In vitro inhibitory effect of gossypol from gossypol-acetic acid, and (+)- and (-)-isomers of gossypol on the growth of Edwardsiella ictaluri.

Abstract

This study was conducted to evaluate the toxic effect of gossypol from gossypol-acetic acid, and (+)- and (-)-isomers of gossypol on the growth of Edwardsiella ictaluri. Inhibitory effect of various concentrations of gossypol on the growth of E. ictaluri was determined. Bacterial recovery was performed by preincubation of bacteria in medium containing various concentrations of gossypol and subsequent activation of bacteria by inoculating on gossypol-free plates. Concentrations of racemic gossypol, (+)-gossypol and (-)-gossypol of 1.5 microg ml(-1) or higher significantly reduced the number of bacterial colonies compared with that of the control. The growth of E. ictaluri was completely inhibited on agar plates supplemented with 3 microg ml(-1), regardless of the forms of gossypol. The inhibitory effect of (+)-gossypol was higher than that of (-)-gossypol or gossypol-acetic acid. Recovery of E. ictaluri was <50% for all three forms of gossypol at concentrations of 5 microg ml(-1). Bacterial recovery remained relatively constant (6.5%) at gossypol concentrations from 10 to 100 microg ml(-1). Complete killing of E. ictaluri was not reached at gossypol levels up to 100 microg ml(-1). Gossypol-acetic acid, and (+)- and (-)-optical isomers have anti-bacterial effect against E. ictaluri. The results suggest the action is bacteriostatic rather than bactericidal. The therapeutic effect of gossypol against E. ictaluri may be useful in controlling enteric septicaemia of catfish.