Coliform detection in cheese is associated with specific cheese characteristics, but no association was found with pathogen detection.

Coliform detection in finished products, including cheese, has traditionally been used to indicate whether a given product has been manufactured under unsanitary conditions. As our understanding of the diversity of coliforms has improved, it is necessary to assess whether coliforms are a good indicator organism and whether coliform detection in cheese is associated with the presence of pathogens. The objective of this study was (1) to evaluate cheese available on the market for presence of coliforms and key pathogens, and (2) to characterize the coliforms present to assess their likely sources and public health relevance. A total of 273 cheese samples were tested for presence of coliforms and for Salmonella, Staphylococcus aureus, Shiga toxin-producing Escherichia coli, Listeria monocytogenes, and other Listeria species. Among all tested cheese samples, 27% (75/273) tested positive for coliforms in concentrations >10cfu/g. Pasteurization, pH, water activity, milk type, and rind type were factors significantly associated with detection of coliforms in cheese; for example, a higher coliform prevalence was detected in raw milk cheeses (42% with >10cfu/g) compared with pasteurized milk cheese (21%). For cheese samples contaminated with coliforms, only water activity was significantly associated with coliform concentration. Coliforms isolated from cheese samples were classified into 13 different genera, including the environmental coliform genera Hafnia, Raoultella, and Serratia, which represent the 3 genera most frequently isolated across all cheeses. Escherichia, Hafnia, and Enterobacter were significantly more common among raw milk cheeses. Based on sequencing of the housekeeping gene clpX, most Escherichia isolates were confirmed as members of fecal commensal clades of E. coli. All cheese samples tested negative for Salmonella, Staph. aureus, and Shiga toxin-producing E. coli. Listeria spp. were found in 12 cheese samples, including 5 samples positive for L. monocytogenes. Although no association was found between coliform and Listeria spp. detection, Listeria spp. were significantly more likely to be detected in cheese with the washed type of rind. Our data provide information on specific risk factors for pathogen detection in cheese, which will facilitate development of risk-based strategies to control microbial food safety hazards in cheese, and suggest that generic coliform testing cannot be used to assess the safety of natural cheese.

When cheese gets the blues: Pseudomonas fluorescens as the causative agent of cheese spoilage.

A bacterial contamination of fresh, low-acid cheese that resulted in production of a blue fluorescent pigment on the surface of the cheese was determined to be caused by Pseudomonas fluorescens biovar IV, a gram-negative bacteria that produces a blue, nondiffusible pigment as well as the soluble pigment pyoverdin, which fluoresces under UV light. Ten isolates collected from contaminated cheese and environmental samples were initially identified as P. fluorescens using 16S rDNA sequencing, but only 8 of the isolates produced blue pigment and fluoresced under UV light when re-inoculated onto fresh, low-acid cheese. The Biolog Metabolic Fingerprint system (Biolog Inc., Hayward, CA) and the Analytical Profile Index (BioMerieux Vitek Inc., Hazelwood, MO) for nonenteric gram-negative species as well as EcoRI ribotyping did not differentiate between the isolates that produced blue color and those that did not. Pulsed field gel electrophoresis with the enzyme XbaI was able to distinguish between the isolates that produced pigment and those that did not and allowed for identification of a specific environmental site (i.e., an overhead cheese vat agitator system) as the likely source of product contamination.

Results from raw milk microbiological tests do not predict the shelf-life performance of commercially pasteurized fluid milk.

Analytical tools that accurately predict the performance of raw milk following its manufacture into commercial food products are of economic interest to the dairy industry. To evaluate the ability of currently applied raw milk microbiological tests to predict the quality of commercially pasteurized fluid milk products, samples of raw milk and 2% fat pasteurized milk were obtained from 4 New York State fluid milk processors for a 1-yr period. Raw milk samples were examined using a variety of tests commonly applied to raw milk, including somatic cell count, standard plate count, psychrotrophic bacteria count, ropy milk test, coliform count, preliminary incubation count, laboratory pasteurization count, and spore pasteurization count. Differential and selective media were used to identify groups of bacteria present in raw milk. Pasteurized milk samples were held at 6°C for 21 d and evaluated for standard plate count, coliform count, and sensory quality throughout shelf-life. Bacterial isolates from select raw and pasteurized milk tests were identified using 16S ribosomal DNA sequencing. Linear regression analysis of raw milk test results versus results reflecting pasteurized milk quality consistently showed low R(2) values (<0.45); the majority of R(2) values were <0.25, indicating small relationship between the results from the raw milk tests and results from tests used to evaluate pasteurized milk quality. Our findings suggest the need for new raw milk tests that measure the specific biological barriers that limit shelf-life and quality of fluid milk products.

