Molecular detection of the index case of a subclinical Salmonella Kentucky epidemic on a dairy farm.

Salmonella enterica commonly colonizes the intestinal tract of cattle and is a leading cause of foodborne illness. A previously described investigation into the prevalence of S. enterica on a dairy farm revealed an 8-year-long asymptomatic S. enterica epidemic caused by serotypes Cerro and Kentucky in the lactating herd. To investigate the source of the S. Kentucky strains, the genomes of two S. Kentucky isolates were sequenced; one collected prior to the epidemic (2004) and one collected during the epidemic (2010). Comparative genomic analysis demonstrated significant polymorphisms between the two strains. PCR primers targeting unique and strain-specific regions were developed, and screening of the archived isolates identified the index case of the asymptomatic S. Kentucky epidemic as a heifer that was raised off-site and transported onto the study farm in 2005. Analysis of isolates collected from all heifers brought onto the farm demonstrated frequent re-introduction of clones of the epidemic strain suggesting transmission of pathogens between farms might occur repeatedly.

Biofilm in milking equipment on a dairy farm as a potential source of bulk tank milk contamination with Listeria monocytogenes.

The objective of this study was to assess the presence of a Listeria monocytogenes-containing biofilm in milking equipment as a potential source of bulk tank milk contamination on a dairy farm where milk contamination had been previously documented. Samples were collected from milking equipment and milking parlor premises on 4 occasions and analyzed for the presence of L. monocytogenes. Pulsed-field gel electrophoresis (PFGE) typing was conducted on L. monocytogenes isolates from the milking equipment, parlor and storage room floors, bulk tank milk, and in-line milk filters. Pieces from milk meters and rubber liners were obtained to visually assess the presence of a biofilm using scanning electron microscopy. A total of 6 (15%), 4 (25%), and 1 (6%) samples were culture-positive for L. monocytogenes in the first, second, and third sample collection, respectively. Two samples were L. monocytogenes hly PCR-positive but were culture-negative in the fourth sample collection. Combined AscI and ApaI restriction analysis yielded 6 PFGE types for 15 L. monocytogenes isolates obtained from milking equipment, parlor, bulk tank milk, and milk filters. A predominant and persistent PFGE type (PFGE type T) was observed among these L. monocytogenes isolates (9/15 isolates). Scanning electron microscopy of samples from the bottom cover of 2 milk meters showed the presence of individual and clusters of bacteria, mainly associated with surface scratches. The presence of a bacterial biofilm was observed on the bottom covers of the 2 milk meters. Prevention of the establishment of biofilms in milking equipment is a crucial step in fulfilling the requirement of safe, high-quality milk.

Dynamics of endemic infectious diseases of animal and human importance on three dairy herds in the northeastern United States.

Endemic infectious diseases in dairy cattle are of significant concern to the industry as well as for public health because of their potential impact on animal and human health, milk and meat production, food safety, and economics. We sought to provide insight into the dynamics of important endemic infectious diseases in 3 northeastern US dairy herds. Fecal samples from individual cows and various environmental samples from these farms were tested for the presence of major zoonotic pathogens (i.e., Salmonella, Campylobacter, and Listeria) as well as commensal bacteria Escherichia coli and enterococci. Additionally, the presence of Mycobacterium avium ssp. paratuberculosis was tested in fecal and serum samples from individual cows. Test results and health and reproductive records were maintained in a database, and fecal, plasma, DNA, and tissue samples were kept in a biobank. All bacteria of interest were detected on these farms and their presence was variable both within and between farms. The prevalence of Listeria spp. and L. monocytogenes in individual fecal samples within farm A ranged from 0 to 68.2% and 0 to 25.5%, respectively, over a period of 3 yr. Within farm B, continuous fecal shedding of Salmonella spp. was observed with a prevalence ranging from 8 to 88%; Salmonella Cerro was the predominant serotype. Farm C appeared less contaminated with Salmonella and Listeria, although in the summer of 2005, 50 and 19.2% of fecal samples were positive for Listeria and L. monocytogenes, respectively. The high prevalence of E. coli (89 to 100%), Enterococcus (75 to 100%), and Campylobacter (0 to 81%) in feces suggested they were ubiquitous throughout the farm environment. Fecal culture and ELISA results indicated a low prevalence of Mycobacterium avium ssp. paratuberculosis infection in these farms (0 to 13.6% and 0 to 4.9% for culture-positive and ELISA-positive, respectively), although the occasional presence of high shedders was observed. Results have major implications for food safety and epidemiology by providing a better understanding of infectious disease dynamics on dairy farms. Comprehensive understanding of these infections may lead to better farm management practices and pathogen reduction programs to control and reduce the on-farm contamination of these pathogens and to prevent their further entry into the food-chain.

