Lack of staphylococcal enterotoxin production among strains of Staphylococcus aureus from bovine mastitis in Denmark

Rapid detection of major enterotoxin genes and antibiotic resistance of Staphylococcus aureus isolated from raw milk in the Yazd province, Iran

INTRODUCTION

Raw milk is a nutrient-rich food, but it may harbour harmful bacteria, such as enterotoxigenic Staphylococcus aureus (S. aureus), which can cause staphylococcal food poisoning. Antibiotic resistance of S. aureus in raw milk can increase the risk of such infections, particularly among susceptible individuals.

OBJECTIVE

This study aimed to investigate the prevalence of enterotoxin genes a, d, g, i and j and the antibiotic resistance of S. aureus isolated from raw milk samples.

METHODS

During a 6-month sampling period, 60 raw milk specimens were obtained from diverse locations in Yazd province, Iran. Antibiogram profiling was conducted via the disc diffusion method. In addition, staphylococcal enterotoxin (SE) genes a, d, g, i, and j were detected through real-time PCR analysis.

RESULTS

Bacteriological assays confirmed the presence of S. aureus in 11 samples (18.3%). All isolates demonstrated 100% resistance to penicillin G but exhibited sensitivity to vancomycin, while resistance to other antibiotics ranged from 36.4% to 45.5%. The prevalence of enterotoxin genes in these strains showed variable distribution, with sea being the predominant SE (45.5%), followed by sed (36.4%), seg (18.2), sej and sei (9.1% each).

CONCLUSIONS

This study discovered the presence of multiple enterotoxins in S. aureus strains obtained from raw milk samples. These strains also demonstrated resistance to a variety of antibiotics. Since enterotoxigenic S. aureus is known to cause human food poisoning, monitoring food hygiene practices, especially during raw milk production, is critical.

Genomic investigation of Staphylococcus aureus isolates from bulk tank milk and dairy cows with clinical mastitis

Staphylococcus aureus is one of the most common pathogens that cause mastitis in dairy cows. Various subtypes, virulence genes and mobile genetic elements have been associated with isolates from bulk tank milk and clinical mastitis. So far, no Danish cattle associated S. aureus isolates have been whole-genome sequenced and further analyzed. Thus, the main objective was to investigate the population structure and genomic content of isolates from bulk tank milk and clinical mastitis, using whole-genome sequencing. This may reveal the origin of strains that cause clinical mastitis. S. aureus isolates from bulk tank milk (n = 94) and clinical mastitis (n = 63) were collected from 91 and 24 different farms, respectively and whole-genome sequenced. The genomic content was analyzed and a phylogenetic tree based on single nucleotide polymorphisms was constructed. In general, the isolates from both bulk tank milk and clinical mastitis were of similar genetic background. This suggests that dairy cows are natural carriers of the S. aureus subtypes that cause clinical mastitis if the right conditions are present and that a broad range of subtypes cause mastitis. A phylogenetic cluster that mostly consisted of ST151 isolates carried three mobile genetic elements that were primarily found in this group. The prevalence of resistance genes was generally low. However, the first ST398 methicillin resistant S. aureus isolate from a Danish dairy cow with clinical mastitis was detected.

Detection of the enterotoxigenic genes (sei,sej) in Staphylococcus aureus isolates from bovine mastitis milk in the West Azerbaijan of Iran

Staphylococcus aureus is a major causative pathogen of clinical and subclinical mastitis of dairy domestic ruminants. This organism produces a variety of extracellular toxins and virulence factors such as enterotoxin SEI and SEJ that contribute to its pathogenic potential. In this study 25 S. aureus isolates obtained from four dairy herds of Urmia region which is located in West Azerbaijan province in Iran. The tested isolates were identified on the basis of the cultural and biochemical properties, as well as amplification of the aroA gene which is specific for S. aureus. All isolates were also analyzed for the presence of the SEI (sei) and SEJ (sej) encoding genes using polymerase chain reaction (PCR). Seven positive isolates were detected for sei, but sej gene was not detected in any of the total number of 25 isolates. The present study revealed that the PCR amplification of the aroA gene could be used as a powerful tool for identification of S. aureus from the cases of bovine mastitis. Results of the present study also showed that the strains of S. aureus which cause mastitis can potentially produce enterotoxin SEI. Overall, our results suggest that it is of special importance to follow the presence of enterotoxin-producing S. aureus in other dairy products, especially for protecting the consumers from staphylococcal food poisoning.

An alternative method for Staphylococcus aureus DNA isolation

This study describes a rapid procedure for the isolation of genomic DNA from Staphylococcus aureus that yielded a good amount of high quality DNA for the amplification of staphylococcal enterotoxins genes (A, B, C, D, and E) and the TSST-1 gene as well as enzymatic restriction (HaeIII) from environmental isolates. With this method, it was possible to detect these genes in a sample containing as little as 10(5) cells with positive PCR reactions obtained from approximately 10pg of DNA in a final reaction volume of 25µl.

