Sources and survival of Listeria monocytogenes on fresh, leafy produce.

Listeria monocytogenes is an intracellular human pathogen which enters the body through contaminated food stuffs and is known to contaminate fresh leafy produce such as spinach, lettuce and rocket. Routinely, fresh leafy produce is grown and processed on a large scale before reaching the consumer through various products such as sandwiches and prepared salads. From farm to fork, the fresh leafy produce supply chain (FLPSC) is complex and contains a diverse range of environments where L. monocytogenes is sporadically detected during routine sampling of produce and processing areas. This review describes sources of the bacteria in the FLPSC and outlines the physiological and molecular mechanisms behind its survival in the different environments associated with growing and processing fresh produce. Finally, current methods of source tracking the bacteria in the context of the food supply chain are discussed with emphasis on how these methods can provide additional, valuable information on the risk that L. monocytogenes isolates pose to the consumer.

Silver nanoparticles: a microbial perspective.

Silver nanoparticles (NPs) are used for a wide range of commercial reasons to restrict microbial growth. The increasing use of silver NPs in modern materials ensures they will find their way into environmental systems. The mode of action which makes them desirable as an antimicrobial tool could also pose a severe threat to the natural microbial balance existing in these systems. Research into the potential environmental threats of silver NPs has mainly focused on particular areas, such as their influence in rivers and estuaries or their effect on organisms such as earthworms and plants. There is a need to focus studies on all aspects of the microbial world and to highlight potential risks and methods of overcoming problems before significant damage is done. This review focuses on the antimicrobial uses, mechanisms of toxicity, and effects on the environment (mainly soil) of silver NPs, illustrating gaps in current knowledge.

Soil contamination with silver nanoparticles reduces Bishop pine growth and ectomycorrhizal diversity on pine roots.

Soil contamination by silver nanoparticles (AgNP) is of potential environmental concern but little work has been carried out on the effect of such contamination on ectomycorrhizal fungi (EMF). EMF are essential to forest ecosystem functions as they are known to enhance growth of trees by nutrient transfer. In this study, soil was experimentally contaminated with AgNP (0, 350 and 790 mg Ag/kg) and planted with Bishop pine seedlings. The effect of AgNP was subsequently measured, assessing variation in pine growth and ectomycorrhizal diversity associated with the root system. After only 1 month, the highest AgNP level had significantly reduced the root length of pine seedlings, which in turn had a small effect on above ground plant biomass. However, after 4 months growth, both AgNP levels utilised had significantly reduced both pine root and shoot biomass. For example, even the lower levels of AgNP (350 mg Ag/kg) soil, reduced fresh root biomass by approximately 57 %. The root systems of the plants grown in AgNP-contaminated soils lacked the lateral and fine root development seen in the control plants (no AgNP). Although, only five different genera of EMF were found on roots of the control plants, only one genus Laccaria was found on roots of plants grown in soil containing 350 mg AgNP/kg. At the higher levels of AgNP contamination, no EMF were observed. Furthermore, extractable silver was found in soils containing AgNP, indicating potential dissolution of silver ions (Ag+) from the solid AgNP.

Effects of long-term contamination of DDT on soil microflora with special reference to soil algae and algal transformation of DDT.

DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) and its principle metabolites, DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene) and DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane) are widespread environmental contaminants but little information is available concerning their effects on non-target microflora (especially microalgae and cyanobacteria) and their activities in long-term contaminated soils. For this reason a long-term DDT-contaminated soil was screened for DDT residues and toxicity to microorganisms (bacteria, fungi, algae), microbial biomass and dehydrogenase activity. Also, five pure cultures isolated from various sites (two unicellular green algae and three dinitrogen-fixing cyanobacteria) were tested for their ability to metabolise DDT. Viable counts of bacteria and algae declined with increasing DDT contamination while fungal counts, microbial biomass and dehydrogenase activity increased in medium-level contaminated soil (27 mg DDT residues kg(-1) soil). All the tested parameters were greatly inhibited in high-level contaminated soil (34 mg DDT residues kg(-1) soil). Species composition of algae and cyanobacteria was altered in contaminated soils and sensitive species were eliminated in the medium and high contaminated soils suggesting that these organisms could be useful as bioindicators of pollution. Microbial biomass and dehydrogenase activity may not serve as good bioindicators of pollution since these parameters were potentially influenced by the increase in fungal (probably DDT resistant) counts. All the tested algal species metabolised DDT to DDE and DDD; however, transformation to DDD was more significant in the case of dinitrogen-fixing cyanobacteria.