Environmental sampling to predict fecal prevalence of Salmonella in an intensively monitored dairy herd.

Although dairy cattle are known reservoirs for salmonellae, cattle that are shedding this organism are often asymptomatic and difficult to identify. A dairy herd that was experiencing a sustained, subclinical outbreak of Salmonella enterica subsp. enterica Cerro was monitored for 2 years. Fecal samples from the lactating cows were collected every 6 to 8 weeks and tested for the presence of Salmonella. Fecal prevalence of Salmonella fluctuated throughout the observation period and ranged from 8 to 88%. Manure composites and water trough samples were collected along with the fecal samples, and bulk milk and milk filters were cultured for the presence of Salmonella on a weekly basis. Over 90% of the manure composites--representing high-animal-traffic areas-were positive at each sampling. Salmonella was detected in 11% of milk samples and in 66% of the milk filters. Results of weekly bulk milk quality testing (i.e., bulk tank somatic cell score, standard plate count, preliminary incubation count) were typically well within acceptable ranges. Milk quality variables had low correlations with herd Salmonella fecal prevalence. When observed over time, sampling period average prevalence of Salmonella in milk filters closely paralleled fecal prevalence of Salmonella in the herd. Based on results of this study, milk filters appear to be an effective method for monitoring shedding prevalence at the herd level. In-line filter testing is also a more sensitive measure of Salmonella, and perhaps other pathogens, in raw milk than testing the milk alone.

Incidence of Escherichia coli O157:H7 and E. coli virulence factors in US bulk tank milk as determined by polymerase chain reaction.

Samples of bulk tank milk from dairies across the United States, taken as part of the National Animal Health Monitoring Dairy 2002 survey, were analyzed for the presence of several genes encoding virulence factors associated with enterohemorrhagic forms of Escherichia coli (EHEC) using real-time and conventional PCR assays. Samples from 859 farms in 21 states were collected and enriched in EC medium at 42.5 degrees C to amplify any E. coli present, and DNA was isolated from the resulting biomass. The eaeA gene encoding intimin, a virulence factor associated with enteropathogenic forms of E. coli and EHEC, was detected in 199 (23%) of the samples. Thirty-six samples (4.2%) were positive for eaeA, the gamma allele of the translocated intimin receptor (gamma-tir), found in EHEC strains of O157:H7, and one or both shiga-like toxin genes (stx1 and stx2), a combination that may be indicative of the presence of O157:H7 EHEC. Testing these 36 samples with a commercially available real-time PCR kit for detection of O157:H7 indicated that 5 samples could be contaminated with O157:H7. A multiplex PCR to detect the presence of fliC, rfbE, and hlyA genes found in O157:H7 reduced to 2 (0.2% of all samples) the number of samples likely to be contaminated with this organism. A strain of O157:H7 (eaeA+, gamma-tir+, stx2+, rfbE+, fliC+, hlyA+) was subsequently isolated from one sample. Thirty-four eaeA-positive samples did not contain detectable gamma-tir but did contain one or both of the stx genes suggesting the presence of EHEC strains other than O157:H7. These results indicate a low incidence of O157:H7 in bulk tank milk but suggest that a risk from other enteropathogenic and EHEC forms of E. coli may exist and that PCR targeting virulence factors associated with highly pathogenic forms of E. coli may be an effective means of detecting potential dangers in raw milk.

Subtyping Listeria monocytogenes from bulk tank milk using automated repetitive element-based PCR.

