A tridecaptin-based fluorescent probe for differential staining of Gram-negative bacteria

The traditional Gram-staining method, which was invented more than a century ago for differentiating bacteria as Gram positive or Gram negative, is still widely practiced in microbiology. However, Gram staining suffers from several problems which can affect the accuracy of the diagnosis. Here, we report a new Gram-negative-specific fluorescent probe, which is based on a narrow-spectrum antibiotic, tridecaptin A1, and allows selective staining of Gram-negative bacteria in different fixed bacterial samples. Solid-phase peptide synthesis was used to prepare the tridecaptin A1-fluorophore conjugate with a single structure. Labeling selectivity of the probe toward Gram-negative bacteria was confirmed by testing against a panel of bacterial species. By combining the use of a previously reported Gram-positive-specific fluorescent probe, we then further showed the capability of the new probe in differential labeling of a number of complex bacterial samples, which included a mouse gut microbiota cultured in vitro, as well as microbiotas collected from the human oral cavity, soil, and crude oil. High labeling selectivity and coverage were observed in most samples. This method offers a new Gram-negative-specific probe with a defined structure, which allows facile fluorescence-based differentiation of Gram-positive and Gram-negative bacteria for further microbial studies.