Pathogenic characteristics of Aeromonas veronii isolated from the liver of a diseased guppy (Poecilia reticulata).

Despite the significant development on their diagnoses and control, aeromonad infection is still a problem in aquaculture. This study described the key bacteriological and pathogenic features of a presumptive Aeromonas sp. isolated from the liver of a diseased guppy (Poecilia reticulata). Molecular identification revealed that the isolate was an Aeromonas veronii (A. veronii PR). It was able to grow in a wide range of temperatures and salt concentrations, and was capable of auto-aggregation and biofilm formation, with temperature as an influencing factor. Some of the extracellular enzymes that may be involved in its virulence include caseinase, gelatinase and lipase. The infection rate was relatively progressive, and fish with prior infection showed marginal resistance to secondary infection. Handling stress differentially influenced the infection kinetics at the early stages; however, the final mortality rates did not significantly differ between the groups. A comparative infection trial revealed that zebrafish (Danio rerio) were more susceptible to A. veronii PR than guppy. The presented intrinsic and extrinsic factors influencing the pathogenicity of A. veronii PR lay the foundation for future research to better understand this pathogen in freshwater ornamental fish aquaculture. SIGNIFICANCE AND IMPACT OF THE STUDY: Aeromonad infections continue to affect the fish farming industry. Several new species of Aeromonads in freshwater ornamental fish have been identified in the last years. In this study, we have characterized an Aeromonas veronii isolate from a diseased guppy. The series of experiments identified the intrinsic and extrinsic factors contributing to the pathogenic characteristics of the isolate. It has been shown to be pathogenic to both guppy and zebrafish. The results offer foundational knowledge in the development of preventive and therapeutic measures to combat this pathogen in the ornamental fish industry.

Short-term feed and light deprivation reduces voluntary activity but improves swimming performance in rainbow trout Oncorhynchus mykiss.

Rainbow trout Oncorhynchus mykiss (~ 180 g, 16 °C and < 5 kg m-3) that were feed deprived and kept in total darkness showed a significant increase in critical swimming speed (U crit) between 1 and 12 days of deprivation (from 3.35 to 4.46 body length (BL) s-1) with no increase in maximum metabolic rate (MMR). They also showed a significant decrease in the estimated metabolic rate at 0 BL s-1 over 12 days which leads to a higher factorial aerobic metabolic scope at day 12 (9.38) compared to day 1 (6.54). Routine metabolic rates were also measured in ~ 90 g rainbow trout that were swimming freely in large circular respirometers at 16 °C. These showed decreasing consumption oxygen rates and reductions in the amount of oxygen consumed above standard metabolic rate (a proxy for spontaneous activity) over 12 days, though this happened significantly faster when they were kept in total darkness when compared to a 12:12-h light-dark (LD) photoperiod. Weight loss during this period was also significantly reduced in total darkness (3.33% compared to 4.98% total body weight over 12 days). Immunological assays did not reveal any consistent up- or downregulation of antipathogenic and antioxidant enzymes in the serum or skin mucus of rainbow trout between 1 and 12 days of feed and light deprivation. Overall, short periods of deprivation do not appear to significantly affect the performance of rainbow trout which appear to employ a behavioural energy-sparing strategy, albeit more so in darkness than under a 12:12-h LD regime.

Bacterial viability differentially influences the immunomodulatory capabilities of potential host-derived probiotics in the intestinal epithelial cells of Atlantic cod Gadus morhua.

AIM: This study explored the effect of heat inactivation on the immunomodulatory capabilities of two potential host-derived probiotics (GP21 and GP12) on the intestinal epithelial cells (IEPC) derived from Atlantic cod. METHODS AND RESULTS: The cells were isolated from the four segments of the gut, namely anterior intestine (AI), mid-intestine (MI), posterior intestine (PI) and rectum (RC). The IEPC cultures were exposed to live or heat-inactivated form of GP21 and GP12 for 24 h. The expression profiles of bacterial defence genes and cytokine genes in the probiotics-exposed IEPCs showed differential patterns. Heat inactivation did not drastically affect the immunomodulatory properties of the probiotics, and this was explicitly typified by the stimulated expression of g-type lysozyme, hepcidin, transferrin and metallothionein in both forms of the bacteria. There was no distinct expression pattern of the interleukin genes during bacterial exposure. This was in contrast to the chemokines where the expression of these genes in IEPCs was down-regulated upon exposure to the heat-inactivated probiotics. Although heat inactivation did not drastically affect the immunomodulatory capabilities of the probiotics, the live form elicited higher immune responses in the IEPCs in most cases. CONCLUSION: This study showed that bacterial viability was a contributing influence, but not a major limiting factor on the immune-related functions of the host-derived probiotics in vitro. SIGNIFICANCE AND IMPACT OF THE STUDY: GP21 and GP12 are beneficial host-derived bacteria and could be utilized as candidate probiotics in cod aquaculture.