Seasonal Trends in Gill Bacterial Flora of Hybrid Tilapia, Oreochromis niloticus × O. aureus

ABSTRACT

Seasonal bacterial flora associated with the gills of hybrid tilapia, Oreochromis niloticus × O. aureus, cultured in earthen ponds were studied quantitatively and qualitatively for the first time in Saudi Arabia at quarterly intervals for one year. The recovered isolates were identified to species level where possible. Total viable counts (TVC) of bacteria in the gills ranged from 9.8±0.9 × 10⁵ to 4.2±3.1× 10⁶ colony forming units (cfu)/g of gills in spring, 3.9±1.7 × 10⁶ to 1.1±2.4 × 10⁷ cfu/g in summer, 6.5±2.3 × 10⁶ to 4.2±2.6 × 10⁷cfu/g in autumn, and 1.9±2.0 × 105 to 1.2±2.9 × 10⁶ cfu/g in winter. In total, 15 bacterial genera were identified from the gills of tilapia and less diversification of bacterial genera (7) was observed in summer. The bacterial population was dominated by Gram-negative rods (77%). In all the seasons, Aeromonas hydrophila, Shewanella putrefaciens, Escherichia coli, and Corynebacterium sp. were the most abundant species with a prevalence of >10%. Aeromonas hydrophila was 26%, significantly higher (P < 0.005) in the population. Pseudomonas fluorescens was present in considerable number only during winter. Pseudomonas sp., Pasteurella sp., and Bacillus sp. were observed in most of the seasons. The variation of ambient seasonal temperature could account for some of the variation in the bacterial population.

KEYWORDS:

• Bacteria
• seasonal variation
• tilapia
• gills
• earthen pond
• Saudi Arabia