High-level multidrug resistance and an unexpected mecA gene detection in Vibrio spp. from Litopenaeus vannamei aquaculture in Vietnam.

The global aquaculture industry, particularly the farming of Pacific white shrimp (Litopenaeus vannamei), faces significant economic challenges due to infectious diseases, with Vibrio spp. being a primary causative agent of vibriosis. The widespread and often indiscriminate use of antibiotics in aquaculture has led to an alarming increase in antimicrobial resistance (AMR) among Vibrio strains, rendering conventional treatments increasingly ineffective. This study aimed to characterize the phenotypic antibiotic resistance profiles and the presence of associated resistance genes in Vibrio spp. isolates obtained from diseased L. vannamei. Our findings reveal a high prevalence of multidrug-resistant (MDR) Vibrio spp., exhibiting resistance to several commonly used antibiotics, including ampicillin, amoxicillin, doxycycline, tetracycline, erythromycin, azithromycin, dalacin (clindamycin), and cefpodoxime. Molecular analysis identified a high frequency of resistance genes such as Carb, SHV, tetA, floR, sulI, sulII, sulIII, and gryA. Notably, despite widespread phenotypic resistance to ampicillin, the Amp, the amp gene was consistently absent, suggesting alternative or intrinsic resistance mechanisms. A unique and highly unusual finding was the detection of a mecA-homologous gene in Vibrio isolate V28, whose genus identity was confirmed by 16S rRNA sequencing. The pervasive nature of AMR in these Vibrio isolates underscores the urgent need for sustainable alternative strategies, such as bacteriophage therapy. This characterization provides critical foundational data essential for the future development of such targeted interventions.