Channel catfish is one of the most cost-effective farm-raised food sources, especially in the southern part of the United States. Edwardsiella ictaluri is a primary challenge facing catfish farmers which cause enteric septicemia of catfish (ESC). Trans-cinnamaldehyde (TC), a key compound found in cinnamon, has gained significant attention for its antimicrobial activity against various bacteria. Our long-term goal is to support the US aquaculture industry by identifying potential interventional strategies to reduce catfish mortality associated with bacterial infections.
To investigate the effects of continuous exposure to TC on the virulent E. ictaluri 93–146 strain, we passaged the bacteria in brain heart infusion (BHI) broth containing a permissive TC concentration of 0.016 µl/ml. E. ictaluri was inoculated into BHI broth with TC and incubated overnight at 30ºC, after which samples were transferred to fresh BHI broth with TC daily for 30 days. A control strain was passaged in BHI without TC. The resulting strain after 30 passages in TC was designated D30-TC, while the control strain was labeled D30-BHI. We next explored the pathogenic potential of E. ictaluri D30-TC and D30-BHI strains in catfish fingerlings using an immersion challenge model. Catfish fingerlings were placed in 40 L tanks and divided into four groups: one infected with E. ictaluri 93-146, one sham-vaccinated with BHI, one infected with the D30-TC strain, and one with the D30-BHI strain. Fish were immersed in water containing 107 CFU/ml for 1 hour, and mortality was recorded daily. Notably, no mortality occurred in the D30-TC group, while 88% and 84.37% mortalities were observed in the wild-type and D30-BHI groups, respectively. The D30-TC vaccinated fish demonstrated a survival rate of 63.64%, compared to 20% in the sham-vaccinated group after 21 days.
To investigate the attenuated virulence of the E. ictaluri D30-TC strain, we analyzed proteomic differences between the D30-TC and parental strains. The proteomic analysis identified 124 upregulated and 141 downregulated proteins (p < 0.05, |fold change| > 2) in the D30-TC strain compared to the wild-type strain. Notably, there was downregulation of proteins associated with the Type III secretion system (T3SS) and Type VI secretion system (T6SS), which are crucial for E. ictaluri virulence (Table 1). This downregulation likely contributes to the decreased pathogenicity observed in the D30-TC strain.