Aqueous extracts or infusions of Ocimum gratissimum showed no activity against the test organisms (Salmonella spp., Shigella sonnei, Shigella schmitzi, Staphylococcus aureus and Escherichia coli). This indicates that the antibacterial principles are not watersoluble. A high phenol content, in this case thymol, of the oil gives a higher antibacterial activity. Saturated aqueous solutions of Ocimum gratissimum oil and of thymol, respectively , showed inhibitory effects on the growth of Salmonella spp., E.coli, Staphylococcus aureus, Shigella sonnei and S.schmitzi. The minimum inhibitory concentration for the oil was 20.0– 27.5% and for thymol 11.0–20.0% depending on the bacteria in question. Thymol was more active than the oil of Ocimum gratissimum, but neither had any appreciable activity against Pseudomonas aeruginosa (MIC > 75%) The aqueous solutions of thymol and Ocimum gratissimum oil were most effective against S.schmitzi ().
The antibacterial effect of Ocimum gratissimum oil is frequently reported. El-Said et al. () found that Ocimum gratissimum oil was active against Escherichia coli, Klebsiella aerogenes, Proteus spp., Salmonella spp., Shigella sonnei, Shigella sonnei, Bacillus subtilis, Sarcina lutea and Staphylococcus aureus. The oil showed no activity against Pseudomonas aeruginosa, an organism which is known to be resistant to many antibacterial agents. Grover and Rao found that the oil of Ocimum gratissimum was active against a number of bacteria. The oil was not active against Klebsiella pneumonias. Ramanoelina et al. () report that the oil of Ocimum gratissimum was tested on eight referied strains of bacteria commonly used for antibiotic measurements and also on twelve other enteropathogenic bacteria strains and showed a large spectrum action. Janssen et al. () determined the maximum inhibitory dilution (MID) of Ocimum gratissimum oil against Escherichia coli and Staphylococcus aureus, and the results were 1:1600 and 1:3200, respectively. According to earlier reports Ocimum gratissimum oil showed no antibacterial activity against Pseudomonas aeruginosa and Klebsiella pneumoniae, but Ndounga and Ouamba showed that the oil had a low inhibitory effect on P.aeruginosa (+) and a quite good inhibitory effect on K.pneumoniae (+ + +). These opposite results are probably due to different techniques used for the testing and different oil compositions. El-Said et al. () used drops of oil on the surface of seeded agar plates, which were incubated and zones of inhibition measured. Grover and Rao used the disc diffusion method with discs clipped in the essential oil and measured inhibition zones. Ndounga and Ouamba also used the disc diffusion method. Janssen et al. () state that a combination of techniques should be used to obtain optimum information of the antibacterial activity. The biogram technique should be applied to investigate which constituent of an oil might be responsible for an observed activity, and the dilution technique to determine the maximum inhibitory dilution-value, which is a more fundamental parameter of antimicrobial activity than the inhibition diameter obtained by the agar overlay technique. The minimum inhibitory concentration of Ocimum gratissimum oil was 312.5 μg/ml for Staphylococcus aureus, Escherichia coli, Salmonella sp., Serratia marcescens, Klebsiella pneumoniae and Proteus vulgaris and 625 μg/ml for Streptococcus faecalis and Pseudomonas aeruginosa ().
The antibacterial effect of Ocimum suave (Ocimum urtidfolium) oil is less investigated. Chogo and Crank report the following rninirnum inhibitory concentrations for Ocimum suave oil: Escherichia coli 900, Micrococcus luteus 700 and Staphylococcus aureus 800 μg/ml. The same values for eugenol (main component of Ocimum suave oil) are 600, 600 and 700 μg/ ml, respectively. The authors also tested Ocimum suave oil from which the eugenol was removed and got inhibitory concentrations > 1000 μg/ml, which according to them indicate that eugenol is the active substance. Janssen et al. () report maximum inhibitory dilutions for Ocimum suave oil against E.coli and S.aureus. For E.coli the values are 1:400 or 1:800 and for S.aureus 1:400, 1:800 or 1:1600 depending on the oil sample. They also present a growth curve of S aureus obtained for dilutions of the essential oil of Ocimum suave, which indicate that mainly the lag times changed and that the growth rates in the logarithmic phase were little influenced by the oil dilutions.
The oil of Ocimum sanctum is surprisingly little investigated with respect to the antibacterial activity considering its extensive use and the research going on in other areas. The essential oil of Ocimum sanctum was active against a number of bacteria, but it showed no activity against Klebsiella pneumoniae. Prasad et al. () found that Ocimum sanctum oil was not active against Salmonella saintpaul, Salmonella spp. and Pseudomonas sp. Compared to the oil of Ocimum gratissimum it seemed to have a higher antibacterial activity. Even 0.2% dilutions of the oil were active against most of the tested bacteria, except for Pseudomonas aeruginosa ().
Ocimum basilicum oil is antibacterial against Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis and Staphylococcus aureus. The highest inhibition diameter, 17.7 mm, was measured with B.subtilis (). Ocimum basilicum oil was mildly active against S.aureus, E.coli, Salmonella sp., Serratia marcescens, Klebsiellapneumoniae and Proteus vulgaris. It showed no activity against Streptococcus faecalis and Pseudomonas aeruginosa. Minimum inhibitory concentrations measured varied from 1250–5000 μg/ml. The antibacterial activity of Ocimum basilicum oil varied according to its origin, i.e. probably due to different composition of the oil. For instance French Ocimum basilicum oil was not active against Staphylococcus aureus while both the Indian and Niazbo Ocimum basilicum oils were active. In general Ocimum basilicum oil seemed to have a higher activity against gram positive than against gram negative bacteria. A chloroform extract of Ocimum basilicum leaves showed antibacterial activity against S.aureus (). A 50% ethanol extract of Ocimum basilicum leaves sho wed antibacterial activity against E.coli, Salmonella enteritidis and Shigella flexneri ().
Essential oils of Ocimum canum and Ocimum trichodon showed antibacterial activity against E.coli, B.subtilis and S.aureus. Maximum inhibitory dilution for Ocimum canum oil was 1:400 for E. coli and 1:800 for S. aureus. Corresponding values for Ocimum trichodon oil were 1:400 and 1:1600, respectively. The essential oil of Ocimum kilimandscharicum was active against a number of bacteria, all of them gram positive. No effect was found against the tested gram negative bacteria.