| Since the antibiotics were discovered and began to be used, the antibiotic-resistance of bacteria have been found and more and more severe, which poses a great threat to the health of human beings and animals. And now research on the bacterial resistance has been a hotspot in the worldwide.21 sites along the Jiulong River were selected to sample the surface water in this study and191 unique antibiotic-resistant bacterial strains were screened on the solid media with different antibiotics. A phylogenetic analysis of 16 S r DNA sequences and operational taxonomic units showed that these strains belonged to 20 genera including the common genera Acinetobacter(25.7%), Aeromonas(16.2%), Comamonas(14.7%) and Pseudomonas(12.0%).Antimicrobial susceptibility testing revealed that the resistant rates to 19 tested antibiotics of 191 isolates ranged from 8.9% to 99.5% and all of these strains were multi-resistant. Overall,the order based on the degree of resistance to eight classes of drugs was β-lactams >rifamycins > aminoglycosides > chloramphenicols > macrolides > tetracyclines >aminoglycosides > quinolones.Polymerase chain reaction was used to detect 23 kinds of resistant genes corresponding to eight classes of antibiotics in 191 isolates and it showed that the bla TEM gene(89.0%) was the most frequent, followed by the arr2/3 gene(66.5%) and the qnr S gene(55.0%). All of 191 strains carried at least one resistant gene. However, the order based on the rates of resistant genes failed to agree with the one of resistant phenotypes with the probable reasons that, I suggested,the resistant genes we selected to detect were far less than the ones actually existed and some genes failed to express normally.In addition, the int I1 gene was found in all isolates, while the int I2 gene was found in 5strains and the int I3 gene was never found. The variable region of class 1 integrons was successfully amplified in seventy out of 191 int I1-positive strains and 31 gene cassette arrays were obtained. The most dominant array was dfr A12-orf F-aad A2 and nine new arrays were also found, including aad A5-orf D, qac H-cml A1-aad A2, aac A4-aad A1-tna A′, aac A4-ere A1-bla OXA-2-aad A1, dfr A5-aac A4-nit1-nit2-cat B3, dfr A17-aac A4-nit1-nit2-cat B3, unknown-aac A4-ere A1-bla OXA-2-dfr A1, dfr A16-bla PSE-1-aad A2-cml A1-aad A1 and dfr A16-bla PSE-1-aad A2-cml A1-orf A′-tna IS4′. 34 kinds of cassettes found in the variable region contained 27 resistant genes, four open reading frames encoding hypothetical proteins, two transposase-like genes and a novel gene. The cassette aad A2 which enabled the strains to be resistant to streptomycin and spectinomycin was the predominant. The variable region of class 2 integron with the same array dfr A1-sat2-aad A1 was successfully amplified in all five int I2-positive isolates. Among them, one strain carried a class 1 integron simultaneously.This study revealed that the antibiotic-resistant bacterial isolates from Jiulong River were diverse and the extent of bacterial resistance was quite severe. Integrons were widespread in these strains, which may be one of the important reasons for multi-resistance. The discovery of the new gene cassette suggested that there would be many other new resistant genes in strains all over the world. This would apparently threaten the health of human beings and the development of husbandry and aquaculture. Thereby, we need to strengthen the control on the use of antibiotics and to find out the valid approach to face this challenge. |
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