• Mayowa Agunbiade Biocatalysis & Technical Biology Research Group, Cape Peninsula University of Technology


Investigations were carried out on cidal effects of crude extract and fractions obtained from the stem bark of Psidium guajava on a panel of seventy-four strains of Escherichia coli and Shigella dysenteriae implicated in diarrhoea and dysentery infections.

A known weight of the powdered sample of the stem bark of the test plant was subjected to cold extraction using methanol and sterile distilled water in ratio 3:2 (v/v). The mixture was left on the laboratory bench for four days with regular agitation after which it was filtered into a clean sterile flask. The supernatant obtained from the solution was concentrated in vacuo using a rotatory evaporator. The aqueous part obtained was then lyophilized to obtain a crude extract of the sample. Fractions were later obtained from the crude extract using different organic solvents in their order of polarity. The crude extract and the fractions were subjected to antibacterial tests against a panel of bacterial strains using the agar well diffusion method. The minimum inhibitory concentrations, minimum bactericidal concentrations and phytochemical analysis of the extracts were determined. In addition, the mode of action of the active fractions which include killing rate, protein and potassium ions leakages were also investigated.

The crude extracts at a concentration of 25 mg/mL exhibited antibacterial activity against all bacterial strains used for this study. On the other hand, all the fractions except the aqueous fraction exhibited antibacterial activities against the panel of bacterial strains at a final concentration of 10 mg/mL. The MIC exhibited by the crude extract against susceptible bacterial strains ranged between 1.56 mg/mL and 12.5 mg/mL. On the other hand, those for the four fractions ranged between 0.31 mg/mL and 5.0 mg/mL. The minimum bactericidal concentrations ranged between 3.13 mg/mL to 12.5 mg/mL and 0.63 mg/mL to 5.0 mg/mL for the crude extract and fractions respectively. The time kill assay revealed that the percentage of the cells killed increase with an increase in the concentrations of the fractions as well as contact time intervals. Finally, the protein and potassium ions leakages from the bacterial cells followed the same trend with that of time-kill assay.

In conclusion, the stem bark extracts of Psidium guajava exhibited appreciable bactericidal effects on bacterial strains associated with diarrhoea and dysentery in humans.

Keywords: Psidium guajava, Antibacterial, Phytochemicals, Killing rate, Protein leakage, Potassium ions leakage.


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