Which two classes of antibiotics contain beta-lactam rings?

• A. Penicillins and Cephalosporins
• B. Macrolides and Tetracyclines
• C. Aminoglycosides and Fluoroquinolones
• D. Sulfonamides and Nitrofurans

Answer: (A)

Beta-lactam antibiotics are defined by the presence of a four-membered beta-lactam ring in their core structure, which is essential for their ability to inhibit enzymes that cross-link the bacterial cell wall (penicillin-binding proteins). Penicillins and cephalosporins both contain this beta-lactam ring, giving them a similar mechanism of action and structural basis. The other classes listed do not have a beta-lactam ring and thus do not share this core feature; they act by different targets such as ribosomal function, DNA replication, or metabolic pathways. So the two classes with beta-lactam rings are penicillins and cephalosporins.