| Swine streptococcicosis is a group of Infectious diseases of different diagnosis types caused by Streptococcus suis. As an important zoonosis pathogen, such bacteria can lead to not only the symptom of swine septicemia, meningitis, arthrophlogosis, pneumonia, endocarditis, polyserositis and focal abscess in swine, but also the infection and death in employed person of hog industry. Antibacterial agents have played an important role in preventing and treating streptococcicosis, yet with the widespread use of those medication, the drug resistance of Streptococcus suis is getting more and more serious, especially against macrolide, tetracyclines,β-lactam, Sulfanilamide and Quinolone antibiotics which were often used in treating Streptococcus.s infections tradtionally.It has become an important problem for pig-raising industry and scientific researchers of many countries about how to control swine streptococcicosis. At present, antibacterial strategy is the therapy based on the MIC. As the development of the research, American scholar Zhao. pointed the drawback of the traditional treatment strategy . A theory was pointed that the MSW should be closed or narrowed as far as possible when selecting and using antibacterial agents and adjusting treatment protocols. Based on the theory, we study if tyloain tartrate combined with amoxicillin sodium can have the effect of narrowing or closing the MSW.In the experiment, the samples were 100 swines'nasal swabs from hoggerys in the three provines of Northeast China. 100 strains of streptococcus suis sensitivity to tylosin tartrate and amoxicillin sodium were tested by K-B test. The result showed that, the resistant rate of tylosin tartrate was 80%; the resistant rate of amoxicillin sodium was 0%,20% strains were sensitive to tylosin tartrate and amoxicillin sodium at the same time, Revealed that resistance of isolates of streptococcus suis was serious against tylosin tartrate and sensitive against amoxicillin sodium.By the United States Committee for Clinical Laboratory Standards(NCCLS), MIC of 20 sensitive strains of Streptococcus suis to tylosin tartrate , amoxicillin sodium and compound tylosin tartrate and amoxicillin sodium were tested, and the FIC index was figured. The result showed that, the FIC of 3 strains (accounts for 15%)>1.0 , which demonstrated inhibitory activity after combinating the two drugs, the MIC of 2 strains for the combination of the two drugs were reduced in different degrees compared with two drug used individually, FIC≤1 indicated that the combination was additive effec against 17 strains(accounts for 85%). Randomly selecte a additive effect strain D10 of tylosin tartrate combined with amoxicillin sodium. To study possible interaction between tylosin tartrate and amoxicillin sodium, time-kill experiments were performed with the clinic sensitive strain. The colony numbers of tylosin tartrate decreased slightly with time. The colony numbers were 5.5log10cfu·mL-1 at 0h and 4.3 log10cfu·mL-1 at 12h, lossed only 1.2log10cfu·mL-1; the colony numbers were 0 at 12h and lossed 5.5log10cfu·mL-1; The colony numbers of tylosin tartrate combined with amoxicillin sodium decreased quickly and were 0 at 8h. The colony numbers of tylosin tartrate combined with amoxicillin sodium decreased exceed sum of two single-agent and loss>2log10 cfu·mL-1compared with most effective amoxicillin sodium at 8h, which indicated synergistic action for tylosin tartrate combined with amoxicillin sodium.Through the agar plate dilution method, the MIC,MIC99 and MPC of tylosin tartrate, amoxicillin sodium and tylosin tartrate combined with amoxicillin sodium were measured respectively against Streptococcus suis D10, the results showed that the MIC,MIC99 ,MPC and SI of tylosin tartrate, amoxicillin sodium,tylosin tartrate combined with amoxicillin sodium were respectively 1.62μg·mL-1,0.56μg·mL-1,0.3\0.45μg·mL-1;1.3098μg·mL-1,0.3553μg·mL-1,0.2187\0.32805μg·mL-1;12.8μg·mL-1,1.835μg·mL-1,0.6144\0.9216μg·mL-1;9.77,5.16,2.81.The combination of the tylosin tartrate and amoxicillin sodium can reduce the MPC of the two drugs used individually and can narrow the MSW of the two drugs used individually, that is the combination can narrow the MSW of the tylosin tartrate and the amoxicillin sodium by 3.5 times and 1.8 times respectively.Through growth curve of colony recovery of different concentration tylosin tartrate,amoxi- cillin sodium and combination of tylosin tartrate and amoxicillin sodium against Streptococcus suis D10, the platform stage shortened compared with individual drug and maked curve moved towards left side.It can be seen that combination of tylosin tartrate and amoxicillin sodium can narrow the MSW of the two drugs used individually.
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