Salmonellae and salmonellosis associated with milk and milk products. A review

Development of Shortened Enrichment Methods for Detection of Salmonella Typhimurium Spiked in Milk

Rapid and accurate testing of pathogenic Salmonella enterica in dairy products could reduce the risk of exposure to the bacterial pathogens for consumers. This study aimed to reduce the assessment time needed for enteric bacteria recovery and quantification in food using the natural growth properties of Salmonella enterica Typhimurium (S. Typhimurium) in cow's milk and efficiently using rapid PCR methods. Over 5 h of 37 °C enrichment, culture and PCR methods measured increases in the non-heat-treated S. Typhimurium concentration at similar rates, with an average increase of 2.7 log10 CFU/mL between the start of enrichment and 5 h. In contrast, no bacteria were recovered by culture after S. Typhimurium in milk received heat treatment, and the number of gene copies of heat-treated Salmonella detected by PCR did not increase with the enrichment time. Thus, comparing culture and PCR data over just 5 h of enrichment time can detect and differentiate between replicating bacteria and dead bacteria.

Integrated Metabolomics and Proteomics Dynamics of Serum Samples Reveals Dietary Zeolite Clinoptilolite Supplementation Restores Energy Balance in High Yielding Dairy Cows

Dairy cows can suffer from a negative energy balance (NEB) during their transition from the dry period to early lactation, which can increase the risk of postpartum diseases such as clinical ketosis, mastitis, and fatty liver. Zeolite clinoptilolite (CPL), due to its ion-exchange property, has often been used to treat NEB in animals. However, limited information is available on the dynamics of global metabolomics and proteomic profiles in serum that could provide a better understanding of the associated altered biological pathways in response to CPL. Thus, in the present study, a total 64 serum samples were collected from 8 control and 8 CPL-treated cows at different time points in the prepartum and postpartum stages. Labelled proteomics and untargeted metabolomics resulted in identification of 64 and 21 differentially expressed proteins and metabolites, respectively, which appear to play key roles in restoring energy balance (EB) after CPL supplementation. Joint pathway and interaction analysis revealed cross-talks among valproic acid, leucic acid, glycerol, fibronectin, and kinninogen-1, which could be responsible for restoring NEB. By using a global proteomics and metabolomics strategy, the present study concluded that CPL supplementation could lower NEB in just a few weeks, and explained the possible underlying pathways employed by CPL.

In vitro efficiency evaluation of phage cocktail for biocontrol of Salmonella spp. in food products

This study used a set of different bacteriophages to control contaminations of Salmonella spp., a major food pathogen. A cocktail of four phages designated based on host range and in vitro lytic assay showed a lower bacteriophage insensitive mutant frequency and considerable stability at 4 °C and 28 °C up to 60 days. The work evaluated the effectiveness of cocktail of four phages in reducing Salmonella spp. in four different food matrices (liquid egg, eggshell, milk, lettuce). A maximum of 1.7 log reduction in Salmonella spp. was achieved upon treatment of liquid eggs with phage cocktail for 72 h at 4 °C. In milk, the application of phage cocktail reduced recoverable Salmonella spp. by 1.9 log and 1.8 log at 28ºC (6 h) and 4ºC (72 h), respectively. A significant 2.9 log reduction of Salmonella spp. was obtained in eggshell after a 6 h incubation and Salmonella spp. was beyond detection level after 24 h at 28ºC. The application of cocktail also reduced Salmonella spp. beyond the detectable level in lettuce after 8 h at 28 °C. Our results indicated considerable stability of phages in different food matrices. Taken together, our findings establish the potential effectiveness of the bacteriophage cocktail as a biocontrol agent for Salmonella spp. in different food matrices.

