High incidence of Listeria monocytogenes in European red smear cheese

The incidence of Listeria and Listeria monocytogenes in European red smear cheese was determined in order to assess whether the lack of recent outbreaks of listeriosis associated with cheese is due to improved hygenic conditions in the dairies. Out of European red-smear cheese samples of various types, 15.8% contained organisms of the genus Listeria, 6.4% of the samples were contaminated with L. monocytogenes, 10.6% with L. innocua, and 1.2% with L. seeligeri. Six cheese samples contained two or more Listeria species, including at least one L. monocytogenes isolate. The incidences of L. monocytogenes in cheeses from various countries were: Italy 17.4%, Germany 9.2%, Austria 10%, and France 3.3%. Listeria were found most frequently in soft and semi-soft cheese. Eight samples contained more than 100 L. monocytogenes cfu/cm2 cheese surface, 2 samples had counts above 10(4) cfu/cm2 cheese surface. Surprisingly, a higher incidence of L. monocytogenes was observed in cheeses made from pasteurized milk (8.0%) than in cheeses manufactured from raw milk (4.8%). Phage-typing of isolated Listeria strains clearly confirmed that (i) contaminations within dairy plants were persistent over a period of several weeks to months and (ii) that cross-contamination within the dairy plant is and important factor. Comparison of our data with past surveys seems to indicate that contamination of red smear soft cheese with L. monocytogenes has not decreased sufficiently over the past 15 years. It is therefore strongly recommended that these products are monitored carefully by cheese-making companies.     

Multistate outbreak of Listeria monocytogenes associated with Mexican-style cheese made from pasteurized milk among pregnant, Hispanic women

Listeriosis is a severe infection caused by Listeria monocytogenes. Since 2004, the Centers for Disease Control and Prevention has requested that listeriosis patients be interviewed using a standardized Listeria Initiative (LI) questionnaire. In January 2009, states and the Centers for Disease Control and Prevention began investigating a multistate outbreak of listeriosis among pregnant, Hispanic women. We defined a case as an illness occurring between October 2008 and March 2009 with an L. monocytogenes isolate indistinguishable from the outbreak strain by pulsed-field gel electrophoresis. We conducted a multistate case-control study using controls that were selected from L. monocytogenes illnesses in non-outbreak-related pregnant, Hispanic women that were reported to the LI during 2004 to 2008. Eight cases in five states were identified. Seven of these were pregnant, Hispanic females aged 21 to 43 years, and one was a 3-year-old Hispanic girl, who was excluded from the study. Seven (100%) cases but only 26 (60%) of 43 controls had consumed Mexican-style cheese in the month before illness (odds ratio, 5.89; 95% confidence interval, 1.07 to ∞; P = 0.04). Cultures of asadero cheese made from pasteurized milk collected at a manufacturing facility during routine sampling by the Michigan Department of Agriculture on 23 February 2009 yielded the outbreak strain, leading to a recall of cheeses produced in the plant. Recalled product was traced to stores where at least three of the women had purchased cheese. This investigation highlights the usefulness of routine product sampling for identifying contaminated foods, of pulsed-field gel electrophoresis analysis to detect multistate outbreaks, and of the LI for providing timely exposure information for case-control analyses. Recalls of contaminated cheeses likely prevented additional illnesses.     

Incidence of Listeria monocytogenes in cheese manufacturing plants from the northeast region of São Paulo, Brazil

The incidence of Listeria monocytogenes in three cheese manufacturing plants from the northeastern region of S?o Paulo, Brazil, was evaluated from October 2008 to September 2009. L. monocytogenes was found in samples from two plants, at percentages of 13.3% (n = 128) and 9.6% (n = 114). Samples of raw and pasteurized milk, water, and Minas Frescal cheese were negative for L. monocytogenes, although the pathogen was isolated from the surface of Prato cheese and in brine from one of the plants evaluated. L. monocytogenes was also isolated from different sites of the facilities, mainly in non-food contact surfaces such as drains, floors, and platforms. Serotype 4b was the most predominant in the plants studied. The results of this study indicate the need for control strategies to prevent the dispersion of L. monocytogenes in the environment of cheese manufacturing plants.     