Short communication: Nalpha-lauroyl-L-arginine ethylester monohydrochloride reduces bacterial growth in pasteurized milk.

Effective strategies for extending fluid milk product shelf-life by controlling bacterial growth are of economic interest to the dairy industry. To that end, the effects of addition of l-arginine, Nalpha-lauroyl ethylester monohydrochloride (LAE) on bacterial numbers in fluid milk products were measured. Specifically, LAE was added (125, 170, or 200 mg/L) to conventionally homogenized and pasteurized 3.25% fat chocolate or unflavored milk products. The treated milks and corresponding untreated controls were held at 6 degrees C and plated on standard plate count agar within 24 h of processing and again at 7, 14, 17, and 21 d of storage. Bacterial counts in all unflavored milk samples treated with LAE remained below the Pasteurized Milk Ordinance limit for grade A pasteurized fluid milk of 4.3 log cfu/mL for the entire 21 d. Bacterial counts in unflavored samples containing 170 and 200 mg/L of LAE were significantly lower than those in the untreated unflavored milk at d 17 and 21 postprocessing. Specifically, bacterial counts in the milk treated with 200 mg/L of LAE were 5.77 log cfu/mL lower than in untreated milk at 21 d postprocessing. Bacterial counts in chocolate milk treated with 200 mg/L of LAE were significantly lower than those in the untreated chocolate milk at d 14, 17, and 21. In chocolate milk treated with 200 mg/L of LAE, bacterial counts were 0.9 log cfu/mL lower than in the untreated milk at 21 d postprocessing. Our results show that addition of LAE to milk can reduce bacterial growth. Addition of LAE is more effective at controlling bacterial growth in unflavored milk than in chocolate milk.

Identification and characterization of elevated microbial counts in bulk tank raw milk.

The bacterial composition of bulk tank milk from 13 farms was examined over a 2-wk period to characterize sudden elevations in the total bacterial count referred to as "spikes." Bulk tank milk samples collected at each pick-up were analyzed for standard plate count, Petrifilm aerobic count, somatic cell count, gram-negative organisms, and streptococci. Twenty standard plate count spikes were observed: 12 associated with streptococci, 4 associated with gram-negative organisms, 2 associated with streptococci and gram-negative organisms, and 2 that were not definitively characterized. Spikes ranged from 14,000 to 600,000 cfu/ml. Streptococcus uberis was isolated as the predominant organism from 11 spikes, and Escherichia coli was isolated from 4 spikes. Statistical analysis of total bacterial counts indicated a high correlation (r = 0.94) between standard plate counts and Petrifilm aerobic count. Regression analysis of standard plate counts and Petrifilm aerobic counts yielded the equation log10 (standard plate count) = 0.73 + 0.85log10 (Petrifilm aerobic count), indicating that the correlation, although strong, is not one to one. In a related pilot study, triplicate bulk tank milk samples were collected and analyzed for total bacterial count and presumptive streptococcus, gram-negative, and staphylococcus counts. Two-way ANOVA of these triplicate data indicated a lack of significant variation among the triplicate samples, suggesting that one sample can reliably gauge the microbial status of the entire bulk tank.

Bacterial tracking in a dairy production system using phenotypic and ribotyping methods.

A systematic sampling plan was designed to collect raw and pasteurized milk samples throughout a single-raw milk source, dairy-processing operation experiencing reduced product shelf lives due to bacterial contamination. The objectives were to track bacterial contamination sources throughout a complete dairy production system and use this information to reduce bacterial spoilage losses in processed fluid products. Over a 5-week period, 233 bacterial isolates were collected, representative of different colony morphologies on psychrotrophic bacteria count (PBC) plates. Forty-five isolates (19%) were obtained from pasteurized milk and 188 (81%) were isolated from raw product. Thirty isolates were identified as Pseudomonas spp. by Gram stain and biochemical methods. Of these, 27 (90%) were postpasteurization isolates and 3 (10%) were raw milk isolates. Automated ribotyping revealed that raw and pasteurized Pseudomonas fluorescens isolates were indistinguishable (similarity index > 0.93), suggesting the possibility of postpasteurization contamination with bacteria from raw product. In the plant, filler nozzles were identified as the primary reservoirs of postpasteurization contamination. Nozzle replacement produced significantly lower finished-product PBCs at 7 days postprocessing (> 4-log reduction) and extended fluid product shelf life.