Interactions and associated resistance development mechanisms between microplastics, antibiotics and heavy metals in the aquaculture environment

Microplastics (MPs), antibiotics and heavy metals unavoidably contaminate the aquaculture environment, with serious ecological and human health consequences. Antibiotic-resistance genes (ARGs) and metal resistance genes (MRGs) are induced in microorganisms in the environment as a defence mechanism against antibiotics and heavy metals respectively. Furthermore, heavy metals in the aquaculture environment will cause bacteria to employ antibiotic resistance through co-selection (co-resistance or cross-resistance) and other potential mechanisms. MPs may also act as potential carriers of multidrug resistance in aquaculture environments because they are hotspots for the enrichment and transmission of antibiotic resistance. Multidrug-resistant (MDR) microorganisms in the aquaculture environment may infiltrate aquatic organisms via the food chain (eg MDR-microorganisms→myctophids→ tuna/ squid/ whales/ seabirds/ seals), and after long-term enrichment, they may gain access to the human body, posing a major risk to aquaculture and human health. As a result, this review article summarises contamination of MPs, heavy metal and antibiotics in global aquaculture settings; discusses the generation of ARGs, MRGs and common selection mechanisms for resistance development and emphasises the role of MPs as hot spots for antibiotic-resistance selective enrichment, as well as potential carriers of multi-drug-resistant genes (MDRGs), in the aquaculture environment, posing an increasing threat to aquaculture and even human health.