CARBON-NITROGEN RATIO INTERACTS WITH INITIAL CONCENTRATION OF TOTAL SOLIDS ON INSECTICIDAL CRYSTAL PROTEIN AND SPORE PRODUCTION IN BACILLUS-THURINGIENSIS HD-73

A response-surface methodology was used to study the effect of carbon: nitrogen ratio (C:N) and initial concentration of total solids (CTS) o n insecticidal crystal protein production and final spore count. Bacil lus thuringiensis var. kurstaki HD-73 was grown in a stirred-tank reac tor using soybean meal, glucose, yeast extract, corn steep solids and mineral sails. Soybean meal and glucose were added according to a cent ral composite experimental design to test C:N ratios ranging from 3:1 to 11:1 and C-TS levels from 60 g/l to 150 g/l. Cry production was qua ntified using sodium dodecyl sulfate/polyacrylamide gel electrophoresi s. The response-surface model, adjusted to the data, indicated that me dia with a C:N of 7:1 yielded the highest relative Cry production at e ach CTS The spore count was higher at low C:N ratio (4:1) and high CTS (near 150 g/l). Specific Cry production varied from 0.6 to 2.2 g Cry/ 10(10) spores. A 2.5-fold increase in C-rs resulted in a six-fold incr ease of protoxin production at a 7:1 C:N ratio. It is concluded that t he best production conditions for Cry and for spores are different and optimization of B. thuringiensis processes should not be done on a sp ore-count basis but on the amount of Cry synthesized.