Note: The antibiotic penicillin targets the enzyme that catalyzes the cross-linking of peptidoglycan. If a gram-positive cell cannot cross-link its cell wall, it no longer serves as an effective barrier. The bacterium then becomes susceptible to osmotic damage and lyses. Most bacteria have developed resistance mechanisms to penicillin, although a few bacteria—including Streptococcus pyogenes, which causes strep throat and some skin infections, and Treponema pallidum, which causes syphilis—are still very sensitive to this antibiotic.