Biotic and Abiotic Properties Mediating Plant Diversity Effects on Soil Microbial Communities in an Experimental Grassland

Plant diversity drives changes in the soil microbial community which may result in alterations in ecosystem functions. However, the governing factors between the composition of soil microbial communities and plant diversity are not well understood. We investigated the impact of plant diversity (plant species richness and functional group richness) and plant functional group identity on soil microbial biomass and soil microbial community structure in experimental grassland ecosystems. Total microbial biomass and community structure were determined by phospholipid fatty acid (PLFA) analysis. The diversity gradient covered 1, 2, 4, 8, 16 and 60 plant species and 1, 2, 3 and 4 plant functional groups (grasses, legumes, small herbs and tall herbs). In May 2007, soil samples were taken from experimental plots and from nearby fields and meadows. Beside soil texture, plant species richness was the main driver of soil microbial biomass. Structural equation modeling revealed that the positive plant diversity effect was mainly mediated by higher leaf area index resulting in higher soil moisture in the top soil layer. The fungal-to-bacterial biomass ratio was positively affected by plant functional group richness and negatively by the presence of legumes. Bacteria were more closely related to abiotic differences caused by plant diversity, while fungi were more affected by plant-derived organic matter inputs. We found diverse plant communities promoted faster transition of soil microbial communities typical for arable land towards grassland communities. Although some mechanisms underlying the plant diversity effect on soil microorganisms could be identified, future studies have to determine plant traits shaping soil microbial community structure. We suspect differences in root traits among different plant communities, such as root turnover rates and chemical composition of root exudates, to structure soil microbial communities.     

Survival of Listeria monocytogenes in Experimentally Infected Mice

Physiological saline was found to be very detrimental to the viability of Listeria monocytogenes. The LD(50) value was substantially reduced when peptone was used as the suspending fluid rather than saline. Normal splenic tissue was not inhibitory to the survival of Listeria. In experimentally infected mice, L. monocytogenes survived for 8 days in the peritoneal cavity and for at least 11 days in the spleen.     

Survival of Salmonella enteritidis and Listeria monocytogenes on salad vegetables

The influence of initial head-spaces of air – 4.9% CO₂/2.1% O₂/93% N₂ and 5% CO₂/5.2% O₂/89.8% N₂ – on Salmonella enteritidis and Listeria monocytogenes, and on microbial association with shredded carrots and lettuce was studied at 4 °C. Both these pathogens survived but did not grow in any vegetable regardless of the packaging system used. Total viable count, lactic acid bacteria and pseudomonads were also monitored. Lactic acid bacteria were the predominant organisms in all samples. The pH dropped significantly during the storage of vegetables.     

Survival of Listeria monocytogenes in milk during high-temperature, short-time pasteurization

Milk from cows inoculated with Listeria monocytogenes was pooled for 2 to 4 days and then heated at 71.7 to 73.9 degrees C for 16.4 s or at 76.4 to 77.8 degrees C for 15.4 s in a high-temperature, short-time plate heat exchanger pasteurization unit. L. monocytogenes was isolated from milk after heat treatment in six of nine pasteurization trials done at 71.7 to 73.9 degrees C and in none of three trials done at 76.4 to 77.8 degrees C. An average of 1.5 to 9.2 L. monocytogenes cells was seen in each milk polymorphonuclear leukocyte before heat treatment in 11 of 12 pasteurization trials. Noticeable degradation of leukocytes with intracellular listeria was detected in unpasteurized milk after 3 days of storage at 4 degrees C, and by 4 days of storage leukocytes had deteriorated to cellular debris, suggesting that holding unpasteurized milk refrigerated for 4 or more days would eliminate a protective effect leukocytes may provide for increasing heat resistance of L. monocytogenes. Results indicate that under the conditions of this study, L. monocytogenes can survive the minimum high-temperature, short-time treatment (71.7 degrees C, 15 s) required by the U.S. Food and Drug Administration for pasteurizing milk.     

Theme 1- Abiotic and Biotic Stresses

In Vitro Cellular &; Developmental Biology - Plant -     

Survival of Listeria monocytogenes in milk during high-temperature, short-time pasteurization.

