Gram-Negative Rods

• Definition: Gram-negative rods (bacilli) are bacteria with a thin peptidoglycan layer sandwiched between an inner and an outer membrane, the latter studded with lipopolysaccharide. The group spans the Enterobacterales (E. coli, Klebsiella, Proteus, Enterobacter, Salmonella, Shigella), the non-fermenting aerobes (Pseudomonas aeruginosa, Acinetobacter, Stenotrophomonas), and the curved/spiral organisms (Helicobacter pylori, Campylobacter, Vibrio).

Why they matter

• Endotoxin-driven sepsis: lipid A is the single structural feature that makes this group the archetypal cause of gram-negative septic shock, distinguishing them from gram-positive exotoxin-mediated shock syndromes.
• Antimicrobial resistance: extended-spectrum β-lactamase (ESBL) and carbapenemase-producing Enterobacterales and difficult-to-treat Pseudomonas are designated urgent/serious threats by the CDC and drive the IDSA's stand-alone antimicrobial resistance treatment guidance.
• Anatomic breadth: they are the leading cause of urinary tract infection, a dominant cause of hospital-acquired and ventilator-associated pneumonia, and the usual flora recovered from intra-abdominal and biliary sepsis.

Epidemiology worth recalling

• E. coli causes the large majority of community-acquired cystitis and pyelonephritis in otherwise healthy women, and is a leading cause of neonatal sepsis and meningitis alongside group B Streptococcus.
• Klebsiella, Enterobacter, Pseudomonas, Acinetobacter cluster in ICU patients, ventilated patients, burn units, and those with recent broad-spectrum antibiotics or indwelling devices.
• H. pylori colonizes a substantial fraction of the world's population, with higher prevalence in immigrants from high-prevalence regions, and is acquired in childhood by fecal-oral/oral-oral spread.
• Host risk factors repeated on exams: diabetes, cirrhosis/alcohol use disorder, neutropenia, cystic fibrosis, urinary catheters, and recent hospitalization.

Classification and staining

• Cell envelope: thin peptidoglycan retains no crystal violet after alcohol decolorization, so organisms take the safranin counterstain and appear pink rods. The outer membrane also excludes vancomycin and many hydrophobic drugs, explaining intrinsic gram-positive-only drug failure.
• Metabolic split: Enterobacterales are facultative anaerobes that ferment glucose; Pseudomonas and Acinetobacter are non-fermenting obligate aerobes.

Culture and biochemical identification

• MacConkey agar: bile salts/crystal violet inhibit gram-positives; lactose fermenters (E. coli, Klebsiella, Enterobacter, Citrobacter — the "lactose is KEE" mnemonic) turn pink, while Salmonella, Shigella, Proteus, and Pseudomonas remain colorless.
• EMB agar: E. coli yields a green metallic sheen.
• Oxidase: detects cytochrome c oxidase; positive in Pseudomonas, Vibrio, Campylobacter, Helicobacter, Neisseria — negative across Enterobacterales.
• Urease: positive in Proteus, H. pylori, and many Klebsiella; the basis for both struvite stone formation and the urea breath test.
• Other spot tests: indole-positive E. coli; H2S production on TSI/Hektoen by Salmonella and Proteus; Campylobacter grows at 42°C in microaerophilic conditions; H. pylori is a microaerophilic curved rod requiring selective media.
• MALDI-TOF mass spectrometry has largely replaced long biochemical panels for species-level identification in U.S. clinical labs, with automated broth microdilution for susceptibility.

Virulence factors beyond LPS

• Adhesins: FimH type 1 fimbriae bind mannosylated uroplakin (cystitis); P fimbriae bind Gal-Gal on uroepithelium (pyelonephritis).
• Capsule: E. coli K1 resists complement and mediates neonatal meningitis; Klebsiella K1/K2 hypermucoviscous strains give a positive string test.
• Exotoxins: Shiga toxin (cleaves 28S rRNA of the 60S ribosome); ETEC heat-labile (cAMP) and heat-stable (cGMP) enterotoxins; Pseudomonas exotoxin A (ADP-ribosylates EF-2, like diphtheria toxin).
• Biofilm/pigment: alginate mucoid conversion in cystic fibrosis; pyocyanin and pyoverdine give blue-green colonies with a grape-like odor.

