Prevalence and characterization of class Ⅰ integron in Aeromonas from pig-fish integrated farm

Aeromonas is regarded as an important pathogen of freshwater animals. However, little is known about genetics of antimicrobial resistance in Aeromonas sp. in Chinese aquaculture. The aim of this study was to investigate the presence of class Ⅰ integron and characterize multidrug-resistant Aermonas sp. isolated from integrated farms. Three hundred and seventeen Aeromonas strains were isolated from pig feces, fish, pond water and sediments. These samples were collected from 5 pig-fish integrated farms in Foshan and Zhaoqing of Guangdong province. All strains were evaluated for resistance to 20 antimicrobials of 7 classes by two fold dilution method. Genomic DNA was extracted with the Omega DNA mini kit. All the isolates were screened for the presence of intI1. All the intI1-positive strains were also amplified sulI, qacE1 fragment and gene cassette region by PCR. Gene cassettes arrays were analyzed by BLAST. Percentages of isolates resistant to 20 antimicrobials were variable. Resistance was most prevalent for ampicillin, sulfamonomethoxine and nalidixic acid. Among 317 Aeromonas isolates, 50 (15.77%) isolates were positive for intI1. The incidence of class 1integron was more prevalent in Aeromonas from fish and pig than that from environmental sources. Comparing the resistance phenotypes represented in integron-positive and integron-negative strains, the results showed that the resistance rates of trimethoprim/sulfamethoxazole, doxycycline, enrofloxacin, neomycin and florfenicol among integron-positive isolates are higher than those from integron-negative strains. All of the integron-positive strains were resistant to more than three classes of agents. There were16 different gene cassettes encoding resistance to aminoglycosides (aadA1, aadA2, aac6-, aacA4), trimethoprim (drfA1, dfrA12, dfrA15, dfrA17, dfrB4), β-lactams(bla_OXA-10, bla_OXA-21), chloramphenicl (catB3, catB8), rifampicin (arr-2, arr-3) and plasmid-mediated quinolone resistance (PMQR) gene aac(6')-Ib-cr. Most of the integron-positive isolates carrying gene cassettes mentioned above corresponded to the resistance profile of antimicrobial agents such as trimethoprim, streptomycin, chloramphenicol. It indicated that there was a close relationship between class Ⅰ integron and multi-drug resistance in Aeromonas isolates. It is concluded that class Ⅰ integron mediated multi-drug resistance was wide spread in Aeromans from different origins of pig-fish integrated farms. Further study on resistance mechanism should be undertaken so as to provide a basis for rational usage of antimicrobial agents in Chinese aquaculture.