Listeria monocytogenes persistence in food-associated environments: epidemiology, strain characteristics, and implications for public health

Over the last 10 to 15 years, increasing evidence suggests that persistence of Listeria monocytogenes in food processing plants for years or even decades is an important factor in the transmission of this foodborne pathogen and the root cause of a number of human listeriosis outbreaks. L. monocytogenes persistence in other food-associated environments (e.g., farms and retail establishments) may also contribute to food contamination and transmission of the pathogen to humans. Although L. monocytogenes persistence is typically identified through isolation of a specific molecular subtype from samples collected in a given environment over time, formal (statistical) criteria for identification of persistence are undefined. Environmental factors (e.g., facilities and equipment that are difficult to clean) have been identified as key contributors to persistence; however, the mechanisms are less well understood. Although some researchers have reported that persistent strains possess specific characteristics that may facilitate persistence (e.g., biofilm formation and better adaptation to stress conditions), other researchers have not found significant differences between persistent and nonpersistent strains in the phenotypic characteristics that might facilitate persistence. This review includes a discussion of our current knowledge concerning some key issues associated with the persistence of L. monocytogenes, with special focus on (i) persistence in food processing plants and other food-associated environments, (ii) persistence in the general environment, (iii) phenotypic and genetic characteristics of persistent strains, (iv) niches, and (v) public health and economic implications of persistence. Although the available data clearly indicate that L. monocytogenes persistence at various stages of the food chain contributes to contamination of finished products, continued efforts to quantitatively integrate data on L. monocytogenes persistence (e.g., meta-analysis or quantitative microbial risk assessment) will be needed to advance our understanding of persistence of this pathogen and its economic and public health impacts.

Campylobacter spp. as a Foodborne Pathogen: A Review

Campylobacter is well recognized as the leading cause of bacterial foodborne diarrheal disease worldwide. Symptoms can range from mild to serious infections of the children and the elderly and permanent neurological symptoms. The organism is a cytochrome oxidase positive, microaerophilic, curved Gram-negative rod exhibiting corkscrew motility and is carried in the intestine of many wild and domestic animals, particularly avian species including poultry. Intestinal colonization results in healthy animals as carriers. In contrast with the most recent published reviews that cover specific aspects of Campylobacter/campylobacteriosis, this broad review aims at elucidating and discussing the (i) genus Campylobacter, growth and survival characteristics; (ii) detection, isolation and confirmation of Campylobacter; (iii) campylobacteriosis and presence of virulence factors; and (iv) colonization of poultry and control strategies.

The role of lactobacilli and probiotics in maintaining vaginal health

BACKGROUND

The vaginal microbiota of healthy women consists typically of a diversity of anaerobic and aerobic microorganisms. Lactobacilli are the most prevalent and often numerically dominant microorganisms and are relevant as a barrier to infection. The capacity of lactobacilli to adhere and compete for adhesion sites in the vaginal epithelium and the capacity to produce antimicrobial compounds (hydrogen peroxide, lactic acid, bacteriocin-like substances), are important in the impairment of colonization by pathogens.

OBJECTIVE

This review summarizes the role of lactic acid bacteria in preventing illness of the host, including bacterial vaginosis, yeast vaginitis, urinary tract infection and sexually transmitted diseases.

CONCLUSIONS

The administration of probiotics that colonize the vaginal tract can be important in maintaining a normal urogenital health and also to prevent or treat infections.

Effects of various sugars added to growth and drying media upon thermotolerance and survival throughout storage of freeze-dried Lactobacillus delbrueckii ssp. bulgaricus

The aim of this research effort was to investigate the role of various sugar substrates in the growth medium upon thermotolerance and upon survival during storage after freeze-drying of Lactobacillus bulgaricus. Addition of the sugars tested to the growth medium, and of these and sorbitol to the drying medium (skim milk) was investigated so as to determine whether a relationship exists between growth and drying media, in terms of protection of freeze-dried cells throughout storage. The lowest decrease in viability of L. bulgaricus cells after freeze-drying was obtained when that organism was grown in the presence of mannose. However, L. bulgaricus clearly survived better during storage when cells had been grown in the presence of fructose, lactose or mannose rather than glucose (the standard sugar in the growth medium). A similar effect could not be observed in terms of thermotolerance; in this case, the growth medium supplemented with lactose was found to yield cells bearing the highest heat resistance. Supplementation of the drying medium with glucose, fructose, lactose, mannose or sorbitol led in most cases to enhancement of protection during storage, to a degree that was growth medium-dependent.

