STEPTRACKERMICROBIOLOGY + NEPHROLOGY | GRAM-NEGATIVE RODS | HIGH-YIELD REVIEW

Enterobacteriaceae

The gut's gram-negative rods — lactose fermenters that cause most UTIs and hospital infections, plus the urease-making stone former.

FOCUS ON
E coli virulence by disease: fimbriae (P pili) → UTI/pyelonephritis; K1 capsule → neonatal meningitis; LPS → septic shock; strains: ETEC (traveler's, LT/ST), EPEC (infants, no toxin), EIEC (invasive, dysentery), EHEC O157:H7 (Shiga-like toxin, no sorbitol fermentation → HUS). Klebsiella: big capsule, currant-jelly sputum, alcoholics/aspiration, abscesses. Proteus: urease ⁺, swarming → struvite stones. Serratia: red pigment, catalase ⁺. ESBL/CRE resistance.
ORGANISM · STRAIN
VIRULENCE
DISEASE
CLUE · TREATMENT
Uropathogenic E coli
P fimbriae bind uroepithelium
Cystitis, pyelonephritis — #1 cause of UTI
Nitrofurantoin, TMP-SMX, fosfomycin; ceftriaxone for pyelo
E coli K1
K1 capsule
Neonatal meningitis (2nd to GBS) and sepsis
Ampicillin + cefotaxime / gentamicin
ETEC
Heat-labile (↑ cAMP) + heat-stable (↑ cGMP) toxins
Traveler's diarrhea (watery)
LT works like cholera toxin · fluids
EPEC
Attaches, effaces microvilli; no toxin
Watery diarrhea in infants (developing countries)
Infant-ward outbreaks
EIEC
Invades colonic mucosa (like Shigella)
Dysentery with fever
—
EHEC O157:H7
Shiga-like toxin (phage); doesn't ferment sorbitol
Bloody diarrhea without fever → HUS (anemia with schistocytes, thrombocytopenia, AKI) in children
Undercooked beef, petting zoos, sprouts · avoid antibiotics
Klebsiella pneumoniae
Large polysaccharide capsule (mucoid colonies), urease ⁺
Lobar pneumonia with currant-jelly sputum in alcoholics, diabetics; aspiration; lung/liver abscesses; UTI; nosocomial
Mucoid colonies · carbapenemase (KPC) strains
Proteus mirabilis
Urease ⁺ → ↑ urine pH; swarming motility
UTI with struvite (Mg-NH₄-PO₄) staghorn calculi
Alkaline urine, ammonia smell
Serratia marcescens
Red pigment; catalase ⁺; slow lactose fermenter
Hospital UTI, pneumonia, IV drug endocarditis
CGD pathogen
Enterobacter · Citrobacter
Inducible AmpC β-lactamase
Nosocomial UTI, pneumonia, bacteremia
Cefepime or carbapenem
ESBL / CRE
Extended-spectrum β-lactamases · carbapenemases (KPC, NDM)
Resistant hospital infections
ESBL → carbapenem · CRE → ceftazidime-avibactam, polymyxins

Mechanism — Proteus makes its own stones

UreaseUrea → NH₃ + CO₂
→
Urine pH ↑
→
Struvite precipitatesMg-NH₄-PO₄
→
Staghorn calculusShelters bacteria

Other urease-positive organisms (Klebsiella, S saprophyticus, Ureaplasma) can do the same.

E coli strains compared

STRAIN
TOXIN
STOOL
WHO
ETEC
LT, ST
Watery
Travelers
EPEC
None
Watery
Infants
EIEC
Invasion
Bloody, fever
Any
EHEC
Shiga-like
Bloody, no fever
Children → HUS

How it's tested

COMMON CONFUSIONEHEC is distinguished in the lab by failure to ferment sorbitol (sorbitol MacConkey) — other E coli ferment it.
THE TRAPAntibiotics in EHEC may increase toxin release and HUS — supportive care only.
YOUR APPROACHLactose fermenter gram ⁻ rod → site (urine, CSF, gut, lung) → virulence factor → strain or species → resistance pattern.

If you see… think…

Most common cause of UTI → E coli (P fimbriae)
Neonatal meningitis, gram ⁻ rod → E coli K1
Watery diarrhea on a trip abroad → ETEC
Child with bloody diarrhea then anemia, low platelets, AKI → EHEC O157:H7 (HUS)
Alcoholic with currant-jelly sputum → Klebsiella
Liver abscess in a diabetic from Asia → hypervirulent Klebsiella
Staghorn calculus + alkaline urine → Proteus (urease)
Red-pigmented colonies in a hospital outbreak → Serratia
Doesn't ferment sorbitol → EHEC
Klebsiella resistant to carbapenems → KPC (CRE)