Sixty-one Listeria monocytogenes strains from raw milk were analyzed with an automated repetitive element-based PCR (rep-PCR) system to examine the utility of this system for serotype grouping and to determine whether specific regional relationships could be identified. Results of the similarity analysis revealed two primary clusters of L. monocytogenes isolates. Cluster 2 exclusively contained serogroup 1/2a isolates; however, two 1/2a isolates were also found in cluster 1. Isolates of serogroups 1/2b, 4b, 3b, and 4c were also in cluster 1. Clusters 1 and 2 were separated at a relative similarity of 86%. Listeria species other than L. monocytogenes (L. ivanovii, L. seeligeri, L. welshimeri, L. grayi, and L. innocua) had similarity scores of less than 80% in pairwise comparisons with the L. monocytogenes isolates. Thus, this method may be useful for species identification once an isolate is characterized as Listeria. When rep-PCR fingerprints of the L. monocytogenes 1/2a isolates were compared, there was no apparent regional grouping. However, discrimination between isolates suggests that the rep-PCR assay might be useful for tracking L. monocytogenes 1/2a and for tracking isolates across regions or within smaller ecological niches. The automated rep-PCR method could not discriminate between serotypes 1/2b and 4b but may be useful for discriminating between 1/2a and other serotypes and for tracking isolates within serotype 1/2a.

Prevalence of Salmonella enterica in bulk tank milk from US dairies as determined by polymerase chain reaction.

Samples of bulk tank milk from dairies across the United States, taken as part of the National Animal Health Monitoring System Dairy 2002 survey, were analyzed for the presence of Salmonella enterica using a commercially available real-time polymerase chain reaction (PCR) kit. Samples from 854 farms in 21 states were collected and enriched in tetrathionate broth to amplify any salmonellae present, and DNA was isolated from the resulting biomass. One hundred one samples (11.8%) were shown to contain Salmonella enterica using the real-time PCR assay, whereas conventional culture techniques detected the pathogen in only 22 (2.6%) of the samples. A conventional PCR assay targeting a different gene from Salmonella enterica confirmed the presence of the organism in 94 of the real-time PCR-positive samples. Thus, assay of milk samples by real-time PCR indicates that the prevalence of Salmonella enterica in US bulk tank milk is substantially higher than previously reported.

High-performance liquid chromatography method for the characterization of rhamnolipid mixtures produced by pseudomonas aeruginosa UG2 on corn oil.

A HPLC method was developed to quantify rhamnolipid species in a bacterial biosurfactant mixture. The biosurfactant mixtures containing mainly 3-[3'-(L-rhamnopyranosyl-oxy)decanoyloxy]decanoic acid (RhC10C10), 3-[3'-(2'-O-alpha-L-rhamnopyranosyl-oxy)decanoyloxy]decanoic acid (Rh2C10C10), 3-[3'-(2'-O-alpha-L-rhamnopyranosyl-oxy)decanoyloxy]dodecanoic acid (Rh2C10C12), and a dehydrogenated variety of the latter, 3-[3'-(2'-O-alpha-L-rhamnopyranosyl-oxy)decanoyloxy]dodecenoic acid (Rh2C10C12-H2), were isolated from Pseudomonas aeruginosa UG2 cultures grown on corn oil as sole carbon. The rhamnolipid species were identified and quantified after their derivatization to the corresponding phenacyl esters. To confirm the reliability of the HPLC method, the biosurfactant mixtures and the HPLC isolated species were further analyzed. Mass spectroscopy (electrospray ionization and atmospheric pressure chemical ionization techniques) was used to confirm their molecular mass, gas chromatography to verify their fatty acid content, and a colorimetric assay to quantify the rhamnose content.

Genetic engineering approach to toxic waste management: case study for organophosphate waste treatment.

Currently, there has been limited use of genetic engineering for waste treatment. In this work, we are developing a procedure for the in situ treatment of toxic organophosphate wastes using the enzyme parathion hydrolase. Since this strategy is based on the use of an enzyme and not viable microorganisms, recombinant DNA technology could be used without the problems associated with releasing genetically altered microorganisms into the environment. The gene coding for parathion hydrolase was cloned into a Streptomyces lividans, and this transformed bacterium was observed to express and excrete this enzyme. Subsequently, fermentation conditions were developed to enhance enzyme production, and this fermentation was scaled-up to the pilot scale. The cell-free culture fluid (i.e., a nonpurified enzyme solution) was observed to be capable of effectively hydrolyzing organophosphate compounds under laboratory and simulated in situ conditions.

Influence of rhamnolipids and triton X-100 on the biodegradation of three pesticides in aqueous phase and soil slurries.

