| Streptococcus suis as a new anthropozoonosis pathogen has been reported in all countries where the pig industry is developed,which can bring about great losses.At present,studies have shown that the resistance of Streptococcus suis to commonly used antibacterial drugs has increased year by year,which has an important impact on the treatment of clinical infections,so routine surveillance of drug resistance is of great significance.This experiment is aimed at the increasing drug resistance of the Streptococcus suis,the main contents are as follows.1.Investigation on the resistance of Streptococcus suis to the common antibioticsIn this experiment,pathogenic bacteria were isolated and identified from suspected Streptococcus suis isolates collected from 8 large-scale pig farms in Zhejiang,Jiangxi,Fujian and Jiangsu in 2018,and the isolates were tested for resistance to common antibiotics.A total of 44 strains of Streptococcus suis were isolated,purified and identified from 64 samples.The minimum inhibitory concentration of 20 antibacterial drugs was determined by the micro-broth dilution method recommended by the American Committee for Clinical Laboratory Standards(CLSI)and the drug resistance analysis was carried out with reference to the drug resistance breakpoint specified by CLSI.The results showed that the resistance rates of 44 isolates to marboftoxacin,tilmicosin,tulathromycin,erythromycin,lincomycin,clindamycin and doxycycline were all above 90%.Erythromycin,lincomycin and doxycycline was as high as 100%,suggesting that this isolate has extremely high resistance to these antibiotics.In addition to the low resistance rates of ceftiofur(2.2%)forβ-lactam antibiotics,the resistance rates of penicillin and amoxicillin were 56.8%and 47.7%,respectively,showing moderate degree resistance.The resistance rates to the amide alcohol antibiotics florfenicol and chloramphenicol were 81.8%and 59.1%,respectively.The resistance rates to enrofloxacin was as high as 88.6%.The resistance rates of spectinomycin and gentamicin to aminoglycosides were all around 50%.The resistance rates to tiamulin and valnemulin were 65.9%and 50%,respectively.Streptococcus suis isolates were more sensitive to human antibiotics vancomycin,linezolid and rifampicin,and the resistance rates are 0%,25%and 4.6%,respectively.The drug resistance spectrum analysis of drug-resistant strains showed that all the isolates were multi-drug resistant strains and the distribution of drug resistance spectrum was complicated,and lincosamides,macrolides and tetracyclines were present in all drug resistance spectrums indicate that these three types of antibiotic resistance often occur simultaneously.It also indicates that the drug resistance situation of Streptococcus suis is more severe and should be taken seriously,and it is necessary to carry out daily surveillance.2.Detection of macrolides,tetracyclines and pleuromutilins resistant genes and study on the horizontal spread of lsa(E)geneFurther,13 drug resistance genes of macrolides,tetracyclines and pleuromutilins were detected in the Streptococcus suis isolates.Only ermB,tetO,tetM and lsa(E)were detected in this study with the rates of 95.5%,59.1%,9.1%and 38.6%,respectively.In this experiment,17 strains of lsa(E)positive strains were tested for conjugation transfer,of which 5 strains were successfully joined,and the frequency was 2.7 × 10-9～3.5 × 10-8.The sensitivity of conjugons and recipient bacteria to pleuromutilins was further compared.The results showed that the sensitivity of all conjugons to tiamulin and valnemulin decreased.The MICs of tiamulin for the five conjugons were increased by at least 4-128 times relative to the recipient bacteria,and the MICs of valnemulin(except one conjugon)were at least 2-128 times higher than that of the recipient bacteria.It was found that the conjugons JSXZPIJ and FJNP9J detected lsa(E),ermB and tetO simultaneously;the conjugon JSXZP2J detected lsa(E)and ermB;the conjugons JS7J and FJNP8J only detected lsa(E),the resistance genes are consistent with drug resistance phenotypes.The results indicate that lsa(E)may be accompanied by co-transfer of other genes,but the genetic elements may be different,which proved that lsa(E)is one of the main reasons for the resistance of Streptococcus suis to pleuromutilins,and the gene can be transmitted horizontally between different strains of Streptococcus suis.In summary,this study tested 20 antibiotic susceptibility of Streptococcus suis isolates in 8 large-scale pig farms,providing practical guidance for clinical prevention and treatment of Streptococcus suis infection in each farm.In particular,the lsa(E)gene which mediates the resistance of Streptococcus suis to pleuromutilins was detected in this experiment,and the gene can be horizontally transmitted between different strains of Streptococcus suis. |
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