Genomic insights of S. aureus associated with bovine mastitis in a high livestock activity region of Mexico

Abstract

Importance: Bovine mastitis, predominantly associated with gram-positive Staphylococcus aureus, poses a significant threat to dairy cows, leading to a decline in milk quality and volume with substantial economic implications.

Objective: This study investigated the incidence, virulence, and antibiotic resistance of S. aureus associated with mastitis in dairy cows.

Methods: Fifty milk-productive cows underwent a subclinical mastitis diagnosis, and the S. aureus strains were isolated. Genomic DNA extraction, sequencing, and bioinformatic analysis were performed, supplemented by including 124 S. aureus genomes from cows with subclinical mastitis to enhance the overall analysis.

Results: The results revealed a 42% prevalence of subclinical mastitis among the cows tested. Genomic analysis identified 26 sequence types (STs) for all isolates, with Mexican STs belonging primarily to CC1 and CC97. The analyzed genomes exhibited multidrug resistance to phenicol, fluoroquinolone, tetracycline, and cephalosporine, which are commonly used as the first line of treatment. Furthermore, a similar genomic virulence repertoire was observed across the genomes, encompassing the genes related to invasion, survival, pathogenesis, and iron uptake. In particular, the toxic shock syndrome toxin (tss-1) was found predominantly in the genomes isolated in this study, posing potential health risks, particularly in children.

Conclusion and relevance: These findings underscore the broad capacity for antibiotic resistance and pathogenicity by S. aureus, compromising the integrity of milk and dairy products. The study emphasizes the need to evaluate the effectiveness of antibiotics in combating S. aureus infections.

Keywords: S. aureus; bovine mastitis; genomics; multi-resistant genes; virulence factors.

Fig. 1. Level of subclinical mastitis per mammary quarter. The bars indicate the number of mammary rooms diagnosed with subclinical mastitis according to the CMT for the different levels of the test (T, 1, 2, and 3). Quarter mammary: PRT, PLT, ART, and ALT.

PRT, posterior right teat; PLT, posterior left teat; ART, anterior right teat; ALT, anterior left teat; CMT, California Mastitis Test.

Fig. 2. Antimicrobial susceptibility of the S. aureus strains isolated from cows exhibiting mastitis. OX, P, E, VA, RA, LZD, LVX, TE, CIP, C, GE, TSX.

OX, oxacillin; P, penicillin; E, erythromycin; VA, vancomycin; RA, rifampin; LZD, linezolid; LVX, levofloxacin; TE, tetracycline; CIP, ciprofloxacin; C, chloramphenicol; GE, gentamicin; TSX, trimetropine-sulfamethoxazole.

Fig. 3. S. aureus associated with mastitis spanning tree. The colored circles represent different STs, and their sizes are proportional to the number of genomes included. The black numbers represent the STs, and the red numbers indicate the allelic difference among connected STs.

Fig. 4. S. aureus phylogenetic tree coupled to a presence/absence antibiotic resistance heatmap. Blued dots on branches represent a bootstrap higher than 85. Name shading indicate the isolation country, and the colorful column shows ST for each isolate. The green dots indicate the gene presence, while the absence of green dots indicates the opposite.

Fig. 5. S. aureus phylogenetic tree coupled to a presence/absence virulence heatmap. The blue dots on the branches represent a bootstrap higher than 85. Name shading indicate the isolation country, and the colorful column shows the ST for each isolate. The green dots indicate gene presence, while the absence of green dots indicates the opposite.