FIGURE 6.

Assay of heme sensitivity for the wild-type L. lactis and ΔLlYybT mutant strains. Late-log cultures of L. lactis wild-type strain, ΔLlYybT mutant (Δ1816), and revertant 1816Rwt strain were plated on GM17 agar plates, and 230 nmol of heme was added to a filter disk. Diameter measurements of growth inhibition zones from triplicate assays are provided below each image with standard deviations in millimeters. Different superscript letters indicate significant differences in zone sizes (p < 0.001), whereas the same superscript letters indicate zones that were not significantly different (p > 0.05). Inactivation of LlYybT gene in the Δ1816 mutant resulted in increased heme sensitivity when compared with the wild type (wt) as determined by a larger diameter of growth inhibition. As expected the revertant strain (1816Rwt), which is derived from Δ1816, but had the inactivation plasmid removed, had an equivalent growth inhibition zone as the wild type. The ring of enhanced growth outside the growth inhibition zone is due to respiration growth in the presence of subinhibitory levels of heme (39).