Vessels operating worldwide need to treat ballast water to inactivate organisms to comply with the Ballast Water Management Convention. We analysed the phenotypic resistance of isolated unidentified aerobically grown bacteria to antimicrobial substances in the uptake water before treatment and the discharge water after treatment on a vessel in operation with a certified UV based ballast water management system (BWMS). We isolated Escherichia coli on selective media and genotyped the identified isolates. All isolates were categorized into clonal and phylogenetic groups based on Polymerase Chain Reaction (PCR) methods. We examined the presence of Plasmid-Mediated Quinolone Resistance (PMQR) genes, betalactamases, adhesins, autotransporters, genes encoding iron uptake proteins, genes associated with immune system evasion, and genotoxins. For all non-clonal isolates, we tested their sensitivity to antimicrobial substances. Resistance to cefotaxime increased after treatment in both, unidentified bacteria and E. coli isolates. PCR analyses showed a decrease in the occurrence of qnrS and ompTAPEC genes and an increase in qnrB, kpsMTII, blaTEM, fyuA, and usp genes in the discharged ballast water. The results of the disk diffusion method indicated an increase in resistance to ampicillin, ceftazidime, and the highest increase occurred in resistance to cefotaxime. The number of Multi-Drug Resistant (MDR) isolates decreased substantially in the discharge water. BWMS effectively eliminate phytoplankton and zooplankton, while UV water treatment may cause different negative changes in bacteria, introducing new threats to human health (microbiome to accumulate resistance genes) and aquaculture (increase diversity and volume of antibiotics used, lower seafood production), hence opening a new issue in global discharges of ballast water.
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