Fate of Listeria monocytogenes during the manufacture and ripening of Parmesan cheese

Parmesan cheese was made from a mixture of pasteurized whole and skim milk that was inoculated to contain ca. 10(4) to 10(5) cells of Listeria monocytogenes/ml. Curd was cooked at 51 degrees C (124 degrees F) for ca. 45 min. During cheese making, maximum numbers of L. monocytogenes appeared just before cooking; at this point, the increase over initial numbers was a .61 to 1.0 order of magnitude. During cooking of curd, the average decrease in numbers of L. monocytogenes was a .22 order of magnitude. During cheese ripening, numbers of L. monocytogenes decreased almost linearly and faster than reported for other hard cheeses. Listeria monocytogenes strain California died faster than did strain V7. Listeria monocytogenes were not detected in cheese after 2 to 16 wk of ripening, depending on the strain of the pathogen and the lot of cheese. Parmesan cheese made in this study was not a favorable medium for survival of L. monocytogenes.     

High-pressure processing of Turkish white cheese for microbial inactivation

High-pressure processing (HPP) of Turkish white cheese and reduction of Listeria monocytogenes, total Enterobacteriaceae, total aerobic mesophilic bacteria, total molds and yeasts, total Lactococcus spp., and total Lactobacillus spp. were investigated. Cheese samples were produced from raw milk and pasteurized milk and were inoculated with L. monocytogenes after brining. Both inoculated (ca. 10(7) to 10(8) CFU/g) and noninoculated samples were subjected to HPP in a high-pressure food processor at 50 to 600 MPa for 5 and 10 min at 25 degrees C. Reductions in L. monocytogenes, total aerobic mesophilic bacteria, Lactococcus spp., and Lactobacillus spp. in both pasteurized- and raw-milk cheese samples and reductions in total molds and yeasts and total Enterobacteriaceae counts in raw-milk cheese samples increased with increased pressure (P < or = 0.05). The maximum reduction of the L. monocytogenes count, ca. 4.9 log CFU/g, was obtained at 600 MPa. Because of the highly inhibitory effect of pasteurization, the total molds and yeasts and total Enterobacteriaceae counts for the cheese samples produced from pasteurized milk were below the detection limit both before and after HPP. There was no significant difference in inactivation of L. monocytogenes, total aerobic mesophilic bacteria, Lactococcus spp., and Lactobacillus spp. under the same treatment conditions for the raw milk and pasteurized milk cheeses and for 5- and 10-min treatment times (P > 0.05). No significant change was detected in pH or water activity of the samples before and after HPP. Our findings suggest that HPP can be used effectively to reduce the microbial load in Turkish white cheese.     

An outbreak of food-borne listeriosis due to cheese in Japan, during 2001

Food-borne outbreaks caused by Listeria monocytogenes have been recognized in US and European countries. Only sporadic cases, of neonatal listeriosis, have been reported in Japan. Since L. monocytogenes has been often isolated from foods in Japan, food-borne outbreaks potentially could have occurred. In February 2001, L. monocytogenes serotype 1/2b was isolated from a washed-type cheese during routine Listeria monitoring of 123 domestic cheeses. Further samples from products and the environments at the plant that produced the contaminated cheese were examined for L. monocytogenes. L. monocytogenes serotype 1/2b was detected in 15 cheese samples, at most probable number that ranged from <30 to 4.6 x 10(9)/100 g, and in environmental samples. Studies with people who had consumed cheese from the plant revealed 86 persons who had been infected with L. monocytogenes. Thirty-eight of those people had developed clinical symptoms of gastroenteritis or the common cold type after the consumption of cheese. Isolates from those patients exhibited the same serotype, pathogenicity for mice and HeLa cells, DNA fingerprinting patterns and PCR amplification patterns. From the epidemiological and genetic evidence, it appeared that the outbreak was caused by cheese. This is the first documented incidence of food-borne listeriosis in Japan.     