From colonization to invasion

• Breach of a barrier is the initiating event: periurethral colonization with ascent along a catheter, aspiration of oropharyngeal flora in a ventilated patient, translocation across ischemic or perforated bowel, or burn/wound inoculation. This is why gram-negative infection is so tightly linked to instrumentation.
• Adhesion precedes disease: fimbrial adhesins anchor uropathogenic E. coli against urine flow; without adherence, micturition clears the inoculum. P-fimbriated strains reach the renal pelvis and produce flank pain and fever rather than isolated dysuria.

The endotoxin cascade — why gram-negative sepsis looks the way it does

• Lipid A → LBP → CD14 → TLR4/MD-2 → NF-κB drives transcription of TNF-α, IL-1, and IL-6. Endotoxin is released in bulk during bacterial lysis, including after the first dose of a bactericidal β-lactam.
• Vasodilation and capillary leak: cytokine-induced inducible nitric oxide synthase produces NO → profound arteriolar dilation → low systemic vascular resistance with compensatory high cardiac output. This is the warm, hyperdynamic shock with bounding pulses and wide pulse pressure that precedes cold, decompensated shock.
• Coagulopathy: cytokines induce endothelial and monocyte tissue factor → thrombin generation, microvascular fibrin deposition, consumption of platelets and factors → disseminated intravascular coagulation with prolonged PT/aPTT, low fibrinogen, elevated D-dimer, schistocytes.
• Organ failure: microvascular thrombosis plus maldistributed flow yields acute tubular necrosis, ARDS, and lactic acidosis from impaired oxygen utilization.

Toxin- and enzyme-specific chains

• Shiga toxin binds Gb3 on renal glomerular endothelium, halting protein synthesis → endothelial injury, platelet microthrombi → the triad of hemolytic anemia, thrombocytopenia, and acute kidney injury.
• Proteus urease hydrolyzes urea to ammonia → urinary alkalinization → precipitation of magnesium ammonium phosphate as staghorn calculi that harbor bacteria and defeat sterilization.
• H. pylori urease buffers gastric acid locally, permitting mucosal colonization; CagA delivered by a type IV secretion system and VacA drive inflammation, and antral-predominant gastritis raises gastrin → acid hypersecretion → duodenal ulcer, whereas corpus-predominant disease causes atrophy, hypochlorhydria, and cancer risk.

Urinary tract

• Cystitis: dysuria, frequency, suprapubic pain without fever; risk factors are female anatomy, sexual activity, spermicide, catheters.
• Pyelonephritis: fever, rigors, flank pain, costovertebral angle tenderness, often with nausea; suggests a P-fimbriated or obstructing process. Recurrent Proteus UTI with alkaline urine and staghorn calculi is the stone-forming variant.

Respiratory tract

• HAP/VAP: new infiltrate, purulent secretions, and worsening oxygenation in a hospitalized or ventilated patient — Klebsiella, Enterobacter, Pseudomonas, Acinetobacter.
• Necrotizing Klebsiella pneumonia: upper-lobe cavitation with thick blood-tinged sputum in patients with alcohol use disorder, diabetes, or aspiration risk.
• Cystic fibrosis: chronic mucoid Pseudomonas colonization with progressive bronchiectasis and declining FEV1.

Gastrointestinal and hepatobiliary

• Traveler's diarrhea (ETEC): watery, non-bloody, self-limited, days after arrival abroad.
• Hemorrhagic colitis (EHEC O157:H7): bloody diarrhea with severe cramping and characteristically little or no fever, following undercooked ground beef, unpasteurized products, or petting-zoo exposure; children and elderly progress to HUS.
• Invasive Klebsiella liver abscess syndrome: fever and right upper quadrant pain in a diabetic patient, with metastatic endophthalmitis or meningitis.
• H. pylori disease: epigastric burning pain — classically relieved by food in duodenal ulcer and worsened by food in gastric ulcer — plus dyspepsia, iron deficiency anemia, and the MALT lymphoma and gastric adenocarcinoma associations.

Pseudomonas-specific syndromes

• Malignant otitis externa in the elderly diabetic; hot tub folliculitis; osteomyelitis after a nail puncture through a sneaker; burn and wound sepsis; ecthyma gangrenosum — a painless necrotic black eschar with an erythematous halo in the neutropenic patient, indicating bacteremic vascular invasion.

Neonatal and bloodstream

• Neonatal sepsis/meningitis: E. coli K1 in the first week of life with temperature instability, poor feeding, bulging fontanelle.
• Bacteremia/septic shock: fever, hypotension, warm extremities early, then multiorgan failure and DIC.

