Characterization of an avirulent pleiotropic mutant of the insect pathogen Bacillus thuringiensis: reduced expression of flagellin and phospholipases.

An avirulent pleiotropic mutant of the insect pathogen Bacillus thuringiensis subsp. gelechiae, isolated by Heierson et al. (A. Heierson, I. Sidén, A. Kivaisi, and H. G. Boman, J. Bacteriol. 167:18-24, 1986) as a spontaneous phage-resistant mutant, was further characterized and found to lack the expression of phosphatidylcholine- and phosphatidylinositol-degrading phospholipase C, beta-lactamase, and flagellin because of the absence of corresponding mRNAs. The avirulent mutant was also found to be less efficient in killing insect cells in vitro than the wild type and to have altered behavior in vivo; wild-type B. thuringiensis does not circulate in the insect hemolymph after injection, whereas the avirulent mutant and nonpathogenic control bacteria remain in circulation. Flagella and motility may be important for virulence in the early stages of an infection; mutants with decreased motility appear less virulent when fed to Trichoplusia ni but not when injected. The 50% lethal doses of wild-type strain Bt13 and avirulent mutant strain Bt1302 were estimated to be 0.52 +/- 0.25 and 2,600 +/- 1,300 CFU per injected larva, respectively.