Evaluating the Effect of Environmental Factors on Pathogen Regrowth in Compost Extract

Pathogenic microorganisms may survive the composting process in low numbers and subsequently regrow to high levels under favorable conditions. The objective of this study was to investigate the regrowth potential of Salmonella spp., Escherichia coli O157:H7, and Listeria monocytogenes in dairy-based composts under different environmental conditions. Water extract of commercially available dairy compost was used as a model system. Cocktails of five rifampin-resistant strains of each pathogen previously grown in reduced nutrient media (1/2 or 1/10 strength of tryptic soy broth, TSB) were inoculated into water extract of compost of different ratios (1:2,1:5, and 1:10, w/v), and then stored at 35°C or 22°C for 7 days. The strains exhibiting greatest survival or regrowth were identified by pulsed-field gel electrophoresis (PFGE). At 22°C, both E. coli O157:H7 and L. monocytogenes multiplied in all compost extracts, whereas Salmonella spp. regrew in both 1:2 and 1:5 compost extracts but not in 1:10. For all three pathogens, incubation at 22°C provides better conditions for regrowth than at 35°C. Both Salmonella and E. coli O157:H7 previously adapted to nutrient-limited broth (1/10 strength of TSB) regrew in compost extracts to higher populations than the control cultures grown previously in full strength of TSB. In the absence of indigenous microorganisms, all three pathogens regrew even in the most diluted sterile compost extract (1:10) with growth potentials ranging from 2.30 to 3.59 log CFU/ml. In nonsterile compost extract with ca. 5 log CFU/ml of background microorganisms, all three pathogens regrew only in the most concentrated compost extract (1:2) with much less population increases ranging from 0.70 to 1.43 log CFU/ml. Compost extract samples of all ages supported the regrowth of both Salmonella and E. coli O157:H7 with population increases ranging from 0.95 to 2.32 log CFU/ml. The PFGE patterns for E. coli O157:H7 isolates from sterile compost extracts matched with either the spinach outbreak strain or an avirulent B6914 strain. These results demonstrated that compost extract of dairy-based compost contained sufficient nutrients for pathogen regrowth. Cultures previously adapted to low nutrient media regrew to higher populations than control cultures; however, indigenous microflora suppressed the pathogen regrowth in compost extract, especially at 35°C.     

Use of zero-valent iron biosand filters to reduce Escherichia coli O157:H12 in irrigation water applied to spinach plants in a field setting

Aims:  Zero‐valent iron (ZVI) filters may provide an efficient method to mitigate the contamination of produce crops through irrigation water. Methods:  A field‐scale system was utilized to evaluate the effectiveness of a biosand filter (S), a biosand filter with ZVI incorporated (ZVI) and a control (C, no treatment) in decontaminating irrigation water. An inoculum of c. 8·5 log CFU 100 ml^?1 of Escherichia coli O157:H12 was introduced to all three column treatments in 20‐l doses. Filtered waters were subsequently overhead irrigated to ‘Tyee’ spinach plants. Water, spinach plant and soil samples were obtained on days 0, 1, 4, 6, 8, 10, 13 and 15 and analysed for E. coli O157:H12 populations. Results:  ZVI filters inactivated c. 6 log CFU 100 ml^?1E. coli O157:H12 during filtration on day 0, significantly (P < 0·05) more than S filter (0·49 CFU 100 ml^?1) when compared to control on day 0 (8·3 log CFU 100 ml^?1). On day 0, spinach plants irrigated with ZVI‐filtered water had significantly lower E. coli O157 counts (0·13 log CFU g^?1) than spinach irrigated with either S‐filtered (4·37 log CFU g^?1) or control (5·23 log CFU g^?1) water. Soils irrigated with ZVI‐filtered water contained E. coli O157:H12 populations below the detection limit (2 log CFU g^?1), while those irrigated with S‐filtered water (3·56 log CFU g^?1) were significantly lower than those irrigated with control (4·64 log CFU g^?1). Conclusions:  ZVI biosand filters were more effective in reducing E. coli O157:H12 populations in irrigation water than sand filters. Significance and Impact of the Study:  Zero‐valent ion treatment may be a cost‐effective mitigation step to help small farmers reduce risk of foodborne E. coli infections associated with contamination of leafy greens.     