The effect of surfactants on the biodegradation of trifluralin and atrazine (by Streptomyces PS1/5) and coumaphos (by degrading consortia from a contaminated cattle dip) in liquid cultures and soil slurries was tested at different concentrations of a rhamnolipid mixture (Rh-mix) and Triton X-100 (TX-100). The extent of trifluralin biodegradation in liquid culture was improved at high concentrations of both surfactants. The extent of atrazine degradation dropped in the presence of either surfactant. Coumaphos biodegradation improved slightly at Rh-mix dosages >3000 microM; however, it was readily inhibited by TX-100 at amounts above the critical micelle concentration. In soil slurries, the extent of both trifluralin and atrazine biodegradation was higher in Hagerstown A (HTA) soil than in Hagerstown B (HTB) soil and was not significantly affected by the presence of either surfactant. The onset of trifluralin biodegradation was retarded at higher concentrations of surfactants. In the absence of surfactant, up to 98% of coumaphos in both soil slurries was transformed. At increasing dosages of Rh-mix, the onset of coumaphos biodegradation was retarded, but the removal efficiency of the pesticide increased. Rh-mix and TX-100 depletion was observed during Streptomyces PS1/5 growth in liquid cultures. Rh-mix concentration also decreased during coumaphos biodegradation, whereas TX-100 concentration was not affected. These results suggest that surfactants, added for the purpose of increasing the apparent water solubility of hydrophobic organic compounds, may have unintended effects on both the rate and extent of biodegradation of the target compounds if the surfactants can also be degraded by the microorganisms in the system.

Using a portable real-time PCR assay to detect Salmonella in raw milk.

The purpose of this study was to determine the efficacy of a portable real-time PCR system in detecting Salmonella spp. in raw milk. The 200 bulk milk samples chosen for this study constituted a subset of the samples for a larger study; this subset contained 24 samples that were culture positive for Salmonella and 176 that were culture negative. Milk was both plated directly on selective agar and plated after enrichment in selective media. Presumptive Salmonella colonies were isolated by direct culturing of five samples, while Salmonella was isolated from the remaining 19 positive samples only after enrichment. Presumptive Salmonella isolates were serotyped, and isolates from 22 samples were confirmed to be Salmonella isolates. PCR assays of culture-positive milk prior to enrichment yielded no evidence of Salmonella. DNA extracts of bacterial pellets from the enriched samples were analyzed for Salmonella by real-time PCR with the Ruggedized Advanced Pathogen Identification Device (RAPID). Fifty-four samples from the enrichment pellets tested positive for Salmonella by real-time PCR. Two samples that tested positive for Salmonella by culture and serotyping tested Salmonella negative by real-time PCR. Serotyping identified isolates from these samples as Salmonella Montevideo. All DNA extracts of Salmonella Montevideo isolates tested positive for Salmonella by real-time PCR. Thirty-three samples tested negative by culture and positive by real-time PCR. These results indicate that the portable real-time PCR system appears to be a useful tool for detecting Salmonella in raw milk. Additionally, the combination of enrichment and real-time PCR techniques used in this study can yield results in 24 h, compared with the 48 to 72 h required for traditional culture.

Effect of nutritional and environmental conditions on the production and composition of rhamnolipids by P. aeruginosa UG2.

The production of rhamnolipid biosurfactants by P. aeruginosa UG2 was examined under different culture conditions. Rhamnolipid yield was affected by the nature of the carbon sources, the nutrient concentrations, pH, and age of the culture. Hydrophobic substrates like corn oil, lard (rich in unsaturated and saturated fat), and long chain alcohols maximized biosurfactant production (100-165 mg/g substrate). Hydrophilic substrates like glucose, and succinic acid delivered poor yields (12-36 mg/g substrate). Rhamnolipid production was greater when N as (NH4)(2)SO4 and trace metals were added in several periodic doses rather than at the beginning of the process. Increased biosurfactant production was seen in cultures maintained at neutral pH relative to cultures allowed to develop acidic conditions (pH = 6.25). Although the level of rhamnolipid production was affected by culture conditions, the distribution of rhamnolipid subspecies did not vary between cultures. A dirhamnolipid species containing two 10 carbon alpha-hydroxy fatty acids [Rh2C10C10] was the most abundant in the mixtures (60.6 mol%), while the levels of the monorhamnolipid [RhC10C10] (20.7 mol%) and two dirhamnolipids [Rh2C10C12 and its dehydro variant Rh2C10C12-H2] (18.7 mol%) were similar. Biosurfactant mixtures produced with corn oil as sole carbon source solubilized the herbicide trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzamine] to a greater extent. This suggests that the presence of incompletely metabolized hydrophobic by-products acting as co-solvents can increase the solubilization capacity of biosurfactant mixtures.