MILK Symposium review: Microbiological quality and safety of milk from farm to milk collection centers in Rwanda

The aim of this study was to generate knowledge on the most important milk quality and safety attributes, including somatic cell count (SCC), total bacterial count (TBC), Escherichia coli, Salmonella, and Brucella spp. antibodies and antibiotic residues in milk in the chain from farm to milk collection center (MCC) in Rwanda. In addition, we investigated farm and management factors associated with high TBC, SCC, and Salmonella counts. Raw milk was sampled at the farm and MCC levels. Milk samples were taken from dairy farms linked to 2 selected MCC in each of the 4 provinces in Rwanda. In total, 406 bulk milk samples from 406 farms and 32 bulk milk samples from 8 MCC were collected and analyzed. Farm milk average SCC varied between 180 × 10³ and 920 × 10³ cells/mL, whereas average SCC in milk samples at MCC varied between 170 × 10³ and 1,700 × 10³ cells/mL. The mean milk TBC of different farms per MCC varied between 1.1 × 10⁶ and 1.6 × 10⁷ cfu/mL, whereas in milk samples from different MCC, the mean TBC ranged between 5.3 × 10⁵ and 2.4 × 10⁸ cfu/mL. The high TBC in milk from MCC suggests proliferation or recontamination of milk by bacteria during transportation. Escherichia coli was detected in 35 of 385 farm milk samples and ranged between 5 cfu/mL and 1.1 × 10⁴ cfu/mL, whereas in milk samples from the MCC, it was detected in 20 out 32 samples varying between 5 cfu/mL and 2.9 × 10³ cfu/mL. Overall farm prevalence of Salmonella in milk samples was 14%, but no milk samples from MCC were positive for Salmonella. Five out of 22 bulk milk samples from different MCC were positive for Brucella spp. antibodies, but no Brucella antibodies were detected in milk samples from farms. The prevalence of antibiotic residues as detected by the Delvotest SP NT (DSM, Delft, the Netherlands) was low: 1.3% in farm milk samples and undetected in MCC milk samples. Lack of a separate milking area was associated with high TBC, whereas offering of supplemental feeds, keeping data of past diseases, and an unhygienic milking area were associated with high SCC. Lack of teat washing before milking was the only factor associated with Salmonella contamination of milk at the farm level. This study indicated high TBC and SCC of milk samples at the farm and MCC levels, which indicates both microbial contamination of milk and poor udder health in dairy cows. Presence of E. coli, Salmonella, and Brucella antibodies in milk was common, but finding antibiotic residues in milk was uncommon.

Survivability of Salmonella and Escherichia coli O157:H7 Pathogens and Food Safety Concerns on Commercial Powder Milk Products

Milk and dairy products are susceptible to the incidence of foodborne illnesses by numerous pathogens, including Listeria monocytogenes, Salmonella spp., Escherichia coli, enteropathogenic Campylobacter jejuni, Yersinia enterocolitica, Cronobacter (Enterobacter sakazakii) and Staphylococcus aureus. Annually Salmonella infections cause approximately 93.8 million cases of gastroenteritis and 155,000 deaths worldwide. Including meat and poultry, dairy products are the most commonly contaminated foods by Salmonella. Studies show that Salmonella, Escherichia coli O157:H7 and Listeria monocytogenes are among the top 5 pathogens causing hospitalization and life-threatening foodborne illnesses. The U.S. Centers for Disease Control and Prevention (CDC) estimated that annually around 1.2 million foodborne illnesses with more than 23,000 hospitalizations, 450 deaths and 130 outbreaks were attributed to Salmonella infection in the U.S. The Salmonella enteric in skim milk powder survived at three months storage, with water activity as low as 0.33. With respect to Escherichia coli O157:H7, it is capable of causing disease at a low dosage, ranging from 5–50 cells. Viable cells of Escherichia coli O157:H7 reportedly survive in infant formula powder for one year at 5 °C. The survivability of Escherichia coli in powder milk was significantly reduced with the synergistic effects of storage time and temperature. The U.S. Dairy Export Council recommends that milk powder should be stored in a cool and dry place, at a temperature not to exceed 27 °C, and a relative humidity not to exceed 65%. Reports have recommended that milk powder products need to be stored in light, oxygen, and moisture-proof containers. In this article, the survival of the major foodborne pathogens including Salmonella and Escherichia coli O157:H7 in powdered milk products from common dairy species such as cow and goats are reviewed.

