Highly bioluminescent Streptococcus thermophilus strain for the detection of diary-relevant antibiotics in milk

Inefficient translational initiation is often the cause of poor foreign gene expression in gram-positive organisms. The expression of bacterial luciferase (lux) genes in Streptococcus thermophilus (bioluminescence) was improved by addressing this problem in two ways; by ribosome-binding site (RBS) replacement, and by enhancing lux RBS access by polymerase chain reaction modification either alone or combined with translational coupling to a truncated upstream open- reading frame (orf') having its own RBS. Lactococcal expression signals were employed for plasmid-based lux expression. The same constructs were used to monitor bioluminescence in Lactococcus lactis, as well as two non-lactic bacterial strains, for comparison. High lux expression was achieved in all four organisms with a heterodimeric thermostable enzyme. Surprisingly, where ready access to the lux RBS was predicted, translational coupling to the lactococcal orf remained a prerequisite for detectable lux expression in L. lactis. In contrast, high bioluminescence in S. thermophilus was independent of coupling. Consistent with these observations, inspection of published gene sequences suggests that RBS "strength" may be a more important factor in translation in S. thermophilus than in L. lactis. Using reduced light production in highly bioluminescent S. thermophilus as an indicator of biocide presence in milk, test times could be significantly shortened compared with a commercial test utilizing the related non-bioluminescent strain. lux genes appear to be sensitive, exponential-phase reporters of gene activity in S. thermophilus, an organism with molecular biology and genetics that remain largely unstudied.