Escherichia coli O157:H7 outbreak linked to raw milk cheese in Quebec, Canada: use of exact probability calculation and casecase study approaches to foodborne outbreak investigation

The analytical studies used to investigate foodborne outbreak are mostly case-control or retrospective cohort studies. However, these studies can be complex to perform and susceptible to biases. This article addresses basic principles of epidemiology, probability, and the use of case-case design to identify the source of an Escherichia coli O157:H7 outbreak linked to raw milk cheese consumption in Quebec, Canada; a small number of cases with the same pulsed-field gel electrophoresis (PFGE) profile were involved. Between 4 December 2008 and 15 January 2009, a cumulative total of 16 E. coli O157:H7 cases with the same PFGE profile were reported to Quebec public health authorities. Among the first six cases reported, three had consumed raw milk cheese from the same producer (cheese A). Raw milk cheese is consumed by about 2 % of the Quebec population. By using the exact probability calculation, it was found that a significantly higher proportion of E. coli O157:H7 cases (with the specific PFGE profile) than expected had consumed cheese A (P < 0.001). These computations were updated during the course of the investigation to include subsequent cases and gave the same results. A case-case study corroborated this result. This article considers alternative statistical and epidemiological approaches to investigate a foodborne outbreak-in particular with an exact probability calculation and case-case comparisons. This approach could offer a fast and inexpensive alternative to regular case-control studies to target public health actions, particularly during a foodborne outbreak.     

Survival of Listeria monocytogenes during the manufacture and ripening of Turkish white cheese

Listeria monocytogenes was enumerated during the manufacture and ripening of Turkish White cheese with particular reference to a) pasteurized milk, b) cheese milk after inoculation with L. monocytogenes (0 h), c) after curd formation (2 h), d) curd after pressing (6 h), e) curd after pH was reduced (17 h), f) curd after salting (32 h), and g) cheeses during ripening. Cheeses were also examined periodically for total solids, moisture and salt contents, pH values and aerobic plate count. An increase in the number of L. monocytogenes was observed during manufacture. Following salting and throughout the storage period, numbers of L. monocytogenes decreased at a rate depending on the salt concentration, starter activity and storage time. The initial microbial number had a significant (P < 0.01) effect on the survival of L. monocytogenes during the storage period.     

Occurrence of Listeria spp. in critical control points and the environment of Minas Frescal cheese processing

Critical control points (CCPs) associated with Minas Frescal cheese (a Brazilian soft white cheese, eaten fresh) processing in two dairy factories were determined using flow diagrams and microbiological tests for detection of Listeria monocytogenes and other species of Listeria. A total of 218 samples were collected along the production line and environment. The CCPs identified were reception of raw milk, pasteurization, coagulation and storage. Thirteen samples were positive for Listeria; 9 samples were Listeria innocua, 2 were Listeria grayi and 2 were L. monocytogenes. In factory A, Listeria was found in 50% of raw milk samples, 33.3% of curd samples, 16.7% of pasteurized milk samples, 16.7% of cheese samples and 25% of rubber pipes used to transport the whey. The microorganism was not obtained from environmental samples in this plant. In factory B, Listeria was found in one sample of raw milk (16.7%) and in three samples of environment (17.6%) and L. monocytogenes was obtained from raw milk (16.7%) and the floor of the cheese refrigeration room (14.3%). Two serotypes, 4b and 1/2a, were observed among the strains of L. monocytogenes isolated, both which are frequently involved in outbreaks of food-borne listeriosis and sporadic cases of the disease all over the world.     

Survival of Listeria monocytogenes during the manufacture and ripening of Swiss cheese.

Rindless Swiss cheese was made from a mixture of pasteurized whole and skim milk that was inoculated to contain 10(4) to 10(5) cfu of Listeria monocytogenes (strain Ohio, California, or V7)/ml. During clotting of milk, numbers of L. monocytogenes remained nearly unchanged. When the curd was heated gradually to attain the cooking temperature (50 degrees C), numbers of L. monocytogenes increased by approximately 40 to 45% over those in inoculated milk. Cooking curd at 50 degrees C (122 degrees F) for 30 to 40 min resulted in resilient curd having a pH of 6.40 to 6.45 and decreased L. monocytogenes by 48% compared with numbers of the pathogen in inoculated milk. After curd was pressed under whey, numbers of L. monocytogenes increased by approximately 52% over those in inoculated milk and reached their maxima at the end of this stage. A sharp decrease in numbers of L. monocytogenes occurred during brining of cheese blocks (7 degrees C for 30 h). The population of L. monocytogenes continued to decrease during cheese ripening. Average D values for strains California, Ohio, and V7 were 29.2, 24, and 22.5 d, respectively. Listeria was not detected (direct plating, and cold enrichment) after 80, 77, and 66 d of ripening of Swiss cheese made from milk inoculated with strains California, Ohio, and V7, respectively. Thus, Swiss cheese made in this study did not permit extended survival of L. monocytogenes.     