Population Dynamics of Listeria monocytogenes, Escherichia coli O157:H7, and Native Microflora During Manufacture and Aging of Gouda Cheese Made with Unpasteurized Milk

ABSTRACT

Cheeses made with unpasteurized milk are a safety concern due to possible contamination with foodborne pathogens. Listeria monocytogenes and Escherichia coli O157:H7 have been implicated in several outbreaks and recalls linked to Gouda cheese made with unpasteurized milk. The U.S. Food and Drug Administration Code of Federal Regulations requires cheeses made with unpasteurized milk to be aged at a minimum of 1.7°C for at least 60 days before entering interstate commerce. The goal of this study was (i) to assess the population dynamics of L. monocytogenes and E. coli O157:H7 during aging of Gouda cheese when the pathogens were inoculated into the unpasteurized milk used for manufacture and (ii) to compare the native microbial populations throughout manufacture and aging. Unpasteurized milk was inoculated with L. monocytogenes at 1 or 3 log CFU/mL or with E. coli O157:H7 at 1 log CFU/mL, and Gouda cheese was manufactured in laboratory-scale or pilot plant-scale settings. Cheeses were stored at 10°C for at least 90 days, and some cheeses were stored up to 163 days. Initial native microflora populations in unpasteurized milk did not differ significantly for laboratory-scale or pilot plant-scale trials, and population dynamics trended similarly throughout cheese manufacture and aging. During manufacture, approximately 81% of the total L. monocytogenes and E. coli O157:H7 populations was found in the curd samples. At an inoculation level of 1 log CFU/mL, L. monocytogenes survived in the cheese beyond 60 days in four of five trials. In contrast, E. coli O157:H7 was detected beyond 60 days in only one trial. At the higher 3-log inoculation level, the population of L. monocytogenes increased significantly from 3.96 ± 0.07 log CFU/g at the beginning of aging to 6.00 ± 0.73 log CFU/g after 150 days, corresponding to a growth rate of 0.04 ± 0.02 log CFU/g/day. The types of native microflora assessed included Enterobacteriaceae, lactic acid bacteria, mesophilic bacteria, and yeasts and molds. Generally, lactic acid and mesophilic bacterial populations remained consistent at approximately 8 to 9 log CFU/g during aging, whereas yeast and mold populations steadily increased. The data from this study will contribute to knowledge about survival of these pathogens during Gouda cheese production and will help researchers assess the risks of illness from consumption of Gouda cheese made with unpasteurized milk.

HIGHLIGHTS

Powdered infant formula as a source of Salmonella infection in infants

Powdered infant formula is not sterile and may be intrinsically contaminated with pathogens, such as Salmonella enterica, that can cause serious illness in infants. In recent years, at least 6 outbreaks of Salmonella infection in infants that have been linked to the consumption of powdered infant formula have been reported. Many of these outbreaks were identified because the Salmonella strains were unique in some way (e.g., a rare serotype) and a well-established Salmonella surveillance network, supported by laboratories capable of serotyping isolates, was in place. Another common feature of the outbreaks was the low level of salmonellae detected in the implicated formula (salmonellae may be missed in routine testing). These outbreaks likely represent only a small proportion of the actual number of Salmonella infections in infants that have been linked to powdered infant formula. Managing this problem requires a multidimensional approach in which manufacturers, regulators, and caregivers to infants can all play a role.

Effect of phage on survival of Salmonella enteritidis during manufacture and storage of cheddar cheese made from raw and pasteurized milk

The ability of Salmonella Enteritidis to survive in the presence of phage, SJ2, during manufacture, ripening, and storage of Cheddar cheese produced from raw and pasteurized milk was investigated. Raw milk and pasteurized milk were inoculated to contain 10(4) CFU/ml of a luminescent strain of Salmonella Enteritidis (lux) and 10(8) PFU/ml SJ2 phage. The milks were processed into Cheddar cheese following standard procedures. Cheese samples were examined for Salmonella Enteritidis (lux), lactic acid bacteria, molds and yeasts, coliforms, and total counts, while moisture, fat, salt, and pH values were also measured. Salmonella Enteritidis (lux) was enumerated in duplicate samples by surface plating on MacConkey novobiocin agar. Bioluminescent colonies of Salmonella Enteritidis were identified in the NightOwl molecular imager. Samples were taken over a period of 99 days. Counts of Salmonella Enteritidis (lux) decreased by 1 to 2 log cycles in raw and pasteurized milk cheeses made from milk containing phage. In cheeses made from milks to which phage was not added, there was an increase in Salmonella counts of about 1 log cycle. Lower counts of Salmonella Enteritidis (lux) were observed after 24 h in pasteurized milk cheese containing phage compared to Salmonella counts in raw milk cheese with phage. Salmonella Enteritidis (lux) survived in raw milk and pasteurized milk cheese without phage, reaching a final concentration of 10(3) CFU/g after 99 days of storage at 8 degrees C. Salmonella did not survive in pasteurized milk cheese after 89 days in the presence of phage. However, Salmonella counts of approximately 50 CFU/g were observed in raw milk cheese containing phage even after 99 days of storage. In conclusion, this study demonstrates that the addition of phage may be a useful adjunct to reduce the ability of Salmonella to survive in Cheddar cheese made from both raw and pasteurized milk.

