"KNOW YOUR BUG" - GNR Identification & Clinical Associations
1. This gram-negative rod is the most common cause of community-acquired urinary tract infections and bacteremia from a urinary source. It is also the most common ESBL-producing organism worldwide.
What is Escherichia coli?
*E. coli is the #1 cause of UTIs and gram-negative bacteremia. The CTX-M family of ESBLs, particularly CTX-M-15, is the most common ESBL enzyme globally and is strongly associated with E. coli sequence type 131.
6. A patient with a Pseudomonas wound infection needs an IV β-lactam. The intern orders ceftriaxone. The attending says this drug has NO antipseudomonal activity. Name the two cephalosporins that DO cover Pseudomonas.
What are ceftazidime (3rd generation) and cefepime (4th generation)?
*Among traditional cephalosporins, only ceftazidime and cefepime have antipseudomonal activity. Ceftriaxone, cefotaxime, and other 3rd-generation cephalosporins do NOT cover Pseudomonas. Cefepime has the added advantage of stability against AmpC β-lactamases.
11. A 72-year-old woman has E. coli bacteremia from a urinary source. The isolate is resistant to ceftriaxone and ciprofloxacin but susceptible to piperacillin-tazobactam and meropenem. The MERINO trial showed this antibiotic class is preferred over piperacillin-tazobactam for ESBL-E bacteremia.
What are carbapenems (e.g., meropenem)?
*The MERINO trial demonstrated that meropenem was superior to piperacillin-tazobactam for definitive treatment of ceftriaxone-resistant E. coli and K. pneumoniae bacteremia (30-day mortality 3.7% vs. 12.3%). Carbapenems remain the standard of care for ESBL-E bloodstream infections. Piperacillin-tazobactam should not be used for ESBL-E bacteremia despite in vitro susceptibility.
16. A patient has a polymicrobial intra-abdominal abscess. The surgeon asks for an antibiotic that covers gram-negatives, gram-positives, AND anaerobes as a single agent. This carbapenem fits the bill — it covers Enterobacterales, Pseudomonas, streptococci, and Bacteroides.
What is meropenem (or imipenem-cilastatin)?
*Antipseudomonal carbapenems (meropenem, imipenem) have the broadest spectrum of any β-lactam class — covering most Enterobacterales, Pseudomonas, Acinetobacter (if susceptible), gram-positives (except MRSA and Enterococcus faecium), and anaerobes. They do NOT cover MRSA, VRE, or atypicals. Imipenem lowers the seizure threshold more than meropenem.
21. A 55-year-old man with a recent ICU stay develops fever. Blood cultures grow a gram-negative rod resistant to ceftriaxone, cefepime, piperacillin-tazobactam, ciprofloxacin, AND meropenem. This pattern of resistance — nonsusceptibility to ALL β-lactams AND fluoroquinolones — defines this resistance phenotype in Pseudomonas.
What is difficult-to-treat resistance (DTR)?
*DTR-Pseudomonas is defined as nonsusceptibility to all first-line agents: all β-lactams (including carbapenems and BL/BLI combinations) AND fluoroquinolones. This definition replaced older MDR/XDR classifications because it better predicts clinical outcomes. Preferred agents for DTR-P. aeruginosa include ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, or cefiderocol.
2. A 60-year-old hospitalized man grows this gram-negative rod from sputum cultures. It is intrinsically resistant to carbapenems and is most commonly treated with TMP-SMX. It is often a colonizer rather than a true pathogen.
What is Stenotrophomonas maltophilia?
*S. maltophilia is intrinsically resistant to carbapenems due to chromosomal metallo-β-lactamases (L1) and a cephalosporinase (L2). First-line treatment includes TMP-SMX, minocycline, levofloxacin, or cefiderocol — IDSA recommends combination therapy with at least 2 active agents for true infections. Always question whether isolation represents colonization vs. infection.
7. This is the only carbapenem that does NOT have antipseudomonal activity. It can be given once daily and is often used for outpatient parenteral antibiotic therapy (OPAT) for susceptible Enterobacterales infections.
What is ertapenem?
