Potential risks of microplastics combined with superbugs: Enrichment of antibiotic resistant bacteria on the surface of microplastics in mariculture system

Microplastics have become emerging pollutants and served as potential vectors for harmful bacteria, while rare information on the emergency and propagation of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) on the surface of microplastics is available. This study investigated the enrichment of ARB, especially multi-antibiotic resistant bacteria (MARS), on the surface of microplastics in mariculture system. Polyethylene terephthalate accounted for the highest proportion (75%) in the collected microplastics. The counts of cultivable ARB in microplastic samples were 6.40 x 10(6)-2.48 x 10(8) cfu/g, which were 100-5000 times higher than those in water samples. The ratios of cultivable ARB to total cultivable bacteria from microplastic samples were higher than those from water samples. High-throughput sequencing showed that the diversity and abundance of cultivable ARB in the microplastic samples was high with the predominant bacterial genera of Vibrio, Muricauda and Ruegeria. Total 160 MARB isolates were obtained and most of isolates were obtained from the microplastic samples. MARB isolates resisting or intermediating to four and three antibiotics accounted for much higher proportions in the microplastic samples, and the higher percentage of antibiotic resistance was to penicillin, sulfafurazole, erythromycin and tetracycline. The dominant multiple antibiotic resistance profile was TET-SFX-ERY-PEN, which accounted for 25.4% in microplastic samples and 23.9% in water samples. In typical MARB isolates, the positive detection rate of ARGs was up to 80.0% in microplastic samples while that was 65.3% in water samples. Five types of class 1 integrons (intI1) associated gene cassette arrays and seven types of gene cassettes were detected in microplastic samples, which were more than those in water samples. These results revealed that microplastics were hazardous pollutants for the enrichment of ARB, especially superbugs, and the spread of antibiotic resistance.