Alkaline peptone water

A total of 120 raw seafood samples of five different food types (fish, n = 35; shellfish, n = 25; shrimp, n = 20; squid, n = 20; and crab, n = 20). All the samples were selected from markets by convenience sampling; they were subsequently stored in sterile bags on ice and were transferred to the laboratory within 2 h. Sample processing was conducted according to the ISO 21872-1: 2017 (Microbiology of the food chain – Horizontal Method for the determination of Vibrio spp.) protocol for the determination of Vibrio spp., with some modifications [26 ]. Briefly, 25 g of individual samples was pre-enriched in alkaline peptone water (Merck, Germany) at 37°C for 6 h. Then, the first enriched cultures were further inoculated into alkaline saline peptone water and were incubated at 37°C for 18 h. Subsequently, the overnight cultures were spread onto selective agar plates, thiosulfate citrate bile salts sucrose, and CHROMagar™ Vibrio (CHROMagar, Paris, France) for Vibrio spp., also the selective media for Aeromonas species (HiMedia, India) and the plates were incubated at 37°C for 18–24 h. The suspected colonies were subjected to conventional biochemical assays for the Vibrio and Aeromonas genera.

Twenty-five g of the collected samples, including fish tissues and organs from both tilapia and mugil and human skin swabs and fecal samples were enriched with alkaline peptone water (Merck, Germany) and incubated under shaking conditions at 37 °C for 24 h. After overnight incubation, serial dilutions were made, then 100 µL aliquots from these dilutions were streaked onto selective Aeromonas agar media (Himedia, Egypt), and the plates were incubated at 37 °C for 24 h under aerobic conditions. The colonies were then examined under a light microscope for a Gram stain and morphological characterization [22 ].
The obtained colonies were tested for catalase and oxidase activity, then characterized biochemically in the usual manner by loading 100 µL of the microbial suspension aseptically onto the analytical API 20 E kits (bioMerieux, France) according to the manufacturer’s instructions.