Comparison of media and sampling locations for isolation of Listeria monocytogenes in queso fresco cheese

Listeriosis associated with Hispanic-style soft cheese is an ongoing public health concern. Although rapid detection methods based on molecular and immunological technologies have been applied successfully for detecting Listeria monocytogenes in foods, obtaining isolates of the pathogen is a critical procedure for epidemiologic studies and regulatory analysis. Oxford agar, a medium recommended by the U.S. Food and Drug Administration Bacteriological Analytical Manual (BAM) to isolate L. monocytogenes from cheese, is unable to differentiate L. monocytogenes from other Listeria species. Hence, two selective isolation media, L. monocytogenes blood agar (LMBA) and Rapid 'L. mono agar (RLMA), were compared with Oxford agar for isolating L. monocytogenes from cheese. Queso fresco cheese was inoculated at 10(0) or 10(1) CFU/g with a five-strain mixture of L. monocytogenes or with the five-strain L. monocytogenes mixture and Listeria innocua. Cheese samples were stored at 21, 12, and 4 degrees C and Listeria counts were determined at 3, 7, and 10 days; 7, 10, 14, 21 days; and 2, 4, 8, and 12 weeks postinoculation, respectively. Surface and interior cheese samples as well as liquid exudate produced during storage were assayed individually to determine differences in Listeria contamination at different sampling locations. L. monocytogenes was more easily differentiated from L. innocua on RLMA than LMBA and Oxford agar. Similar L. monocytogenes counts (ca. 10(4) CFU/g) were obtained on the last sampling day on the surface and interior of cheese samples (P > 0.05) for all storage temperatures and both initial inoculation levels, but smaller cell numbers were detected in the exudate produced during storage. In addition, simultaneous inoculation of L. innocua with L. monocytogenes did not affect the final L. monocytogenes counts in the cheese. The amount of exudate released from the cheese and decrease of pH correlated with storage temperature. More exudate was produced and a greater decrease of pH occurred at 21 degrees C than at 12 or 4 degrees C. Our results indicate that RLMA is a suitable medium for isolating L. monocytogenes from queso fresco cheese. Higher counts of L. monocytogenes were obtained from surface and interior samples of cheese than from the exudate of the cheese during storage. In addition, pH may be a useful indicator of improperly stored queso fresco cheese.     

Molecular Subtyping and Tracking of Listeria monocytogenes in Latin-Style Fresh-Cheese Processing Plants

Latin-style fresh cheeses, which have been linked to at least 2 human listeriosis outbreaks in the United States, are considered to be high-risk foods for Listeria monocytogenes contamination. We evaluated L. monocytogenes contamination patterns in 3 Latin-style fresh-cheese processing plants to gain a better understanding of L. monocytogenes contamination sources in the manufacture of these cheeses. Over a 6-mo period, 246 environmental samples were collected and analyzed for L. monocytogenes using both the Food and Drug Administration (FDA) method and the Biosynth L. monocytogenes detection system (LMDS). Finished cheese samples from the same plants (n = 111) were also analyzed by the FDA method, which was modified to include L. monocytogenes plating medium (LMPM) and the L. monocytogenes confirmatory plating medium (LMCM) used in the LMDS method. Listeria monocytogenes was detected in 6.3% of cheese and 11.0% of environmental samples. Crates, drains, and floor samples showed the highest contamination rates, with 55.6, 30.0, and 20.6% L. monocytogenes positive samples, respectively. Finished products and food contact surfaces were positive in only one plant. The FDA method showed a higher sensitivity than the LMDS method for detection of L. monocytogenes from environmental samples. The addition of LMPM and LMCM media did not further enhance the performance of the FDA method for L. monocytogenes detection from finished products. Molecular subtyping (PCR-based allelic analysis of the virulence genes actA and hly and automated ribotyping) was used to track contamination patterns. Ribotype DUP-1044A, which had previously been linked to a 1998 multistate human listeriosis outbreak in the United States, was the most commonly identified subtype (20/36 isolates) and was isolated from 2 plants. This ribotype was persistent and widespread in one factory, where it was also responsible for the contamination of finished products. We hypothesize that this ribotype may represent a clonal group with a specific ability to persist in food processing environments. While previous listeriosis outbreaks were linked to Latin-style fresh cheeses made from unpasteurized milk, the presence of this organism in pasteurized cheese products illustrates that persistent environmental contamination also represents an important source of finished product contamination.     

