EVALUATION OF PROTECTION POTENTIAL OF AN ANTAGONISTIC ISOLATE OF Pseudomonas mosselii AGAINST INFECTION OF VIRULENT Aeromonas hydrophila IN CHANNEL CATFISH Ictalurus punctatus

Motile Aeromonas septicemia (MAS), caused by virulent Aeromonas hydrophila (vAh), has severely impacted catfish farming in the southeastern United States since 2009. To date, conditions that trigger the onset of MAS outbreaks remain largely unknown and control of MAS has been a challenge issue. According to the observation of fish specialists, fish mortalities occurred in a pond from onset (≤10 fish) to outbreak (˃20,000 fish) in just a few days. Consistent with this observation, laboratory experiments found that fish mortality stated at 6-8 h and peaked with 24-48 h post challenge with vAh. The rapid development of the disease makes the use of antibiotic therapy and other intervention strategies problematic.

In this study, we investigated the effect of an isolate of Pseudomonas mosselii (Pm) on vAh infection. Pm was originally isolated from the gill of  a  channel catfish that survived a lethal challenge of vAh. Assays in vitro showed that Pm was inhibitory to growth of vAh on LB agar plate (Fig. 1 left panel). The cell lysate of Pm produced inhibitory zone on vAh cell lawn (Fig. 1, right panel). Co-inoculation vAh and Pm  in water had no significant effects on vAh growth and infection; however, Pm protected fish against vAh infection when fish were pre-immersed in Pm-propagated water for 15 min prior to challenge (Fig. 2). The relative percentage of survival of fish was approximately 79%, compared with the control. The mechanism of protection could be due to pre-occupation of infection sites by P m and antagonistic to the establishment of infection by vAh.

Whether the isolate of P. mosselii can be used as a probiotic agent for control MAS is under investigation.