Prevalence, antibiotic resistance, virulence and genetic diversity of Staphylococcus aureus isolated from bulk tank milk samples of U.S. dairy herds

Abstract

Background Colonization of dairy cows by Staphylococcus aureus (S. aureus), especially those which are multi-drug resistant and toxin producing, is a concern for animal health and well-being as well as public health. The objective of this study was to investigate the prevalence, antibiotic resistance, gene content and virulence determinants of S. aureus in bulk tank milk samples (BTM) from U.S. dairy herds. Results BTM samples were collected, once in winter and once in summer, from 189 U.S. dairy herds. Of 365 BTM samples cultured, the sample and herd prevalence of S. aureus in BTM was 46.6% (170 of 365 samples) and 62.4% (118 of 189 herds), respectively. Among a subset of 138 S. aureus isolates that were stored for further analysis, 124 were genome sequenced after being confirmed as S. aureus using phenotypic tests. The most commonly identified antimicrobial resistance-associated gene was norA (99.2%) and mecA gene responsible for methicillin resistance (MRSA) was identified in one isolate (0.8%). The most frequently detected putative virulence genes were aur (100%), hlgB (100%), hlgA, hlgC, hlb (99.2%), lukE (95.9%) and lukD (94.3%). In the 53 staphylococcal enterotoxin positive isolates, sen (37.9%), sem (35.5%), sei (35.5%) and seg (33.1%) were the most frequently detected enterotoxin genes. Among the 14 sequence types (ST) and 18 spa types identified, the most common was ST2187 (20.9%) and t529 (28.2%), respectively. The most predominant clone was CC97 (47.6%) followed by CC unknown (36.3%). The single MRSA isolate belonged to ST72-CC8, spa type t126 and was negative for the tst gene but harbored all the other virulence genes investigated. Conclusion Our findings indicated a high prevalence of S. aureus in BTM of U.S. dairy herds, with isolates showing little evidence of resistance to antibiotics commonly used to treat mastitis. However, isolates often carried genes for the various enterotoxins. This study identified predominant genetic clones. Despite lower prevalence, the presence of MRSA and multi-drug resistant strains in BTM poses a significant risk to animal and public health if their number were to increase in dairy environment. Therefore, it is necessary to continuously monitor the use of antibiotics in dairy cows.

Molecular characterization of multi-drug-resistant Staphylococcus aureus in mastitis bovine milk from a dairy farm in Anhui, China

“…The resistance of S. aureus to penicillin G, SXT, ciprofloxacin, norfloxacin, polymyxin B, clindamycin, clarithromycin, and erythromycin was significant, which indicates that it is unsuitable to employ those antibiotics in this region. In current findings, blaZ, ermC, rpoB, and ant(4')-Ia are successfully amplified, which is not in agreement with previous studies in which S. aureus harboring norA, aph (3 ′ )-Ia, mecA, ant (6)-la, aph (3 ′ ) III, mph(C), msr(A), norA, and blaZ (32). β-lactamases (blaZ) is one of the important enzymes which resist penicillin (48,52); the current results are in line with a previous study about blaZ in S. aureus (50).…”

“…This variability between different study results could be attributed to the differences in geographical, study population, farm hygienic and management practices (such as breeds, farm size, the practice of hand milking, and absence of dry cow therapy), milking methods, and instruments employed (11, 42). Herein, the four virulence genes of hla, nuc, clfa, and eta were examined in the current S. aureus bacteria, which differed from the six genes named sea, seb, seg, sei, selp, and tsst1 in a previous study in S. aureus isolated from tank milk samples in Turkey (43), and twenty-two genes (aur, splA, hlgC, hlb, lukE, seg, sei, sem, sen, seo, seu, seb, sec, sed, seh, lukD, sek, splB, hlgA, sel, seq, and hlgB) from milk samples in the US (32). Exfoliative toxin (eta) genes were rarely detected in S. aureus isolated from mastitis positive dairy animals.…”

Molecular characterisation of virulence genes in Staphylococcus aureus associated with clinical bovine mastitis

“…Screening was performed for the presence of PVL in 18 isolates of S. aureus from bovine mastitis and none of them was found to have the presence of the gene. Similarly, absence of PVL was observed in other studies (Patel et al, 2021;Prashanth et al, 2011). Varying occurrence of PVL in bovine mastitis is observed throughout India, as high as 41.6 per cent (Mitra et al, 2013) and as low as 10.53 per cent (Shrivastava et al, 2018).…”

Methicillin-resistant Staphylococcus aureus in China: a multicentre longitudinal study and whole-genome sequencing

“…Analysing the genotypic characteristics of MRSA clones is valuable for understanding MRSA evolution and dissemination [ 1 , 7 ]. Whole-genome sequencing allows for a comprehensive analysis of the epidemiology and genomic repertoire of S. aureus and provides in-depth insights into the evolution of particular MRSA clones [ 6 , 8 ]. While few studies have characterized extensive collections of MRSA isolates from China using whole-genome sequencing [ 9 , 10 ].…”