Short Communication: Quantification of the Transmission of Microorganisms to Milk via Dirt Attached to the Exterior of Teats

Pathogens and spoilage microorganisms can be transmitted to milk via dirt (e.g., feces, bedding material, soil, or a combination of these) attached to the exterior of the cows' teats. To determine the relevance of this pathway and to perform quantitative microbial risk analysis of the microbial contamination of farm tank milk (FTM), it is important to know the amount of dirt transmitted to milk via the exterior of teats. In this study at 11 randomly selected Dutch farms the amount of dirt transmitted to milk via the exterior of teats is determined using spores of mesophilic aerobic bacteria as a marker for transmitted dirt. The amount of transmitted dirt to milk varied among farms from approximately 3 to 300 mg/L, with an average of 59 mg/L. The usefulness of the data for microbial risk analyses is briefly illustrated using the contamination of FTM with spores of butyric acid bacteria as a case study. In a similar way the data can be used to identify measures to control the contamination of FTM with other microorganisms or chemical residues.

Minimizing the Level of Butyric Acid Bacteria Spores in Farm Tank Milk

A year-long survey of 24 dairy farms was conducted to determine the effects of farm management on the concentrations of butyric acid bacteria (BAB) spores in farm tank milk (FTM). The results were used to validate a control strategy derived from model simulations. The BAB spore concentrations were measured in samples of FTM, feces, bedding material, mixed corn and grass silage fed to cows in the barn, and soil. In addition, a questionnaire was used to gather farm management information such as bedding material used and teat cleaning method applied. The average BAB spore concentration in FTM was 2.7 log10 spores/L, and 33% of the FTM samples exceeded a concentration of 3 log10 spores/L. Control of the average spore concentration in mixed silage fed was the only aspect of farm management that was significantly related to the concentration of BAB spores in FTM. Farms that fed mixed silage with the lowest average BAB spore concentrations (3.4 log10 spores/g) produced FTM with the lowest average concentration (2.1 log10 spores/L). The efficiency of farm management in controlling the BAB spore concentration in FTM depended to a large extent on the ability of farmers to prevent incidents with elevated BAB spore concentrations in mixed silage (>5 log10 spores/g) and not on the average BAB spore concentration in mixed silage across the year. The survey showed that farmers should aim for a concentration in mixed silage of less than 3 log10 spores/g and should prevent the concentration from exceeding 5 log10 spores/g to ensure a concentration in FTM of less than 3 log10 spores/L. These results correspond with the previously reported model simulations.

Minimizing the Level of Bacillus cereus Spores in Farm Tank Milk

In a year-long survey on 24 Dutch farms, Bacillus cereus spore concentrations were measured in farm tank milk (FTM), feces, bedding material, mixed grass and corn silage, and soil from the pasture. The aim of this study was to determine, in practice, factors affecting the concentration of B. cereus spores in FTM throughout the year. In addition, the results of the survey were used in combination with a previously published modeling study to determine requirements for a strategy to control B. cereus spore concentrations in FTM below the MSL of 3 log10 spores/L. The B. cereus spore concentration in FTM was 1.2 +/- 0.05 log10 spores/L and in none of samples was the concentration above the MSL. The spore concentration in soil (4.9 +/- 0.04 log10 spores/g) was more than 100-fold higher than the concentration in feces (2.2 +/- 0.05 log10 spores/g), bedding material (2.8 +/- 0.07 log10 spores/g), and mixed silage (2.4 +/- 0.07 log10 spores/g). The spore concentration in FTM increased between July and September compared with the rest of the year (0.5 +/- 0.02 log10 spores/L difference). In this period, comparable increases of the concentrations in feces (0.4 +/- 0.03 log10 spores/g), bedding material (0.5 +/- 0.05 log10 spores/g), and mixed silage (0.4 +/- 0.05 log10 spores/g) were found. The increased B. cereus spore concentration in FTM was not related to the grazing of cows. Significant correlations were found between the spore concentrations in FTM and feces (r = 0.51) and in feces and mixed silage (r = 0.43) when the cows grazed. The increased concentrations during summer could be explained by an increased growth of B. cereus due to the higher temperatures. We concluded that year-round B. cereus spores were predominantly transmitted from feeds, via feces, to FTM. Farmers should take measures that minimize the transmission of spores via this route by ensuring low initial contamination levels in the feeds (<3 log10 spores/g) and by preventing growth of B. cereus in the farm environment. In addition, because of the extremely high B. cereus spore concentrations in soil, the contamination of teats with soil needs to be prevented.