*Ertapenem covers most Enterobacterales (including ESBL producers) and anaerobes but lacks activity against Pseudomonas and Acinetobacter. Meropenem, imipenem, and doripenem are the antipseudomonal carbapenems. Ertapenem's once-daily dosing makes it ideal for OPAT.
12. A patient has Enterobacter cloacae bacteremia. The isolate is susceptible to ceftriaxone, cefepime, and meropenem. The attending avoids ceftriaxone and chooses cefepime. This property of cefepime — being a weak AmpC inducer AND resistant to AmpC hydrolysis — makes it the preferred cephalosporin for AmpC-producing organisms.
What is stability against AmpC β-lactamase hydrolysis (formation of stable acyl-enzyme complexes)?
*Cefepime is both a weak inducer of AmpC AND stable against AmpC hydrolysis, making it the preferred cephalosporin for infections caused by E. cloacae, K. aerogenes, and C. freundii. Ceftriaxone and ceftazidime are susceptible to AmpC hydrolysis and should be avoided even if the isolate initially tests susceptible. If cefepime MIC is ≥16 µg/mL, switch to a carbapenem.
17. A patient with hospital-acquired pneumonia is started on piperacillin-tazobactam. This combination antibiotic covers Enterobacterales, Pseudomonas, many anaerobes, and some gram-positives. The tazobactam component inhibits this class of β-lactamases.
What are class A β-lactamases (including many ESBLs like TEM, SHV, and some CTX-M enzymes)?
Tazobactam inhibits many class A serine β-lactamases but does NOT reliably inhibit AmpC (class C) or OXA-type (class D) β-lactamases. This is why piperacillin-tazobactam is unreliable for AmpC-producing organisms and ESBL-E bacteremia. Piperacillin-tazobactam covers: Enterobacterales, Pseudomonas, anaerobes (including Bacteroides), and streptococci. It does NOT cover MRSA or Acinetobacter.
22. A patient is started on ceftriaxone for an E. coli UTI. The culture also grows Enterococcus faecalis. The attending notes that cephalosporins have an intrinsic gap in coverage against this genus of gram-positive organisms, regardless of generation.
What is Enterococcus?
*ALL cephalosporins — from 1st to 5th generation — have an intrinsic gap against Enterococcus species. This is a classic board question. If Enterococcus is suspected or confirmed (e.g., biliary infections, endocarditis, complicated UTI), ampicillin (for E. faecalis) or vancomycin must be added. Piperacillin-tazobactam DOES cover E. faecalis, which is one advantage over cephalosporins for empiric abdominal coverage.
3. Blood cultures from a patient with a biliary drain grow this gram-negative rod. The lab reports it is resistant to ceftriaxone but was initially susceptible. The organism is known for inducible AmpC β-lactamase production that can emerge during cephalosporin therapy.
What is Enterobacter cloacae?
*E. cloacae complex, Klebsiella aerogenes, and Citrobacter freundii are the "moderate risk" AmpC producers — resistance to 3rd-generation cephalosporins can emerge in ~20% of infections even when the isolate initially tests susceptible. Avoid ceftriaxone for serious infections with these organisms.
8. This antibiotic class works by inhibiting the 30S ribosomal subunit and is concentration-dependent. It covers most aerobic gram-negative rods including Pseudomonas but carries risks of nephrotoxicity and ototoxicity. Gentamicin, tobramycin, and amikacin belong to this class.
What are aminoglycosides?
*Aminoglycosides are bactericidal, concentration-dependent antibiotics with excellent gram-negative coverage. They are dosed once daily to maximize peak concentration (Cmax/MIC ratio). They have NO anaerobic activity. Tobramycin has the best anti-Pseudomonal activity among traditional aminoglycosides; amikacin has the broadest gram-negative spectrum due to resistance to many aminoglycoside-modifying enzymes.
13. A patient with uncomplicated cystitis has a urine culture growing ESBL-producing E. coli susceptible to nitrofurantoin and TMP-SMX. The resident asks if a carbapenem is needed. For uncomplicated cystitis caused by ESBL-E, carbapenems are NOT necessary - these oral agents are preferred.
What are nitrofurantoin, TMP-SMX, or a fluoroquinolone (if susceptible)?
