Abstract: This study explored the effects of low-concentration azithromycin on protein expression, capsular polysaccharide and drug susceptibility of Streptococcus suis type 2, so as to lay a foundation for further study on the effects of low-concentration antibiotics in the environment on microorganisms. After S. suis type 2 was exposed to low-concentration azithromycin, the key differentially expressed proteins were screened by proteomic iTRAQ technology. At the same time, the capsular polysaccharide content and drug susceptibility of S. suis type 2 were determined. One hundred and seventy-four differentially expressed proteins were identified, accounting for 11.6% of the total identified proteins. Most differentially expressed proteins were involved in catalytic and metabolic processes. Most differentially expressed proteins belonged to membrane proteins. Among them, three capsular polysaccharide proteins, nine ABC transporters, one 50S ribosomal protein, one ribosomal RNA methyltransferase and one DNA gyrase were up-regulated; eight capsular polysaccharide proteins, four 50S ribosomal proteins, four 30S ribosomal proteins, one ribosomal RNA methyltransferase and one DNA polymerase IV were down regulated. When S. suis type 2 was exposed to low-concentration azithromycin, S. suis type 2 had reduced sensitivity to a variety of antibiotics. The drug sensitivity of the surviving strains recovered after returning to normal culture. When S. suis type 2 was exposed to low-concentration azithromycin, the content of capsular polysaccharide did not change significantly. S. suis type 2 adapted to the selection pressure of azithromycin by changing the expression of its own proteome. When coexisting with low-concentration azithromycin, S. suis type 2 started the efflux pump system. The efflux effect would increase the risk of bacteria producing multiple drug resistance.

Key words: Streptococcus suis type 2, azithromycin, capsular polysaccharide, drug susceptibility