Molecular mechanisms of antibiotic inhibition on microbial dissimilatory iron reduction

Microbe-mediated iron cycling plays a pivotal role in biogeochemical processes. However, the impact of trace-level antibiotics on microbially mediated dissimilatory iron reduction (DIR) remains unexplored. In this study, we investigated the effects of four typical antibiotics on DIR mediated by Shewanella oneidensis MR-1, with a focus on their inhibitory effects on three extracellular electron transfer (EET) pathways: 1) direct EET, 2) soluble redox mediator-dependent EET, and 3) nanowire-mediated EET. Our findings demonstrate a concentration-dependent decline in DIR activity with increasing ceftizoxime concentrations, culminating in complete suppression at 64 μg/L. Polymyxin disrupts the cell membrane, causing structural damage that subsequently impairs the electron transport chain (ETC), leading to a reversible reduction in DIR activity. In contrast, ofloxacin and tetracycline directly down-regulated genes associated with electron production and transfer, thereby suppressing both electron transport system activity and the synthesis of NADH dehydrogenase and c-type cytochromes. This irreversibly disrupts ETC function, blocking S. oneidensis MR-1 from conducting EET and impairing DIR activity. This finding reveals antibiotic-induced alterations in microbial iron metabolism and provides new insights into their potential impact on the environmental iron cycling.