Description benzylpenicillin (penicillin g) is narrow spectrum antibiotic used to treat infections caused by susceptible bacteria. it is a natural penicillin antibiotic that is administered intravenously or intramuscularly due to poor oral absorption. penicillin g may also be used in some cases as prophylaxis against susceptible organisms.indication :for use in the treatment of severe infections caused by penicillin g-susceptible microorganisms when rapid and high penicillin levels are required such as in the treatment of septicemia, meningitis, pericarditis, endocarditis and severe pneumonia.mechanism of action by binding to specific penicillin-binding proteins (pbps) located inside the bacterial cell wall, penicillin g inhibits the third and last stage of bacterial cell wall synthesis. cell lysis is then mediated by bacterial cell wall autolytic enzymes such as autolysins- it is possible that penicillin g interferes with an autolysin inhibitor.