Final answer:
Penicillin has a narrow spectrum of activity and is effective against gram-positive and limited numbers of gram-negative bacteria.
Step-by-step explanation:
Penicillin has a narrow spectrum of activity. It is effective against gram-positive bacteria and a limited number of gram-negative bacteria. It is commonly used to treat infections such as diphtheria, gonorrhea, pneumonia, syphilis, and certain types of boils. Penicillin G is the earliest and most widely used form of penicillin, although it cannot be taken orally due to its instability in acidic conditions. Instead, oral penicillins such as penicillin V, ampicillin, and amoxicillin are used.