Heat inactivation of Salmonella spp. in fresh poultry compost by simulating early phase of composting process

Aims: The purpose of this study was to determine the effect of moisture on thermal inactivation of Salmonella spp. in poultry litter under optimal composting conditions. Methods and Results: Thermal inactivation of Salmonella was studied in fresh poultry compost by simulating early phase of composting process. A mixture of three Salmonella serotypes grown in Tryptic soy broth with rifampin (TSB‐R) was inoculated in fresh compost with 40 or 50% moisture at a final concentration of c. 7 log CFU g^?1. The inoculated compost was kept in an environmental chamber which was programmed to rise from room temperature to target composting temperatures in 2 days. In poultry compost with optimal moisture content (50%), Salmonella spp. survived for 96, 72 and 24 h at 50, 55 and 60°C, respectively, as compared with 264, 144 and 72 h at 50, 55 and 60°C, respectively, in compost with suboptimal moisture (40%). Pathogen decline was faster during the come‐up time owing to higher ammonia volatilization. Conclusions: Our results demonstrated that Salmonella spp. survived longer in fresh poultry compost with suboptimal moisture of 40% than in compost with optimal moisture of 50% during thermophilic composting. High nitrogen content of the poultry compost is an additional factor contributing to Salmonella inactivation through ammonia volatilization during thermal exposure. Significance and Impact of the Study: This research validated the effectiveness of the current composting guidelines on Salmonella inactivation in fresh poultry compost. Both initial moisture level and ammonia volatilization are important factors affecting microbiological safety and quality of compost product.     

Variable agronomic practices, cultivar, strain source and initial contamination dose differentially affect survival of Escherichia coli on spinach

Aims: Greenhouse and field trials were conducted under different agronomic practices and inoculum doses of environmental Escherichia coli and attenuated E. coli O157:H7, to comparatively determine whether these factors influence their survival on leaves and within the rhizosphere. Methods and Results: Hydroponic conditions: E. coli spray‐inoculated at log 4 CFU ml^?1 was recovered from leaf surfaces at a mean population of 1·6 log CFU g^?1 at 15 days. E. coli O157:H7 sprayed at log 2 or 4 CFU ml^?1 levelled off on spinach leaf surfaces at a mean average population of 1·4 log CFU g^?1 after 14 days, regardless of initial dose. Quantitative recovery was inconsistent across leaf developmental age. Field conditions: Average populations of E. coli O157:H7 spray‐inoculated at log 1·45 or 3·4 CFU m^?2 levelled off at log 1·2 CFU g^?1 over a 14‐day period. Pathogen recovery from leaves was inconsistent when compared to regularly positive detection on basal shoot tissue. Pathogen recovery from soil was inconsistent among sampling locations. Moisture content varied up to 40% DW and was associated with 50% (P < 0·05) decrease in positive locations for E. coli O157:H7 but not for E. coli. Conclusions: Overall, similar populations of environmental E. coli and E. coli O157:H7 were recovered from plants despite differences in inoculum dose and agronomic conditions. Strain source had a significant impact on the quantitative level and duration of survival on leaves and in soil. Water availability appeared to be the determinant factor in survival of E. coli and E. coli O157:H7; however, E. coli showed greater environmental fitness. Significance and Impact of the Study: Persistence of surrogate, indicator E. coli and E. coli O157:H7, irrespective of variable growing conditions in spinach is predominantly limited by water availability, strain source and localization within the plant. These findings are anticipated to ultimately be adopted into routine and investigative pathogen testing protocols and mechanical harvest practices of spinach.     

