Figure 6. Survival rates of Td₃₅₄₀₅, Tde471^mut, and Tde471^com strains in serum.

The strains were co-incubated with either 25% normal human serum (NHS) or heat-inactivated human serum (HIS) for 1 h at 37°C. The number of surviving bacteria was counted using a Petroff Hausser counting chamber. The survival rates were calculated as follows: the total number of living cells in NHS divided by the total number of living cells in HIS. Cell counting was repeated in triplicate with at least three independent samples, and the results are expressed as the mean ± SEM. The data were analyzed by one-way ANOVA followed by Tukey’s multiple comparison at p < 0.01.