Pathophysiology
Clinical meaning
Antibiotic resistance is a critical global health threat that the NP must understand to prescribe antimicrobials effectively and participate in stewardship efforts. Resistance mechanisms are broadly classified into four categories. Enzymatic inactivation: Bacteria produce enzymes that chemically modify or destroy antibiotics. Beta-lactamases hydrolyze the beta-lactam ring of penicillins and cephalosporins. Extended-spectrum beta-lactamases (ESBLs) are plasmid-mediated enzymes (primarily CTX-M, TEM, SHV variants) that hydrolyze extended-spectrum cephalosporins (ceftriaxone, cefotaxime, ceftazidime) and aztreonam but are inhibited by beta-lactamase inhibitors (clavulanate, tazobactam, avibactam). ESBL-producing organisms (predominantly E. coli and Klebsiella pneumoniae) are increasingly common in community-acquired UTIs and bacteremia. Carbapenemases (KPC, NDM, OXA-48, VIM, IMP) hydrolyze virtually all beta-lactams including carbapenems โ the last-resort antibiotics for gram-negative infections. Carbapenem-resistant Enterobacteriaceae (CRE) carry mortality rates of 40-60% in bloodstream infections. Target modification: Bacteria alter the antibiotic target so the drug can no longer bind. Methicillin-resistant S. aureus (MRSA) produces an altered penicillin-binding protein (PBP2a) encoded by the mecA gene that has low affinity for all beta-lactams. Vancomycin-resistant Enterococcus (VRE) modifies the D-Ala-D-Ala peptidoglycan target to D-Ala-D-Lac, reducing vancomycin binding affinity 1000-fold. Efflux pumps: Bacteria express...