| Streptococcus suis infection is a worldwide problem in the swine industry which is a very important zoonotic agent associated with a variety of diseases including meningitis, septicaemia, arthritis, endocarditis and pneumonia not only in pigs but also in humans. It is one of important measures in treating and preventing them by antibacterials. Macrolides have antibacterial activity to gram positive coccus and mycoplasma simultaneously, with little noxious property and low price, which have applied for human and animals. Macrolides such as erythromycin, tylosin are very commonly used in pig husbandry and have satisfactory effectiveness on Streptococcus suis. Recently, many countries have reported that resistance of S. suis to macrolide increases quickly and has different resistance mechanism and phenotype, which creats a severe threaten to treatment for such deseases. The resistance genes reported recently includes ermA, ermB, ermC, mefA and mutations in 23S rRNA and/or ribosomal protein. Among these macrolide-resistant determinants, ermB gene coding cMLSB resistance appears to be predominant.A total of 28 S. suis isolated from different pig farms in northeast of China (Heilongjiang province, Liaoning province and Jilin province) were collected from nasal cavities and identified by Gram staining, biochemical event and Lancefield serogroup. Their resistance and phenotypes to macrolides were tested by microdilution and double-disk diffusion methods, and the drug-resistant strains were screened simultaneously. It showed that most of the strains belonged to D serogroup, and 57.1% of the strains were highly virulent. MICs of macrolides against S. suis are between 0.25μg/ml and >64μg/ml. The rate of resistance to erythromycin, azithromycin, tylosin was 92.8%, 92.8% and 89.3% respectively, among which the high-level resistant strains were 84.6%,84.6% and 100%. Constitutive MLSB resistance was the main phenotype. The rate of resistance to other 9 antimicrobial agents was 35.7% ~100%, and resistance rate to only chlormycetin and ciprofloxacin was below 50%, all of the strains were multidrug resistant (3~9) strains. It indicates severity of S. suis resistance in the area.From point of view of molecular biology illuminated mechanism of macrolide antibiotic resistance, screening for the presence of erm (ermA, ermB, ermC) and mef A/E genes by PCR. Of the total of 28 strains, 57.1% of the strains harbored ermA gene, 92.8% of the strains possessed ermB gene, 53.6% of the strains had ermA and ermB gene simultaneously, and all of the 28 strains are negative for ermC or mefA/E genes. Partial erm sequences amplified have registered in GenBank. Sequence of ermA and ermB genes of S.suis isolates had a rather high homology with the sequence of erm gene, transposon and plasmids of S.aureus, S.pyogenesis, S. pneumoniae and E. faecalis pulished in GeneBank. Nucleotide sequence comparison represented a 83%~100% homology in ermA and 98%~100% in ermB with other sequences in GenBank. Amino acid sequences of ermA vary more frequently than those of ermB. 11 sequences of ermA mutated simultaneously at 34 sites, while 7 sequences of ermB were identical and only differed by 1~3 amino acids in the remaining strains. No causative relationship could be found between gene mutations and macrolides resistance. The study indicated very high resistance rates and levels against macrolides in northeast of China, which was mediated by ribosome methylation encoded by the ermA and/or ermB gene. A general exchange of erm genes could exist among different generic bacteria. |
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