Survival of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium in manure and manure‐amended soil under tropical climatic conditions in Sub‐Saharan Africa

Aims: To establish the fate of Escherichia coli O157:H7 and Salmonella Typhimurium in manure and manure‐amended agricultural soils under tropical conditions in Sub‐Saharan Africa. Methods and Results: Survival of nonvirulent E. coli O157:H7 and Salm. Typhimurium at 4 and 7 log CFU g^?1 in manure and manure‐amended soil maintained at ≥80% r.h. or exposed to exclusive field or screen house conditions was determined in the Central Agro‐Ecological Zone of Uganda. Maintaining the matrices at high moisture level promoted the persistence of high‐density inocula and enhanced the decline of low‐density inocula in the screen house, but moisture condition did not affect survival in the field. The large majority of the survival kinetics displayed complex patterns corresponding to the Double Weibull model. The two enteric bacteria survived longer in manure‐amended soil than in manure. The 7 log CFU g^?1E. coli O157:H7 and Salm. Typhimurium survived for 49–84 and 63–98 days, while at 4 log CFU g^?1, persistence was 21–28 and 35–42 days, respectively. Conclusions: Under tropical conditions, E. coli O157:H7 and Salm. Typhimurium persisted for 4 and 6 weeks at low inoculum density and for 12 and 14 weeks at high inoculum density, respectively. Significance and Impact of the Study: Persistence in the tropics was (i) mostly shorter than previously observed in temperate regions thus suggesting that biophysical conditions in the tropics might be more detrimental to enteric bacteria than in temperate environments; (ii) inconsistent with published data isothermally determined previously hence indicating the irrelevance of single point isothermal data to estimate survival under dynamic temperature conditions.     

Inactivation of Salmonella Typhimurium,Escherichia coli O157:H7 and Listeria monocytogenes on alfalfa seeds by the combination treatment of vacuumed hydrogen peroxide vapour and vacuumed dry heat

We evaluated the combined effects of vacuumed hydrogen peroxide vapour (VHPV) and vacuum-sealed dry heat (vacuum heat, VH) to inactivate food-borne pathogens (Salmonella Typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes) on alfalfa seeds. Alfalfa seeds inoculated with food-borne pathogens were sequentially treated initially with 1·0 ml of 0 or 30% VHPV for 1 min and later with dry heat (DH) or VH for 2 h, and the rate of seed germination was evaluated. The combination treatment decreased the populations of three food-borne pathogens below the limit of detection (1·0 log CFU per gram) on alfalfa seeds without decreasing germinability. The sequential treatment using VHPV and VH greatly reduced the total treatment time needed to inactivate pathogens on alfalfa seeds by more than 5 log CFU per gram. These results demonstrate that a combination of VHPV and VH has potentially employed as a new method for pasteurization of alfalfa seeds without affecting their germinability.     

Microbiological analysis of composts produced on South Carolina poultry farms

Aims: The purpose of this study was to determine whether the methods used in compost operations of small and medium‐sized poultry forms resulted in the production of an amendment free of foodborne pathogens. Methods and Results: Nine compost heaps on five South Carolina poultry farms were surveyed at different stages of the composting process. Compost samples were analysed for coliforms and enriched for Escherichia coli O157:H7, Salmonella spp. and Listeria monocytogenes. The waste materials and composting practices differed among the surveyed farms. On two farms, new materials were added to heaps that had previously completed the active composting phase. Five compost heaps did not reach an internal temperature of 55°C, and c. 62% of all internal samples in the first composting phase contained moisture contents <40%. Escherichia coli was detected in 63% of the surface samples (n = 38) and 9·8% of the internal samples (n = 82) from the first composting phase, as compared with 16·7% of the surface samples (n = 12) and 0% internal samples (n = 24) from the second composting phase. Salmonella was detected in 26 and 6·1% of all surface and internal samples collected from heaps in the first composting phase, respectively, but was absent in all compost samples undergoing a second composting phase. The predominant Salmonella serotypes were Thompson, Montevideo and Anatum. Neither E. coli O157:H7 nor L. monocytogenes was detected in any of the samples. Conclusions: Our results indicate that the conditions at the compost surface are suitable for pathogen survival, and the complete composting process can result in the elimination of pathogens in poultry wastes. Significance and Impact of the Study: This research provides information regarding the effectiveness of the composting practices and microbiological quality of poultry compost produced by small‐ and medium‐sized farms. Ensuring the safety of compost that may be applied to soils should be an integral part of preharvest food safety programme.     

