The inhibitory effect of azadirachtin on Bacillus subtilis, Escherichia coli, an Paenibacillus larvae, the causative agent of American foulbrood in the honeybee, Apis mellifera L

American foulbrood disease (AFB) of honeybees, caused by the bacterium Paenibacillus larvae, continues to be a major concern to the beekeeping industry worldwide. The disease has been under control for 4 decades, a result achieved largely by using a single antibiotic-oxytetracycline hydrochloride. Recent reports of P. larvae resistance to the antibiotic have prompted researchers to test other materials that exhibit antibiotic properties and to assess their possible use in foulbrood disease control. The present study was conducted to determine if azadirachtin, the active component of the extract of the neem tree (Azadirachta indica), and formulated azadirachtin had inhibitory effects on cultures of P. larvae. Gram-positive Bacillus subtilis and gram-negative Escherichia coli were also tested for comparison. The experimental results showed that formulated azadirachtin completely inhibited vegetative growth of P larvae at a concentration of 5 microgram/ml and reduced the growth of B. subtilis and E. coli at 5 and 25 microgram/ml in TMYGP agar. At 50 microgram/ml vegetative growth of E. coli was completely inhibited and B. subtilis significantly inhibited. Nonformulated azadirachtin affected E. coli most, with 33% inhibition of growth at 0.05 microgram/ml and complete inhibition at 50 microgram/ml. B. subtilis and P larvae were less susceptible, with B. subtilis barely surviving 50 microgram/ml, while P larvae demonstrated about 33% of growth at the same concentration. A linear dose response was observed between concentrations of azadirachtin from 0.1 to 5.0 microgram/ml and spore germination of P. larvae, with a complete inhibition of spore germination at 0.60 microgram/ml. The difference in response of P. larvae to nonformulated and formulated azadirachtin suggests that proprietary "inert" ingredients in formulated azadirachtin significantly inhibit vegetative growth and spore germination in the bacterium. Research on the effect of azadirachtin on the honeybees is needed to evaluate the feasibility of using the compound for control of AFB.