A. Bhunia, J. Med. Microbiol. 48:117-124, 1999). In this study, expression of LAP in L. monocytogenes at various growth temperatures (25, 37, and 42 degrees C) and in various growth phases was determined by performing an enzyme-linked immunoassay (ELISA) and Western blotting with a specific monoclonal antibody (monoclonal antibody H7). The ELISA and Western blot results indicated that there was a significant increase in LAP expression over time only at 37 and 42 degrees C and that the level of LAP expression was low during the exponential phase and high during the stationary phase. In contrast, there were not significant differences in LAP expression between the exponential and stationary phases at 25 degrees C. Examination of the adhesion of L. monocytogenes cells from exponential-phase (12-h) or stationary-phase (24-h) cultures grown at 37 degrees C to Caco-2 cells revealed that there were not significant differences in adhesion. Although expression of L. monocytogenes LAP was different at different growth temperatures and in different growth phases, enhanced expression did not result in increased adhesion, possibly because only a few LAP molecules were sufficient to initiate binding to Caco-2 cells.

Non-specific resistance mechanisms to listeriosis: implications for experimental and naturally occurring infection

Use of murine listeriosis as an experimental model has greatly increased our understanding of the complex interplay of cells and mediators in non-specific antibacterial resistance (innate immunity). Important contributions made with this experimental model include demonstrating the ability of inflammatory cytokines (i.e. IFN-gamma, IL-1 alpha, TNF-alpha) to protect against bacterial infection, and illustrating the rapidity of the host cytokine response (detectable within 1 h of challenge) during bacterial infection. Most experimental studies of host defense against Listeria monocytogenes (L. monocytogenes) have used a parenteral challenge (i.v. or i.p.). This ignores the pathogenesis of naturally occurring listeriosis, which usually results from ingestion of Listeria-contaminated food products. In this review, we will include consideration of the host-pathogen interactions that occur when L. monocytogenes invades through its natural portal of entry, the gastrointestinal tract. Although resistance to facultative intracellular pathogens, such as L. monocytogenes, was formerly thought to revolve exclusively around the T helper cell/macrophage axis, more recent evidence indicates that neutrophils are able to ingest and kill L. monocytogenes and prevent the unrestricted multiplication of listeriae in parenchymal cells. Exploring the mechanisms involved in this process will provide new insights into the communication between leukocytes and tissue cells in host defense.

Requirements for invasion of epithelial cells by Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans, an oral bacterium implicated in human periodontal disease, was recently demonstrated to invade cultured epithelial cells (D. H. Meyer, P. K. Sreenivasan, and P. M. Fives-Taylor, Infect. Immun. 59:2719-2726, 1991). This report characterizes the requirements for invasion of KB cells by A. actinomycetemcomitans. The roles of bacterial and host factors were investigated by using selective agents that influence specific bacterial or host cell functions. Inhibition of bacterial protein synthesis decreased invasion, suggesting the absence of a preformed pool of proteins involved in A. actinomycetemcomitans invasion. Inhibition of bacterial and eukaryotic energy synthesis also decreased invasion, confirming that A. actinomycetemcomitans invasion is an active process. Bacterial adherence to KB cells was indicated by scanning electron microscopy of infected KB cells. Further, the addition of A. actinomycetemcomitans-specific serum to the bacterial inoculum reduced invasion substantially, suggesting a role for bacterial attachment in invasion. Many of the adherent bacteria invaded the epithelial cells under optimal conditions. Inhibitors of receptor-mediated endocytosis inhibited invasion by A. actinomycetemcomitans. Like that of many facultatively intracellular bacteria, A. actinomycetemcomitans invasion was not affected by eukaryotic endosomal acidification. These are the first published observations describing the requirements for epithelial cell invasion by a periodontopathogen. They demonstrate that A. actinomycetemcomitans utilizes a mechanism similar to those used by many but not all invasive bacteria to gain entry into eukaryotic cells.