A new metabolic cell-wall labelling method reveals peptidoglycan in Chlamydia trachomatis

Peptidoglycan (PG), an essential structure in the cell walls of the vast majority of bacteria, is critical for division and maintaining cell shape and hydrostatic pressure. Bacteria comprising the Chlamydiales were thought to be one of the few exceptions. Chlamydia harbour genes for PG biosynthesis and exhibit susceptibility to ‘anti-PG’ antibiotics yet attempts to detect PG in any chlamydial species have proven unsuccessful (the ‘chlamydial anomaly’). We used a novel approach to metabolically label chlamydial PG using d-amino acid dipeptide probes and click chemistry. Replicating Chlamydia trachomatis were labelled with these probes throughout their biphasic developmental life cycle, and the results of differential probe incorporation experiments conducted in the presence of ampicillin are consistent with the presence of chlamydial PG-modifying enzymes. These findings culminate 50 years of speculation and debate concerning the chlamydial anomaly and are the strongest evidence so far that chlamydial species possess functional PG.

Fig. 1. Fluorescent labelling of intracellular C. trachomatis PG. 

a–e, Differential interference contrast (DIC) (a) and fluorescent (b–e) microscopy of L2 cells infected for 18 h with C. trachomatis in the presence of the dipeptide probe EDA-DA (1 mM). Subsequent binding of the probe to an azide modified Alexa Fluor 488 (green) was achieved via click chemistry. Antibody to MOMP (red) was used to label chlamydial EBs and RBs. DAPI (blue) was used for nuclear staining. b–e show a merge of all three fluorescent channels. Boxes indicate location of chlamydial inclusions, and magnification of the boxes is provided in c–e. Fluorescent images are maximum intensity projections of deconvoluted z-stacks. 

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