Identification of sites of injury in Lactobacillus bulgaricus during heat stress

Heat resistance of Lactobacillus bulgaricus in skimmed milk at 62 degrees, 64 degrees, 65 degrees and 66 degrees C was studied. The response to increasing temperatures in this range was not linear, with temperatures at 65 degrees C and above giving a lower survival rate than would be predicted from experiments at lower temperatures. To identify sites of injury at these temperatures, chemical markers were used. Heating at 64 degrees C and below resulted in damage to the cytoplasmic membrane. At temperatures of 65 degrees C and above chemical markers also indicated damage in the cell wall and proteins. Using differential scanning calorimetry analysis of whole cells of Lact. bulgaricus seven main peaks were observed (1-51, m1-61, m2-73, n-80, p-89, q-100, r-112 degrees C). Three of these peaks (l(r), m(r) and p(r)) were the result of reversible reactions. Analysis of cell fractions identified the cell structure involved in giving rise to each of the three reversible peaks; l(r), cell membrane lipids, m(r), ribosomes, and p(r), DNA. The evidence presented in this paper shows that irreversible reactions in the cell ribosomes are a critical site of damage in Lact. bulgaricus during heat stress in liquid media at 65 degrees C and above.

Characterization of two bacteriocins produced by Pediococcus acidilactici isolated from "Alheira", a fermented sausage traditionally produced in Portugal

Lactic acid bacteria were isolated from "Alheira" sausages that have been sampled from different regions in Portugal. The sausages were produced according to different recipes and with traditional starter cultures. Two isolates (HA-6111-2 and HA-5692-3) from different sausages were identified as strains of Pediococcus acidilactici. Each strain produces a bacteriocin, designated as bacHA-6111-2 and bacHA-5692-3. Both bacteriocins are produced at low levels after 18 h of growth in MRS broth (3200 AU/ml against Enterococcus faecium HKLHS and 1600 AU/ml against Listeria innocua N27). BacHA-6111-2 and bacHA-5692-3 are between 3.5 kDa and 6.5 kDa in size, as determined by tricine-SDS-PAGE. Complete inactivation or significant reduction in antimicrobial activity was observed after treatment of cell-free supernatants with proteinase K, pronase and trypsin. No change in activity was recorded when treated with catalase. Both bacteriocins are sensitive to treatment with Triton X-114 and Triton X-100, but resistant to Tween 20, Tween 80, SDS, Oxbile, NaCl, urea and EDTA. The bacteriocins remained stable after 2 h at pH 6.0. A decrease in antibacterial activity was recorded after 60 min at 100 degrees C. After 60 min at 80 degrees C, 60 degrees C and 25 degrees C the antibacterial activity against L. innocua N27 decreased by 25%. Addition of bacHA-6111-2 and bacHA-5692-3 (1600 AU/ml) to a mid-log (5-h-old) culture of L. innocua N27 inhibited growth for 7 h. Addition of the bacteriocins (3200 AU/ml) to a mid-log (5-h-old) culture of E. faecium HKLHS repressed cell growth. The bacteriocins did not adhere to the surface of the producer cells. Both strains contain a 1044 bp DNA fragment corresponding in size to that recorded for pediocin PA-1. Sequencing of the fragments from both bacteriocins revealed homology to large sections of pedA (188 bp), pedB (338 bp) and pedC (524 bp) of pediocin PA-1 and the bacteriocins are considered similar to pediocin PA-1.

Bacteriocin production by spray-dried lactic acid bacteria

AIMS

Cell survival and antagonistic activity against Listeria innocua, Listeria monocytogenes and Staphylococcus aureus were investigated after spray-drying three bacteriocin-producing strains of lactic acid bacteria: Carnobacterium divergens, Lactobacillus salivarius and Lactobacillus sakei.

