Pyocyanin reversibly inhibits Chlamydia by impairing newly formed elementary body infectivity and delaying the developmental cycle.

Chlamydia is an obligate intracellular bacterial pathogen responsible for sexually transmitted infections and blindness worldwide. In our search for antichlamydial agents from natural sources, we identified pyocyanin, a phenazine derivative from Pseudomonas aeruginosa. Previous studies showed that pyocyanin disables elementary body (EB) infectivity at low concentrations and inhibits intracellular growth at higher concentrations. In the present study, we show that its antichlamydial activity is primarily attributed to the intracellular developmental stage. Complete inhibition of detectable infectious progeny was achieved at 16 μM at the standard 36 hpi endpoint, while its effect on the infection stage reached only approximately 60% inhibition. Two independent delayed treatment assays revealed that pyocyanin reduces the infectivity of newly formed EBs without reducing their number. Furthermore, pyocyanin delays rather than arrests the chlamydial developmental cycle, and its inhibitory effect is reversible, requiring sustained drug exposure for optimal efficacy. These findings position pyocyanin among a growing class of antichlamydial agents that target distinct steps of the pathogen's developmental cycle and support its potential as a chemical probe for chlamydial biology and a lead compound for drug development.