Fluorescent reporter probes for TPase-mediated PG cross-linking. (A) Fluorescent stem pentapeptide mimics resembling the donor strand (FSPPM) are proposed to incorporate via D,D-TPase activity as shown for S. aureus PG; FSPPMs could theoretically also incorporate via L,D-TPase activity if the terminal D-Ala were cleaved first by a CPase. (B) Fluorescent stem tetrapeptide mimics resembling the donor strand (FSTPM), which lack the terminal D-Ala of FSPPMs, are proposed to incorporate exclusively via L,D-TPase activity.