Enhanced biodegradation of diesel oil by a newly identified Rhodococcus baikonurensis EN3 in the presence of mycolic acid.

AIMS: The aim of the present study was to isolate and characterize a bacterium, strain EN3, capable of using diesel oil as a major carbon and energy source, and to analyse the enhancement of diesel oil degradation by this organism using synthetic mycolic acid (2-hexyl-3-hydroxyldecanoic acid). METHOD AND RESULTS: An actinomycete with the ability to degrade diesel oil was isolated from oil contaminated soil and characterized. The strain had phenotypic properties consistent with its classification in the genus Rhodococcus showing a 16S rRNA gene similarity of 99.7% with Rhodococcus baikonurensis DSM 44587(T). The ability of the characterized strain to degrade diesel oil at various concentrations (1000, 5000, 10 000 and 20 000 mg l(-1)) was determined. The effect of synthetic mycolic acid on the biodegradation of diesel oil was investigated at the 20 000 mg l(-1) concentration; the surfactant was added to the flask cultures at three different concentrations (10, 50 and 100 mg l(-1)) and degradation followed over 7 days. Enhanced degradation was found at all three concentrations of the surfactant. In addition, the enhancement of diesel oil degradation by other surfactants was observed. CONCLUSIONS: The synthetic mycolic acid has potential for the remediation of petroleum-contaminated sites from both an economic and applied perspective as it can stimulate biodegradation at low concentrations. SIGNIFICANCE AND IMPACT OF THE STUDY: This study showed that the synthesized mycolic acid can be used for potential applications in the bioremediation industries, for example, in oil spill clean-up, diesel fuel remediation and biostimulation.

Influence of phenol on the biodegradation of pyridine by freely suspended and immobilized Pseudomonas putida MK1.

AIMS: To study the effect of co-contaminants (phenol) on the biodegradation of pyridine by freely suspended and calcium alginate immobilized bacteria. METHODS AND RESULTS: Varying concentrations of phenol were added to free and calcium alginate immobilized Pseudomonas putida MK1 (KCTC 12283) to examine the effect of this pollutant on pyridine degradation. When the concentration of phenol reached 0.38 g l(-1), pyridine degradation by freely suspended bacteria was inhibited. The increased inhibition with the higher phenol levels was apparent in increased lag times. Pyridine degradation was essentially completely inhibited at 0.5 g l(-1) phenol. However, immobilized cells showed tolerance against 0.5 g l(-1) phenol and pyridine degradation by immobilized cell could be achieved. CONCLUSIONS: This works shows that calcium alginate immobilization of microbial cells can effectively increase the tolerance of P. putida MK1 to phenol and results in increased degradation of pyridine. SIGNIFICANCE AND IMPACT OF THE STUDY: Treatment of wastewater stream can be negatively affected by the presence of co-pollutants. This work demonstrates the potential of calcium alginate immobilization of microbes to protect cells against compound toxicity resulting in an increase in pollutant degradation.

A method to increase silver biosorption by an industrial strain of Saccharomyces cerevisiae.