Review of studies on the thermal resistance of Salmonellae

Heat resistance data for different serotypes of Salmonella enterica in different food products and laboratory media are reviewed. From all D-values reported, the highest heat resistance of Salmonella was in liquid eggs and liquid egg yolks. The equation from a line drawn through the highest D-values, and above all values reported, was log D-value = 11.7 - 0.188T degrees C. From this equation, the calculated z-value was 5.3 degrees C (9.5 degrees F), and a process at 71degrees C (160 degrees F) will require 1.2 s to inactivate 1 log of Salmonella cells. This calculation did not include data that evaluated the heat resistance after stress conditions or data for Salmonella Senftenberg. The heat resistance of Salmonella is highly influenced by the strain tested, the type of experiment (log reduction versus end-point), culture conditions prior to the experiment, heating menstruum, and recovery conditions. Heat resistance data for Salmonella are still nonexistent or scarce in chicken meat, fruit juices, and aquacultured fish.

Salmonellae, salmonellosis, and dairy foods: a review

Salmonellae continue to be a major concern for the dairy industry because these bacteria have caused recent outbreaks of illness and have been isolated from various dairy products in the market place. Salmonellae are generally not heat resistant and normally grow at 35 to 37 degrees C, but they can grow at much lower temperatures, provided that the incubation time is suitably extended. To minimize problems, foods should be held at or below 2 to 5 degrees C at all times. Both conventional and rapid methods are available to isolate salmonellae from dairy foods and to identify the bacteria. Salmonellae behave differently in different kinds of cheese: they survived in ripening Cheddar cheese for up to 7 mo at 13 degrees C and for 10 mo at 7 degrees C; in coldpack cheese food for several weeks, depending on the pH and preservative used; and in Domiati cheese 13 to 36 d, depending on the manufacturing process used. When Mozzarella cheese was made, temperatures of stretching and molding (60 degrees C) killed all salmonellae present, but, in cottage cheese, survival of the pathogen depended on the cooking temperature of curd. Spray drying of skim milk killed substantial numbers of salmonellae, but some survivors remained. Butter readily supported growth of salmonellae at room temperature, and neither freezing nor refrigeration for brief periods eliminated salmonellae from butter. Use of appropriate hygienic procedures, e.g., Hazard Analysis Critical Control Point system, during processing should reduce the likelihood of salmonellosis outbreaks associated with dairy foods.

Toxin production by Bacillus cereus dairy isolates in milk at low temperatures

A total of 136 strains of Bacillus cereus isolated from milk and cream were evaluated for toxin production based on HeLa S3, Vero, and human embryonic lung (HEL) cell cytotoxicity in vitro. HEL cell monolayers were more susceptible than the other two cell lines. The percentage of isolates exhibiting HEL cytotoxicity was similar (43.0 and 48.4%) when the strains were grown in brain heart infusion broth containing 0.1% glucose (BHIG) at 7 and 24 h, respectively, at 30 degrees C. In milk, only 21.8% of isolates showed HEL cytotoxicity at 7 h, and the number increased significantly to 73.2% at 24 h at 30 degrees C. Further, 102 toxin-positive isolates were acclimatized to grow at 8 degrees C in milk. Ninety-four (92.2%) of the strains produced HEL cytotoxicity of various degrees with no strict correlation to bacterial cell numbers and also elicited vascular permeability reaction in rabbit skin. Under aerated growth conditions (agitation, 200 rpm) B. cereus elicited cytotoxicity in BHIG and in milk at temperatures of 30, 15, and 8 degrees C. However, in nonaerated (stagnant) cultures toxin production was diminished (BHIG) or completely lost (milk) at all temperatures. Toxin production at 8 degrees C was evaluated in two different types of commercial cardboard milk packages by inoculation with a potent toxigenic dairy isolate. No detectable HEL cytotoxicity was observed in milk in any of the packages either at stagnant conditions or during mechanical shaking. However, the same strain produced cytotoxin in whipped cream at 8 degrees C.

