The cell-penetrating peptide, Pep-1, has activity against intracellular chlamydial growth but not extracellular forms of Chlamydia trachomatis.

OBJECTIVES In the course of studies to identify novel treatment strategies against the pathogenic bacterium, Chlamydia, we tested the carrier peptide, Pep-1, for activity against an intracellular infection. METHODS Using a cell culture model of Chlamydia trachomatis infection, the effect of Pep-1 was measured by incubating the peptide with extracellular chlamydiae prior to infection, or by adding Pep-1 to the medium at varying times after infection, and assaying for inhibition of inclusion formation. RESULTS Pep-1 had a concentration-dependent effect on chlamydial growth with 100% inhibition of inclusion formation at 8 mg/L peptide. There was a window of susceptibility during the chlamydial developmental cycle with a maximal effect when treatment was begun within 12 h of infection. Pep-1 treatment caused a severe reduction in the production of infectious progeny even when started later, when the effect on inclusion formation was minimal. Furthermore, electron micrographs showed a paucity of progeny elementary bodies (EBs) in the inclusion. In contrast, pre-incubation of EBs with Pep-1 prior to infection did not affect inclusion formation. Taken together, these findings indicate that the antichlamydial effect was specific for the intracellular stage of chlamydial infection. By comparison, Pep-1 had no antimicrobial activity against Escherichia coli and Staphylococcus aureus or the obligate intracellular parasite, Toxoplasma gondii. CONCLUSIONS Pep-1 has antichlamydial activity by preventing intracellular chlamydial growth and replication but has no effect on extracellular chlamydiae.