Nine plant essential oils (EOs) were studied for their anti-bacterial effect on spores and vegetative cells of Clostridium botulinum 62A and Bacillus cereus T. Cedar oil at the concentration of 300 ppm was the most active against B. cereus T and C. botulinum spores and vegetative cells. Spores were generally less resistant than vegetative cells, except for the essential oils of eucalyptus and camomile (300 ppm), which demonstrated a significant activity against B. cereus T and C., botulinum vegetative cells. In general, spores of Bacillus were more sensitive to these essential oils than Clostridium spores. For B. cereus T, EO of rosemary (greater than or equal to 170 ppm), eucalyptus (greater than or equal to 450 ppm), orange (greater than or equal to 300 ppm), savage carrots (greater than or equal to 400 ppm) and artemisia (greater than or equal to 500 ppm) were partially sporicidal; whereas, EO of camomile and grapefruit (greater than or equal to 300 ppm) were sporostatic. In the case of C. botulinum 62A, eucalyptus, camomile, savage carrots, vervain, grapefruit and orange oils were sporostatic at the tested concentrations. However, EO of cedar (greater than or equal to 300 ppm) was partially sporicidal. These EOs inhibited specifically one or more stage(s) of the spore cycle depending on the EO and its concentration. Eucalyptus, orange, artemisia and cedar oils (300 ppm) and grapefruit, savage carrots and vervain (400 ppm) inhibited the growth of C. botulinum 62A spores by blocking their germination in L-alanine. The inhibition of spore germination of C. botulinum 62A by EOs (at greater than or equal to 300 ppm) was by inhibition of commitment-to-germinate of spores. However, the outgrowth was not sensitive to cedar, eucalyptus, vervain and savage carrots oils at the concentration of 400 ppm. (C) 1997 Academic Press Limited.
