It has since become the most widely used antibiotic to date and is still used for many [Gram-positive bacteria]?l infections.
Penicllin works by inhibiting the formation of [peptidoglycan cross links]? in the bacterial cell wall by directly competing for the reaction site (the [beta-lactam ring]? of penicillin is a chemical analogue for the enzyme that links the peptidoglycan molecules in bacteria) and thus preventing the bacteria from multiplying (or rather causing cell lysis? or death when the bacteria tries to divide)
Antibiotic resistance to penicillin is now common amongst many hospital acquired bacteria. The resistance to penicillin has been partly (maybe mostly) due to the rise of beta-lactamase producing bacteria which secrete an enzyme that breaks down the beta-lactam ring of penicillin, rendering it harmless to the bacteria.