Microbiological and chemical analysis of neem (Azadirachta indica A. Juss) extracts: new data on antimicrobial activity

The antimicrobial effects of extracts of neem seed (Azadirachta indica A. Juss.) were investigated using microbial growth inhibition assays. A laboratory-prepared neem. seed extract along with a commercially available formulated product, were characterized using HPLC, and shown to be effective against a range of bacteria in an agar diffusion assay. The active ingredient, i.e., the unformulated seed extract of the commercial product, also showed activity and this was further investigated in a biochromatogram, using the sensitive bacterium Bacillus mycoides. Results showed antibacterial activity as three discrete inhibition zones that did not correspond to the R-f of the major neem. metabolites, azadirachtin, nimbin and salannin. This suggests that these compounds were not antibacterial. The colony radial growth rates of the fungal pathogens that cause 'take-all' and 'snow mould' disease were both significantly affected when the commercial, unformulated, neem seed extract was incorporated into the growth medium. Experiments in liquid culture suggested that the effect was fungistatic. Conidial germination of the commercially important obligate pathogen Sphaerotheca fuliginea (powdery mildew) was reduced to 11%. The results show that neem. seed extracts possess antimicrobial activity with notable effects on some fungal phytopathogens. This work demonstrates that neem seed extracts have potential for controlling both microbial and insect pests.