The occurrence of Listeria species in milk and dairy products: a national survey in England and Wales

A total of 4172 samples of milk, cheese and other dairy products were examined over a 1-year period for the presence of Listeria species. Strains of Listeria were found most frequently in soft, ripened cows milk cheese; 63 out of 769 (8.2%) samples contained Listeria monocytogenes, 25 samples contained species other than L. monocytogenes, and 18 samples contained both L. monocytogenes and other Listeria spp. Eleven samples of pasteurized cows milk (1.1%) from four dairies contained L. monocytogenes, and other Listeria spp. were isolated from a further five samples. Goats and ewes milk and their products, yogurt, cream and ice cream also occasionally contained Listeria spp. Levels of Listeria were usually low, but 20 samples of cheese contained more than 1000 cfu/g. Most strains of L. monocytogenes belonged to serotype 1/2 (58%) or serotype 4b (33%).     

Evaluation of Listeria innocua as a suitable indicator for replacing Listeria monocytogenes during ripening of Camembert cheese

The suitability of Listeria innocua for use as an indicator for replacing Listeria monocytogenes during the cheese-making and ripening of Camembert cheese was evaluated. Pasteurized whole milk inoculated with either L. innocua or L. monocytogenes was used to make Camembert cheese, which were ripened in three stages. All cheese was ripened in three stages: room temperature (∼20 °C) and relative humidity of 60% for 36 h; 12 °C and relative humidity of 93% for 2 weeks; and 7 °C and relative humidity of 85% for 3 weeks. Results showed that population values of L. innocua and L. monocytogenes on day 1 were 7.16 and 6.11 log₁₀ CFU/g, respectively, which declined to 6.54 and 5.45 log₁₀ CFU/g, respectively, during subsequent 20 days. Thereafter, L. innocua and L. monocytogenes populations increased to 7.38 and 6.06 log₁₀ CFU/g on day 35 of ripening, respectively. During ripening, surface and interior of cheeses were analysed for populations of L. innocua and L. monocytogenes , respectively. The data were collected on day 1, 5, 10, 15, 20, 25, 30, and 35 of ripening. Generally, the growth of L. innocua and L. monocytogenes is faster in surface than in centre. Top centre, bottom centre and bottom surface locations had similar population values during ripening. There were no significant differences ( P  > 0.05) between batch and section of cheese. The ripening time and locations had significant effect ( P  < 0.05) on the survival and growth of L. innocua and L. monocytogenes . The trends of survival and growth of L. innocua and L. monocytogenes were similar. These results indicated that L. innocua can be considered as an indicator for L. monocytogenes during ripening of Camembert cheese.     

