This study evaluates the effects of a broad-spectrum antibiotic (florfenicol) on antibiotic resistance genes (ARGs) and bacterial community structure in aquatic environments. We constructed an indoor aquatic microcosm model, adding different concentrations of florfenicol (0.1, 1, 10, 100 mg L⁻¹), and water and sediment samples were collected after 0, 7, 30, and 60 days. qPCR and 16S rDNA amplicon sequencing were used to study the changes in the ARGs and bacterial community structure of the collected samples. The results show that the inclusion of florfenicol resulted in an increased abundance of the floR and optrA genes. Adding 100 mg L⁻¹ florfenicol to the water increased the abundance of optrA gene copies with the maximum on the Day 7, and increased the abundance of floR gene copies with the maximum on Day 30. Adding 100 mg L⁻¹ florfenicol to the sediment increased the abundance of floR and optrA genes by one order of magnitude on Day 60. Meanwhile, the average number of operational taxonomic units (OTUs) in the water samples was 257, and the average number of OTUs in sediment samples was 823. The bacterial community diversity and richness in sediments were higher than those in water. The difference between the maximal and minimal values of the Shannon diversity index in the water and sediment samples was 4.36 and 1.95, respectively. The effect of florfenicol on the bacterial community structure in water was much higher than that in sediment. At 30 days, the diversity index and richness index of the florfenicol treatment groups with 1 and 10 mg L⁻¹ concentrations began to increase; at 60 days, the diversity and richness indices of the 100 mg L⁻¹ florfenicol treatment group began to increase. The samples at the same sampling time in the sediments clustered closer together. The results of this study provide a scientific basis for guiding the rational use of florfenicol in aquaculture, maintaining a healthy and stable microecological environment in aquaculture, and provide theoretical data for environmental ecological risk assessment and safety management caused by microbial resistance under the abuse of florfenicol.

Florfenicol is one of the most widely used antibiotics in aquaculture and veterinary clinics because of its low side effects and strong bactericidal effect. A total of 45~60% of florfenicol is not absorbed by the animal body and accumulates in the aquatic environment through a variety of pathways, which affects denitrification. Indoor aquatic microcosm models were constructed and sediment samples were collected at different florfenicol concentrations (0.1, 1, 10, and 100 mg/L) on days 0, 7, 30, and 60 to extract the microbial genome DNA and determine the water properties. qPCR and amplicon sequencing were used to study the dynamic changes in the nirS gene and nirS-type denitrification community structure, diversity, and abundance, respectively. The results showed that high florfenicol stress influenced the sediment’s physicochemical properties, reducing conductivity, alkaline dissolved nitrogen, and organic matter content. In addition, the abundance of nirS, a functional denitrification gene, increased obviously with increased florfenicol concentrations but decreased the diversity of nirS-type denitrification microorganisms. Proteobacteria was the dominant denitrifying phylum in the sediment. Our study provides a scientific basis for the rational use of florfenicol in aquaculture to maintain a healthy and stable microecological environment and also provides a preliminary understanding of the response characteristics of water denitrifying microorganisms to florfenicol exposure.

Background: Bacteriocins are peptides that demonstrate antimicrobial activity and are produced by bacteria. Bacillus velezensis is considered a reliable producer of bacteriocins, with a strong antimicrobial spectrum and antimicrobial activity. In this study, B. velezensis G7 isolated from mangrove plant roots was used as the experimental strain. Whole-genome sequencing (WGS) analysis was performed to annotate functional genes. Results: The results demonstrated that B. velezensis G7 has a genome size of 3,894,836 bp, a GC content of 46.67%, and predicts 3,715 CDSs, 10 rRNA genes, and 83 tRNA genes. Gene annotation revealed pathways related to bacteriocin synthesis and immunity. Identified 8 bacteriocin and secondary metabolite biosynthetic gene clusters in B. velezensis G7, including Sactipeptides, LCI, Amylocyclicin, ComX4, Mersacidin, Lanthipeptide, Bacilysin, and NRPS. The crude protein from B. velezensis G7 shows broad-spectrum antibacterial activity, with the strongest inhibition against Listeria monocytogenes and the weakest against Staphylococcus aureus . Conclusions: The genome of B. velezensis G7 was analyzed, and bacteriocin-related genes were identified. The results suggest that B. velezensis G7 has the potential to produce bacteriocins, thereby providing a theoretical basis for future studies on bacteriocin production in B. velezensis . In addition, these findings may facilitate further advances in bacteriocin research and bioinformatics analysis.