Epinephrine-induced potentially lethal arrhythmia during arthroscopic shoulder surgery: a case report.

Arthroscopic shoulder surgery performed on a healthy female could have resulted in a fatal outcome when the epinephrine present in the arthroscopic irrigating solution contributed to the onset of ventricular tachycardia requiring defibrillation during surgery. During this procedure, the shoulder was infiltrated with 30 mL of a 1:100,000 solution of epinephrine into the subacromial space and glenhumeral joint. Subsequently, instrumentation of the glenhumeral joint by the orthopedic surgeon with a standard arthroscopy trocar resulted in a 0.5-cm size lesion to the posterior humeral cortex. Minutes after the start of the surgical procedure, the patient displayed an abrupt onset of ventricular tachycardia and hypertension. These signs and symptoms suggested an intraosseous infusion of both infiltrated and irrigation solution containing epinephrine through the lesion in the humeral cortex. Approximately 800 mL of a .01 mg/mL concentration of irrigation solution containing epinephrine was used. A diagnosis of epinephrine-induced ventricular tachycardia was made. The arthroscopic irrigating solution was immediately discontinued and lidocaine, 100 mg intravenously, was administered; however, the patient's cardiac rhythm degenerated into a sustained ventricular tachycardia that was unresponsive to pharmacologic intervention. A full code was called; the surgeon, anesthesia team, and operating room personnel succesfully provided advanced cardiac life support and cardioverted the patient back into a sinus rhythm with no untoward effects.

Antimicrobial resistance of Salmonella enterica isolates from bulk tank milk and milk filters in the United States.

Salmonella isolates were recovered from bulk tank milk as part of the National Animal Health Monitoring System (NAHMS) Dairy 2002 and 2007 surveys. In-line milk filters were also tested in the 2007 survey. The objective of this study was to determine the prevalence of antimicrobial resistance among Salmonella enterica isolates from bulk milk and milk filters in the NAHMS Dairy 2002 and 2007 surveys and to further characterize resistant isolates. Susceptibilities to 15 antibiotics were determined for 176 Salmonella isolates of 26 serotypes using an automated antimicrobial susceptibility system. Resistant isolates were screened by PCR for the presence of the extended-spectrum ß-lactamase (bla(CMY)) gene and class I integrons and further characterized by pulsed-field gel electrophoresis. Thirty isolates (17.0%) representing six S. enterica serotypes exhibited resistance to at least one antimicrobial agent (serotypes Newport [14 of 14 isolates exhibited resistance], Dublin [7 of 7], Typhimurium [3 of 5], Kentucky [4 of 22], Anatum [1 of 13], and Infantis [1 of 2]). Twenty isolates (11.4%), including all 14 Newport, 3 Dublin, 2 Typhimurium, and 1 Infantis isolate, displayed the typical multidrug-resistant, bla(CMY)-positive (MDR-AmpC) phenotype which included resistance to ampicillin, chloramphenicol, streptomycin, sulfonamide, and tetracycline, plus resistance to amoxicillin-clavulanic acid and extended-spectrum cephalosporins. Five of the MDR-AmpC isolates carried class I integrons (2.8%). Two-enzyme (XbaI and BlnI) pulsed-field gel electrophoresis discerned clades within serotypes and, together with the resistance profiles, identified strains that appeared to have persisted temporally and geographically. These results suggest that there is a low but appreciable risk of infection with MDR Salmonella from consumption of nonpasteurized milk and dairy products.

Factors associated with Salmonella presence in environmental samples and bulk tank milk from US dairies.