Initial evaluation

• Gram stain of a sterile-site specimen is the fastest orienting test: pink rods in blood, CSF, pleural, or peritoneal fluid immediately narrows empiric therapy.
• Urinalysis: pyuria with positive leukocyte esterase, plus positive nitrite — nitrite reflects bacterial nitrate reductase, present in Enterobacterales but absent in Pseudomonas and Enterococcus, a frequently tested discriminator.
• Blood cultures before antibiotics (two sets, separate sites) in any suspected bacteremia, per the Surviving Sepsis Campaign, which also calls for lactate measurement and antimicrobials within the first hour for septic shock.

Confirmatory testing

• Culture is the gold standard. Quantitative urine culture is conventionally interpreted at ≥10^5 CFU/mL for clean-catch specimens, with lower thresholds accepted in symptomatic or catheterized patients. Species identification is by MALDI-TOF, with susceptibilities reported per CLSI breakpoints.
• Respiratory specimens: sputum or, in ventilated patients, endotracheal aspirate/BAL culture. The ATS/IDSA HAP/VAP guideline favors non-invasive sampling with semiquantitative cultures.
• Imaging: contrast CT for suspected intra-abdominal or renal abscess; obtain renal imaging when pyelonephritis fails to defervesce, to exclude obstruction or emphysematous infection.

Syndrome-specific tests

• EHEC: stool culture on sorbitol-MacConkey agar (O157 is sorbitol-non-fermenting) plus a Shiga toxin immunoassay or nucleic acid test, since non-O157 Shiga toxin strains are missed by culture alone. Monitor CBC, smear, and creatinine for HUS.
• H. pylori: the American College of Gastroenterology recommends active-infection testing — urea breath test or stool antigen — not serology, which cannot distinguish current from past infection. PPIs must be held roughly two weeks and antibiotics/bismuth about four weeks beforehand to avoid false negatives. If endoscopy is performed (alarm features, age-based criteria), biopsy for rapid urease testing and histology. Test of cure is mandatory, at least four weeks after therapy.
• Resistance detection: ESBL is inferred from ceftriaxone non-susceptibility; carbapenemase testing (e.g., modified carbapenem inactivation, molecular assays) distinguishes serine carbapenemases from metallo-β-lactamases and directly changes drug selection.

Directed therapy by syndrome

• Uncomplicated cystitis: per the IDSA uncomplicated cystitis guideline, nitrofurantoin, TMP-SMX (where local resistance is low), or fosfomycin; fluoroquinolones are reserved because of FDA-labeled tendon, neuropathy, and aortic risks. Nitrofurantoin and fosfomycin do not achieve renal parenchymal levels and must not be used for pyelonephritis.
• Pyelonephritis/complicated UTI: a third-generation cephalosporin such as ceftriaxone, or a fluoroquinolone when susceptibility supports it; source control for obstruction or abscess.
• HAP/VAP and sepsis: ATS/IDSA and the Surviving Sepsis Campaign advise early broad-spectrum coverage guided by local antibiograms and risk factors for multidrug resistance (prior IV antibiotics within 90 days, prior resistant isolate, septic shock), then aggressive de-escalation on culture data.
• Pseudomonas: an antipseudomonal β-lactam (piperacillin-tazobactam, cefepime, ceftazidime, meropenem); combination therapy is used empirically in shock or neutropenia to broaden the chance of an active drug, not for synergy, and is narrowed once susceptibilities return.

Resistance patterns that change the choice

• AmpC inducible organisms (Enterobacter, Citrobacter freundii, Serratia): avoid third-generation cephalosporins even if susceptible in vitro — cefepime or a carbapenem is preferred (IDSA resistant gram-negative guidance).
• ESBL producers: carbapenems for bacteremia and serious infection; for ESBL cystitis alone, narrower oral agents such as nitrofurantoin or TMP-SMX are acceptable.
• Carbapenem-resistant Enterobacterales: newer β-lactam/β-lactamase inhibitor combinations (ceftazidime-avibactam, meropenem-vaborbactam, imipenem-relebactam); metallo-β-lactamase producers require cefiderocol or ceftazidime-avibactam plus aztreonam.
• Difficult-to-treat Pseudomonas: ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, or cefiderocol.