Milk from cows inoculated with Listeria monocytogenes was pooled for 2 to 4 days and then heated at 71.7 to 73.9 degrees C for 16.4 s or at 76.4 to 77.8 degrees C for 15.4 s in a high-temperature, short-time plate heat exchanger pasteurization unit. L. monocytogenes was isolated from milk after heat treatment in six of nine pasteurization trials done at 71.7 to 73.9 degrees C and in none of three trials done at 76.4 to 77.8 degrees C. An average of 1.5 to 9.2 L. monocytogenes cells was seen in each milk polymorphonuclear leukocyte before heat treatment in 11 of 12 pasteurization trials. Noticeable degradation of leukocytes with intracellular listeria was detected in unpasteurized milk after 3 days of storage at 4 degrees C, and by 4 days of storage leukocytes had deteriorated to cellular debris, suggesting that holding unpasteurized milk refrigerated for 4 or more days would eliminate a protective effect leukocytes may provide for increasing heat resistance of L. monocytogenes. Results indicate that under the conditions of this study, L. monocytogenes can survive the minimum high-temperature, short-time treatment (71.7 degrees C, 15 s) required by the U.S. Food and Drug Administration for pasteurizing milk.     

The Influence of Biotic and Abiotic Factors on Psammic Rotifers in Artificial and Natural Lakes

The effect of sand grain size, porosity and the abundance of algae on the community structure of psammic rotifers was tested in three anthropogenic lakes (Upper Silesia) and compared with three natural ones (West Pomerania). The structure of grain size in the studied beaches of artificial and natural lakes was similar but differences were found in the abundance of algae. The structure of the psammic rotifer community in Upper Silesian lakes was different from natural lakes in West Pomerania. The structure of psammic rotifer communities in anthropogenic reservoirs and lakes (West Pomerania) differed in numbers of species and numbers of psammobiont species, and also diversity index. The most altered structure of rotifer assemblages was observed in the psammon rotifers of the strongly contaminated Dzierżno Duże Lake.     

Linking Soil Biotic and Abiotic Factors to Apple Replant Disease: a Greenhouse Approach

Apple replant disease (ARD) is a frequently occurring plant disease, which causes retarded growth and mortality of young apple trees in replanted orchards. The aetiology is not well understood, but soil‐borne micro‐organisms are often discussed as primary causal agents of the replant problem. A greenhouse study was conducted in Laimburg, Italy, with orchard soils from the region, with the aim of obtaining information about the influence of soil biotic and abiotic factors on the aetiology of the disease. Apple rootstocks (M9) were planted into soils cultivated with apple trees that were either fumigated with chloropicrin or not fumigated, as well as mixtures of fumigated and non‐fumigated soils. In addition, uncultivated soils (from the inter‐row, from a fallow plot and from a meadow) were taken as controls. Various parameters were measured after 62 days in a controlled pot assay. Soils fumigated with chloropicrin resulted in higher apple shoot growth and lower microbial biomass carbon than non‐fumigated soils. Uncultivated soils had generally the highest microbial biomass carbon and the highest ergosterol contents. No considerable differences between basal respiration, ergosterol content, pH, electrical conductivity, and most nutrient and metal contents were observed between fumigated and non‐fumigated soils. Denaturing gradient gel electrophoresis gels of DNA extracted from the soils revealed differences in the fungal, bacterial and actinobacterial communities of the different soils, indicating significant shifts in microbial community composition after chloropicrin treatment. This study indicates biotic factors in soil to be a causal agent of apple replant disease.     

Survival and heat resistance of Listeria monocytogenes after exposure to alkali and chlorine

A strain of Listeria monocytogenes isolated from a drain in a food-processing plant was demonstrated, by determination of D values, to be more resistant to the lethal effect of heat at 56 or 59 degrees C following incubation for 45 min in tryptose phosphate broth (TPB) at pH 12.0 than to that of incubation for the same time in TPB at pH 7.3. Cells survived for at least 6 days when they were suspended in TPB at pHs 9.0, 10.0, and 11.0 and stored at 4 or 21 degrees C. Cells of L. monocytogenes incubated at 37 degrees C for 45 min and then stored for 48 or 144 h in TPB at pH 10.0 were more resistant to heat treatment at 56 degrees C than were cells stored in TPB at pH 7.3. The alkaline-stress response in L. monocytogenes may induce resistance to otherwise lethal thermal-processing conditions. Treatment of cells in 0.05 M potassium phosphate buffer (pH 7.00 +/- 0.05) containing 2.0 or 2.4 mg of free chlorine per liter reduced populations by as much as 1.3 log(10) CFU/ml, while treatment with 6.0 mg of free chlorine per liter reduced populations by as much as 4.02 log(10) CFU/ml. Remaining subpopulations of chlorine-treated cells exhibited some injury, and cells treated with chlorine for 10 min were more sensitive to heating at 56 degrees C than cells treated for 5 min. Contamination of foods by L. monocytogenes cells that have survived exposure to processing environments ineffectively cleaned or sanitized with alkaline detergents or disinfectants may have more severe implications than previously recognized. Alkaline-pH-induced cross-protection of L. monocytogenes against heat has the potential to enhance survival in minimally processed as well as in heat-and-serve foods and in foods on holding tables, in food service facilities, and in the home. Cells surviving exposure to chlorine, in contrast, are more sensitive to heat; thus, the effectiveness of thermal processing in achieving desired log(10)-unit reductions is not compromised in these cells.     