Modelling the fate of Listeria monocytogenes during manufacture and ripening of smeared cheese made with pasteurised or raw milk

The dynamics of the physicochemical characteristics of foods help to determine the fate of pathogens throughout processing. The aim of this study was to assess the behaviour of Listeria monocytogenes during cheesesmaking and ripening and to model the growth observed under the dynamic conditions of the cheese. A laboratory scale cheese was made in 4 independent replicates from pasteurised or raw cow's milk, artificially contaminated with L. monocytogenes. No growth of L. monocytogenes occurred during raw milk cheese-making, whereas growth did occur in pasteurised milk. During ripening, growth occurred in raw milk cheese, but inactivation occurred in pasteurised milk cheese. The behaviour observed for L. monocytogenes was modelled using a logistic primary model coupled with a secondary cardinal model, taking into account the effect of physicochemical conditions (temperature, pH, water activity and lactate). A novel statistical approach was proposed to assess the optimal growth rate of a microorganism from experiments performed in dynamic conditions. This complex model had an acceptable quality of fit on the experimental data. The estimated optimum growth rates can be used to predict the fate of L. monocytogenes during cheese manufacture in raw or pasteurized milk in different physicochemical conditions. The data obtained contributes to a better understanding of the potential risk that L. monocytogenes presents to cheese producers (growth on the product, if it is contaminated) and consumers (the presence of high numbers) and constitutes a very useful set of data for the completion of chain-based modelling studies.     

Listeria monocytogenes in Gorgonzola: subtypes, diversity and persistence over time

L. monocytogenes represents a primary concern in the production of Gorgonzola, a Protected Designation of Origin (PDO) Italian blue-veined cheese produced only in the Piedmont and Lombardy regions. L. monocytogenes isolates (N=95) obtained from Gorgonzola rinds, paste, and production/ripening environments were serotyped and then genotyped using Pulsed Field Gel Electrophoresis (PFGE). The goal of this study was to investigate the variability of L. monocytogenes PFGE-types across different PDO Gorgonzola manufacturers (N=22). The majority of the strains (88%) were serotyped as 1/2a. PFGE identified 2 major pulse-types grouping 62 strains, detected from different plants and years, suggesting the presence of persistent and niche-adapted L. monocytogenes. In 9 plants, environmental strains shared the same pulse-types with strains from rinds or paste, suggesting a possible transmission pathway. Encouragingly, L. monocytogenes was retrieved from only 1 paste, indicating that production processes were under control in 21 plants. In the remaining plant, un-effective pasteurization or cross-contamination during production processes could be the cause of the contamination. Consequently, it is imperative that producers operate under the total respect of the Good Manufacturing Practices and following the principles of the Hazard Analysis Critical Control Point plans, in order to contain contamination throughout the whole processing.     

PRESENCE OF LISTERIA IN MEXICAN CHEESES

The presence of Listeria was investigated in ripened cheeses (Chihuahua, Manchego) and fresh cheese (Panela) from street vendors and retail stores in Mexico City. Cheeses were tested for Listeria, pH, NaCl, moisture and fat. Listeria selective cold enrichment was used to recover Listeria from positive samples. Fresh cheese had the lowest pH and NaCl contents and the highest moisture Chihuahua and Manchego cheeses made with pasteurized milk were negative for Listeria. Panela cheese samples were the most contaminated. The presence of Listeria was 65% in fresh cheeses: Listeria murrayi 20% , Listeria inoccua 15% , Listeria grayi 15%, and Listeria monocytogenes 15%. L. monocytogenes serotypes 1/2a, 1/2b and 4b were isolated from positive samples.     

Occurrence and Antimicrobial Susceptibility of Listeria monocytogenes Isolated from Brined White Cheese in Jordan

Listeria monocytogenes is a serious foodborne pathogen that has been isolated from different dairy food products. Several foodborne outbreaks of listeriosis have been associated with consumption of cheese. The aims of this study were to determine the occurrence of L. monocytogenes and Listeria spp. in brined white cheese (BWC) sold in Jordan, and to determine the susceptibility of isolated L. monocytogenes to antimicrobials. Three hundred and fifty samples of 5 different types of BWC (akkawi, boiled, halloumi, pasteurized, and shellal) were collected from a local market in Jordan. The ISO (11290-1) procedure was followed for isolation and identification of Listeria spp. from cheese samples and a polymerase chain reaction (PCR) technique was used for confirmation of L. monocytogenes isolates. The VITEK2 automated system was used for testing antimicrobial susceptibility of L. monocytogenes isolates. The overall prevalence of Listeria spp. in cheese sample was 27.1%. L. monocytogenes was isolated from 39 (11.1%) samples. Other isolated species were L. grayi (6.9%), L. innocua (2%), L. ivanovii (4%), L. seeligeri (2%), and L. welshimeri (0.3%). The pH values and salt concentrations of L. monocytogenes positive cheese samples ranged from 5.10 to 6.32 and 5.64 to 13.16, respectively. L. monocytogenes isolates were sensitive or intermediate susceptible to imipenem, gentamicin, linezolid, teicoplanin, vancomycin, fusidic acid, trimethoprim/sulfamethoxazole, benzylpenicillin, ciprofloxacin, erythromycin, tetracycline, and rifampicin, but resistant to fosfomycin, oxacillin, and clindamycin.     

