Antibacterial activity of watermelon (Citrullus lanatus) seed against selected microorganisms

This study was aimed at evaluating the effect of extraction methods on the antibacterial activity of Citrullus lanatus seed extract. C. lanatus (watermelon) is a popular fruit consumed all over the world. Three solvents were used for the extraction process: chloroform, methanol and distilled water while two extraction conditions- cold extraction and Soxhlet extraction (coded as hot in this study) were employed. Antibacterial activity of the seed extracts was determined by agar well diffusion method. The seed extracts were tested against clinical isolates including Staphylococcus sp., Escherichia coli, Proteus sp., Klebsiella sp. and a type Pseudomonas aeruginosa (ATCC 27853). It was observed that the cold methanol extracts had the highest antibacterial effect on Staphylococcus sp. followed by hot methanol extract while cold chloroform extract showed no antibacterial activity. In the presence of P. aeruginosa only the hot methanol and chloroform extracts showed significant antibacterial potentials (p≤0.05). Also, saponins which have been implicated in antimicrobial activity were found to be present in moderate and high concentrations in the hot and cold methanol extracts respectively. Results of this study reveal that the kind of solvent employed as well as the conditions for extraction (cold maceration and Soxhlet extraction) influenced the efficacy of the extract against specific test organisms. Furthermore, the presence of saponins may have influence the relatively high zone of inhibition recorded with cold and hot methanol extracts against some of the test organisms.    Key words: Watermelon seed, antibacterial, Soxhlet extraction, cold maceration, solvents.