METHODS AND RESULTS

Bacterial cell concentrates were spray-dried and stored at 4 degrees C and 18 degrees C and 0.3% ERH (equilibrium relative humidity). Enumeration and antagonistic activity were evaluated before and after spray-drying and at regular intervals during storage.

CONCLUSIONS

A higher survival rate was obtained when survival was performed at 4 degrees C. With the exception of Carnobacterium divergens which lost the inhibitory activity against Staph. aureus after drying, antagonistic production was not affected by the process nor by the storage. Of the three species studied, Lact. salivarius showed the highest resistance to the spray-drying and storage processes.

SIGNIFICANCE AND IMPACT OF THE STUDY

Spray-drying is a potentially useful process for large scale production of dried powders containing viable organisms with antagonistic activity against pathogens.

Problems in identifying predictors and correlates of weight loss and maintenance: implications for weight control therapies based on behaviour change

Weight management is a dynamic process, with a pre-treatment phase, a treatment (including process) phase and post-treatment maintenance, and where relapse is possible during both the treatment and maintenance. Variability in the statistical power of the studies concerned, heterogeneity in the definitions, the complexity of obesity and treatment success, the constructs and measures used to predict weight loss and maintenance, and an appreciation of who and how many people achieve it, make prediction difficult. In models of weight loss or maintenance: (i) predictors explain up to 20-30% of the variance; (ii) many predictors are the sum of several small constituent variables, each accounting for a smaller proportion of the variance; (iii) correlational or predictive relationships differ across study populations; (iv) inter-individual variability in predictors and correlates of outcomes is high and (v) most of the variance remains unexplained. Greater standardization of predictive constructs and outcome measures, in more clearly defined study populations, tracked longitudinally, is needed to better predict who sustains weight loss. Treatments need to develop a more individualized approach that is sensitive to patients' needs and individual differences, which requires measuring and predicting patterns of intra-individual behaviour variations associated weight loss and its maintenance. This information will help people shape behaviour change solutions to their own lifestyle needs.

Cellular injuries of spray-dried Lactobacillus spp. isolated from kefir and their impact on probiotic properties

The injuries caused by spray drying (SD) of three potential probiotic lactobacilli isolated from kefir grains and the impact on some probiotic properties, were evaluated. Results demonstrated that Lactobacillus plantarum 83114 and L. kefir 8321 showed a slight reduction of viability (0.11 and 0.29 log CFU/ml respectively) after SD process, and L. kefir 8348 was found to be more sensitive to the process with a reduction in viability of 0.70 log CFU/ml. Neither membrane damage, evaluated by increased sensitivity to NaCl, lysozyme, bile salt and penicillin G, nor changes in acidifying activity in MRS and milk by lactobacilli were detected after SD. L. plantarum 83114 and L. kefir 8321 after SD did not lose their capacity to adhere to intestinal cells. Nevertheless, L. kefir 8348 showed a significant loss of adhesion capacity after SD. In addition, rehydrated spray-dried L. kefir 8321 retained the ability to protect against Salmonella invasion of intestinal cells. This effect was observed when L. kefir is co-incubated with Salmonella before invasion assay. This work shows that the membrane integrity evaluated by indirect methods and some probiotic properties of lactobacilli isolated from kefir did not change significantly after SD, and these powders could be used in functional foods applications.

Preservation of probiotic strains isolated from kefir by spray drying

AIMS

This work aims to investigate the survival of Lactobacillus kefir CIDCA 8348, Lactobacillus plantarum CIDCA 83114 and Saccharomyces lipolytica CIDCA 812, all isolated from kefir, during spray drying and subsequent storage.

METHODS AND RESULTS

Micro-organisms were grown in De Man, Rogosa, Sharpe (MRS) or yeast medium (YM) medium and harvested in the stationary phase of growth. The thermotolerance in skim milk (D and Z values), the survival of spray drying at different outlet air temperatures and subsequent storage in different conditions during 150 days were studied. The resistance to the heat treatments was higher in Lact. plantarum compared to Lact. kefir and S. lipolytica. The three micro-organisms studied varied considerably in their ability to survive to spray drying processes. Lactobacillus plantarum showed the highest survival rate for all the tested outlet air temperatures and also to the further storage in the dried state. The survival rates of Lact. kefir and S. lipolytica through drying and subsequent storage in the dried state decreased when the drying outlet air temperatures increased.

