cephalosporin, any of a group of more than 20 antibiotics derived from species of fungi of the genus Cephalosporium and closely related chemically to penicillin. Cephalosporins, e.g., cefaclor (Ceclor), act against both gram-positive and gram-negative bacteria (see Gram's stain) by inhibiting bacterial cell wall synthesis. They are widely used to treat gonorrhea, meningitis, and staphylococcal and streptococcal infections in patients who cannot use penicillin. Overuse of cephalosporins has led to increased bacterial resistance to the drugs (see drug resistance.)