Species Diversity, Antimicrobial Susceptibility, and Provisional Epidemiological Cut-Off Values of Vibrio spp. Isolated From Pacific White Shrimp (Litopenaeus vannamei) Farms in Southern Thailand.

Abstract

Vibrio spp. are the most common opportunistic pathogens in shrimp farms, and drug-resistant Vibrio spp. pose a major threat to both shrimp and human health. The current study investigated species diversity and antimicrobial susceptibility of Vibrio spp. from Pacific white shrimp (Litopenaeus vannamei) in Southern Thailand. Shrimp were collected from ten intensive farms with a history of disease outbreaks. Vibrio spp. were isolated from the hepatopancreas, identified to species level via 16S rRNA sequencing, and tested for minimum inhibitory concentrations (MICs) against eight antibiotics. A total of 27 Vibrio species were recovered. V. parahaemolyticus was the most frequently recovered species among the selected isolates, followed by V. alginolyticus and V. harveyi. Overall, the isolates were susceptible to oxytetracycline, doxycycline, ciprofloxacin, enrofloxacin, and florfenicol, with MIC₅₀ ranging from ≤ 0.25 to 2 μg/mL. Conversely, most Vibrio spp. were resistant to amoxicillin, ceftriaxone, and sulfadimethoxine, with MIC₅₀ values of at least 128 μg/mL. To our knowledge, this study provides the first evidence of high-level ceftriaxone resistance in Vibrio spp. from farmed shrimp. To distinguish fully susceptible bacteria (wild type) from those that develop acquired resistance, epidemiological cut-off (CO_WT) was determined. The CO_WT for florfenicol was calculated as ≤ 4 μg/mL; when this threshold was applied to the Vibrio spp. isolates, 83.10% were classified as wild type. Our findings highlight the need for strict regulation of antibiotic use in shrimp aquaculture to minimize the emergence and spread of antibiotic-resistant bacteria.