Abstract

In vitro antibacterial time kill studies of extracts of Helichrysum longifolium was assessed using twenty-three bacteria species made up of eleven Gram positive and twelve Gram negative strains. All test bacteria were susceptible to the methanol extract, while none was susceptible to the aqueous extract. Two of the test bacteria were susceptible to the ethyl acetate extract, while ten and seven were susceptible to the acetone and chloroform extracts respectively at the test concentration of 5 mg/ml. The minimum inhibitory concentrations (MICs) ranged between 0.5 and 5.0 mg/ml, for acetone and methanol extracts; 0.1-5.0 mg/ml for chloroform extract and 5.0 mg/ml for the ethyl acetate extract; while minimum bactericidal concentrations (MBCs) ranged between 1.0  and >5 mg/ml for all the extracts. Average log reductions in viable cell counts for all the extracts ranged between 0.1 Log₁₀ and 7.5 Log₁₀ cfu/ml after 12 h interaction at 1 × MIC and 2 × MIC. Most of the extracts were rapidly bactericidal at 2 × MIC achieving a complete elimination of most of the test organisms within 12 h exposure time.

Key words: Helichrysum longifolium, crude extract, MIC, time-kill.