Figure 6.

Schematic of evolved strain BLA3-E1 removing the growth dependence on exogenous d-alanine

The elimination of alanine racemase in strain BLA3 blocks the endogenous d-alanine synthesis pathway, which results in the growth dependence on exogenous d-alanine. The evolved strain BLA3-E1 produces additional endogenous d-alanine by activating dormant alternative d-alanine synthesis pathway, improves the utilization of exogenous d-alanine by enhancing the expression of d-alanine transporter and reduces the consumption of d-alanine by blocking the incorporation of d-alanine into teichoic acid.