Screening of blue crab (Portunus pelagicus L.) eyestalk extracts for growth inhibition of Pectobacterium carotovorum subsp. carotovorum (Hauber et al. 1998) (Erwinia carotovora) Jones (Holland) and Ralstonia solanacearum (Yabuuchi et al. 1995) (Pseudomona

Abstract

Crustacean eyestalks are known to contain neurosecretory hormones that inhibit molting, growth and gonadal development. Along this framework, this study was conducted to: screen crude eyestalk extracts from the blue crab Portunus pelagicus for possible growth inhibition on two bacterial pathogens Ralstonia solanacearum and Pectobacterium carotovorum subsp. carotovorum, and determine effective concentrations of the crude extract against the pathogens. The screening was done through bioassay using the top agar inoculation technique with 70% ethanol as the extracting solvent and streptomycin as the positive control. Different concentrations of blue crab extract and streptomycin were obtained using serial dilution.

Results showed inhibitory effects of the blue crab eyestalk extracts against the two pathogens R. Solanacearum and P. carotovorum at bacterial populations ranging from 4.63 x 10⁶ – 5.70 x 10⁶ for the former and 5.20 x 10⁶ – 5.95 x 10⁶ for the latter. However, inhibitory effects of the eyestalk extracts were apparently lower than that of streptomycin and seem to be comparable to growth inhibition at lower concentrations of streptomycin. Moreover, the mixed molt and reproductive stages of the eyestalk source crabs may have diluted the concentrations of eyestalk hormones. Among two pathogens, P. carotovorum appeared more responsive to the different concentrations of eyestalk extracts. It showed greater inhibition by the extract at the maximum concentration of 44.5 mg mL^-1 and had a lower minimum inhibitory concentration (MIC) of 1.39 mg mL^-1.

Results further suggest that blue crab eyestalk extracts can be used as a possible control agent against the growth of bacterial pathogens P. carotovorum and R. solanacearum. However, further verification of the study should be done to determine eyestalk extract effect at higher pathogen populations.