CONCLUSIONS

Spray drying is a suitable method to preserve micro-organisms isolated from kefir grains. A high proportion of cells were still viable after 80 days of storage at refrigerated temperatures.

SIGNIFICANCE AND IMPACT OF STUDY

It is the first report about spray-dried probiotic strains isolated from kefir grain and contributes to the knowledge about these micro-organisms for their future application in novel dehydrated products.

Foci of contamination of Listeria monocytogenes in different cheese processing plants

Listeria monocytogenes is a ubiquitous bacterium widely distributed in the environment that can cause a severe disease in humans when contaminated foods are ingested. Cheese has been implicated in sporadic cases and in outbreaks of listeriosis worldwide. Environmental contamination, in several occasions by persistent strains, has been considered an important source of finished product contamination. The objectives of this research were to (i) evaluate the presence of L. monocytogenes within the factory environments and cheeses of three processing plants, artisanal producer of raw ewe's milk cheeses (APC), small-scale industrial cheese producer (SSI) and industrial cheese producer (ICP) each producing a distinct style of cheese, all with history of contamination by L. monocytogenes (ii) and identify possible sources of contamination using different typing methods (arsenic and cadmium susceptibility, geno-serotyping, PFGE). The presence of markers specific for 3 epidemic clones (ECI-ECIII) of L. monocytogenes was also investigated. Samples were collected from raw milk (n = 179), whey (n = 3), cheese brining solution (n = 7), cheese brine sludge (n = 505), finished product (n = 3016), and environment (n = 2560) during, at least, a four-year period. Listeria monocytogenes was detected in environmental, raw milk and cheese samples, respectively, at 15.4%, 1.1% and 13.6% in APC; at 8.9%, 2.9% and 3.4% in SSI; and at 0%, 21.1% and 0.2% in ICP. Typing of isolates revealed that raw ewe's milk and the dairy plant environment are important sources of contamination, and that some strains persisted for at least four years in the environment. Although cheeses produced in the three plants investigated were never associated with any case or outbreak of listeriosis, some L. monocytogenes belonging to specific PFGE types that caused disease (including putative epidemic clone strains isolated from final products) were found in this study.

Effect of the pH of growth on the survival of Lactobacillus delbrueckii subsp. bulgaricus to stress conditions during spray-drying

AIMS

The aim of this study was to optimize survival of Lactobacillus delbrueckii subsp. bulgaricus during spray-drying and subsequent storage through optimizing the pH of growth conditions.

METHODS AND RESULTS

Cell concentrates previously grown without or with pH controlled were spray-dried and stored at 20 degrees C and heat treated at 57 degrees C. Cells grown under noncontrolled pH were more resistant to both drying and heating than cells grown under controlled pH but no significant differences were observed during storage. The intracellular proteins profile of cells grown under both conditions was studied by two-dimensional SDS-polyacrylamide gel electrophoresis. Eight proteins were identified using automated mass spectrometry (MS) and tandem mass spectrometry (MS/MS) data acquisition. Of the identified proteins, only cochaperonin GroES corresponded to a known heat shock protein (HSP). The other proteins identified are proteins involved in glycolysis. For cells grown under noncontrolled pH the expression of the Hsp70, GroES and GroEL, measured by Western blotting, was enhanced.

CONCLUSIONS

The higher resistance of cells grown under noncontrolled pH correlates with the enhanced production of heat shock proteins.

SIGNIFICANCE AND IMPACT OF THE STUDY

Growth of L. bulgaricus under controlled pH (commonly used by the starter cultures production industry) results in cells more sensitive to stresses frequently encountered by the cells during starter cultures preparation/storage/utilization.

Food handlers as potential sources of dissemination of virulent strains of Staphylococcus aureus in the community

Food handlers may constitute a reservoir of virulent strains of Staphylococcus aureus and may be vehicles of their transmission to food. One hundred and sixty-two volunteers were assessed for the presence of S. aureus on the hands and in the nose. S. aureus was isolated by routine procedures, and the isolates were tested for susceptibility against a panel of nine antimicrobial agents. The isolates were further characterized by SmaI-PFGE profiling and the presence of virulence factors.

