Effects of Ocimum gratissimum L essential oil at subinhibitory concentrations on virulent and multidrug-resistant Shigella strains from Lagos, Nigeria.

Ocimum gratissimum leaf extracts have been extensively demonstrated to be effective against the various aetiologic agents of diarrhoea, including Shigellae. However, the mechanism of the shigellocidal action of this plant remains to be understood. This study investigated the effects of O. gratissimum essential oil (EO) at subinhibitory concentrations of 0.75 and 1.0 microg/ml on virulence and multidrug-resistant strains of 22 Shigella isolates from Nigeria. Compared with untreated Shigella strains, O. gratissimum EO caused significant decreases (p<0.01) in extracellular protease activity, o-lipopolysaccharide rhamnose content and incidence of invasiveness mediated as keratoconjunctivitis in guinea pig. The disparity in extracellular protease activity and o-lipopolysacharide rhamnose between the two treatment groups was also found to be significant (p<0.05), suggesting greater anti-virulent effects of O. gratissimum oil at 1.0 microg/ml. Antibiotic susceptibility testing revealed that the EO of O. gratissimum reduced the MICs of antibiotics to which Shigellae showed resistance by 9.8-53.1% and fluoroquinolones by 18.2-45.5%. The results of this study strongly suggest inhibition of extracellular protease and expression of O-LPS rhamnose in Shigellae by O. gratissimum EO. The future use of O. gratissimum- antibiotic combinations as a therapeutic measure against shigellosis is discussed.