Survival of Listeria monocytogenes in sea water and effect of exposure on thermal resistance

Survival, recoverability and sublethal injury of two strains of Listeria monocytogenes , Scott A and an environmental strain KM, on exposure to sea water at 12·8 or 20·8 °C was determined using in situ diffusion chambers. Plate counts were used to assess recoverability and injury while 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) reduction was used to determine respiratory activity. T₉₀ values (times for 10-fold decreases in numbers of recoverable cells) on non-selective medium (trypticase soya agar with 0·6% yeast extract) at 12·8 and 20·8 °C were 61·7 and 69·2 h for L. monocytogenes Scott A, and 103·0 and 67·0 h for L. monocytogenes KM, respectively. On selective medium (Oxford agar), T₉₀ values at 12·8 and 20·8 °C were 60·6 and 56·9 h for L. monocytogenes Scott A, and 83·0 and 65·9 h for L. monocytogenes KM, respectively. With Scott A, the percentage of sublethally injured cells at 12·8 and 20·8 °C was 1·7 and 17·7%, respectively, while for KM the values were 19·0 and 1·6%, respectively. The fraction of cells reducing CTC but which were not recoverable on plating progressively increased on exposure to sea water. Listeria monocytogenes KM challenged at 58 °C showed an apparent increase in heat resistance after exposure to sea water at 20·8 °C for 7 d ( D ₅₈= 2·64 min) compared with before exposure ( D ₅₈= 1·24). This increase in thermal resistance was not apparent at temperatures greater than 63 °C, and analysis of the best-fit regression lines fitted to the thermal data obtained from the two cell populations indicated that their thermal resistance was not significantly different ( P > 0·05) over the temperature range tested (58–62 °C).     

Survival and Heat Resistance of Listeria monocytogenes after Exposure to Alkali and Chlorine

A strain of Listeria monocytogenes isolated from a drain in a food-processing plant was demonstrated, by determination of D values, to be more resistant to the lethal effect of heat at 56 or 59°C following incubation for 45 min in tryptose phosphate broth (TPB) at pH 12.0 than to that of incubation for the same time in TPB at pH 7.3. Cells survived for at least 6 days when they were suspended in TPB at pHs 9.0, 10.0, and 11.0 and stored at 4 or 21°C. Cells of L. monocytogenes incubated at 37°C for 45 min and then stored for 48 or 144 h in TPB at pH 10.0 were more resistant to heat treatment at 56°C than were cells stored in TPB at pH 7.3. The alkaline-stress response in L. monocytogenes may induce resistance to otherwise lethal thermal-processing conditions. Treatment of cells in 0.05 M potassium phosphate buffer (pH 7.00 ± 0.05) containing 2.0 or 2.4 mg of free chlorine per liter reduced populations by as much as 1.3 log₁₀ CFU/ml, while treatment with 6.0 mg of free chlorine per liter reduced populations by as much as 4.02 log₁₀ CFU/ml. Remaining subpopulations of chlorine-treated cells exhibited some injury, and cells treated with chlorine for 10 min were more sensitive to heating at 56°C than cells treated for 5 min. Contamination of foods by L. monocytogenes cells that have survived exposure to processing environments ineffectively cleaned or sanitized with alkaline detergents or disinfectants may have more severe implications than previously recognized. Alkaline-pH-induced cross-protection of L. monocytogenes against heat has the potential to enhance survival in minimally processed as well as in heat-and-serve foods and in foods on holding tables, in food service facilities, and in the home. Cells surviving exposure to chlorine, in contrast, are more sensitive to heat; thus, the effectiveness of thermal processing in achieving desired log₁₀-unit reductions is not compromised in these cells.     

Survival of Listeria monocytogenes in yogurt during storage at 4°C

Cows' milk was inoculated with ca 10³and 10⁷cfu/ml Listeria monocytogenes. After fermentation at 42°C for 0–5 h, the yogurt was stored at 4°C. Low and high inocula survived for 48 h and 7 d, respectively; L. monocytogenes cells were not detectable by direct plating or cold-enrichment after 5 and 15 d, respectively. In low inoculum samples, initial pH at the time of refrigeration was 4·9; the final pH at the time of last sampling was 4·2. In the samples with high inoculum the pH decreased from 5·0 to 4·2.     