Effective inactivation of food pathogens Listeria monocytogenes and Salmonella enterica by combined treatment of hypericin-based photosensitization and high power pulsed light

Aims: The aim of this study was to evaluate the inactivation efficiency of Listeria monocytogenes ATC_L3C 7644 and Salmonella enterica serovar Typhimurium strain DS88 by combined treatment of hypericin (Hyp)‐based photosensitization and high power pulsed light (HPPL). Methods and Results: Cells were incubated with Hyp (1 × 10^?5 or 1 × 10^?7 mol l^?1) in PBS and illuminated with a light λ = 585 nm. For the combined treatment, bacteria were, after photosensitization, exposed to 350 pulses of HPPL (UV light dose = 0·023 J cm^?2). Fluorescence measurements were performed to evaluate optimal time for cell–Hyp interaction. Results indicate that Hyp tends to bind both Listeria and Salmonella. After photosensitization treatment, Listeria population was reduced 7 log, whereas Salmonella was inactivated just 1 log. Electron photomicrograps of Salmonella and Listeria confirmed that photosensitization induced total collapse of the Listeria cell wall, but not that of Salmonella. After combined photosensitization–HPPL treatment, the population of Listeria was diminished by 7 log and Salmonella by 6·7 log. Conclusions: Listeria can be effectively inactivated by Hyp‐based photosensitization (7 log), whereas Salmonella is more resistant to photosensitization and can be inactivated just by 1 log in vitro. Combined treatment of photosensitization and pulsed light inactivates effectively (6·7–7 log) both the Gram‐positive and the more resistant to photosensitization Gram‐negative bacteria. Significance and Impact of the Study: A new approach to combat Gram‐positive and Gram‐negative bacteria is proposed, combining photosensitization with high power pulsed light.     

Antilisterial activity of lactose monolaurate in milk,drinkable yogurt and cottage cheese

Lactose monolaurate (LML) was previously found to be an antimicrobial against Listeria monocytogenes in culture medium at concentrations between 3 and 5 mg ml^?1. In this study, the microbial inhibitory activity of LML in dairy products inoculated with a 5‐strain cocktail of clinical isolates of L. monocytogenes was investigated. Addition of LML at a concentration of 5 mg ml^?1 resulted in 4·4, 4·0 and 4·2 log reductions in 0·5% fat, 1% fat and 3·25% fat milks, respectively; 4·1, 4·4, and 3·5 log reductions in nonfat, 1% fat, and 1·5% fat yogurts, respectively; and 4·0 log reductions in both nonfat and 2% fat cottage cheese. The inhibitory effect of LML was only observed at 37°C and not 5°C. Experiments suggest that both the lauric acid and the esterified lactose moiety of LML play roles in the growth inhibition.

Significance and Impact of the Study

A novel sugar ester, lactose monolaurate, inhibited the growth of a five‐strain cocktail of Listeria monocytogenes in milk, yogurt and cottage cheese. This is the first report of the use of a sugar ester to inhibit the growth of Listeria in food systems.     

