The cephalosporins are semisynthetic antibiotics derived from products of various microorganisms, including Cephalosporium and Streptomyces. All cephalosporins have a 7-aminocephalosporanic acid composed of a di-hydrothiazine ring fused to a p-lactam ring (Fig. 45.3).As with the penicillins, the cephalosporin p-lactam ring is the chemical group associated with antibacterial activity. The different pharmacological, pharmacokinetic, and antibacterial properties of individual cephalosporins result from substitution of various groups on the basic molecule. Cephalosporins also vary in acid stability and p-lactamase susceptibility. Table 45.2 shows the large number of available cephalosporins.
The p-lactamases (penicillinases) inactivate some cephalosporins but are much less efficient than are the cephalosporinases (p-lactamases specific for the cephalosporins). Resistance to cephalosporins also results from modification of microbial PBPs.
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