Monocytogenes Survival Research Articles

Evidence for expressional crosstalk between the central virulence regulator PrfA and the stress response mediator ClpC in Listeria monocytogenes.

Virulence is a multifactorial trait which depends on the coordinated expression of many bacterial products, hence it is to be expected that the regulatory circuits that control the relevant genetic determinants are somehow interconnected. Two pleiotropic regulatory elements acting at different levels, the transcription factor PrfA which controls virulence gene expression and the potential chaperone ClpC which is involved in tolerance to environmental stress, are required for Listeria monocytogenes survival within the host. We analyzed the influence of PrfA on clpC expression in L. monocytogenes. clpC transcription is maximal under heat-shock conditions, i.e. at 42 degrees C, and is very weak or undetectable at 37 degrees C. In a prfA* mutant which constitutively overexpresses PrfA and PrfA-dependent virulence genes, clpC transcription dropped to basal levels during exponential growth at 42 degrees C. This repression was not observed during stationary phase, indicating growth phase-dependent regulation of clpC. Culture in charcoal-treated medium, which triggers in wild-type strains the transcriptional activation of the PrfA regulon, also caused a strong downregulation of clpC. Moreover, in a prfA deletion mutant, clpC transcription during exponential growth at 37 degrees C was clearly enhanced, reaching the same high levels of the wild-type at 42 degrees C. Overall, our results indicate that clpC expression is negatively controlled at the transcriptional level, directly or indirectly, by the central virulence regulator PrfA.

Listeria monocytogenes Survival in Soil and Incidence in Agricultural Soils

The effects of soil type, inoculum level, microbial competition, fertilizers, and carbon and nitrogen supplementation on Listeria monocytogenes survival in soil were examined by utilizing soil columns in ABS (acrylonitrile butadiene styrene) plastic piping. Sandy soil yielded a lower level of L. monocytogenes survival than clay loam or sandy loam soils. L. monocytogenes numbers decreased from high inoculum levels, increased when inoculum levels were low, and reached higher levels more quickly in autoc1aved soil. Soil amended with solid chicken manure supported a higher population than soil amended with either liquid hog manure or inorganic nitrogen-phosphorus-potassium fertilizer, but only when microbial competitors had been reduced. Carbon and nitrogen supplementation had no effect on the population of L. monocytogenes. In a field survey analyzed by using a 3-tube most probable number procedure with enrichment, Listeria spp. were present in 1 of 13 cultivated fields and 6 of the 13 surrounding uncultivated meadows.

The relationship between ecophysiology, indigenous microflora and growth of Listeria monocytogenes in grass silage.

The combined effect of the physical and chemical parameters (oxygen tension, pH and dry matter) influencing Listeria monocytogenes growth and survival in silage were simultaneously studied in a model in vitro system. Ensiled grass was exposed to a range of low oxygen concentrations, 0-5% v/v, and their effect was recorded with respect to acidification and microbial population dynamics of the epiphytic microflora, i.e. lactic acid bacteria, enterobacteria, yeasts, moulds and L. monocytogenes in grasses pre-inoculated with the latter. Listeria monocytogenes survival depended on the establishment of a fine balance between the physico-chemical and microbiological characteristics, i.e. oxygen tension, dry matter, pH, grass and microbiological quality. In all grasses ensiled, an oxygen concentration of 1.0% or greater sustained L. monocytogenes growth, below this level growth was shown to be principally dependent on the rate and quality of the fermentation. In most grasses 0.5% oxygen prolonged survival, whereas 0.1% and 0% oxygen caused L. monocytogenes to die off. In very poor quality grass with a restricted fermentation L. monocytogenes survival was prolonged even under anaerobic conditions.

Listeria monocytogenes Survival Model Validated in Simulated Uncooked‐Fermented Meat Products for Effects of Nitrite and pH

ABSTRACTPrevious modeling studies in broth cultures demonstrated that acidity and nitrite increased the inactivation rate of Listeria monocytogenes. To validate this effect during storage of simulated uncooked‐fermented meat products, lean beef was ground with salt, adjusted to pH 4.0–5.1, and treated with nitrite at 0–300 μg/mL. Samples were immediately inoculated with L. monocytogenes (107 CFU/g) and survivors were enumerated over 21 days storage at 37°C. The time to achieve a four log decline as greatly affected by pH, ranging from 21 days at pH 5.0 to < 1.0 day at pH 4.0. Growth occurred at pH 5.1 after a long lag period. Nitrite additions did not affect survival, suggesting that the effective concentration was the rapidly decreasing residual nitrite level.

Fate of Listeria monocytogenes in raw and cooked ground beef with meat processing additives

The effect of sodium lactate (1.8% w/w), sodium erythorbate (0.1% w/w), kappa-carrageenan (1% w/w), and the alginate meat binder (0.4% w/w, sodium alginate; 0.6% w/w lactic acid; and 0.075% w/w calcium carbonate) on Listeria monocytogenes survival and growth was determined in raw and cooked ground beef stored aerobically at 4°C. There was no significant ( P > 0.05) increase in numbers of L. monocytogenes during storage of raw ground beef. However, L. monocytogenes numbers were generally lower in treatments with sodium lactate, and higher in sodium erythorbate compared to controls and meat with other additives. Increases in total aerobic plate counts were less pronounced in raw meat formulated with sodium lactate and alginate meat binder than with other additives. Cooking meat with initial inoculum levels of 6.52 to 7.03 L. monocytogenes log CFU/g to 65°C resulted in lower destruction (0.56 and 1.18 log CFU/g) in samples with added alginate meat binder and kappa-carrageenan, respectively, compared to the control. Survivors (2.11–3.73 log CFU/g) decreased initially and then increased slightly, but not significantly ( P > 0.05), during storage (4°C, 6 days) of the cooked products.

Cell-mediated immunity to intestinal infection.

Specified pathogen-free B6D2F1 mice were orally infected with various doses of Listeria monocytogenes. Oral inocula containing more than 2.5 X 10(8) live organisms consistently initiated infection in the Peyer's patches (PP) of the small intestine. At lower doses the infection was sporadic, with many mice showing no apparent infection in the PP. The PP appeared to be the only site of tissue invasion and L. monocytogenes survival in the intestinal tissues, as no organisms were recovered from mucosa dissected free of all visible PP. Within the PP, the bacteria multiplied and the infection then disseminated to the mesenteric lymph node (MLN), liver, and spleen. However, bacteria were almost completely eliminated from all tissues, both systemic and gut-associated by 6 days postinfection. Mice given a primary L. monocytogenes infection by the oral route were highly resistant to subsequent intravenous or oral challenge. Likewise, sublethal intravenous infection rendered mice highly resistant to subsequent oral infection. In addition, lymphocytes from the PP, MLN, and spleens of mice recovering from a primary oral infection were able to adoptively transfer immunity to normal recipients. Finally, after oral infection, mice did not display peripheral delayed hypersensitivity to L. monocytogenes antigens until the organisms had penetrated to the spleen.