Limitation in the detection of Listeria monocytogenes in food in the presence of competing Listeria innocua

Aims: Detectability of Listeria monocytogenes at 10⁰ CFU per food sample in the presence of Listeria innocua using standard microbiological detection was evaluated and compared with the real‐time PCR‐based method. Methods and Results: Enrichment in half‐Fraser broth followed by subculture in Fraser broth according to EN ISO 11290‐1 was used. False‐negative detection of 10⁰ CFU L. monocytogenes was obtained in the presence of 10¹ CFU L. innocua per sample using the standard detection method in contrast to more than 10⁵ CFU L. innocua per sample using real‐time PCR. Identification of L. monocytogenes on the chromogenic medium by the standard procedure was impossible if L. innocua was able to overgrow L. monocytogenes by more than three orders of magnitude after the enrichment in model samples. These results were confirmed using naturally contaminated food samples. Conclusions: Standard microbiological method was insufficient for the reliable detection of 10⁰ CFU L. monocytogenes in the presence of more than 10⁰ CFU of L. innocua per sample. On the other hand, if the growth of L. monocytogenes was sufficient to reach the concentration equal to the detection limit of PCR, the amount of the other microflora present in the food sample including L. innocua was not relevant for success of the PCR detection of L. monocytogenes. Significance and Impact of the Study: After the enrichment, the PCR detection is more convenient than the standard one as PCR detection is not compromised by other present microflora.     

Effect of continuous ohmic heating to inactivate Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes in orange juice and tomato juice

Aims: The purpose of this study was to investigate the efficacy of continuous ohmic heating for reducing Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes in orange juice and tomato juice. Methods and Results: Orange juice and tomato juice were treated with electric field strengths in the range of 25–40 V cm^?1 for different treatment times. The temperature of the samples increased with increasing treatment time and electric field strength. The rate of temperature change for tomato juice was higher than for orange juice at all voltage gradients applied. Higher electric field strength or longer treatment time resulted in a greater reduction of pathogens. Escherichia coli O157:H7 was reduced by more than 5 log after 60‐, 90‐ and 180‐s treatments in orange juice with 40, 35 and 30 V cm^?1 electric field strength, respectively. In tomato juice, treatment with 25 V cm^?1 for 30 s was sufficient to achieve a 5‐log reduction in E. coli O157:H7. Similar results were observed in Salm. Typhimurium and L. monocytogenes. The concentration of vitamin C in continuous ohmic heated juice was significantly higher than in conventionally heated juice (P < 0·05). Conclusions: Continuous ohmic heating can be effective in killing foodborne pathogens on orange juice and tomato juice with lower degradation of quality than conventional heating. Significance and Impact of the Study: These results suggest that continuous ohmic heating might be effectively used to pasteurize fruit and vegetable juices in a short operating time and that the effect of inactivation depends on applied electric field strengths, treatment time and electric conductivity.     

Reduction of Escherichia coli O157:H7 on radish seeds by sequential application of aqueous chlorine dioxide and dry‐heat treatment

Aims: To assess the effectiveness of sequential treatments of radish seeds with aqueous chlorine dioxide (ClO₂) and dry heat in reducing the number of Escherichia coli O157:H7. Methods and Results: Radish seeds containing E. coli O157:H7 at 5·5 log CFU g^?1 were treated with 500 μg ml^?1 ClO₂ for 5 min and subsequently heated at 60°C and 23% relative humidity for up to 48 h. Escherichia coli O157:H7 decreased by more than 4·8 log CFU g^?1 after 12 h dry‐heat treatment. The pathogen was inactivated after 48 h dry‐heat treatment, but the germination rate of treated seeds was substantially reduced from 91·2 ± 5·0% to 68·7 ± 12·3%. Conclusions: Escherichia coli O157:H7 on radish seeds can be effectively reduced by sequential treatments with ClO₂ and dry heat. To eliminate E. coli O157:H7 on radish seeds without decreasing the germination rate, partial drying of seeds at ambient temperature before dry‐heat treatment should be investigated, and conditions for drying and dry‐heat treatment should be optimized. Significance and Impact of the study: This study showed that sequential treatment with ClO₂ and dry‐heat was effective in inactivating large numbers of E. coli O157:H7 on radish seeds. These findings will be useful when developing sanitizing strategies for seeds without compromising germination rates.     

