Objective   Physiological functions of the Sec pathway subunit secD of Aeromonas hydrophila were studied.

  Method  An secD-knockout A.hydrophila was constructed by homologous recombination with pRE112 suicide vector. Effects of the genetic deletion on the physiological phenotypes including hemolytic and extracellular protease activities were determined on sheep’s blood and skim milk agar plates; on the biofilm formation, by crystal violet staining combined with SpectraMax® i3; on the bacterial acid-base and hyperosmolarity tolerance, according to the growth curve; and on the minimal inhibitory concentration (MIC) of antibiotics, using the broth microdilution method.

  Result   Compared to wild-type, the constructed ∆secD A.hydrophila showed no significant differences in growth and tolerance to pH 5, 6, 7, and 8. The activities of hemolytic and extracellular protease, as well as the resistance to high osmotic pressure or pH 9 were enhanced. The biofilm forming ability significantly declined. And the resistance to antibiotics varied, as the MIC increased 4 times on oxytetracycline, tetracycline, enoxacin, or meropenem and two times on ciprofloxacin and norfloxacin, but decreased 2 times on erythromycin and 4 times on cefotaxime.

  Conclusion  The Sec pathway subunit secD was confirmed to be involved in the transport of virulence factors and resistance to antibiotics in A.hydrophila. It could be a key element in studies for developing antimicrobial applications and prevention of disease infected by the pathogen.