Concentrations of Butyric Acid Bacteria Spores in Silage and Relationships with Aerobic Deterioration

Germination and growth of spores of butyric acid bacteria (BAB) may cause severe defects in semihard cheeses. Silage is the main source of BAB spores in cheese milk. The objectives of the study were to determine the significance of grass silages and corn silages as sources of BAB spores and to investigate the relationships between high concentrations of BAB spores in corn silage and aerobic deterioration. In the first survey, samples were taken from various locations in silos containing grass and corn silages and from mixed silages in the ration offered to the cows on 21 farms. We demonstrated that the quantity of BAB spores consumed by cows was determined by a small fraction of silage with a high concentration of spores (above 5 log10 BAB/g). High concentrations were most often found in corn silage within areas with visible molds (69% of the samples). Areas with visible molds in grass silage and surface layers of corn silage contained, respectively, 21 and 19% of the cases of concentrations above 5 log10 BAB spores/g. Based on these results, we concluded that currently in the Netherlands, corn silage is a more important source of BAB than is grass silage. In a second survey, 8 corn silages were divided into 16 sections and each section was studied in detail. High concentrations of BAB spores were found in only the top 50 cm of these 8 silages. Elevated concentrations of BAB spores were associated with different signs of aerobic deterioration. In 13% of the sections in corn silage with more than 5 log10 yeasts and molds/g, more than 5 log10 BAB spores/g were found. Sections with a temperature of more than 5 degrees C above ambient temperature contained, in 21% of the cases, more than 5 log10 BAB spores/g. Concentrations above 5 log10 BAB spores/g were measured in 50% of the sections with a pH above 4.4. All sections with a pH above 4.4 also showed a temperature that was more than 5 degrees C above ambient temperature and a concentration of yeasts and molds above 5 log10 cfu/g. Based on these results, we postulated that high concentrations of BAB spores in corn silage are the result of oxygen penetration into the silage, resulting in aerobic deterioration and the formation of anaerobic niches with an increased pH just below the surface. Growth of BAB in these anaerobic niches with an increased pH caused the locally high concentrations of BAB in corn silage.

Predictive Modeling of Bacillus cereus Spores in Farm Tank Milk During Grazing and Housing Periods

The shelf life of pasteurized dairy products depends partly on the concentration of Bacillus cereus spores in raw milk. Based on a translation of contamination pathways into chains of unit-operations, 2 simulation models were developed to quantitatively identify factors that have the greatest effect on the spore concentration in milk. In addition, the models can be used to determine the reduction in concentration that could be achieved via measures at the farm level. One model predicts the concentration when soil is the source of spores, most relevant during grazing of cows. The other model predicts the concentration when feed is the main source of spores, most relevant during housing of cows. It was estimated that when teats are contaminated with soil, 33% of the farm tank milk (FTM) contains more than 3 log(10) spores/L of milk. When feed is the main source, this is only 2%. Based on the predicted spore concentrations in FTM, we calculated that the average spore concentration in raw milk stored at the dairy processor during the grazing period is 3.5 log(10) spores/L of milk and during the housing period is 2.1 log(10) spores/L. It was estimated that during the grazing period a 99% reduction could be achieved if all farms minimize the soil contamination of teats and teat cleaning is optimized. During housing, reduction of the concentration by 60% should be feasible by ensuring spore concentrations in feed below 3 log(10) spores/g and a pH of the ration offered to the cows below 5. Implementation of these measures at the farm level ensures that the concentration of B. cereus spores in raw milk never exceeds 3 log(10) spores/L.

Improving Farm Management by Modeling the Contamination of Farm Tank Milk with Butyric Acid Bacteria

Control of contamination of farm tank milk (FTM) with the spore-forming butyric acid bacteria (BAB) is important to prevent the late-blowing defect in semi-hard cheeses. The risk of late blowing can be decreased via control of the contamination level of FTM with BAB. A modeling approach was applied to identify an effective control strategy at the farm level. The simulation model developed was based on a translation of the contamination pathway into a chain of unit operations. Using various simulations, the effects of factors related to feed quality, feed management, cattlehouse hygiene, and milking practices on the contamination level of FTM were evaluated. Contamination level of silage was found to be the most important factor. When silage contains on average less than 3 log10 BAB/g, a basic pretreatment of udder teats before milking (approximately 75% removal of attached spores) is sufficient to assure an FTM contamination level below 1 BAB/mL. When silage contains more than 5 log10 BAB/g, it should not be fed, because it then becomes almost impossible to assure an FTM contamination level below 1 BAB/mL. Measures aimed at improving cattlehouse hygiene, the contamination via soil, and the contamination level of other feeds contribute only marginally to the control of the contamination of FTM with BAB. Application of the modeling methodology could be beneficial for the control of the contamination of FTM with other microorganisms such as Bacillus cereus.