*Uncomplicated cystitis is the ONE scenario where ESBL-E can be treated with simple oral agents based on susceptibility. Carbapenems are reserved for serious/systemic ESBL-E infections. This is a key antibiotic stewardship principle - using the narrowest effective agent preserves carbapenems for when they are truly needed.
18. A patient with a severe penicillin allergy (anaphylaxis) has a serious Pseudomonas infection. The team needs a non-β-lactam IV antibiotic with antipseudomonal activity. This fluoroquinolone, given IV, covers Pseudomonas and can be used in penicillin-allergic patients.
What is ciprofloxacin (or levofloxacin)?
*Ciprofloxacin has the best antipseudomonal activity among fluoroquinolones. Levofloxacin also has Pseudomonal coverage but is slightly less potent. Moxifloxacin does NOT cover Pseudomonas. For severe penicillin allergy with Pseudomonas infection, non-β-lactam options include ciprofloxacin, aminoglycosides, or aztreonam (which is a β-lactam but does not cross-react with penicillin allergy).
23. A 68-year-old woman with recurrent UTIs is prescribed ciprofloxacin. She develops bilateral Achilles tendon pain. This well-known adverse effect of fluoroquinolones carries an FDA black box warning and is more common in patients over 60, those on corticosteroids, and organ transplant recipients.
What is tendinopathy/tendon rupture?
*Fluoroquinolone black box warnings include tendinitis/tendon rupture, peripheral neuropathy, CNS effects, and exacerbation of myasthenia gravis. Risk factors for tendon rupture include age >60, concurrent corticosteroids, and solid organ transplant. Fluoroquinolones should be reserved for infections where no safer alternative exists. Other notable side effects include QTc prolongation, C. difficile infection, and aortic aneurysm/dissection.
4. This non-fermenting gram-negative rod is the most common cause of hospital-acquired pneumonia in ICU patients on mechanical ventilation. It is intrinsically resistant to many antibiotics and has a propensity to develop resistance during therapy.
What is Pseudomonas aeruginosa?
*P. aeruginosa has multiple intrinsic resistance mechanisms including efflux pumps, low outer membrane permeability, and chromosomal AmpC. Antipseudomonal β-lactams include piperacillin-tazobactam, ceftazidime, cefepime, aztreonam, and carbapenems (meropenem, imipenem — but NOT ertapenem). Always use antipseudomonal agents when Pseudomonas is suspected.
9. This β-lactam is a monobactam — the only one in clinical use. It covers aerobic gram-negative rods (including Pseudomonas) but has NO gram-positive or anaerobic activity. It is safe to use in patients with penicillin allergy. It is uniquely stable against metallo-β-lactamases.
What is aztreonam?
*Aztreonam is the only monobactam. It has a unique β-lactam ring structure that does not cross-react with penicillin or cephalosporin allergies (except ceftazidime, which shares a side chain). Its stability against metallo-β-lactamases (NDM, VIM, IMP) makes it a key component of combination therapy (aztreonam + avibactam) for MBL-producing organisms.
14. A hospitalized patient has Pseudomonas aeruginosa pneumonia. The isolate is susceptible to piperacillin-tazobactam, cefepime, and meropenem. IDSA guidance recommends using a non-carbapenem β-lactam over a carbapenem when both are active, for this stewardship reason.
What is carbapenem-sparing (preserving carbapenem activity to prevent emergence of carbapenem resistance)?
*When P. aeruginosa is susceptible to both traditional β-lactams (pip-tazo, ceftazidime, cefepime) AND carbapenems, IDSA recommends the non-carbapenem β-lactam to preserve carbapenem activity. Unnecessary carbapenem use drives selection of carbapenem-resistant organisms. This is a core antibiotic stewardship principle.
19. A patient has a complicated UTI caused by an ESBL-producing E. coli. The isolate is susceptible to TMP-SMX and levofloxacin. Rather than using a carbapenem, IDSA guidance now prefers these oral agents for ESBL-E pyelonephritis/complicated UTI when susceptibility is confirmed.
What are TMP-SMX, ciprofloxacin, or levofloxacin?
*Updated IDSA 2024 guidance no longer recommends carbapenems as first-line for ESBL-E pyelonephritis or complicated UTI if TMP-SMX or a fluoroquinolone is active. This is a major shift toward carbapenem stewardship. Carbapenems are still preferred for ESBL-E bacteremia and critically ill patients. This change aligns IDSA with ESCMID guidelines.
