THE INHIBITORY EFFECT OF AZADIRACHTIN ON BACILLUS-SUBTILIS, ESCHERICHIA-COLI, AND 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 beekeepi ng industry worldwide. The disease has been under control for 4 decade s, a result achieved largely by using a single antibiotic-oxytetracycl ine hydrochloride. Recent reports of P. larvae resistance to the antib iotic have prompted researchers to test other materials that exhibit a ntibiotic properties and to assess their possible use in foulbrood dis ease control. The present study was conducted to determine if azadirac htin, the active component of the extract of the neem tree (Azadiracht a indica), and formulated azadirachtin had inhibitory effects on cultu res of P. larvae. Grampositive Bacillus subtilis and gram-negative Esc herichia coli were also tested for comparison. The experimental result s showed that formulated azadirachtin completely inhibited vegetative growth of P. larvae at a concentration of 5 mu g/ml and reduced the gr owth of B. subtilis and E. coli at 5 and 25 mu g/ml in TMYGP agar. At 50 mu g/ml, vegetative growth of E. coli was completely inhibited and B. subtilis significantly inhibited. Nonformulated azadirachtin affect ed E. call most, with 33% inhibition of growth at 0.05 mu g/ml and com plete inhibition at 50 mu g/ml. B. subtilis and P. larvae were less su sceptible, with B. subtilis barely surviving 50 mu g/ml, while P. larv ae demonstrated about 33% of growth at the same concentration. A linea r dose response was observed between concentrations of azadirachtin fr om 0.1 to 5.0 mu g/ml and spore germination of P. larvae, with a compl ete inhibition of spore germination at 0.60 mu g/ml. The difference in response of P. larvae to nonformulated and formulated azadirachtin su ggests that proprietary ''inert'' ingredients in formulated azadiracht in significantly inhibit vegetative growth and spore germination in th e bacterium. Research on the effect of azadirachtin on the honeybees i s needed to evaluate the feasibility of using the compound for control of AFB. (C) 1998 Academic Press.