Mechanism of antibacterial action of n-hexane and ethyl acetate soluble fractions of Psidium guajava']java leaves against Salmonella enterica serovar Typhi

As a popular plant used traditionally in the treatment of typhoid fever, leaf extracts of Psidium guajava reportedly inhibit typhoidal salmonella in vitro and in vivo. This study explored the antibacterial mechanism involved in the bactericidal activity of n-hexane and ethyl acetate soluble fractions of P. guajava leaves against S. Typhi. S. Typhi was cultured in Mueller Hinton Broth in the presence and absence of the extracts. The time course of bactericidal action was measured by kill-curve analysis. The antibacterial mechanism was elucidated by analyzing the effect of divalent cations (Ca2+ and Mg2+) on the minimum inhibitory concentrations of the fractions against S. Typhi. Cellular leakage was monitored by measuring extracellular levels of protein, 260 nm-absorbing materials, pyruvic acid, potassium ions and inorganic phosphate in treated bacteria cells. DNA damage was measured using single-cell gel electrophoresis. The extracts significantly (P < 0.05) increased the extracellular concentration of protein, 260 nm-absorbing materials, pyruvic acid, potassium ions and inorganic phosphate in bacterial cultures. Scanning electron micrographs showed that the bacterial cells were destroyed. There was also considerable DNA damage. It can be concluded that n-hexane and ethyl acetate soluble fractions of P. guajava exert their bactericidal actions against S. Typhi by distorting the membrane permeability leading to leakages of vital biomolecules as well as morphological and DNA damages.