Prevalence and expression of staphylococcal enterotoxin genes in Staphylococcus aureus isolated from food poisoning outbreaks

Genomic Insights into Methicillin-Resistant Staphylococci and Mammaliicocci from Bulk Tank Milk of Dairy Farms in Serbia

The potential risk to human and animal health provides a rationale for research on methicillin-resistant staphylococci (MRS) and mammaliicocci (MRM) in dairy herds. Here, we aimed to estimate their occurrence in the bulk tank milk (BTM) samples collected in 2019-2021 from 283 bovine dairy farms in the Belgrade district. We used whole-genome sequencing to characterize the obtained isolates and assess their genetic relatedness. A total of 70 MRS/MRM were recovered, most frequently Staphylococcus haemolyticus and Mammaliicoccus sciuri. Five clusters of 2-4 genetically related isolates were identified and epidemiological data indicated transmission through, e.g., farm visits by personnel or milk collection trucks. Most MRSA isolates belonged to the typical livestock-associated lineage ST398-t034. One MRSA isolate (ST152-t355) harbored the PVL-encoding genes. Since MRS/MRM isolates obtained in this study frequently harbored genes conferring multidrug resistance (MDR), this argues for their role as reservoirs for the spread of antimicrobial resistance genes. The pipeline milking system and total bacterial count >100,000 CFU/mL were significantly associated with higher occurrences of MRS/MRM. Our study confirms that BTM can be a zoonotic source of MRS, including MDR strains. This highlights the urgent need for good agricultural practices and the continuous monitoring of MRS/MRM in dairy farms.

A food poisoning caused by ST7 Staphylococcal aureus harboring sea gene in Hainan province, China

ST7 Staphylococcus aureus is highly prevalent in humans, pigs, as well as food in China; however, staphylococcal food poisoning (SFP) caused by this ST type has rarely been reported. On May 13, 2017, an SFP outbreak caused by ST7 S. aureus strains occurred in two campuses of a kindergarten in Hainan Province, China. We investigated the genomic characteristics and phylogenetic analysis of ST7 SFP strains combined with the 91 ST7 food-borne strains from 12 provinces in China by performing whole-genome sequencing (WGS). There was clear phylogenetic clustering of seven SFP isolates. Six antibiotic genes including blaZ, ANT (4′)-Ib, tetK, lnuA, norA, and lmrS were present in all SFP strains and also showed a higher prevalence rate in 91 food-borne strains. A multiple resistance plasmid pDC53285 was present in SFP strain DC53285. Among 27 enterotoxin genes, only sea and selx were found in all SFP strains. A ФSa3int prophage containing type A immune evasion cluster (sea, scn, sak, and chp) was identified in SFP strain. In conclusion, we concluded that this SFP event was caused by the contamination of cakes with ST7 S. aureus. This study indicated the potential risk of new emergencing ST7 clone for SFP.

Co-Occurrence of <i>Staphylococcus aureus</i> and Ochratoxin A in Pasteurized Milk

Pathogens and mycotoxins are serious public health risks for humans and food safety in milk. This study concentrated on detecting <i>Staphylococcus aureus</i> and Ochratoxin A (OTA) in 210 pasteurized milk from ten urban Beijing districts to suggest the co-occurrence of <i>S. aureus</i> with toxin-producing genes and OTA in milk and the possible risk. <i>S. aureus</i> was identified by physiological and biochemical experiments and molecular biology experiments, and enterotoxin genes were identified by PCR. OTA was detected by LC-MS/MS. The study found 29 isolates of <i>S. aureus</i>, of which 17.24% had the sea gene encoding enterotoxin A. OTA was detected in 31 out of 120 samples and the maximum amount of detection was 18.8 μg/kg. The results of this study indicate that when failing to guarantee the cold chain, the presence of <i>S. aureus</i> with enterotoxin genes in milk will present a risk to food safety. Furthermore, the high detection rates and levels of OTA in milk suggest that OTA is a hidden risk. The co-occurrence of <i>S. aureus</i> and OTA in milk is a food safety concern and there is a need to control the occurrence of these two biohazards in milk.

Microcalorimetric Investigations of Reversible Staphylococcal Enterotoxin Unfolding

Staphylococcal food poisoning (SFP) is a common food-borne illness often associated with contamination during food handling. The genes for Staphylococcal enterotoxin (SE) isoforms SEA and SEB are frequently detected in human nasal Staphylococcus aureus isolates and these toxins are commonly associated with SFP. Past studies described the resistance of preformed SE proteins to heat inactivation and their reactivation upon cooling in foods. Full thermodynamic analyses for these processes have not been reported, however. The thermal stabilities of SEA, SEB, and SEH and reversibility of unfolding in simple buffers were investigated at pH 4.5 and pH 6.8 using differential scanning calorimetry (DSC). SEA and SEB unfolding was irreversible at pH 6.8 and at least partially reversible at pH 4.5 while SEH unfolding was irreversible at pH 4.5 and reversible at pH 6.8. Additional studies showed maximum refolding for SEB at pH 3.5–4.0 and diminished refolding at pH 4.5 with increasing ionic strength. SE-stimulated secretion of interferon-gamma by human peripheral blood mononuclear cells was used to assess residual SE biological activity following heat treatments using conditions matching those used for DSC studies. The biological activities of SEB and SEH exhibited greater resistance to heat inactivation than that of SEA. The residual activities of heat-treated SEB and SEH were measurable but diminished further in the presence of reconstituted nonfat dry milk adjusted to pH 4.5 or pH 6.8. To different extents, the pH and ionic strengths typical for foods influenced the thermal stabilities of SEA, SEB, and SEH and their potentials to renature spontaneously after heat treatments.

