Penicillin is a type of antibiotic that treats bacterial infections like pneumonia and strep throat. Some bacteria have become resistant to penicillin, making the drug less effective. Penicillins are one of the oldest and most widely used classes of antibiotics, belonging to the β-lactam family. They’re highly effective against Penicillin, antibiotic derived from Penicillium mold, one of the first antibiotics to be discovered. Penicillins were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. They are still widely used today for various bacterial infections, though many types of bacteria have developed resistance following extensive use. Penicillin is a group of antibiotics (medications used to treat bacterial infections) that work in a specific way to destroy bacteria in your body. Types of penicillin and drugs closely related to them are called “penicillins.” Penicillins are antibiotics used to treat bacterial infections that are derived from the antibiotic penicillin. Penicillin antibiotics stop bacteria from multiplying by preventing bacteria from forming the walls that surround them. Penicillins are a type of antibiotic derived from Penicillium fungi. An antibiotic is a type of medicine that inhibits the growth of, or kills, bacteria. Penicillin G (also called benzylpenicillin) was discovered by accident in 1928.