Occurrence of enterotoxic Staphylococcus aureus in raw milk from yaks and cattle in Mongolia

Staphylococcal food poisoning is considered one of the leading foodborne illnesses in humans worldwide and is associated with contaminated foods of animal origin, such as milk and dairy products. In this study, we investigated the occurrence of staphylococci and the enterotoxigenic properties of Staphylococcus aureus isolated from raw milk from yaks (Bos mutus) and cattle in Mongolia. Staphylococci were isolated from 72 (74%) of the 97 raw milk samples. Of the samples containing staphylococci, 69% (50 of 72) were from yaks and 30.5% (22 of 72) were from cattle. S. aureus was detected in 10% of yak (7 of 72) and 21% of cattle (15 of 72) milk samples. Staphylococcal enterotoxin C was detected in 23% (5 of 22) of the S. aureus strains investigated, based on the reverse passive latex agglutination technique. Three of the five enterotoxigenic strains were from yaks and two were from cattle. None of the S. aureus strains tested produced staphylococcal enterotoxins A, B, or D. To our knowledge, this is the first report of the occurrence of staphylococci and enterotoxigenic S. aureus in milk from yaks and cattle in Mongolia.

Enterotoxin production by Staphylococcus aureus isolated from mastitic cows

Staphylococcus aureus is an important cause of mastitis in cows. The ability of S. aureus strains to produce one or more enterotoxins in milk and dairy products is linked to staphylococcal food poisoning. To determine whether staphylococci causing bovine mastitis could cause human foodborne intoxication, the production of staphylococcal enterotoxins A through D (SEA, SEB, SEC, and SED) by 160 S. aureus isolates was evaluated with the use of a reverse passive latex agglutination enterotoxin kit. All S. aureus strains were isolated over a 9-month period from 2,343 routine submissions of a composite quarter collection of individual mastitic cows at 18 dairy farms in the San Joaquin Valley in California. Prior to enterotoxin detection, isolates were grown by a method that enhances the in vitro synthesis of enterotoxin. Twenty-two of 160 S. aureus isolates produced enterotoxin. Seven produced SEC, 12 produced SED, and 3 produced both SEC and SED. None of the isolates produced SEA or SEB.

Geographical variation in the presence of genes encoding superantigenic exotoxins and beta-hemolysin among Staphylococcus aureus isolated from bovine mastitis in Europe and USA

The object was to examine the geographical variation in the presence of superantigenic exotoxins and beta-hemolysin among epidemiologically independent Staphylococcus aureus isolates from bovine mastitis. A total of 462 S. aureus isolates from nine European countries and USA were examined for the presence of genes encoding staphylococcal enterotoxins A-E, and H, toxic shock toxin-1 (TSST-1), and beta-hemolysin, and 128 of these were examined for exfoliative toxins A and B. The detection was done by PCR. Phenotypic methods were used to confirm the PCR-results. None of the 128 isolates carried the genes for exfoliative toxin A or B. The total proportion of isolates in which superantigenic exotoxins were detected varied from 2% (one isolate) of the Danish isolates to 65% (32 isolates) of the Norwegian isolates. This marked and highly significant geographical variation was also present for the individual exotoxins. The genes encoding enterotoxin C, TSST-1, and enterotoxin D were the most common superantigens. The present and earlier studies demonstrate that the superantigenic exotoxins that were investigated in this study, do not play a role in the pathogenesis of bovine S. aureus mastitis. In contrast to the geographical variation among superantigenic exotoxins, 97% of the isolates were PCR-positive for and/or produced beta-hemolysin on 5% calf blood agar. Except for three isolates, the Norwegian isolates were PCR-negative, but positive on 5% calf blood agar. Sequence variation in the primer regions in the beta-hemolysin encoding gene of the Norwegian isolates is suggested, and should be investigated further. The consistent presence of beta-hemolysin suggests that this factor, or a co-existing gene correlated to beta-hemolysin, may be an active virulence factor in the pathogenesis of bovine S. aureus mastitis.

Differences between Danish bovine and human Staphylococcus aureus isolates in possession of superantigens

PCR-assays for the detection of staphylococcal enterotoxins A-E, and H, toxic shock toxin 1, and exfoliative toxins A and B were evaluated against phenotypic methods, and performed well. Four hundred and fourteen isolates of Staphylococcus aureus from Danish cases of bovine mastitis were screened for genes encoding these superantigens. One hundred isolates from Danish human carriers were also included in the study. In contrast to the frequent presence of genes encoding and in vitro expression of superantigens among the human carrier isolates, only one of 414 isolates from bovine mastitis carried the genes encoding enterotoxin C and toxic shock toxin-1. These results further support the hypothesis that the bovine and human S. aureus reservoirs constitute two separate sub-populations of the species S. aureus. The results also show that these superantigens are generally not present in Danish S. aureus isolates from bovine mastitis, and thus play no essential role in the pathogenesis of bovine S. aureus mastitis.

Production of enterotoxins and toxic shock syndrome toxin by Staphylococcus aureus isolated from bovine mastitis in Brazil

The production of toxic shock syndrome toxin 1 (TSST-1) and enterotoxins (SE) A, B, C and D by bovine mastitis isolates of Staphylococcus aureus was evaluated by immunodiffusion using the Optimum-Sensitivity Plate method. S. aureus strains were isolated from bovine mastitis in 23 dairy herds in the state of Minas Gerais, Brazil, during 1994-9. Of 127 isolates, 83 (65.04%) produced one or several toxins, and among them production of SE was found in 54 (43.0%) isolates, of which 1138 (29.09%) secreted enterotoxin identified as type D. TSST-1 was found in 5829 (45.723.0%) isolates.