Genomic Characterization of a Yellow-Colony AHPND-Associated Vibrio parahaemolyticus Carrying a Conserved Chromosome II Sucrose-Utilization Region

Acute hepatopancreatic necrosis disease (AHPND) is associated with Vibrio spp. carrying pVA1-type plasmids encoding PirA/PirB toxins, but culture-based screening may be affected by atypical colony coloration on thiosulfate–citrate–bile salt–sucrose (TCBS) agar. This study characterized a yellow-colony AHPND-associated Vibrio parahaemolyticus strain, designated as 25-018, isolated from cultured Penaeus vannamei in Thailand, using TCBS colony observation, API 20E sucrose testing, species-specific and toxin gene PCR, whole-genome sequencing, antimicrobial susceptibility testing, and shrimp challenge. In TCBS agar culture and API tests, strain 25-018 and a non-AHPND V. alginolyticus comparator formed yellow colonies and were sucrose-positive, whereas AHPND-associated V. parahaemolyticus and V. campbellii comparators formed green colonies and were sucrose-negative. The 5.58 Mb genome of strain 25-018 contained an intact 4027-bp chromosome II scrA-scrK-scrB region sharing 98.14% nucleotide identity with sucrose-positive V. parahaemolyticus, and a 69,082 bp pVA1-related plasmid carrying pirA and pirB. Chromosomal resistance determinants included blaCTX-M-55 and blaCARB-21, and the isolate was resistant to ampicillin and cefepime. Experimental exposure of P. vannamei and P. monodon produced AHPND-consistent hepatopancreatic lesions. These findings show that yellow colony formation can occur in AHPND-associated V. parahaemolyticus and that TCBS colony color alone should not exclude isolates from further molecular evaluation.