Rickettsia Typhi Peptidoglycan Mapping with Data-Dependent Tandem Mass Spectrometry

Rickettsia species are the etiological agents for many arthropod-borne illnesses. R. typhi is a Gram-negative obligate intracellular pathogen responsible for endemic typhus. Subunits of peptidoglycan are pattern recognition receptor ligands partially responsible for activation of the innate immune response to bacterial insult. Host detection of R. typhi cell envelope glycoconjugates remains poorly defined. In this work, peptidoglycan from R. typhi was digested and the resultant subunits were analyzed by two different, complementary, sample preparation methods each of which was subjected to LC/MS analysis to infer structure. R. typhi peptidoglycan was determined to be similar to most other Gram-negative bacteria, with mDAP-type muropeptide subunits. However, additional alanine residues were observed elongating the muropeptide stems, rather than the glycine residues usually observed in Gram-negative bacterial peptidoglycan. Despite this deviation, R. typhi, and likely all Rickettsiae, contain a murein layer that should agonize host cellular peptidoglycan receptors and be susceptible to peptidoglycan-targeting antimicrobials.