CLOSTRIDIAL STRAIN DEGENERATION

Strain degeneration, the loss of the capacity to produce solvents and form spores, typically occurs when Clostridium acetobutylicum and rela ted clostridia are repeatedly subcultured in batch culture or grown in continuous culture, as opposed to being grown from germinated, heat-t reated spores. Several mechanisms for degeneration have been identifie d thus far. (i) Degeneration can be caused by excessive acidification of the culture during exponential growth. We present data interpreted to mean that C. beijerinckii (formerly C. acetobutylicum) NCIMB 8052 c ells ferment glucose to acetic and butyric acids at an uncontrolled ra te, so that, during rapid growth, the rate of acid production can exce ed the rate of induction of the solventogenic pathway enzymes. As a re sult, the medium pH drops to bactericidal levels, and the cells cannot switch to solventogenesis and sporulation. The clostridia seem to be poised either to produce excess acids, or to initiate solventogenesis, depending on small differences in the rates of growth. (ii) We have i solated transposon-insertion mutants of C. beijerinckii NCIMB 8052 tha t are resistant to degeneration, suggesting the involvement of a regul atory region of the clostridial chromosome. (iii) involvement of a glo bal regulatory gene has been inferred in C. beijerinckii NCIMB 8052 wh ich degenerates irreversibly in chemostat culture. (iv) Impairment of butanol formation due to a defect in NADH generation has been reported in an oligosporogenous strain which can revert to the non-degenerate phenotype. (v) In continuous culture, degenerate cells may be selected because they continue to divide, while the non-degenerate cells stop dividing and start differentiating.