penicillin is effective on gram positive bacteria by acting on the proteins which are cross-link in the peptidoglycan layers. chlamydia is caused by a gram negative bacteria, the strains are mostly like that. so, gram neg bacteria will be resistant to penicillin by making their own enzyme to degrade penicillin. also, bacteria can alter their peptidoglycan layer to hide their penicillin binding proteins, so making penicillin ineffective. that is is why penicillin cannot be used for chlamydia. I found this