RESULTS

The prevalence of S. aureus was 19.8% in the nose and 11.1% on the hands; 6.2% of the individuals carried S. aureus both in their noses and hands, and three individuals had the same strain (PFGE type) in the nose and on the hands. Although 82% of the isolates were resistant to at least one antibiotic, none demonstrated the presence of either mecA gene or resistance to oxacillin (none identified as MRSA). Sixty-eight percent of the isolates from the nose and hands possessed enterotoxin genes. This study revealed a high prevalence of antibiotic resistance and virulence determinants among the isolates, including not only classical and novel enterotoxin genes but also major virulence factors such as tst. Potential dissemination of these strains in the community is a matter of concern.

Acinetobacter spp. in food and drinking water - A review

Acinetobacter spp. has emerged as a pathogen of major public health concern due to their increased resistance to antibiotics and their association with a wide range of nosocomial infections, community-acquired infections and war and natural disaster-related infections. It is recognized as a ubiquitous organism however, information about the prevalence of different pathogenic species of this genus in food sources and drinking water is scarce. Since the implementation of molecular techniques, the role of foods as a source of several species, including the Acinetobacter baumannii group, has been elucidated. Multidrug resistance was also detected among Acinetobacter spp. isolated from food products. This highlights the importance of foods as potential sources of dissemination of Acinetobacter spp. between the community and clinical environments and reinforces the need for further investigations on the potential health risks of Acinetobacter spp. as foodborne pathogens. The aim of this review was to summarize the published data on the occurrence of Acinetobacter spp. in different food sources and drinking water. This information should be taken into consideration by those responsible for infection control in hospitals and other healthcare facilities.

Mini-review: Staphylococcus epidermidis as the most frequent cause of nosocomial infections: old and new fighting strategies

Staphylococcus epidermidis is nowadays regarded as the most frequent cause of nosocomial infections and indwelling medical device-associated infections. One of the features that contributes to the success of this microorganism and which is elemental to the onset of pathogenesis is its ability to form biofilms. Cells in this mode of growth are inherently more resistant to antimicrobials. Seeking to treat staphylococcal-related infections and to prevent their side effects, such as the significant morbidity and health care costs, many efforts are being made to develop of new and effective antistaphylococcal drugs. Indeed, due to its frequency and extreme resistance to treatment, staphylococcal-associated infections represent a serious burden for the public health system. This review will provide an overview of some conventional and emerging anti-biofilm approaches in the management of medical device-associated infections related to this important nosocomial pathogen.

Effect of various growth media upon survival during storage of freeze-dried Enterococcus faecalis and Enterococcus durans

AIMS

The effects of three different growth media (MRS, M17 and Lee's) on survival during freeze-drying and subsequent storage of six strains of Enterococcus faecalis and two strains of E. durans were investigated.

METHODS AND RESULTS

Distinct Enterococcus spp. strains were grown on M17, MRS and Lee's broth, freeze-dried and stored at 20 degrees C in air under darkness. At regular intervals throughout storage, freeze-dried samples were rehydrated and then plated on M17 agar.

CONCLUSIONS

A higher survival rate during storage of dried E. durans was obtained when growth occurred in MRS. The same effect was not observed, however, for the majority of E. faecalis strains, which clearly survived better in the dried state when this organism had been grown in M17 or Lee's medium.

SIGNIFICANCE AND IMPACT OF STUDY

The survival of the dried Enterococcus spp. tested during storage was shown to be strain-specific and dependent on the growth medium.