Ascomycetous yeast species recovered from grapes damaged by honeydew and sour rot

Aims: To identify ascomycetous yeasts recovered from sound and damaged grapes by the presence of honeydew or sour rot. Methods and Results: In sound grapes, the mean yeast counts ranged from 3·20 ± 1·04 log CFU g^?1 to 5·87 ± 0·64 log CFU g^?1. In honeydew grapes, the mean counts ranged from 3·88 ± 0·80 log CFU g^?1 to 6·64 ± 0·77 log CFU g^?1. In sour rot grapes counts varied between 6·34 ± 1·03 and 7·68 ± 0·38 logCFU g^?1. Hanseniaspora uvarum was the most frequent species from sound samples. In both types of damage, the most frequent species were Candida vanderwaltii, H. uvarum and Zygoascus hellenicus. The latter species was recovered in high frequency because of the utilization of the selective medium DBDM (Dekkera/Brettanomyces differential medium). The scarce isolation frequency of the wine spoilage species Zygosaccharomyces bailii (in sour rotten grapes) and Zygosaccharomyces bisporus (in honeydew affected grapes) could only be demonstrated by the use of the selective medium ZDM (Zygosaccharomyces differential medium). Conclusions: The isolation of several species only from damaged grapes indicates that damage constituted the main factor determining yeast diversity. The utilization of selective media is required for eliciting the recovery of potentially wine spoilage species. Significance and Impact of the Study: The impact of damaged grapes in the yeast ecology of grapes has been underestimated.     

Modelling and predicting the simultaneous growth of Listeria monocytogenes and spoilage micro-organisms in cold-smoked salmon

Aims: To evaluate and model the simultaneous growth of Listeria monocytogenes and spoilage micro-organisms in cold-smoked salmon. Methods and Results: Growth kinetics of L. monocytogenes, lactic acid bacteria (LAB), Enterobacteriaceae, enterococci and Photobacterium phosphoreum were determined in two series of challenge tests with sliced and vacuum-packed cold-smoked salmon (SVP-CSS). The product contained a high level of smoke components and at 2°C levels of L. monocytogenes increased <100-fold in 193 days. Without the addition of spoilage micro-organisms, L. monocytogenes reached ca 10⁸ CFU g⁻¹ at 5, 10, 17·5 and 25°C. Inoculation with spoilage micro-organisms reduced this level to 10²–10⁴ CFU g⁻¹. LAB dominated the spoilage microfora of SVP-CSS and competition between LAB and L. monocytogenes in SVP-CSS was appropriately described by a simple expansion of the Logistic model. This interaction model aided in predicting the growth of L. monocytogenes in naturally contaminated SVP-CSS when it was used in combination with expanded versions of existing secondary models for L. monocytogenes and LAB. Conclusions: Temperature, water activity/NaCl, simultaneous growth of LAB, smoke components and to a lesser extent lactate and pH control growth of L. monocytogenes in SVP-CSS. These factors must be included in mathematical models to predict growth of the pathogen in this product. Significance and Impact of the Study: The suggested predictive model can be used to support assessment and management of the human health risk due to L. monocytogenes in SVP-CSS.     

The evaluation of the microbial safety of fresh ready‐to‐eat vegetables produced by different technologies in Italy