Special situations

• Shiga toxin-producing E. coli: supportive care and volume; the IDSA infectious diarrhea guideline advises against antibiotics and antimotility agents, which increase HUS risk.
• H. pylori: the ACG favors bismuth quadruple therapy (PPI, bismuth, tetracycline, metronidazole) given clarithromycin resistance; clarithromycin triple therapy only where resistance is known to be low and there is no prior macrolide exposure.
• Prevention: no vaccine exists for E. coli, Klebsiella, or Pseudomonas; ACIP-recommended typhoid vaccine covers Salmonella Typhi, and Hib conjugate vaccine covers the gram-negative coccobacillus H. influenzae type b.

• Oxidase is the pivot point: an oxidase-positive, non-lactose-fermenting, obligately aerobic gram-negative rod with blue-green pigment and a grape-like odor in a burn, neutropenic, or cystic fibrosis patient is Pseudomonas aeruginosa. Enterobacterales are oxidase-negative.
• Bloody diarrhea after undercooked beef → do not give antibiotics. The single best next step is supportive hydration plus stool Shiga toxin testing and monitoring of CBC and creatinine; antibiotics and loperamide increase HUS risk. The distractor answer is ciprofloxacin.
• Nitrofurantoin is a cystitis-only drug. Choosing it for a febrile patient with flank pain is a classic trap — it does not reach renal tissue concentrations.
• Enterobacter, Citrobacter, Serratia harbor inducible AmpC: a third-generation cephalosporin can select for resistance mid-treatment even when the initial report says susceptible; cefepime or a carbapenem is the safer choice.
• Urease is the shared thread between Proteus (alkaline urine, staghorn/struvite calculi, swarming motility) and H. pylori (urea breath test, ammonia-mediated acid buffering).
• Serology cannot confirm eradication of H. pylori. After treatment, use urea breath test or stool antigen at least four weeks later, off PPI — a repeatedly tested ACG point.
• Painless black necrotic eschar with erythematous rim in a febrile neutropenic patient is ecthyma gangrenosum; obtain blood cultures and start antipseudomonal coverage immediately.
• Neonatal meningitis in the first week: E. coli K1 capsule and group B Streptococcus are the two organisms to name; the capsule, not endotoxin, explains complement resistance and CNS invasion.
• Warm shock with wide pulse pressure and high cardiac output points to lipid A/TLR4-mediated vasodilation — do not confuse this hemodynamic picture with the low-output, cold shock of cardiogenic failure.

• Enterobacteriaceae (E. coli, Klebsiella, Proteus) are most common gram-negative rods clinically
• Possess lipopolysaccharide (LPS) endotoxin in outer membrane → triggers sepsis/shock
• Facultative anaerobes; ferment glucose; produce catalase (positive)
• Oxidase-negative (key distinguishing feature from Pseudomonas/Vibrio)
• Common causes of UTIs, pneumonia, intra-abdominal infections, bacteremia

• E. coli: Most common cause of community UTI, neonatal meningitis (K1 capsule), traveler's diarrhea (ETEC)
• Klebsiella: Nosocomial pneumonia (mucoid colonies), liver abscesses in alcoholics
• Proteus: "Swarming" motility on agar; urea-positive (urease+); chronic UTIs with struvite stones
• Pseudomonas aeruginosa: Opportunistic in immunocompromised; green/blue pigment; respiratory infections
• Enterobacter/Citrobacter: Nosocomial infections; intrinsic ampicillin resistance

1. 1.Confusing oxidase testing: Enterobacteriaceae are oxidase-negative; Pseudomonas/Vibrio are oxidase-positive—critical for differentiation
2. 2.Assuming ampicillin coverage: Most nosocomial gram-negative rods have intrinsic or acquired β-lactamase resistance; use 3rd-generation cephalosporins or broader agents empirically in hospital settings
3. 3.Missing Pseudomonas in non-fermenter cases: Remember it's gram-negative rod but oxidase-positive and non-fermenter—often missed if only checking glucose fermentation

• Community-acquired UTI (E. coli): Nitrofurantoin or TMP-SMX (oral)
• Pneumonia/sepsis (empiric): 3rd-generation cephalosporin (ceftriaxone) ± aminoglycoside or fluoroquinolone
• ESBL producers: Carbapenem (meropenem, ertapenem)
• Pseudomonas: Antipseudomonal β-lactam (piperacillin-tazobactam, cefepime) + aminoglycoside/fluoroquinolone
• Adjust based on culture/sensitivity