Pathogenicity and drug resistance characterization of Streptococcus agalactiae isolated from dairy cows

Streptococcus agalactiae, commonly known as S. agalactiae, is a critical zoonotic pathogen that significantly reduces milk yield and product quality and poses a significant risk to public health. Although S. agalactiae is increasingly recognised as a principal agent causing milkborne infections, research dedicated to this pathogen in dairy cattle has been less extensive than that of other pathogens. This study aimed to examine the antibiotic resistance profiles of S. agalactiae derived from dairy cows and assess its pathogenicity using validated in vivo models. The findings contribute essential scientific insights into the realm of environmental antibiotic resistance research. The resistance of S. agalactiae isolates to drugs was assessed using the broth microdilution technique. Additionally, PCR analysis was used to identify six important virulence genes. The study revealed that S. agalactiae was fully susceptible to streptomycin, meropenem, ciprofloxacin, clindamycin, cefquinome, and cloxacillin in general laboratory settings and within milk samples. However, among the antibiotics tested, tetracycline exhibited the highest level of resistance, with rates reaching 70%. Penicillin showed a resistance level of 50%, followed by doxycycline at 30%. Additionally, the resistance rates for apramycin and cefoxitin were both 20%, whereas florfenicol resistance was observed at a rate of 10%. All isolates of S. agalactiae carried the cfb gene. However, it is noteworthy that only one isolate possessed this gene exclusively, while the other nine isolates shared a uniform set of four additional virulence genes. The study highlighted the significant impact of these virulence factors on the pathogenic behaviour of S. agalactiae from dairy sources. This was demonstrated by the high mortality rates observed in experimental infections using Galleria mellonella (G. mellonella) larvae and mouse models. These findings contribute to understanding the relationship between the pathogenic properties of S. agalactiae and the virulence genes it carries.