Phenotypic and Molecular Characterization of Bovine Mastitis Milk Origin Bacteria and Linkage of Intramammary Infection With Milk Quality

Mastitis is a multi-etiological complex disease of dairy cows and negatively affects the quality and quantity of milk. Milk is a nutritious food for human being; milk quality is negatively affected by intramammary infection of dairy cows. A total of 300 milk samples were collected from mastitis dairy cows irrespective of parity and stage of lactation, 235 (78.33%) samples were culturally positive and yielded 1,100 bacterial isolates. Staphylococcus aureus was found to be the prime etiological agent involved in the mastitis of dairy cows, followed by Escherichia coli and other environmental pathogens. On the molecular characterization of isolates obtained from the milk culture, various toxic genes such as nuc, seb, hla, stx1, stx2, hly, and Sagl were found on different isolated bacteria. Milk somatic cell counts (SCC) were found to be directly related to the severity of mastitis. On drawing the SCC correlation with milk components, it was found that SCC had a significant negative correlation with fat, lactose, solid not fat (SNF), and ash. It was concluded that mastitis-affected milk contains numerous pathogenic bacteria, toxins, and reduced milk quality, which is unfit for human consumption.

The effect of sodium chloride and temperature on the levels of transcriptional expression of staphylococcal enterotoxin genes in Staphylococcus aureus isolates from broiler carcasses

Staphylococcus aureus is one of the most common pathogens associated with food poisoning, which is caused by the ingestion of food contaminated with staphylococcal enterotoxins (SE). Our study aims at evaluating the occurrence and expression of five SE genes (sea, seb, sec, sed, and see) in S. aureus previously isolated from broiler carcasses. Besides that, it also presents an in vitro analysis of the effects of sodium chloride and temperature on the levels of transcriptional expression. A total of 30 S. aureus isolates were investigated for the presence of SEs by PCR assay. The expression level and the effects of sodium chloride (2.5% NaCl), as well as temperature (8 ºC and 12 ºC), on the transcriptional expression, were evaluated by a quantitative reverse transcription PCR (RT-qPCR). Twelve isolates carried at least one of the SE genes. Among them, five representative isolates presented transcriptional expression for at least one gene. Both sodium chloride and low temperatures interfered with the expression of the SE genes, decreasing their values. However, one isolate displayed relative expression 2.25 times higher for sed gene than S. aureus FRI 361 in optimal conditions (p < 0.05), demonstrating their toxigenic potential even under salt stress. There was no evidence of enterotoxin gene expression at 8 ºC.

Hydroxysafflor Yellow A Inhibits Staphylococcus aureus-Induced Mouse Endometrial Inflammation via TLR2-Mediated NF-kB and MAPK Pathway

The present study is designed to investigate the effect of hydroxysafflor yellow A (HYA) on Staphylococcus aureus (S. aureus)-induced mouse endometrial inflammation and to explore its molecular mechanism. We established a mouse endometritis model by intrauterine injection of S. aureus and intrauterine injection of HYA for treatment. Immunohistochemistry, immunofluorescence, and Western blot were used to detect protein expression in uterine tissue, and qPCR was used to measure mRNA expression. HYA could significantly weak uterine pathological changes caused by S. aureus and reduce MPO activity, CD45, CD3, and ED-1 protein expression in uterine tissues of S. aureus-infected mice. Similarly, HYA also significantly decreased S. aureus induced the increase in TNF-α, IL-1β, and IL-6 in uterine tissue. In vivo, we found that knockdown of TLR2 was very important could significantly reduce S. aureus induced the elevated expression of TNF-α, IL-1β, and IL-6 in mEECs. Importantly, in terine tissues of S. aureus-infected mice, HYA significantly decreased the ratio of p-p65/p65, p-IKBα/IKBα, p-p38/p38, p-Erk/Erk, and p-JNK/JNK expression. HYA displays anti-inflammatory effects on S. aureus mouse endometrial inflammation, and this effect might be related to HYA which could block TLR2-mediated NF-kB and MAPK pathway.