24. A patient with neutropenic fever is started on cefepime. 48 hours later, he develops confusion, myoclonus, and a non-convulsive seizure. His creatinine has risen from 1.0 to 2.5 mg/dL. This cephalosporin is well known for causing neurotoxicity, especially in patients with renal impairment when doses are not adjusted.
What is cefepime?
*Cefepime neurotoxicity (encephalopathy, myoclonus, seizures, non-convulsive status epilepticus) occurs most commonly in patients with renal insufficiency who receive unadjusted doses. Cefepime trough levels >20 µg/mL are associated with neurotoxicity. Always adjust cefepime dosing for renal function. If neurotoxicity is suspected, obtain an EEG and check cefepime trough levels. Symptoms are reversible with dose reduction or discontinuation.
5. This gram-negative coccobacillus is a leading cause of ventilator-associated pneumonia and wound infections in military/trauma settings. Carbapenem-resistant strains are classified by the WHO as a critical priority pathogen. The most common carbapenemase it produces is OXA-23.
What is Acinetobacter baumannii?
*CRAB (carbapenem-resistant A. baumannii) is notoriously difficult to treat. Sulbactam-durlobactam (FDA-approved 2023) is now the preferred agent. Sulbactam itself has intrinsic activity against Acinetobacter by binding PBP1 and PBP3 — durlobactam protects it from β-lactamase hydrolysis.
10. This oral antibiotic achieves high urinary concentrations but has NO systemic tissue penetration, making it useful ONLY for uncomplicated cystitis. It retains activity against many ESBL-producing E. coli.
What is nitrofurantoin?
*Nitrofurantoin is effective for uncomplicated cystitis (including ESBL-E) but should NEVER be used for pyelonephritis, bacteremia, or any systemic infection because it does not achieve adequate serum or tissue levels. It is also contraindicated with CrCl <30 mL/min. It has no activity against Proteus, Morganella, Serratia, or Pseudomonas.
15. This class of antibiotics was previously the backbone of CRE and CRAB treatment but has been largely replaced by novel β-lactam/β-lactamase inhibitor combinations due to significant nephrotoxicity and inferior clinical outcomes. Colistin and polymyxin B belong to this class.
What are polymyxins?
*Polymyxins disrupt the gram-negative outer membrane by binding lipopolysaccharide (LPS). They were the "last resort" for CRE and CRAB before newer agents became available. They have been largely replaced by ceftazidime-avibactam, meropenem-vaborbactam, and sulbactam-durlobactam, which show better outcomes and less toxicity. Polymyxins still have a role in combination regimens for CRAB when other options are unavailable.
20. A patient grows Serratia marcescens from blood cultures. The isolate is susceptible to ceftriaxone. Unlike Enterobacter cloacae, this organism is at LOW risk for clinically significant AmpC induction (<5%). IDSA guidance says treatment can be guided by susceptibility results for this group of organisms.
What are the low-risk AmpC producers (Serratia marcescens, Morganella morganii, Providencia spp.)?
*Not all AmpC producers are created equal. S. marcescens, M. morganii, and Providencia spp. have a <5% risk of clinically significant AmpC induction — treatment can follow standard susceptibility results. In contrast, E. cloacae, K. aerogenes, and C. freundii have ~20% risk of AmpC emergence and should generally NOT be treated with 3rd-generation cephalosporins even if initially susceptible.
25. A patient with a complicated intra-abdominal infection is started on ceftriaxone. The attending adds metronidazole. Ceftriaxone covers most Enterobacterales but has a gap in coverage against this group of organisms that are commonly found in intra-abdominal infections — metronidazole fills this gap.
What are anaerobes (e.g., Bacteroides fragilis)?
*Ceftriaxone (and most cephalosporins) have poor anaerobic coverage. For intra-abdominal infections, metronidazole is added to cover Bacteroides fragilis and other anaerobes. Alternatives with built-in anaerobic coverage include piperacillin-tazobactam, carbapenems, and ampicillin-sulbactam. Metronidazole also covers C. difficile (oral formulation) and certain protozoal infections (Giardia, amebiasis).