Important vectors for Listeria monocytogenes transmission at farm dairies manufacturing fresh sheep and goat cheese from raw milk

The aim of this study was to determine the transmission routs of Listeria spp. in dairy farms manufacturing fresh cheese made from ovine and caprine raw milk and to evaluate the impact of Listeria monocytogenes mastitis on raw milk contamination. Overall, 5,799 samples, including 835 environmental samples, 230 milk and milk product samples, and 4,734 aseptic half-udder foremilk samples were collected from 53 dairy farms in the dairy intensive area of Lower Austria. Farms were selected for the study because raw milk was processed to cheese that was sold directly to consumers. A total of 153 samples were positive for Listeria spp., yielding an overall prevalence of 2.6%; L. monocytogenes was found in 0.9% of the samples. Bulk tank milk, cheese, and half-udder samples were negative for Listeria spp. Because none of the sheep and goats tested positive from udder samples, L. monocytogenes mastitis was excluded as a significant source of raw milk contamination. L. monocytogenes was detected at 30.2% of all inspected farms. Swab samples from working boots and fecal samples had a significantly higher overall prevalence (P < 0.001) of L. monocytogenes (15.7 and 13.0%, respectively) than did swab samples from the milk processing environment (7.9%). A significant correlation was found between the prevalence of L. monocytogenes in the animal and in the milk processing environment and the silage feeding practices. Isolation of L. monocytogenes was three to seven times more likely from farms where silage was fed to animals throughout the year than from farms where silage was not fed to the animals.     

Efficacy of direct plating media for recovering Listeria monocytogenes from foods

Studies were done to evaluate 14 direct plating media for their suitability to recover Listeria monocytogenes strain Scott A from pasteurized whole milk, chocolate ice cream mix, Brie cheese and raw cabbage. Healthy cells were inoculated into foods to achieve viable populations of 10(2), 10(4), or 10(6) cells/ml(g). Bind's Acriflavine Agar, Trypaflavine Nalidixic Acid Serum Agar, Listeria Transport Enrichment Agar, Doyle and Schoeni Selective Enrichment Agar (DSSEA) and Modified DSSEA were not suitable for recovering L. monocytogenes from milk and Brie cheese and were therefore not evaluated as direct plating media for recovering the organism from ice cream mix and cabbage. McBride Listeria Agar (MLA), Gum Base Nalidixic Acid Tryptone Soya Agar (GBNTSA), Modified Despierres Agar (MDA) and Modified MLA (MMLA) performed best for recovering all inoculum populations from milk and ice cream mix. Enumeration of L. monocytogenes on several test media was complicated by the growth of large numbers of background microflora present in cabbage and Brie cheese, especially at the lowest test inoculum (10(2)/g). Generally, complete recovery of L. monocytogenes from Brie cheese and cabbage was attained on media when the inoculum population was greater than or equal to 10(4) cells/g. For Brie cheese, MLA, MDA, MMLA and Dominguez Rodriguez Isolation Agar were superior for recovering L. monocytogenes, while GBNTSA, DLEA, MDA and MMLA were best for recovering the organism from cabbage. Results of this experiment indicate that direct-plating procedures, without prior enrichment, can successfully be utilized for recovering L. monocytogenes from foods such as pasteurized milk and ice cream mix which contain low populations of background microflora. However, recovery of L. monocytogenes from foods such as raw cabbage and Brie cheese, which contain high populations of other microorganisms, was not satisfactory using the direct-plating procedures evaluated in this investigation.