Only a handful of cephalosporins have reliable activity against Pseudomonas aeruginosa. The core agents are ceftazidime (third generation), cefepime (fourth generation), ceftolozane-tazobactam, ceftazidime-avibactam, and cefiderocol. Most other cephalosporins, including popular options like ceftriaxone and cefazolin, have no meaningful pseudomonal coverage.
The Classic Options: Ceftazidime and Cefepime
Ceftazidime is the original antipseudomonal cephalosporin. It’s a third-generation agent that has been a workhorse for Pseudomonas infections for decades. Cefepime, a fourth-generation cephalosporin, offers similar pseudomonal activity while also covering a broader range of other gram-negative bacteria, including some that produce AmpC enzymes and resist third-generation cephalosporins. Both are given intravenously for serious infections.
Cefoperazone is another third-generation cephalosporin with Pseudomonas activity, though it is far less commonly used in the United States and not as widely available as ceftazidime.
Because cefepime and ceftazidime kill bacteria based on how long the drug stays above a critical concentration (time-dependent killing), hospitals increasingly use extended infusions rather than standard 30-minute infusions. Infusing cefepime over three hours, for example, keeps the drug at effective levels for a larger portion of the dosing interval, which improves its ability to clear Pseudomonas, especially strains with borderline susceptibility.
Newer Combination Agents
Two newer cephalosporin combinations extend coverage to Pseudomonas strains that resist older drugs.
Ceftolozane-tazobactam pairs a novel cephalosporin with a beta-lactamase inhibitor. It was designed with multidrug-resistant Pseudomonas specifically in mind and is FDA-approved for complicated urinary tract infections, complicated intra-abdominal infections, and hospital-acquired pneumonia. In a study of patients with multidrug-resistant Pseudomonas infections (most of whom had pneumonia), ceftolozane-tazobactam achieved clinical success in 71% of cases. Resistance did emerge during treatment in about 14% of patients, a reminder that even newer agents are not bulletproof.
Ceftazidime-avibactam combines the classic antipseudomonal cephalosporin with avibactam, an inhibitor that neutralizes several types of beta-lactamase enzymes Pseudomonas can produce. In clinical trials for urinary tract infections, cure rates reached roughly 90%, comparable to carbapenems. For hospital-acquired pneumonia, clinical cure was about 69%. This combination is particularly valuable against strains that produce certain carbapenemases, though it does not cover strains with metallo-beta-lactamases.
Cefiderocol: The Siderophore Cephalosporin
Cefiderocol works differently from every other cephalosporin on this list. It is a siderophore cephalosporin, meaning it hijacks the iron-transport channels that bacteria use to pull iron into their cells. Pseudomonas actively draws cefiderocol inside, which bypasses the outer membrane barrier that blocks many other antibiotics. Once inside, it disrupts cell wall synthesis like a traditional cephalosporin.
This mechanism gives cefiderocol activity against Pseudomonas strains that resist nearly everything else, including strains producing metallo-beta-lactamases. In testing, 100% of metallo-beta-lactamase-producing Pseudomonas isolates were susceptible to cefiderocol. The FDA has approved it for complicated urinary tract infections and ventilator-associated pneumonia caused by highly resistant gram-negative bacteria. It is generally reserved for infections where other options have failed or where susceptibility testing shows resistance to first-line agents.
Cephalosporins That Do Not Cover Pseudomonas
This is an equally important list. First- and second-generation cephalosporins (cefazolin, cephalexin, cefuroxime, cefoxitin) have no Pseudomonas activity. Among third-generation agents, ceftriaxone, the most widely prescribed cephalosporin in hospitals, does not cover Pseudomonas. Ceftaroline, the fifth-generation cephalosporin designed for MRSA, also has no pseudomonal coverage. Anti-MRSA cephalosporins as a class lack activity against Pseudomonas species.
Why Pseudomonas Resists Most Cephalosporins
Pseudomonas aeruginosa is intrinsically resistant to many antibiotics because of a combination of defenses. Its outer membrane is unusually impermeable, limiting how much drug can enter the cell. It runs efflux pumps that actively push antibiotics back out. And it carries a chromosomal gene encoding AmpC, a beta-lactamase enzyme that breaks down many cephalosporins before they can work.
What makes resistance worse over time is that mutations can cause Pseudomonas to overproduce AmpC. This overproduction is considered the main driver of resistance to antipseudomonal cephalosporins in clinical strains. Some mutations widen the enzyme’s active site, allowing it to break down even bulkier cephalosporins like ceftazidime and cefepime that were designed to evade it. Other mutations enlarge the substrate binding pocket of the enzyme, making it capable of hydrolyzing drugs it previously could not. These changes can occur during a course of treatment, which is why repeat susceptibility testing matters in serious Pseudomonas infections.
The interplay between AmpC production and efflux pumps (particularly one called MexAB-OprM) creates layered resistance. A strain might test susceptible on paper but still respond poorly if both mechanisms are active simultaneously. This is one reason clinicians often use higher doses or extended infusions when treating confirmed Pseudomonas infections, even with susceptible isolates.
Quick Reference by Generation
- Third generation: Ceftazidime (yes), cefoperazone (yes), ceftriaxone (no), cefotaxime (no)
- Fourth generation: Cefepime (yes)
- Fifth generation: Ceftaroline (no)
- Newer combinations: Ceftolozane-tazobactam (yes), ceftazidime-avibactam (yes)
- Siderophore: Cefiderocol (yes, including many resistant strains)
For empiric coverage of suspected Pseudomonas infections, cefepime and ceftazidime remain the standard first-line cephalosporin choices. The newer agents are typically reserved for confirmed multidrug-resistant or carbapenem-resistant strains, guided by susceptibility results.