Objective Although the policies for reducing or replacing antibiotics have been in place for many years, the use of antibiotics in the early stages of piglet rearing remains an important aspect of disease prevention and treatment in intensive farming operations. The study of the differences in antibiotic resistance and virulence of Escherichia coli isolated from different parts (intestinal tract and mammary gland) of sows with antibiotics in piglet stage in farms can provide a scientific basis for the optimization of antibiotic use in intensive farms.

Method This study recorded the antibiotic usage of sows in a intensive Luochuan pig farm in Zhanjiang City, Guangdong Province, from the birth to around 420 days of age of piglets. Colostrum (within 2 h after delivery), regular milk (24-72 h after delivery) and fresh fecal samples from the anus were collected from healthy sows of the same batch at approximately 420 days of age. Suspected E. coli isolates were obtained by using MacConkey agar plates. Antibiotic susceptibility was analyzed through antibiotic sensitivity test, and the presence of virulence genes was examined by PCR.

Result A total of 704 suspected E. coli isolates were obtained by using MacConkey agar plates, including 300 isolates from fecal samples, 220 from colostrum, and 184 from regular milk. Antibiotic sensitivity test revealed that the resistance rate of fecal E. coli to ampicillin and tetracycline was 96.3% (289/300), with a resistance rate of 63.6%(7/11) to other common antibiotics, and an overall resistance rate of 98.7% (296/300). The resistance rate of colostrum E. coli to ampicillin and tetracycline was 99.5% (219/220), with a resistance rate of 100% (1/1) to other common antibiotics, and an overall resistance rate of 100% (220/220). The resistance rate of regular milk E. coli to ampicillin and tetracycline was 40.8%(75/184), with a resistance rate of 67.9% (74/109) to other common antibiotics, and an overall resistance rate of 81.0% (149/184). A total of 39 non-antibiotic-resistant E. coli strains were identified. PCR detection showed that most of the 39 non-antibioticresistant E. coli strains did not contain common virulence genes associated with porcine pathogenic E. coli (89.74%, 35/39). Additionally, only 2.86% (1/35) of non-antibiotic-resistant regular milk E. coli strains carried virulence genes, while 75.0% (3/4) of non-antibiotic-resistant fecal E. coli strains carried virulence genes.

Conclusion From the stage where antibiotics are used to prevent and control diseases in piglets to their growth into primiparous sows, a small number of non-virulent and nonantibiotic-resistant E. coli strains are still isolated from the anus and mammary glands. Among them, the proportion of nonvirulent and non-antibiotic-resistant E. coli in feces is extremely low, accounting for only 0.33% (1/300). No non-virulent and non-antibiotic-resistant E. coli are isolated from colostrum within 2 hours after delivery, accounting for 0% (0/220). However, a relatively higher proportion of non-virulent and non-antibiotic-resistant E. coli is found in regular milk collected 24-72 h after delivery, accounting for 18.48% (34/184).