Occurrence and survival of verocytotoxin-producing Escherichia coli O157 in raw cow's milk in The Netherlands

From May through November 1997, 1,011 samples of raw milk from bulk storage tanks were examined for the presence of verocytotoxin-producing Escherichia coli of serogroup O157 (O157 VTEC) by immunomagnetic separation following selective enrichment. The samples originated from 1,011 different dairy herds located throughout the Netherlands. O157 VTEC was not isolated from any of the milk samples examined. Additionally, survival of O157 VTEC in raw and UHT-sterilized cow's milk at 7 and 15 degrees C was studied, both in the absence and presence of an activated lactoperoxidase-thiocyanate-hydrogen peroxide system (LPS). Results indicated that the O157 VTEC strain tested was able to grow in raw milk at 7 degrees C as well as at 15 degrees C. Naturally occurring amounts of thiocyanate and hydrogen peroxide in the raw milk tested were not sufficient to activate the LPS. Although the LPS exhibited an antimicrobial activity against O157 VTEC in LPS-activated sterilized milk, O157 VTEC populations were not (or not as obviously) reduced in LPS-activated raw milk. Possibly background microflora were more sensitive to the LPS than the O157 VTEC test strain. It was concluded that raw milk contaminated with O157 VTEC will remain a hazard if kept at 7 or 15 degrees C. Effective pasteurization and avoiding postpasteurization contamination are necessary to ensure the safety of milk.

Isolation and characterisation of Bacillus cereus from pasteurised milk in household refrigerators in The Netherlands

The incidence and some characteristics (carbohydrate metabolism, growth profiles, haemolysin production and enterotoxin production) of Bacillus cereus, in pasteurised, low-fat (1.5%) milk, in household refrigerators in the Netherlands was investigated. In 247 (74%) of the 334 milk samples analyzed, the mesophilic aerobic counts were between 50 and 5000 per millilitre. B. cereus could be isolated from 133 (40%) of the samples. In general the B. cereus counts were low; numbers of less than five per millilitre were observed in 258 (77%) of the samples. As expected, both the mesophilic aerobic counts and levels of B. cereus increased with increasing storage temperatures in the refrigerator and prolonged storage times. In total, 143 presumptive B. cereus colonies were isolated. According to the ISO confirmation tests and the carbohydrate patterns (API 50 CHB) 134 (94%) of these isolates were confirmed to be B. cereus. Of these 134 isolates 20% fermented lactose and 53% of the 106 strains tested were able to grow at 7 degrees C. These percentages are much higher than expected for strains isolated from non-dairy products, suggesting that strains can adapt to environmental conditions in milk. All 106 strains tested, produced haemolysin, 27% showed the discontinuous haemolytic pattern characteristic for haemolysin BL, possibly a virulence factor. Of the 37 B. cereus isolates tested for enterotoxin production 27 (73%), 28 (76%) and 26 (70%) were found to be enterotoxigenic (as determined by the Western immunoblot technique, polymerase chain reaction (PCR) and Vero cell assays, respectively). Isolates unable to ferment lactose, produced less enterotoxin in comparison with those able to utilize lactose. Although only a few outbreaks of food poisoning caused by B. cereus in milk (products) have been reported, most strains isolated from these products are able to produce enterotoxins and may represent a health hazard.

Effect of exogenous proline, betaine, and carnitine on growth of Listeria monocytogenes in a minimal medium

Three Listeria monocytogenes strains isolated from food or food-processing environments were used to assess the response of this species to salinity in a chemically defined minimal medium. Growth in a minimal medium containing five essential amino acids and glucose as a carbon and energy source was comparable to growth in a rich medium (brain heart infusion broth). In the absence and presence of 3% NaCl the final cell numbers reached in minimal medium were 10(9) and 10(7) CFU/ml, respectively. Growth under the latter conditions could not be detected by spectrophotometry by measuring A660. Apparently, this technique was not suitable for these experiments since the detection level was > 10(7) CFU/ml. Exogenously added proline (10 mM), trimethylglycine (betaine) (1 mM), and beta-hydroxy-gamma-N-trimethyl aminobutyrate (carnitine) (1 mM) significantly stimulated growth under osmotic stress conditions in minimal medium at both 37 and 10 degrees C. Betaine and carnitine are present in foods derived from plants and animals, respectively. Therefore, these compounds can contribute significantly to growth of L. monocytogenes in various foods at high osmolarities.