China Animal Husbandry and Veterinary Medicine ›› 2025, Vol. 52 ›› Issue (3): 1301-1316.doi: 10.16431/j.cnki.1671-7236.2025.03.032

• Basic Veterinary Medicine • Previous Articles    

Isolation and Identification of Lactogenic Streptococcus agalactiae and Characterisation of Drug Resistance and Virulence and Genome Analysis

GAO Jiaojiao1,2, ZHENG Nan3, SHAO Wei2, CHEN He1, MA Xianlan1, ZHAO Yankun1,2,3   

  1. 1. Institute of Quality Standards & Testing Technology for Agro-Products, Xinjiang Academy of Agricultural Sciences, Ministry of Agriculture and Rural Affairs-Laboratory of Quality and Safety Risk Assessment for Agro-Products, Key Laboratory of Agro-Products Quality and Safety of Xinjiang, Urumqi 830091, China;
    2. Xinjiang Meat and Milk Herbivore Nutrition Laboratory, College of Animal Science, Xinjiang Agricultural University, Urumqi 830052, China;
    3. Key Laboratory for Quality & Safety Control for Milk and Dairy Products of Ministry of Agriculture and Rural Affairs, Institute of Animal Science of CAAS, Beijing 100193, China
  • Received:2024-05-23 Published:2025-02-22

PDF (PC)

15

Abstract: 【Objective】 Streptococcus agalactiae was one of the main pathogens causing mastitis in dairy cows,and it was of great significance to clarify the resistance,virulence and genomic properties of Streptococcus agalactiae for the study of the resistance mechanism of Streptococcus agalactiae.【Method】 The test was carried out by inoculating milk samples on blood plates for isolation and purification by plate scribing method,and the isolates were identified by morphological observation,Gram staining microscopy and molecular biology methods.The minimum inhibitory concentration (MIC) of the isolates was detected by micro broth dilution method,8 drug resistance genes and 14 virulence genes of the isolates were detected by PCR method,and the isolates were analyzed for drug resistance,virulence genes,and genome function using whole-genome sequencing technology.【Result】 In this study,two strains of bacteria were successfully isolated from 36 samples,which showing white,round,smooth and raised colonies on blood agar plates,and purple spherical Gram-positive organisms were seen on microscopic examination.The biochemical identification showed that the isolates were β-hemolytic and the contact enzyme test was negative,and the PCR amplification showed two bands of 1 500 bp in size as expected,and the sequencing result showed that the similarity with Streptococcus agalactiae ATCC 13813 was ＞99.93%,so the isolates were identified as Streptococcus agalactiae at the molecular level,and were designated as S1 and S2.The results of serotyping and molecular typing showed that the serotype of S1 strain was type Ⅱ and molecular typing was ST312,whereas the serotype of strain S2 was type Ⅲ and molecular typing was ST309.Drug sensitivity tests showed that strains S1 and S2 exhibited high sensitivity to ampicillin,amoxicillin/clavulanic acid,oxacillin,cefphalothin,sulfisoxazole and compound sulfamethoxazole.However,strain S1 was resistant to tetracycline and clindamycin,whereas S2 was resistant to tetracycline,erythromycin and clindamycin.In addition,tetO and ermB resistance genes were detected in strain S2,whereas other resistance genes were not found.In terms of virulence genes,cfb,bca,iagA,bibA,cspA,pavA,sip, hylB and fbsA were detected in both strains S1 and S2.Genomic analysis showed that strain S1 had a circular chromosome of 2.44 Mb in size,with a GC content of 36.34%,and were predicted to contain 2 448 genes.Strain S2 had a ring chromosome of 2.56 Mb in size with a GC content of 36.59% and was predicted to contain 2 576 genes.By constructing a homologous phylogenetic tree,it was found that strain S1 had the highest consistency with the Streptococcus agalactiae NCTC13947 strain from UK; S2 was most closely related to Streptococcus agalactiae NCTC8184 strain from UK.【Conclusion】 In this study,two strains of Streptococcus agalactiae S1 and S2 were isolated from 36 samples of dairy cows with mastitis and found to be sensitive to a variety of antibiotics,but also showed resistance.Genomic analysis revealed their resistance and virulence genes,providing valuable data for further studies on the resistance and virulence of Streptococcus agalactiae.

Key words: Streptococcus agalactiae; drug resistance genes; virulence factors; genomics

CLC Number: