The capacity to produce hydrogen sulfide in streptococcus intermedius

Objectives: Hydrogen sulfide (H₂S) is a toxic gas that induces the modification and release of hemoglobin in erythrocytes. The protein encoded by lcd catalyzes b-elimination of L-cysteine to produce H₂S. Streptococcus intermedius belongs to the anginosus group of streptococci together with S. anginosus and S. constellatus, both of which were shown to have a high capacity to produce H₂S. In contrast, the capacity in S. intermedius remains to be elucidated. In this study, we evaluated the capacity of H₂S production in S. intermedius and determined its molecular basis. Methods: The capacity of crude extract to produce H₂S was compared among 29 strains of S. intermedius, 6 strains of S. gordonii, 6 strains of S. anginosus and 6 strains of S. constellatus. The lcd gene was sequenced and cloned, and the encoded protein was purified for enzymatic characterization. The quantification of lcd were performed by real-time PCR. Results: The crude extracts from S. intermedius strains mostly have a high capacity to produce H₂S, whereas 3 of 29 crude extracts from S. intermedius strains had no significant capacity. Expectedly, lcd from the three strains, which were S. intermedius strains ATCC 27335, IMU151 and IMU202, contained mutations or small deletions. The capacity in strain ATCC 27335 was restored by repairing lcd in its genomic DNA. The kinetic properties of the purified protein encoded by the repaired lcd gene were comparable to those of native proteins from H₂S-producing strains, while the truncated protein from strain ATCC 27335 had no enzymatic activity. However, real-time PCR analysis indicated that lcd in strains ATCC 27335, IMU151 and IMU202 is transcribed and regulated in a manner similar to that in the H₂S-producing strains. Conclusion: Molecular basis of H₂S production in S. intermedius was determined.