The objective of this study was to evaluate herd characteristics and management practices associated with presence of Salmonella in the farm environment and in bulk tank milk (BTM) in US dairy herds. Herd management data, environmental culture, BTM and in-line milk filter polymerase chain reaction results for Salmonella from 260 US dairy herds surveyed during the National Animal Health Monitoring System Dairy 2007 study were analysed. Herd characteristics and management practices were screened by univariate analysis, and selected variables were used to construct a logistic regression model to identify factors associated with the presence of Salmonella in environmental samples. To identify factors associated with the occurrence of Salmonella in BTM and milk filters, a priori selected variables that were related to milking procedures were analysed univariately and a logistic regression model was constructed. The presence of Salmonella in the farm environment was associated with location of the operation in the East (OR = 4.8; CI: 1.9-11.6), not using a broadcast manure spreader (OR = 3.2; CI: 1.4-7.5), use of bovine somatotropin (BST) (OR = 2.7; CI: 1.5-5.0) and use of anionic salts (OR = 2.2; CI: 1.2-3.9). In the final multivariable model, herds with fewer than 100 cows were at decreased odds (OR = 0.3; CI: 0.1-0.7) of being culture positive for Salmonella as were herds with between 100 and 499 cows (OR = 0.4; CI: 0.2-0.8) compared with herds having 500 or more cows. The presence of culture-positive environmental samples and herd size were significantly associated with Salmonella BTM contamination. The herd-level factors identified in this study were in agreement with prior studies but also identified other potential factors that can be targeted in Salmonella control programmes.

Risk factors associated with the presence of viable Listeria monocytogenes in bulk tank milk from US dairies.

The objective of the study was to screen a large number of herd management practices and herd characteristics from US dairies to identify herd-level risk factors associated with the presence of Listeria monocytogenes in bulk tank milk (BTM). A total of 71 variables was univariately evaluated for their association with the presence of L. monocytogenes in BTM. Results from the univariate analysis indicated that using automatic take offs and having an open herd management increased the risk of BTM contamination with L. monocytogenes, while storing manure in outside pens not accessible to cattle decreased the risk. These variables, however, were not sustained in the multivariable model, which indicated that the presence of L. monocytogenes in BTM was significantly associated with region of the country (farms in the southeast and northeast were six and four times more likely respectively, to have BTM contamination than farms in the west) and number of milking cows (herds with >500 milking cows were five times more likely to have BTM contamination than herds with <100 milking cows). In conclusion, our results suggest that risk factors associated with BTM contamination are different depending on the geographical region and herd size of the operation.

A mathematical model of the dynamics of Salmonella Cerro infection in a US dairy herd.

We developed a mathematical model of the transmission dynamics of salmonella to describe an outbreak of S. Cerro infection that occurred in a Pennsylvania dairy herd. The data were collected as part of a cooperative research project between the Regional Dairy Quality Management Alliance and the Agricultural Research Service. After the initial detection of a high prevalence of S. Cerro infection in the herd, a frequent and intensive sampling was conducted and the outbreak was followed for 1 year. The data showed a persistent presence of S. Cerro with a high prevalence of infection in the herd. The dynamics of host and pathogen were modelled using a set of nonlinear differential equations. A more realistically distributed (gamma-distributed) infectious period using multiple stages of infection was considered. The basic reproduction number was calculated and relevance to the intervention strategies is discussed.

Longitudinal study of a clonal, subclinical outbreak of Salmonella enterica subsp. enterica serovar Cerro in a U.S. dairy herd.

Salmonellae are a major group of foodborne pathogens known to affect both humans and animals. Dairy cattle are a known reservoir of these bacteria and human Salmonella infections have been associated with the consumption of improperly processed or contaminated dairy products. Many of the over 2500 known serotypes of Salmonella are known to infect cattle, resulting in asymptomatic to fatal salmonellosis. This study describes the course of a Salmonella outbreak and subsequent endemic infection on a dairy farm in Pennsylvania. The outbreak was initially detected when a few cows with clinical symptoms and one fatality were found to be infected with Salmonella enterica subsp. enterica serovar Typhimurium var. Copenhagan. Based upon sampling of the farm environment, Salmonella Typhimurium var. Copenhagan was succeeded within 3 months by Salmonella enterica subsp. enterica serovar Kentucky. Salmonella enterica subsp. enterica serovar Cerro ultimately supplanted Typhimurium var. Copenhagan and Kentucky in individual animals and environmental samples and persisted in the herd at high prevalence for almost 2 years. Since there were no obvious clinical consequences of the Salmonella Cerro infection, these data suggest that some serotypes of S. enterica subsp. enterica can behave as commensal organisms in dairy cattle and illustrate the difficulties of controlling Salmonella in milk production systems. The consistent finding of Salmonella in the environment reinforces the potential for human exposure to this pathogen and the need to understand the dynamics and ecology of Salmonella in dairy production settings.