Biofloc possess a probiotic effect and has a role on mechanisms related to the immune response in shrimp (Aguilera-Rivera et al., 2018, 2014;). We reported a vibriosis outbreak in a biofloc and clear seawater rearing system. To treat it, oxytetracycline was administered only in clear seawater tanks, but the results were not successful. To understand the mechanisms related to the susceptibility or resistance to Vibrio of L. vannamei juveniles reared in biofloc or clear seawater during a vibriosis outbreak, we isolated and identified Vibrio species, and expression of pathogenicity and oxytetracycline resistance-related genes of Vibrio strains.
Juvenile L. vannamei previously reared in biofloc (13.98 ± 2.94 g) and clear seawater (11.34 ± 1.54 g) presented mortalities (25 and 80%, respectively). Moderate and severe signs of disease were observed in shrimp reared in clear seawater, where a treatment with oxytetracycline was applied in the feed with only occasionally positive results. Shrimp reared in biofloc were observed to have less multifocal melanization in the exoskeleton and redness in appendages. These signs disappeared without the addition of antibiotics to treat the possible infection.
Bacterial isolation and molecular identification of strains were performed. V.
harveyi, V. rotiferianus, Shewanella sp., Photobacterium sp. and P. damselae were identified for both rearing systems. Only Shewanella sp. was identified in shrimp reared under biofloc conditions.
Presence of pathogenicity (toxR) and oxytetracycline resistance (tet(A), tet(B), tet(C), tet(D), tet(E), tet(G)) genes were identified in five strains previously isolated from hepatopancreas of shrimp L. vannamei reared in biofloc and clear seawater system.
Although the occurrence of vibriosis in shrimp reared in biofloc is possible, the microbial dynamics in the system help to control the development of bacterial diseases produced by Vibrio. In comparison to the clear seawater rearing system, the presence of Shewanella sp. only in biofloc could be related to less severe lesions in shrimp. Although oxytetracycline was used for the treatment of vibriosis in the clear seawater system, expression of tet(B), tet(C) and tet(D) genes confirmed the inefficiency of the antibiotic we observed by the presence of disease signs and mortality in shrimp