The manganese-containing superoxide dismutase in Streptococcus thermophilus AO54

Lactic acid bacteria (LAB) constitute a commercially important group of microorganisms that include members of the genus Pediococcus, Lactococcus, Lactobacillus, and Streptococcus. Many contain oxygen-utilizing enzymes; consequently, partially reduced, highly reactive oxygen intermediates may be generated during this aerobic metabolism. These reactive oxygen intermediates include the superoxide radical (O₂·−), hydrogen peroxide (H ₂O₂), and the hydroxyl radical (HO·), and present a threat to the microorganism. However, many of these LAB have evolved antioxidant defense systems to cope with this oxidative stress. Among these defense systems are enzymes capable of scavenging these reactive oxygen intermediates.; Superoxide dismutases (EC 1.15.1.1) are metalloenzymes that catalyze the conversion of the superoxide anion into hydrogen peroxide and molecular dioxygen. Some LAB, although not all, produce endogenous superoxide dismutase (SOD). In this study, the gene encoding for the manganese-containing SOD ( sodA) from the commercially important LAB Streptococcus thermophilus was cloned and sequenced. The sodA gene was found to encode a 201 amino acid polypeptide that is very similar (88% identical) to SodA from Streptococcus mutans. This gene was functionally expressed in Escherichia coli, and was able to rescue the growth of a sodAsodB mutant in the presence of paraquat. A sodA mutant in S. thermophilus proved to be more sensitive than the wild type to growth under aerobic conditions and in the presence of hydrogen peroxide. Moreover, it appears that S. thermophilus is capable of using exogenous manganese chloride as a defense against oxidative stress.; The role of this enzyme in Lactobacillus acidophilus, a lactic acid bacterium that lacks endogenous SOD, was also examined. The sodA gene from S. thermophilus was cloned into L. acidophilus and was expressed under anaerobic and aerobic (with and without shaking) conditions. The presence of the gene slowed down growth rates under these conditions; this might be due to the energetic and metabolic cost of expressing the foreign protein. While sodA did not provide any more protection against hydrogen peroxide, it remains to be seen whether the gene can enhance the probiotic benefit of this microorganism.