Tackling HIV in resource poor countries

Focusing on prevention of HIV will not prevent the deaths of the millions already infected. The international community must adopt a strategy that links treatment and prevention

Survival of Listeria monocytogenes with different antibiotic resistance patterns to food-associated stresses

The ongoing rise of antibiotic resistant microbial pathogens has become one of the major public health threats worldwide. Despite all the effort and actions taken so far, a proliferation of antibiotic resistant (AR) and multi-antibiotic resistant (MAR) strains is still observed, including in foodborne pathogens. This trend has been also noted recently for isolates of Listeria monocytogenes, a species that, although remaining largely sensitive to clinically relevant antimicrobials, has been reported to develop increased tolerance to antibiotics, particularly in isolates recovered from the food-chain. In this study we compared the ability of MAR (n=8), AR (n=18) and antibiotic susceptible (AS, n=11) L. monocytogenes strains from food and clinical origin to survive to different environmental stress conditions, including temperature (58°C), acidic stress (1% v/v lactic acid, pH3.5), and osmotic stress (37% w/v NaCl). The presence of antibiotic active efflux among MAR and AR strains, and its role on L. monocytogenes tolerance to different antimicrobial compounds was also investigated, namely; hydrogen peroxide; organic acids (acetic, citric and lactic); nisin; benzalkonium chloride (BC); and, sodium nitrite. While no significant differences were observed in the survival of the 37 strains exposed to high temperature (58°C), overall the mean logarithmic reduction of clinical strains was statistically lower after acid and salt exposure than that observed for strains of food origin; but both food and clinical strains resistant to two or three antibiotics were significantly less susceptible to acid (lactic acid 1% v/v) and osmotic stresses (37% w/v NaCl) when compared to AS strains. Using the EtBr-agar Cartwheel method, it was possible to detect efflux pumps in three of the 26 MAR and AR isolates, including one control strain; the active efflux in theses isolates was proven to be associated with fluoroquinolone resistance, and possible extrusion of BC and hydrogen peroxide. The mechanisms responsible for the possible correlation between resistance to antibiotics and to acid or salt stress in L. monocytogenes have yet to be understood.

Persistent and non-persistent strains of Listeria monocytogenes: A focus on growth kinetics under different temperature, salt, and pH conditions and their sensitivity to sanitizers

This study aimed to investigate the effect of different conditions, including temperature (37 °C, 22 °C, and 4 °C), NaCl concentrations (2.5%, 4%, and 8%), and acidity (pH = 5), on the growth response of persistent and non-persistent isolates of Listeria monocytogenes. The resistance to two common sanitizers (benzalkonium chloride and hydrogen peroxide) was also investigated. A selected group of 41 persistent and non-persistent L. monocytogenes isolates recovered from three cheese processing plants during a previous longitudinal study was assembled. Average lag time was similar for persistent and non-persistent isolates grown at 37 °C, 22 °C and 4 °C but significantly shorter (p < 0.05) for persistent isolates grown at 2.5%, 4% and 8% NaCl, and at pH 5. Average growth rates were significantly higher (p < 0.05) for persistent than for non-persistent isolates when grown at 22 °C, 2.5%, 4% and 8% NaCl, and at pH 5. These results suggest that persistent strains may be better adapted to grow under stressful conditions frequently encountered in food processing environments than non-persistent strains. No relation between persistence and resistance to the tested sanitizers was found.

Recovery of heat-injured Listeria innocua

Listeria innocua was subjected to thermal inactivation and the extent of heat-injured cells was quantified. Cultures were heated in liquid medium for different times, using temperatures in the range of 52.5 to 65.0 degrees C, and plated on Tryptic Soy Agar with 0.6% yeast extract (TSAYE) used as non-selective medium and on TSAYE plus 5% NaCl (TSAYE+NaCl) and Palcam agar with selective supplement (Palcam agar) as selective media. The difference observed in counts in non-selective and in selective media gave an indication of cell injury during the heat treatment. D- and z- values were calculated for all conditions considered. For each temperature, D-values obtained using non-selective recovery procedures were higher than the ones obtained using the two selective media. When comparing the selective media, it can be concluded that Palcam agar allowed recovery and growth of thermally injured cells and so it was less inhibitor than TSAYE+NaCl. Another important result was the influence of temperature on the degree of cellular injury. As temperature increases, the degree of heat-injured cells also increases, and consequently concern has to be taken with the temperature and the counting medium used in food processing studies. The results of this work clearly demonstrated that selective media used for Listeria monocytogenes enumeration/detection might not be suitable for the recovery of heat-injured cells, which can dangerously underestimate the presence of this foodborne pathogen.