| According to the difference of capsule antigen characteristic, Streptococcus suis divids into 35 serum types,including 1~34 and 1/2 serum type.In all of these types, the type1/ 2, 1,2,7,9 and 14 could cause the pigs to be ill, in which Streptococcus suis 2 is the most popular and broadest,and it is also strongest to swines and humans. The Streptococcicosis is the infectious disease of different types of clinical diagnosis which causes by many different groups of Streptococcus. Streptococcus suis could cause meningitis, arthritis, pneumonia, endocarditis, multiple serositis and local abscess and so on, it could also infect and be lethal to the human.It is one kind of important pathogeny of human and livestock. The antibacterials play an important role in the prevention and treatment of the Streptococcicosis. But it causes a terrible result that bacterial resistance is growing gradually .Tetracyclines antibiotics have been found in produce of Streptomyces or semi-synthetic produce, which belongs to a class of broad-spectrum alkaline antibiotic.They could inhibit the atypical microbial growth including G+ and G- bacteria, Chlamydia, Mycoplasma, Rickettsia and protozoa. In recent years, with the increasing phenomenon of an unreasonable application of tetracyclines, drug-resistance bacteria mutates and increases continuously. The emergence of resistance limites their use.Researchers started to study the molecular mechanism of bacterium against tetracyclines to resolve the problem of drug resistance. Now, antibiotic efflux and ribosomal protection are confirmed to the main molecular mechanism.The study of resistance mechanism of Streptococcus suis against tetracyclines is much less. The medicine mechanism to this kind of antibiotic mainly caused by two kinds of ribosome protection protein TetM and TetO. They rely on GTP to hydrolyze tetracyclin from the 30S subunit, which causes resistance .Both of them are transferable factors.In the experiment, the cotton pledgets of nasal cavity from swines were collected from hoggerys in the three provinces of Northeast China.By isolation and purification,12 strains were gram-positive and chain coccus. Lancefield serum group and biochemical identification of Streptococcus suis, resulted show that 12 strains were serum D-type, a total of 10 kinds of biochemical reactions to be found. We used K-B test to decide the antimicrobial sensitivity of Streptococcus suis to nine kinds of commonly used antimicrobial drug, including tetracycline, chloramphenicol, gentamicin, erythromycin and cotrimoxazole, ciprofloxacin, penicillin G, oxacillin and ampicillin. The results showed that all the strains had a high degree of resistance to nine kinds of antimicrobial agents, and which were multiple drug resistance. The resistant spectrum of Streptococcus suis is 4 to 9.And the maximum resistant spectrum is 8-resistant strains. By the critical concentration as a standard of the United States Committee for Clinical Laboratory Standards (NCCLS),we used microdilution method to determine the resistance of Streptococcus suis against three kinds of tetracycline ,which were doxycycline, tetracycline hydrochloride and chlortetracycline hydrochloride. The results showed that the resistance of Streptococcus suis against doxycycline,tetracycline hydrochloride, chlortetracycline hydrochloride was very high, the resistant rates were 100%, 100%, 91.7%, at the same time, MIC50 and MIC90 of Streptococcus suis against three tetracycline antibiotics were larger than 64μg/ml, the ranges of resistance were 4~> 64,16~> 64,2~>64μg/ml. We used Kunming mouse to test the pathogenicity of 12 Streptococcus suis, in each group we used 5 Kunming mouse. The results showed that 6 strains had high pathogenicity, 5 strains were moderately pathogenicity, and only one had no pathogenicity. The total death rate is 91.7%.In this study, PCR (polymerase chain reaction, PCR) had been used to amplify the resistant genes of tetracycline antibiotic, including tetO and tetM gene ,which clarified the rate of detection,the relationship between the tetracycline-resistant gene point mutations and amino acid mutations and the resistance. 12 strains of Streptococcus suis were positive to tetM genes, but no one harbored tetO gene. The tetM genes nucleotide sequence had 93%~100% homologous genes with the published sequences in GenBank. The mutations of amino acids concentrated on 150~556 sites, and all the 12 strains had a common mutation in 432 site,which was V→I. So the determinate point of the resistance of Streptococcus suis to tetracyclines was the 432 site in tetM gene.
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