Butanol :: Biology, Bacteria

Many species belonging to the genus Clostridium are categorized as strict anaerobes because neither they nor key enzymes inside them are able to function normally during aerobic culti-vations, and stringent anaerobic conditions are required for their growth (1). Despite the gen-eral understanding about the sensitivity of clostridial species to O2, some strains possess me-tabolic mechanisms for withstanding, to some degree, the presence of dissolved oxygen (DO), and this phenomenon has been reviewed(2,3). Clostridium acetobutylicum strain ATCC 824, a widely investigated model for studies on the fermentative production of butanol, sus-tained growth after a shift from anaerobic to microoxic conditions (0-0.2 % DO)at the mid-exponential phase of growth, however, it ceased to grow on flushing the broth with a gas mixture of 5 % O2 / 95 % N2 (4).The O2-tolerance of C. acetobutylicum strain ATCC 824 in microoxic conditions has been attributed to the production of O2-induced polypeptides, wh ich are presumed to play a protective role against toxic activated oxygen species such as superoxide anion and hydrogen peroxide (4).Studies on the oxygen stress-responses in the obligate anaerobes C. acetobutylicum and C.aminovalericum revealed the upregulation of gene clusters coding for O2-scavenging enzymes, including NADH : rubredoxin oxidoreductase (EC 1.18.1.1), glutathione peroxidase (EC 1.11.1.9), thiol peroxidase (EC 1.11.1.1), alkylhydroperoxide reductase (EC 1.11.1.15), and superoxide dismutase (5).Based on the findings of Kawasaki et al. (4,5), Hillman et al. (6) reported that C. acetobutylicum strain ATCC 824 perRmutants, survived aerobic incubation on agar plates for up to 50 days (6). The per mutants also survived oxidative stress in aerated liquid cultures for up to 3 hours before a decrease in cell viability was observed (6). However, at this stage (after 3 hours) if no further O2 was added to the broth culture, the perR mutants were able to reestablish anaerobic c onditions by rapidly consuming DO in the broth (6). These reports (4,5,6) however, did not investigate the effects of oxidative stress on the production of solvents. Despite the presence of metabolic mechanisms in some clostridia for surviving short-term oxidative stress, these appear to be insufficient for establishing long-term survival under aerobic conditions. Literature on the effects of oxidative stress on butanol-producing bacteria, all of which are strict anaerobes, remains limited. A fuller understanding of the effects of such stressants on metabolite production will provide insights into stress-specific responses in bu-tanol-producing bacteria and may reveal strategies for improving the yield of solvents.