Quality of powdered substitutes for breast milk with regard to members of the family Enterobacteriaceae

Members of the family Enterobacteriaceae were cultured from 52.5% of 141 milk substitute infant formulas which were obtained in 35 countries. The concentration did not exceed a level of 1 CFU/g in any product. The species which were isolated most frequently were Enterobacter agglomerans, cloacae, Enterobacter sakazakii, and Klebsiella pneumoniae. If infections due to these organisms occur, it can be useful to include a check of the hygienic precautions which are taken during the preparation and storage of the formula. Milk powders without members of the Enterobacteriaceae might offer extra protection to the newborn if some multiplication does occur in the formula.

Salmonella ealing infections associated with consumption of infant dried milk

In an outbreak of Salmonella ealing infections in November and December, 1985, the proportion of infants infected was in excess of the normal distribution pattern and the cases were geographically widespread. All the infected infants had been fed with a dried-milk product from one manufacturer. Despite intensive efforts to isolate S ealing from packets of the product, it was found only in low numbers in 4 of 267 sealed packets. The source of infection was traced to the factory spray-drier, which had a hole in its inner lining, allowing escape of powder and its return from contaminated insulation material. The plant was closed and the equipment scrapped and later replaced. It is recommended that raw milk and whey, which frequently contain salmonellae, should not be allowed onto the site of milk-drying plants.

Enterobacteriaceae, coliforms, faecal coliforms and salmonellas in raw ewes' milk

Thirty-nine samples of freshly drawn ewes' milk collected at 13 farms, and 120 samples of raw ewes' milk collected on arrival at a dairy were examined. Farm samples had geometric mean counts of 4.4 X 10(2) Enterobacteriaceae/ml, 3.9 X 10(2) coliforms/ml and 2.0 X 10(2) faecal coliforms/ml, whereas the respective mean counts were 6.2 X 10(3)/ml, 5.4 X 10(3)/ml and 1.3 X 10(3)/ml for dairy samples. Salmonellas were not detected by enrichment procedures in any of the 159 samples examined. Escherichia coli (47.5% strains), Enterobacter cloacae (17.7%), Ent. agglomerans (11.3%), Hafnia alvei (6.5%) and Klebsiella oxytoca (6.0%) were the predominant species in 434 Enterobacteriaceae strains isolated from farm samples. Levels and species of Enterobacteriaceae found in the present work in raw ewes' milk imply a considerable risk of early blowing in cheese-making from unpasteurized milk.

Laboratory studies on salmonella-contaminated cheese involved in a major outbreak of gastroenteritis

A major outbreak of gastroenteritis was traced to Cheddar cheese contaminated with Salmonella typhimurium. There were no significant differences in pH values of the contaminated (mean pH 5.31) and non-contaminated (mean pH 5.39) cheese. The isolation rates of Salm. typhimurium were about the same when cheese samples were homogenized in lactose broth, lactose broth containing 1% Tween 80, or in aqueous 2% sodium citrate. Salmonella typhimurium was isolated regardless of preenrichment in lactose broth, but required selective enrichment in selenite cystine or tetrathionate brilliant green broth. There were no marked differences in the isolation rates obtained with different selective enrichment media, or after incubation at 36 degrees and 43 degrees C for 24 or 48 h. Contaminated samples of cheese failed to yield Salm. typhimurium consistently despite large and multiple samplings; samples from the interior of cheese blocks yielded positive results more frequently than the samples from the exterior. The number of Salm. typhimurium in factory sealed blocks as well as in samples obtained from the homes of known cases of salmonellosis was found to range from less than 3/100 g to 9/100 g of cheese. The infective dose of Salm. typhimurium in contaminated cheese was probably no greater than 10(4) organisms, and a rapid decline in numbers of Salm. typhimurium must have occurred subsequent to the outbreak.

