What type of antibiotic is used primarily against gram-positive bacteria?

Penicillin is primarily used against gram-positive bacteria due to its mechanism of action and its structure. It targets the bacterial cell wall, specifically inhibiting the enzyme responsible for cross-linking peptidoglycan layers, which is an essential component of the cell wall in bacteria. Gram-positive bacteria have a thick peptidoglycan layer, making them particularly susceptible to the effects of penicillin.

In contrast, cephalosporins can target both gram-positive and some gram-negative bacteria but are more versatile and not exclusively used for gram-positive infections. Tetracyclines are broad-spectrum antibiotics that can act against both types of bacteria but are not specifically focused on gram-positive organisms. Aminoglycosides are primarily effective against gram-negative bacteria and require oxygen for uptake into the bacterial cell, which limits their action against many gram-positive species. Thus, the specific affinity of penicillin for gram-positive bacteria makes it the correct choice in this instance.