The study was conducted to investigate the variation of density and composition of bacteria in mud in My Thanh river including the upper part (Nhu Gia), middle part (My Thanh 1), and lower part (My Thanh 2). Mud samples were collected once a month from July 2017 to June 2018 at low tide. Bacteria density was determined by plate counting. The results indicated that the total bacterial population was at its peak in Nhu Gia (5,3×104 CFU/g), and at the lowest in My Thanh 2 (5,3×104 CFU/g), and it tended to decrease from January to June. Bacillus and Lactobacillus density was high in Nhu Gia and low in My Thanh 2. Bacillus density was stable throughout the months of the year at all 3 sampling sites, but the density decreased when salinity increased. In My Thanh 2, Bacillus density was proved significantly lower than in Nhu Gia; specifically, in May (8.3 × 103 CFU/g) in My Thanh 2 versus (1,9 × 106 CFU/g) in Nhu Gia. Lactobacillus had the lowest density (3.4 × 102 CFU/g) in June and the highest (2.7 × 105 CFU/mL) in February. Besides, the total Vibrio density tended to increase with the rise of salinity during the year and exceeded 103CFU/g in My Thanh 2. The density of V. harveyi, V. parahaemolyticus in My Thanh 2 and My Thanh 1 was higher than Nhu Gia’s. The higher the salinity, the greater the concentration of Vibrio, V. harvey and V. parahaemolyticus and it may lead to risk for animals.
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