Prevalence, multidrug resistance patterns, virulence and antimicrobial resistance genes of emerging MDR Aeromonas hydrophila strains retrieved from subclinical bovine mastitis

Aeromonas hydrophila poses a significant public health concern globally. This study provides a detailed characterization of this pathogen isolated from bovine subclinical mastitis cases, focusing on its prevalence, antibiotic resistance trends, multidrug-resistant (MDR) profiles, and genetic determinants of its virulence and resistance. Approximately 800 quarter milk samples were gathered from 200 dairy cows from commercial farms in Cairo, Egypt. The prevalence of subclinical mastitis among the examined milk samples was 42.5% (340/800). Besides, the prevalence of A. hydrophila was 20.8% (71/340). PCR proved that the aerA gene (100%) was the prominent inherited virulence gene among the recovered A. hydrophila strains, followed by the alt (53.3%), ast (45.1%), ser (29.6%), act (26.8%), and hylA (22.5%) genes. The A. hydrophila strains were resistant to amoxicillin (100%), tetracycline (90.1%), amoxicillin/clavulanic acid (85.9%), sulfamethoxazole/trimethoprim (84.5%), ceftriaxone (83.1%), erythromycin (76.1%), and gentamycin (53.5%). In contrast, the tested A. hydrophila strains were sensitive to norfloxacin, which displayed significant antimicrobial efficiency. Moreover, 28.2% of the tested A. hydrophila strains were XDR to 8 antimicrobial classes and harbored the blaTEM, blaCTX−M−1, blaOXA−1, tetA, aadA1, and sul1 genes. Additionally, 15.5% of the isolated strains were MDR to 6 classes and were found to carry the blaTEM, blaCTX−M−1, blaOXA−1, tetA, and sul1 genes. Also, 14.1% of the A. hydrophila strains were MDR to 5 antimicrobial classes and carried the blaTEM, blaCTX−M−1, blaOXA−1, tetA, and sul1 genes. This work provides inaugural documentation of MDR and XDR A. hydrophila emerging from subclinical bovine mastitis in Egypt, signaling a substantial public health risk. The aerA, alt, ast, and ser virulence genes and the blaTEM, tetA, blaOXA−1, sul1, blaCTX−M−1, and aadA1 resistance genes are frequently linked with MDR and XDR A. hydrophila strains of milk origin.