Ag+ biosorption by an industrial strain of Saccharomyces cerevisiae was investigated. Older (96 h old) biomass had half the biosorption capacity of younger (24 h old) biomass (0.187 and 0.387 mmol Ag+/g dry mass respectively). Comparisons of cell walls isolated from biomass of either age indicated that chemical composition and Ag+ biosorption capacity varied little over the time span examined and that cell walls from either age of culture had small Ag+ biosorption capacities compared to whole cells of a similar age. Silver-containing precipitates were observed both on the cell wall and within the cell, indicating that intracellular components sorbed Ag+. The concentration of these precipitates within the cell appeared visually to decrease with age in Ag(+)-exposed cells. Incorporation of L-cysteine into the growth medium resulted in biomass with increased silver biosorption capacities, protein and sulphydryl group content. Increasing the concentration of L-cysteine in the growth medium from 0 to 5.0 mM increased silver biosorption from 0.389 to 0.556 mmol Ag+/g dry mass. Isolated cell walls of biomass grown in supplemented media also showed a possible link between silver biosorption capacities, protein and sulphydryl group content. No precipitates were observed in silver-exposed biomass that had been grown in the presence of 5.0 mM L-cysteine.

Metal cation uptake by yeast: a review.

This review addresses metal uptake specifically by yeast. Metal uptake may be passive, active or both, depending on the viability of the biomass, and is influenced by a number of environmental and experimental factors. Uptake is typically accompanied by a degree of ion exchange and, under certain conditions, may be enhanced by the addition of an energy source. Intracellularly accumulated metal is most readily associated with the cell wall and vacuole but may also be bound by other cellular organelles and biomolecules. The intrinsic biochemical, structural and genetic properties of the yeast cell along with environmental conditions are crucial for its survival when exposed to toxic metals. Conditions of pH, temperature and the presence of additional ions, amongst others, have varying effects on the metal uptake process. We conclude that yeasts have contributed significantly to our understanding of the metal uptake process and suggest directions for future work.

Influence of petroleum hydrocarbon contamination on microalgae and microbial activities in a long-term contaminated soil.

Petroleum hydrocarbons are widespread environmental pollutants. Although biodegradation of petroleum hydrocarbons has been the subject of numerous investigations, information on their toxicity to microorganisms in soil is limited, with virtually no work conducted on soil algae. We carried out a screening experiment for total petroleum hydrocarbons (TPH) and their toxicity to soil algal populations, microbial biomass, and soil enzymes (dehydrogenase and urease) in a long-term TPH-polluted site with reference to an adjacent unpolluted site. Microbial biomass, soil enzyme activity, and microalgae declined in medium to high-level (5,200-21,430 mg kg(-1) soil) TPH-polluted soils, whereas low-level (<2,120 mg kg(-1) soil) pollution stimulated the algal populations and showed no effect on microbial biomass and enzymes. However, inhibition of all the tested parameters was more severe in soil considered to have medium-level pollution than in soils that were highly polluted. This result could not be explained by chemical analysis alone. Of particular interest was an observed shift in the species composition of algae in polluted soils with elimination of sensitive species in the medium to high polluted soils. Also, an algal growth inhibition test carried out using aqueous eluates prepared from polluted soils supported these results. Given the sensitivity of algae to synthetic pollutants, alteration in the algal species composition can serve as a useful bioindicator of pollution. The results of this experiment suggest that chemical analysis alone is not adequate for toxicological estimations and should be used in conjunction with bioassays. Furthermore, changes in species composition of algae proved to be more sensitive than microbial biomass and soil enzyme activity measurements.

Changes in microbial properties associated with long-term arsenic and DDT contaminated soils at disused cattle dip sites.

This work examined the effect of long-term arsenic (As) and DDT contamination on soil microbial properties at 11 cattle dip sites in northern New South Wales, Australia. Total As in the surface (0-10 cm) soils from these sites ranged from 34 to 2941 mg As kg(-1) soil and hexane-extractable DDT concentrations ranged between 2.9 and 7673 mg DDT kg(-1) soil. The concentrations of water and oxalate-extractable As were positively correlated with total As. Oxalate-extractable As was more strongly correlated (r(2)=0.87) with total As than water-extractable As (r(2)=0.34). A weak positive relationship was observed between the level of nutrient (organic carbon and nitrogen) and microbial biomass C (r(2)=0.61 and 0.45, respectively). There was a highly significant difference between the microbial properties of polluted and unpolluted sites (P<0.001). In comparison to unpolluted soils, fungal counts, microbial biomass C, and respiration were dramatically reduced (P<0.05) in polluted soils. However, the bacterial population between polluted and unpolluted soils were not different (P<0.05). The results of this study suggested that (a) long-term contamination of soils adjacent to former cattle dipping sites by As and DDT adversely affected soil microbial properties with the fungal populations being the most sensitive and (b) there was little regeneration of microbiota despite 25 years of field ageing of the soils.