Aims: The study was performed to evaluate the safety of whole and RTE vegetables and to investigate the effectiveness of different preventive strategies for the quality assurance of RTE vegetables collected from three Italian production systems. Producer 1, applied a strict system in compliance with GAP‐ GMP – HACCP, Producer 2 used chlorine disinfection at a second washing step, and Producer 3 using a physical microbial stabilization. Methods: During the period 2005–2007, a total of 964 samples including whole vegetables and RTE salads, collected from three different producers in central Italy, were analysed to quantify the aerobic mesophilic count (AMC) and Escherichia coli, and for the presence of Salmonella spp, Listeria monocytogenes, E. coli O157:H7, hepatitis A virus and Norovirus (NoV). Results: None of the whole vegetable samples were positive for L. monocytogenes, E. coli O157:H7, HAV and NoV; however, a low prevalence of Salmonella was found. No pathogens were detected with cultural methods in any of the RTE vegetables analysed, only two RTE samples were positive for L. monocytogenes with PCR, but were not confirmed by the cultural method. The median values of AMC in RTE vegetables measured 24 h after packaging were statistically different among the 3 producers (5·4 × 10⁶, 1·5 × 10⁷ and 3·7 × 10⁷ CFU g^?1, respectively; P = 0·011). The lowest level was detected in Producer 1. Conclusion: The products that were processed applying rigorously GAP, GMP and HACCP showed a better microbiological quality than those processed with chemical or physical stabilization. Study Significance and Impact: The results of the study evidenced the efficacy of GAP, GMP and HACCP in improving microbiological quality of whole and RTE vegetables.     

Kinetic model‐based prediction of the persistence of Salmonella enterica serovar Typhimurium under tropical agricultural field conditions

Aim: Present a kinetic model‐based approach for using isothermal data to predict the survival of manure‐borne enteric bacteria under dynamic conditions in an agricultural environment. Methods and Results: A model to predict the survival of Salmonella enterica serovar Typhimurium under dynamic temperature conditions in soil in the field was developed. The working hypothesis was that the inactivation phenomena associated with the survival kinetics of an organism in an agricultural matrix under dynamic temperature conditions is for a large part due to the cumulative effect of inactivation at various temperatures within the continuum registered in the matrix in the field. The modelling approach followed included (i) the recording of the temperature profile that the organism experiences in the field matrix, (ii) modelling the survival kinetics under isothermal conditions at a range of temperatures that were registered in the matrix in the field; and (iii) using the isothermal‐based kinetic models to develop models for predicting survival under dynamic conditions. The time needed for 7 log CFU g^?1Salmonella Typhimurium in manure and manure‐amended soil to reach the detection limit of the enumeration method (2 log CFU g^?1) under tropical conditions in the Central Agro‐Ecological Zone of Uganda was predicted to be 61–68 days and corresponded with observed CFU of about 2·2–3·0 log CFU g^?1, respectively. The Bias and Accuracy factor of the prediction was 0·71–0·84 and 1·2–1·4, respectively. Conclusions: Survival of Salm. Typhimurium under dynamic field conditions could be for 71–84% determined by the developed modelling approach, hence substantiating the working hypothesis. Significance and Impact of the Study: Survival kinetic models obtained under isothermal conditions can be used to develop models for predicting the persistence of manure‐borne enteric bacteria under dynamic field conditions in an agricultural environment.     

The development of Escherichia coli and Listeria monocytogenes variants resistant to high‐pressure carbon dioxide inactivation

Aims: The objective of this study was to investigate whether bacterial cells could develop resistance (as a part of their adaptation strategy) to high‐pressure CO₂ (HPCD) inactivation. Methods and Results: Alternating cycles of exposure to pressurized CO₂ (10·5 MPa, 35°C, 400 min^?1, 70% working volume ratio during 10 min) and re‐growth of the surviving subpopulation were used to investigate possible increases in the resistance of Escherichia coli and Listeria monocytogenes to HPCD. The results show an increased resistance of both pathogens tested after seven cycles of inactivation. Increase in the resistance after 15 cycles resulted in a difference of 2·4 log CFU ml^?1 in log N₀/N_i when parental (N₀) and treated cultures (N_i) of E. coli and L. monocytogenes were compared. Conclusions: Current findings indicate the ability of micro‐organisms to adapt to HPCD preservation technology. Significance and Impact of the Study: The occurrence of HPCD‐resistant micro‐organisms could pose a new hazard to the safety and stability of HPCD‐processed foods.     

