Biodistributions of l,d‑Transpeptidases in Gut Microbiota Revealed by In Vivo Labeling with Peptidoglycan Analogs

By catalyzing a 3–3 cross-link in peptidoglycan, l,d-transpeptidases (Ldts) can cause resistance to β-lactams in some pathogens in vitro. However, the prevalence of Ldt and Ldt-mediated responses to different β-lactams in vivo have never been explored. Here, we apply an in vivo metabolic labeling strategy to study their biodistributions and Ldt-induced bacterial responses to β-lactams in the mouse gut microbiota. A tetrapeptide-based fluorescent probe that functions as a substrate for Ldts in Gram-positive bacteria efficiently labels ∼18% of total gut bacteria after gavage, suggesting Ldts’ high prevalence in gut microbiota. The cellular distributions of 3–3 cross-links on three gut bacterial species were then identified with the aid of fluorescence in situ hybridization to identify the bacterial taxa. After oral administration of two β-lactams, ampicillin and meropenem, only the latter efficiently inhibits the tetrapeptide labeling, suggesting that Ldts may be able to cause resistance to some β-lactams in the mammalian gut.