Analysis of ammonia-oxidizing bacteria populations in acid forest soil during conditions of moisture limitation.

Ammonia-oxidizer numbers decreased under conditions of moisture limitation in litter, fermentation and humus layers of forest soil in the field, but the extent of regrowth after rehydration varied between layers. Nitrosospira 16S rRNA genes were amplified from all layers, regardless of moisture content or soil pH which varied between 4.1 and 5.2. Nitrosomonas spp. were detected less often, but appeared to exhibit more rapid recovery than the Nitrosospira spp. when drought conditions were relieved by rainfall.

Diversification of Pseudomonas corrugata 2140 produces new phenotypes altered in GC-FAME, BIOLOG, and in vitro inhibition profiles and taxonomic identification.

Bacteria are known to rapidly produce new phenotypes, but it is unclear how phenotype "plasticity" relates to studies on the population ecology of bacteria in complex environments. We characterised a collection of 14 spontaneous phenotype variants, derived from in vitro and in vivo cultures (wheat roots) of Pseudomonas corrugata 2140, using fatty acid methyl ester profiles (GC-FAME), carbon substrate utilisation (BIOLOG), and in vitro inhibition against seven soil microorganisms. All three phenotype profiles indicated marked differences between some variants and the parent isolate. Some variant types were classified taxonomically by GC-FAME as different species to their wild-type parent, and up to a Euclidian distance of 11 from their parent. Taxonomic identification by the BIOLOG assay was more consistent; however, use of 22 carbon sources were altered (lost or gained) in one or more variants. All variant types had a reduced ability to inhibit one or more test organisms, depending on the variant and test organism. Hierarchical cluster analysis of variants using GC-FAME, BIOLOG, and inhibition profiles produced different groupings. The ability of variants to cross taxonomic boundaries specified by the GC-FAME and BIOLOG libraries at the species level has implications for both taxonomy and the ecological study of bacterial communities.

Microbial activity and phospholipid fatty acid pattern in long-term tannery waste-contaminated soil.

We investigated the phospholipid fatty acid (PLFA) pattern and dehydrogenase activity (DHA) in soil samples from three sites (designated as low, medium, and high based on the level of chromium) in a long-term (25 years after last waste input) tannery waste-contaminated area rich in Cr. Soil samples, collected from different soil depths (0-100 cm), at each site were used in this study. In general, soil samples from all three contaminated sites had elevated pH, electrical conductivity, organic carbon (OC), total Cr, and hexavalent Cr [Cr(VI)]. The maximum total Cr concentration in surface soils (0-10 cm) at the highly contaminated site was 102 gkg(-1), with 4.6 mgkg(-1) present as the bioavailable water-soluble Cr. More than 50% of soluble Cr was in the form of Cr(VI) (2.7 mgkg(-1)). DHA (normalized to OC) was inhibited to a greater extent in soil samples from the highly contaminated site than in low- and medium-contaminated soil samples. PLFA analyses of surface soils indicated that there was a shift in PLFA patterns. PLFAs specific for bacteria (i15:0, a15:0, 15:0, i16:0, a17:0, and cy17:0) decreased significantly (P<0.01) with an increase in Cr contamination. Among the bacterial PLFAs, 15:0, i16:0 and a17:0 had a significant negative correlation with contamination including bioavailable Cr(VI) in soil solution. To our knowledge, this is the first report of alterations in the PLFA profile in soils due to long-term tannery waste pollution.