The penicillins are a large group of bactericidal compounds. They can be subdivided and classified by their chemical structure and spectrum of activity. The structure common to all penicillins is a p-lactam ring fused with a thiazolidine nucleus (Fig. 45.1).The antimicrobial activity of penicillin resides in the p-lactam ring. Splitting of the p-lactam ring by either acid hydrolysis or p-lactamases results in the formation of penicilloic acid, a product without antibiotic activity. Addition of various side chains (R) to the basic penicillin molecule creates classes of compounds with the same mechanism of action as penicillin but with different chemical and biological properties. For example, some analogues are resistant to hydrolysis by acid or p-lactamase; some have an extended the spectrum of antibacterial activity; and others show improved absorption from the intestinal tract.

Penicillins may be classified into four groups: natural penicillins (G and V), antistaphylococcal (penicillinase-resistant) penicillins, aminopenicillins, and antipseudo-monal penicillins. Natural penicillins have therapeutic effects limited to streptococci and a few gram-negative organisms. The antistaphylococcal (penicillinase-resist-ant) penicillins treat infections caused by streptococci and staphylococci but do not affect MRSA. The amino-penicillins are effective against streptococci, enterococci, and some gram-negative organisms but have variable activity against staphylococci and are ineffective against P. aeruginosa. The antipseudomonal penicillins retain activity against streptococci and possess additional effects against gram-negative organisms, including various Enterobacteriaceae and Pseudomonas.

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