Comprehensive Virulence Gene Profiling of Bovine Non-aureus Staphylococci Based on Whole-Genome Sequencing Data

Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from milk in dairy cattle worldwide. The virulence factors (VFs) and mechanisms by which these bacteria cause udder infection are not fully known. We determined the distribution and associations of 191 VFs in 25 NAS species and investigated the relationship between VFs and disease. Although the overall number of VFs was not associated with disease severity, increasing numbers of toxin and host immune evasion genes specifically were associated with more severe disease outcomes. These findings suggest that the development of disease and the interactions of VFs with the host are complex and determined by the interplay of genes rather than just the presence of virulence genes. Together, our results provide foundational genetic knowledge to other researchers to design and conduct further experiments, focusing on understanding the synergy between VFs and roles of individual NAS species in IMI and characterizing species-specific effects on udder health.

Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from intramammary infection (IMI) in dairy cattle. Virulence factors (VFs) and mechanisms by which NAS cause IMI are not fully known. Herein, we analyzed the distribution of 191 VFs in 441 genomes of 25 NAS species, after classifying VFs into functional categories: adherence (n = 28), exoenzymes (n = 21), immune evasion (n = 20), iron metabolism (n = 29), and toxins (n = 93). In addition to establishing VF gene profiles, associations of VF genes between and among functional categories were computed, revealing distinctive patterns of association among VFs for various NAS species. Associations were also computed for low, medium, and high somatic cell count (SCC) and clinical mastitis (CM) isolates, demonstrating distinctive patterns of associations for low SCC and CM isolates, but no differences between high SCC and CM isolates. To determine whether VF distributions had any association with SCC or CM, various clustering approaches, including complete linkages, Ward clustering, and t-distributed stochastic neighbor embedding, were applied. However, no clustering of isolates representing low SCC, medium SCC, or high SCC or CM was identified. Regression analysis to test for associations with individual VF functional categories demonstrated that each additional toxin and host immune evasion gene increased the odds of having high SCC or CM, although an overall increase in the number of VFs was not associated with increased SCC or occurrence of CM. In conclusion, we established comprehensive VF gene profiling, determined VF gene distributions and associations, calculated pathogenic potentials of all NAS species, and detected no clear link between VF genes and mastitis.

IMPORTANCE Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from milk in dairy cattle worldwide. The virulence factors (VFs) and mechanisms by which these bacteria cause udder infection are not fully known. We determined the distribution and associations of 191 VFs in 25 NAS species and investigated the relationship between VFs and disease. Although the overall number of VFs was not associated with disease severity, increasing numbers of toxin and host immune evasion genes specifically were associated with more severe disease outcomes. These findings suggest that the development of disease and the interactions of VFs with the host are complex and determined by the interplay of genes rather than just the presence of virulence genes. Together, our results provide foundational genetic knowledge to other researchers to design and conduct further experiments, focusing on understanding the synergy between VFs and roles of individual NAS species in IMI and characterizing species-specific effects on udder health.

Short communication: Characterization of enterotoxin-producing Staphylococcus aureus isolated from mastitic cows

Staphylococcus aureus is not only a common cause of bovine mastitis, but also an agent of food poisoning in humans. In an attempt to determine whether staphylococci causing bovine mastitis could also cause food poisoning, 60 isolates of presumed S. aureus were isolated in the period between March and August 2017 from 3,384 routine, composite, quarter milk samples of individual cows raised on 12 dairy farms in central Italy. Seventeen out of 60 isolates were confirmed as S. aureus after coagulase, thermonuclease, and biochemical tests. These isolates were analyzed by PCR for the presence of the nuc, sea, seb, sec, sed, and see genes. The positive isolates were nuc, 100% (17); sea, 35.29% (6); seb, 5.88% (1); sec, 5.88% (1); sed, 29.41% (5); and see, 47.06% (8). The isolates were also tested with 2 enzyme immunoassay diagnostic kits, one for the screening detection of the production of staphylococcal enterotoxins (SEA, SEB, SEC, SED, SEE) and one for the detection of specific enterotoxin produced by each isolate. Seven out of 17 (41.18%) were enterotoxin producers: 7 produced SEA (41.18%), 1 SEB (5.88%), 1 SEC (5.88%), 5 SED (29.41%), and 6 SEE (35.29%). To further characterize the isolates, they were analyzed by the Kirby Bauer test for susceptibility to 13 antimicrobials (ampicillin, ciprofloxacin, kanamycin, tetracycline, gentamicin, methicillin, nalidixic acid, erythromycin, amoxicillin/clavulanic acid, streptomycin, vancomycin, neomycin, and enrofloxacin), and we detected resistance to ampicillin (52.94%), nalidixic acid (70.59%), erythromycin (5.88%), and amoxicillin/clavulanic acid (17.65%). The isolates were sensitive to the main classes of antimicrobials used for the treatment of bovine subclinical mastitis. The presence of enterotoxin-producing isolates of S. aureus in bovine milk means that a temperature abuse or a breakdown in the thermal treatment of the milk could present a food safety risk, particularly if all enterotoxigenic isolates could potentially produce SEA in milk.