H. Cells of wildtype (WT), ΔnlpI, ΔmepS and ΔnlpIΔmepS containing multicopy plasmids with lacZ were grown onto CPRG indicator agar to assay envelope integrity. CPRG (yellow) cannot penetrate intact Gram-negative envelopes. Its conversion by intracellular β-galactosidase to CPR (red) indicates loss of envelope integrity. 