Protective effect of casein toward Salmonella typhimurium in acid-milk

Lactic acid is the inhibitory agent in yoghurt responsible for the inhibition of Salmonella typhimurium. Casein, however, may exert a protective effect toward the survival of the salmonella in acid-milk products. Salmonella typhimurium was found to die-off 21.2% more rapidly in 18-h yoghurt-whey than in 18-h yoghurt at 37 degrees C with a pH of 3.85 and 1.42% lactic acid. When casein was added to yoghurt-whey, the die-off rate of the salmonellas was reduced to that found in yoghurt. The rate remained unchanged when 4.8% sodium caseinate was added to the whey. When 0 to 14% casein was added to the acid-whey the die-off rate changed from 9.7 to 24.0 min/log reduction of cells, respectively. There was a direct correlation between the increase in casein concentration and length of survival of the salmonellas. At a pH of 3.85, 4.2 or 4.5, the die-off rate was 6.5, 13.0 or 40 min/log reduction of cells in milk containing 1.42% lactic acid, and was 4.0, 10.0 or 33.3 min/log reduction, respectively, in whey with 1.42% lactic acid. Thus, the protective effect of casein toward Salm. typhimurium increased as the pH increased. This indicated that casein exerts a protective effect on Salm. typhimurium in acid dairy products and the degree of protection depends on the casein concentration, the form of the casein molecule and the pH.

Elucidation of the inhibitory factors of yogurt against Salmonella typhimurium

The inhibitory nature of yogurt against contaminating microorganisms has been studied extensively. Nevertheless, the factors responsible for the death of Salmonella typhimurium in yogurt have not been elucidated. An understanding of these factors is important for the determination of yogurt's safety to consumer health. Yogurt fermented for 18 h at 42 C had a stable environment with the following conditions: pH 3.85, oxidation-reduction potential -80 mV, lactic acid concentration 158 mM, and acetic acid concentration 3.7 mM. Under these conditions, lactic acid was responsible for virtually all of yogurt's bactericidal activity against S. typhimurium at 37 C. Die-off rates were observed when these conditions were reproduced artificially in milk (artificial milk yogurt) and when lactic acid was added back to 18-h yogurt from which acids were removed by passage of the whey through a Dowex 1 (Cl-) anion exchange column (cationic yogurt). Factors that augmented lactic acid inhibition of S. typhimurium were low pH and low oxidation-reduction potential. The die-off rate of S. typhimurium was more rapid in yogurt whey (yogurt minus the casein fraction) than in whole yogurt, indicating that the casein fraction was partially able to protect Salmonella.

Association between raw milk and human Salmonella dublin infection

Between 1971 and 1975 the mean annual incidence of human Salmonella dublin infection in California increased more than five-fold. Investigation of the increase showed an association with exposure to raw milk in 44 out of 113 cases. Of these 44 patients, 35 had used certified raw milk from a single dairy. Faecal swabs confirmed S dublin infection in the dairy herd and the milk, and so a pasteurisation order was issued. S dublin appears to be an unusually invasive and life-threatening salmonella serotype: 65% of isolations were obtained from non-faecal specimens (mainly blood cultures), 89 patients (80%) were admitted to hospital and 22 patients died. Almost three-quarters of the patients were aged 20 or over, and half had serious underlying diseases, particularly leukaemias and lymphomas. Five patients presented with infected vascular lesions that included aneurysms with abscesses and infections of previous arterial graft sites. The public's increasing desire for a "health food" such as raw milk should be tempered with an appreciation of its attendant risk to health.

Antimicrobial action of some citrus fruit oils on selected food-borne bacteria

The antimicrobial properties of essential oils, terpineol, and orange oil, in particular, varied according to the type of bacteria tested. Terpineol and other terpeneless fractions of citrus oils appeared to have greater inhibitory effect on food-borne bacteria than the other citrus oils or derivatives. Gram-positive bacteria were, in general, more sensitive to essential oils than gram-negative bacteria. Terpineol extended the shelf life of commercially pasteurized skim milk, low-fat milk, and whole milk for more than 56 days at 4 C. Orange oil extended the shelf life of skim milk and low-fat milk for the same period.