Mastitis is one of the major endemic diseases of dairy cattle. It occurs as a result of the inflammation of the mammary gland and udder tissue; leads towards less milk yield, quality as well as sever financial crisis. Staphylococcus aureus, a Gram-positive bacterium is the causal agent of this disease and probably mastitis is produced by shedding the pathogens and their toxins into milk1. Unfortunately; S. aureus has developed resistance against antibiotics because of the over-prescription and misuse of antimicrobials.
The β-lactams are being extensively used to treat bovine mastitis but the effectiveness of this treatment has reduced because of β-lactamase encoded blaZ that hydrolyze penicillins2. Methicillin/oxacillin resistance is another β-lactam resistance mechanism, which is the consequences of low-affinity penicillin-binding protein (PBP2a) encoded by mecA gene3. Moreover; there are wide arrays of virulence determinants associated with the pathogenicity of S. aureus4. These determinants consist of enzymes and cytotoxins such as hemolysins (α, β, γ and δ), nucleases, lipases, proteases, collagenases and hyaluronidases. Furthermore, some S. aureus strains also produce exoproteins including Panton-Valentine leukocidin (PVL), toxic shock syndrome toxin-1 (TSST-1), exfoliative toxins (ETA and ETB) and staphylococcal enterotoxins (SEA-E, G-I)5.
Amal Awad and colleagues conducted a study to assess the antimicrobial resistance and incidence of S. aureus in mastitic milk. Molecular characterization of the isolated strains was conducted by employing PCR targeting thermonuclease (nuc) and coagulase (coa) genes, β-lactamases (mecA and blaZ) and virulence associated genes (TSST-1 gene, tst and Panton-Valentine leukocidin gene, pvl). In this experiment; scientists collected 200 quarter milk samples from 3 dairy farms at Dakahliya (n = 2) and Damietta (n = 1) Governorates, Egypt from September to December 2016. Afterwards; Amal and team conducted the conventional culturing and Polymerase Chain Reaction (PCR) assays targeting nuc (thermonuclease) and coa (coagulase) genes. Isolates were examined for its vulnerability against 14 antimicrobial agents through disk diffusion technique. Moreover, all the isolates were tested to assess occurance of β-lactamases (blaZ, mecA) and virulence linked (pvl and tst) genes via PCR.
Scientists noted the presence of S. aureus carrying pvl gene which exhibits the personnel sources of contamination to dairy farms. Therefore; to prevent this infection, hygienic measures should be adopted as well as unofficial persons should be restrained from entering to dairy farms. Moreover, high resistance against β-lactams (broadly used by veterinary experts) and toxin producing S. aureus in dairy farms create a huge trouble for public health. Therefore, monitoring the appearance of multidrug resistant strains of S. aureus in dairy farms is indispensable to control this pathogen and the other interlinked zoonotic risks.
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17 November, 2019