Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 14, 2008
Publication Date: March 15, 2008
Citation: Martin, P.A.W., Mongeon, E.A., Gundersen-Rindal, D.E. 2008. Microbial combinatorics: a simplified approach to isolating insecticidal bacteria. Biocont. Sci.Technol. 18:291-305.
Interpretive Summary: Using bacteria specifically pathogenic to insects to control pests is a good alternative to chemical control. However, it is a time-consuming and expensive process to find the appropriate bacteria to use. We developed an easier and more effective way to identify bacteria that can be used to attack and control insects. The process directly tests for toxicity of the bacteria to the insect and allows simultaneous screening of multiple types of bacteria. This procedure can even be used to screen soils for useful chemicals produced by bacteria. The procedure would be useful to scientists who are trying to identify and use bacteria to control insects.
Bacteria, such as Bacillus thuringiensis Berliner, can successfully control pest insects that damage food crops, vector diseases, and defoliate trees. Isolation of these bacteria has been conventionally from soil and sporadically from dead insects. These observations were combined to isolate insecticidal bacteria from soil using the insect as the selective agent. Instead of the time-consuming process of isolating single strains of bacteria from soil and then testing each strain for insect toxicity, the process was reversed. The bacteria present in each soil sample were grown together on solid laboratory media, and then tested as mixtures for the ability to kill insects. Manduca sexta (Linnaeus) (Lepidoptera: Sphingidae), tobacco hornworm larvae was used as a model insect. Bacterial mixtures from 17 of 20 soils tested killed 100% of the larvae within 96 h. Unexpectedly, the bacteria isolated from dead larvae were indistinguishable by colony type. When these bacteria were grown in pure culture and re-tested for toxicity to hornworm larvae, 19 of 20 cultures killed M. sexta larvae and 15 of 19 bacterial strains isolated from these dead larvae were identical by biochemical tests and antibiotic sensitivities to the bacteria fed these larvae. Thus, Koch’s postulates confirmed that these bacteria were responsible for the death of the insects. This approach, combinatorial microbiology, can save thousands of insects and associated time and resources over methods which first isolate bacteria and then test for toxicity.