Influence of planktonic and sessile Listeria monocytogenes on Caenorhabditis elegans

Listeria monocytogenes is the etiologic agent of listeriosis, a food-borne disease affecting humans and a variety of animals. In order to combat this pathogen, it is crucial to have an understanding of its natural interplay with the environment. For this reason, the free soil nematode Caenorhabditis elegans was focused upon because of its shared natural habitat with Listeria and its potential as a model organism for Listeria pathogenesis. Previous studies have generated some contradictory results on Listeria’s ability to kill C. elegans, making additional interaction studies such as this more attractive. In our study, we carried out a series of killing assays in a systematic manner using different Listeria strains under different growth conditions. In addition to studying the effects of planktonic cells, we examined the interaction between C. elegans and sessile listerial cells. Our findings suggest that, rather than causing infection and death, L. monocytogenes may extend the life span of C. elegans. This indicates that Listeria is not pathogenic to C. elegans. We also found that C. elegans can feed and ingest sessile cells, as well as carry the pathogen in its gut, implying that C. elegans could be a vehicle for L. monocytogenes spread in the environment.     

Antioxidant Defenses in Fish: Biotic and Abiotic Factors

Oxygen in its molecular state O₂, is essential for many metabolic processes that are vital to aerobic life. Aerobic organisms cannot exist without oxygen, which nevertheless is inherently dangerous to their lives. Like all aerobic organisms, fish are also susceptible to the effects of reactive oxygen and have inherent and effective antioxidant defenses that are well described in the literature. This review investigates the influence of different biotic and abiotic factors (age, phylogenetic position, feeding behavior, environmental factors, oxygen, temperature, presence of xenobiotics) on antioxidant defenses in fish. Studies of antioxidant activity in fish open a number of novel research lines providing greater knowledge of fish physiology, which will benefit various aspects of fish farming and artificial production.     

Survival of Bactericidal Antibiotic Treatment by a Persister Subpopulation of Listeria monocytogenes

Listeria monocytogenes can cause the serious infection listeriosis, which despite antibiotic treatment has a high mortality. Understanding the response of L. monocytogenes to antibiotic exposure is therefore important to ensure treatment success. Some bacteria survive antibiotic treatment by formation of persisters, which are a dormant antibiotic-tolerant subpopulation. The purpose of this study was to determine whether L. monocytogenes can form persisters and how bacterial physiology affects the number of persisters in the population. A stationary-phase culture of L. monocytogenes was adjusted to 10⁸ CFU ml⁻¹, and 10³ to 10⁴ CFU ml⁻¹ survived 72-h treatment with 100 μg of norfloxacin ml⁻¹, indicating a persister subpopulation. This survival was not caused by antibiotic resistance as regrown persisters were as sensitive to norfloxacin as the parental strain. Higher numbers of persisters (10⁵ to 10⁶) were surviving when older stationary phase or surface-associated cells were treated with 100 μg of norfloxacin ml⁻¹. The number of persisters was similar when a ΔsigB mutant and the wild type were treated with norfloxacin, but the killing rate was higher in the ΔsigB mutant. Dormant norfloxacin persisters could be activated by the addition of fermentable carbohydrates and subsequently killed by gentamicin; however, a stable surviving subpopulation of 10³ CFU ml⁻¹ remained. Nitrofurantoin that has a growth-independent mode of action was effective against both growing and dormant cells, suggesting that eradication of persisters is possible. Our study adds L. monocytogenes to the list of bacterial species capable of surviving bactericidal antibiotics in a dormant stage, and this persister phenomenon should be borne in mind when developing treatment regimens.     

Effects of Soil Abiotic and Biotic Factors on Tree Seedling Regeneration Following a Boreal Forest Wildfire

Ecosystems - Wildfire disturbance is important for tree regeneration in boreal ecosystems. A considerable amount of literature has been published on how wildfires affect boreal forest regeneration....     

Survival of Listeria monocytogenes in raw milk treated in a pilot plant size pasteurizer

The survival of Listeria monocytogenes in raw milk treated in a pilot plant size pasteurizer was investigated. Raw milk was inoculated with different initial concentrations of L. monocytogenes and heated at temperatures ranging from 69 degrees to 73 degrees C. Listerias were not isolated from any of the milk samples immediately after thermal treatment. They were isolated, however, from 46.6% of heated samples (none from samples heated at 73 degrees C) after variable periods at refrigeration temperature. The results suggest that a low number of listerias survive some thermal treatments, but a cold enrichment is necessary to repair the thermally injured cells and detect these organisms in milk. The importance of the isolation technique in the recovery of listerias from pasteurized milk samples is discussed.