Bacterial load estimation of Aeromonas hydrophila in experimentally infected Indian catfish, Clarias magur and its correlation with disease progression
摘要
Motile Aeromonas septicaemia (MAS) disease caused by Aeromonas hydrophila, represents a significant bacterial pathogen that severely impacts the production of Clarias magur. A species-specific SYBR green I-based real-time quantitative PCR (qPCR) assay targeting the AscU gene of the type-III secretion system was developed and optimized for A. hydrophila. The qPCR demonstrated an efficiency of 97.01%, sensitivity of 10 copies/μL and repeatability. To comprehend the pathogenesis, magur were intraperitoneally injected with A. hydrophila with LD₅₀ dose of 1 × 105 CFU/fish, and bacterial loads were quantified from 1 to 144 hours post-infection (hpi). An exponential increase in bacterial load was observed from 1 to 48 hpi, followed by a marked decline at 72 hpi, reaching the lowest levels at 144 hpi. At 1 hpi, bacterial load increased by 10-fold in the internal organs indicating rapid colonization, replication and dissemination leading to systemic infection. The highest bacterial load was documented in the intestine, followed by the kidney, spleen and liver. Likewise, the gill harboured the highest bacterial load, followed by the muscle, fin, skin, and eye. Correlation of bacterial load in organs with disease progression revealed that the pathogenesis of A. hydrophila occurs in distinct stages: an incubation period between 0 and 3 hpi, a prodromal phase from 3 to 8 hpi, an illness phase between 8 and 72 hpi, a decline phase from 72 to 144 hpi, and a convalescence period beyond 144 hpi. The established method offers a reliable tool for the quantification, diagnosis, and epidemiological monitoring of A. hydrophila infection in clinical samples.