Bacterial species in raw and cured compost from a large-scale urban composter

Summary Raw and cured compost samples from a large-scale urban composter were studied over a period of eight months to gain information on bacterial species present. Total viable, aerobic heterotrophic bacteria, lactose-positive bacteria, antibiotic and metal-resistant bacteria and thermophilic bacteria were enumerated. Both raw and cured compost samples contained metal and antibiotic-tolerant bacteria (–1 compost) as well as high numbers (as high as Log 7.4 CFU g^–1 dry weight compost) of thermophilic bacteria isolated by growth at 55 °C. Selected colonies were also identified using the Biolog 95 substrate identification system.Escherichia coli andSalmonella spp. were not detected in compost samples.     

Antimicrobial activity of lemongrass oil against Salmonella enterica on organic leafy greens

Aims: We investigated the antimicrobial effectiveness of lemongrass essential oil on organic leafy greens, romaine and iceberg lettuces and mature and baby spinach, inoculated with Salmonella Newport. The influences of exposure times and abuse temperatures on bacterial survival were also investigated. Methods and Results: Leaf samples were washed, inoculated with Salm. Newport (6‐log CFU ml^?1) and dried. Inoculated leaves were immersed in solutions containing 0·1, 0·3 or 0·5% lemongrass oil in phosphate‐buffered saline for 1 or 2 min and then individually incubated at 4 or 8°C. Samples were taken at day 0, 1 and 3 for the enumeration of survivors. Compared to the PBS control, romaine and iceberg lettuces, and mature and baby spinach samples showed between 0·6–1·5‐log, 0·5–4·3‐log, 0·5–2·5‐log and 0·5–2·2‐log CFU g^?1 reductions in Salm. Newport by day 3, respectively. Conclusions: The antimicrobial activity of lemongrass oil against Salm. Newport was concentration and time dependent. The antimicrobial activity increased with exposure time; iceberg samples treated for 2 min generally showed greater reductions (P < 0·05) than those treated for 1 min (c. 1‐log reduction difference for 0·3 and 0·5% treatments). Few samples showed a difference between refrigeration and abuse temperatures. Significance and Impact of the Study: This study demonstrates the potential of lemongrass oil solutions to inactivate Salm. Newport on organic leafy greens.     

Comparison of selected disinfectants efficiency against <Emphasis Type="Italic">Listeria monocytogenes</Emphasis> biofilm formed on various surfaces

Listeria monocytogenes is a main etiological factor of listeriosis, spread mainly by food products. In recent years, an increasing number of patients with listeriosis and an augmentation in L. monocytogenes antibiotic resistance, e.g. to penicillin and ampicillin, has been reported. The aim of the study was to characterise the L. monocytogenes strains isolated from fish-processed food products. Species identification, based on the multiplex-PCR reaction, was performed, and the genetic similarity of the isolates was analysed with the RAPD technique. The strains, in the form of planktonic cells and a biofilm, were subjected to drug-susceptibility analysis, and the effect of disinfectants on the bacillus cells was evaluated. All of the analysed strains were of the Listeria monocytogenes species. Three genetically distant strains were detected, i.e. Lm I, Lm II and Lm III. Approximately 66.6% penicillin-resistant and 66.6% cotrimoxazole-resistant strains were found. No erythromycin-resistant strain was detected. The Lm II strain was simultaneously resistant to four antibiotics, i.e. penicillin, ampicillin, meropenem and cotrimoxazole. The strongest biofilm was formed on aluminium foil and the weakest on rubber. The tested disinfectant antibiofilm effectiveness was related to the type of surface. The most effective agent was paracetic acid and hydrogen peroxide (elimination rate 5.10–6.62 log CFU?×?cm^?2 and 5.70–7.39 log CFU?×?cm^?2 after 1- and 5-min exposure, respectively) and the least—sodium hydroxide (elimination rate 0.52–1.20 log CFU?×?cm^?2 and 0.98–1.81 log CFU?×?cm^?2 after 1- and 5-min exposure, respectively). Further studies on a greater number of L. monocytogenes strains are recommended.