Detection and Enumeration of Four Foodborne Pathogens in Raw Commingled Silo Milk in the United States

Abstract

A nationwide survey was conducted to obtain qualitative and quantitative data on bacterial contamination of raw commingled silo milk intended for pasteurization. The levels of total aerobic bacteria, total coliforms, Enterobacteriaceae, Escherichia coli, and Staphylococcus aureus were determined using the TEMPO system. The prevalence rates and levels of presumptive Bacillus cereus, E. coli O157:H7, Listeria monocytogenes, and Salmonella spp. were determined in 214 samples. B. cereus was detected in 8.91% of samples, at 3.0 to 93 CFU/ml. E. coli O157:H7 was detected in 3.79 to 9.05% of samples, at <0.0055 to 1.1 CFU/ml, depending on the assay utilized. Salmonella spp. were recovered from 21.96 to 57.94% of samples, at <0.0055 to 60 CFU/ml. L. monocytogenes was detected in 50.00% of samples, at <0.0055 to 30 CFU/ml. The average log-transformed counts of total viable bacteria were slightly lower in samples containing no pathogens. No correlation was observed between the levels of organisms detected with the TEMPO system and the presence or levels of any pathogen except E. coli O157:H7. A higher average log-transformed count of total viable bacteria was observed in samples positive for this organism. The high prevalence rates of target pathogens may be attributed to a variety of factors, including detection methods, sample size, and commingling of the milk in the silo. The effects of commingling likely contributed to the high prevalence rates and low levels of target pathogens because of the inclusion of milk from multiple bulk tanks. The high prevalence rates also may be the result of analysis of larger sample volumes using more sensitive detection methods. These quantitative data could be utilized to perform more accurate risk assessments and to better estimate the appropriate level of protection for dairy products and processing technologies.

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

“…By comparison, D'Amico and colleagues (D'Amico et al, 2008;D'Amico and Donnelly, 2010) reported that between 32 and 39% of raw milk used for making cheese in Vermont (United States) contained coliforms at >10 cfu/ mL. Other US studies reported even higher percentages of raw milk with coliforms above this concentration, ranging from 56% (Jayarao and Wang, 1999) to 80% (Pantoja et al, 2009) and 87% in a more recent study (Jackson et al, 2012). These results, together with results from our study, strongly suggest that raw milk is a very important source of coliforms in cheese made from unpasteurized milk.…”

Dynamics of Escherichia coli Virulence Factors in Dairy Herds and Farm Environments in a Longitudinal Study in the United States

“…STEC strains or stx virulence factors have also been detected at low prevalence in milk from U.S. farms (5,8,9,69) and European farms (20,70,71). E. coli O157 or the associated virulence factors have also been detected in raw bulk milk at prevalences in the range of 0.2% to 9.1% (10,72). Other surveys, however, detected no E. coli O157:H7 in raw milk in the United States (6,35,69,73) and abroad (43,74).…”

“…Most past surveys assessed prevalence of generic or pathogenic E. coli, but concentrations were determined in only a few studies, for instance, in milk (72,83). One study on fecal shedding by calves observed concentrations of E. coli O157 up to 10 6 CFU/g (84).…”

Raw and Heat-Treated Milk: From Public Health Risks to Nutritional Quality

“…However, so far, no correlation has been identified. A recent survey carried out in the United States [30] demonstrated that coliform counts are not an index of the presence of B. cereus, E. coli O157:H7, L. monocytogenes and Salmonella spp. and that subsequently coliform testing of raw milk intended for human consumption cannot be used as a reliable tool for public health risk screening [30,31].…”