Presence of Aminoglycoside and β-Lactam-Resistant Pseudomonas aeruginosa in Raw Milk of Cows

Pseudomonas aeruginosa is a ubiquitous environmental bacterium that causes a variety of infections in humans and animals. Although antibiotic resistance in livestock has been extensively documented, continuous surveillance remains crucial for tracking emerging resistance trends and assessing control measures. During 2017 and 2018, 234 strains of P. aeruginosa were identified from 1063 strains of pathogenic and nonpathogenic bacteria isolated from raw milk of healthy and mastitis cows. In this study, 132 convenience P. aeruginosa isolates were recovered and tested for antimicrobial susceptibility and the presence of antimicrobial resistance genes and virulence factors. Antimicrobial susceptibility testing revealed that these P. aeruginosa isolates were resistant to three (gentamicin, tobramycin, and ceftazidime) out of eight antibiotics. Real-time PCR targeting 21 antibiotic resistance genes indicated that aminoglycoside modifying enzyme (AME) gene ant(3″)-I was most frequently identified in both antimicrobial-resistant and -susceptible P. aeruginosa isolates, followed by aac(6′)-II and aac(6′)-Ib. The β-lactamase encoding gene, blaPDC, was mainly identified in susceptible P. aeruginosa isolates. Virulence factors screening revealed the presence of exoS, exoT, exoU, pyo, aprA, toxA, plcH, algD, lasB, lasI, lasR, rh1L, and rh1R in resistant isolates, with the detection rates ranging from 16.7% to 88.9%. Additionally, next-generation sequencing was conducted on three resistant isolates to validate these findings. This study showed the antibiotic resistance of P. aeruginosa in raw milk samples from large-scale dairy farms in Jiangsu and Shandong provinces, China.