Cephalosporins and Beta-Lactamase Inhibitor Combinations

First-generation agents (cefazolin, cephalexin) have excellent activity against methicillin-susceptible Staphylococcus aureus and streptococci, with limited gram-negative coverage restricted to community-acquired Escherichia coli, Proteus mirabilis, and Klebsiella pneumoniae. No meaningful activity against Enterococcus, methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa, or anaerobes.

Cefazolin is the preferred agent for surgical prophylaxis across most specialties and the drug of choice for methicillin-susceptible Staphylococcus aureus infections in patients who cannot tolerate antistaphylococcal penicillins. It has uniquely low cross-reactivity with penicillins because its 7-position side chain is structurally unrelated to any penicillin side chain. Cephalexin is the oral agent for outpatient skin infections.

Second-generation agents fall into two clinically distinct subgroups. The true cephalosporins (cefuroxime) extend coverage to Haemophilus influenzae and Moraxella catarrhalis while retaining gram-positive activity, and are used for outpatient respiratory infections. The cephamycins (cefoxitin, cefotetan) add anaerobic coverage including Bacteroides fragilis; they are used for intra-abdominal and gynecologic infections.

Third-generation agents (ceftriaxone, cefotaxime, ceftazidime) have substantially expanded gram-negative spectrum including most Enterobacteriaceae, Haemophilus influenzae, and Neisseria gonorrhoeae, at the cost of reduced antistaphylococcal potency compared to first-generation agents.

Ceftriaxone is the most clinically important: once-daily dosing (long half-life of approximately 8 hours), excellent cerebrospinal fluid penetration, and partial biliary elimination making it usable without dose adjustment in renal impairment. It is the standard of care for bacterial meningitis caused by susceptible organisms, community-acquired pneumonia requiring hospitalization, and most gram-negative bacteremias from susceptible organisms. Ceftazidime is the third-generation agent with antipseudomonal activity but weaker gram-positive coverage; it is now primarily used in the ceftazidime-avibactam combination for carbapenem-resistant organisms.

Cefepime is the only widely used fourth-generation agent. It has enhanced stability against chromosomal AmpC-producing organisms, covers Pseudomonas aeruginosa, and covers most Enterobacteriaceae. It retains gram-positive activity comparable to first-generation agents. It is a first-line empiric agent for febrile neutropenia and nosocomial infections where Pseudomonas is a concern. Cefepime is susceptible to extended-spectrum beta-lactamase hydrolysis.

Cefepime accumulates in renal impairment and can cause neurotoxicity: non-convulsive status epilepticus, myoclonus, and encephalopathy. Unexplained encephalopathy in a patient receiving cefepime warrants dose review and consideration of an alternative agent.

Ceftaroline covers methicillin-resistant Staphylococcus aureus, methicillin-susceptible Staphylococcus aureus, streptococci, and most Enterobacteriaceae, but lacks antipseudomonal and anaerobic coverage. Ceftolozane-tazobactam is a novel cephalosporin-inhibitor combination with enhanced antipseudomonal activity and stability against AmpC-overproducing Pseudomonas; it is not active against extended-spectrum beta-lactamase producers or carbapenem-resistant organisms.

The historically cited 10% cross-reactivity rate between penicillins and cephalosporins is not supported by current evidence and should not guide prescribing decisions. Immunologic and clinical challenge data consistently show that true cross-reactivity is mediated by shared 7-position side chains, not by the shared beta-lactam ring. The actual cross-reactivity rate between structurally dissimilar penicillins and cephalosporins is approximately 1 to 2%.

Cephalosporins that share a 7-position side chain with amoxicillin (cefadroxil, cefprozil) carry meaningfully higher cross-reactivity risk in patients with documented amoxicillin allergy and should be avoided or used with caution. Cefazolin has the lowest penicillin cross-reactivity risk of any cephalosporin because its side chain is structurally unrelated to any penicillin; it may be used for surgical prophylaxis in most penicillin-allergic patients after appropriate allergy risk stratification.

Most cephalosporins are eliminated predominantly by renal excretion and require dose adjustment in renal impairment. Ceftriaxone is the major exception: approximately 40% is eliminated by biliary secretion, and the remainder renally. This dual elimination means ceftriaxone does not require dose adjustment in renal impairment and is preferred for gram-negative bacteremia in patients with acute kidney injury. Ceftriaxone should be avoided in neonates (risk of bilirubin displacement) and not co-administered with calcium-containing intravenous solutions due to precipitation.

Cerebrospinal fluid penetration is adequate for ceftriaxone, cefotaxime, and cefepime when meningeal inflammation is present, supporting use for bacterial meningitis caused by susceptible organisms. Ceftazidime also achieves adequate cerebrospinal fluid concentrations and can be used for gram-negative meningitis including Pseudomonas.

Cefepime accumulation in renal impairment causes dose-dependent neurotoxicity: non-convulsive status epilepticus, myoclonus, confusion, and impaired consciousness. Unexplained encephalopathy in a patient receiving cefepime should prompt dose review and electroencephalogram evaluation; discontinuation and substitution of an alternative agent is often required.