STEPTRACKERMICROBIOLOGY + GASTROENTEROLOGY | ENTERIC PATHOGENS | HIGH-YIELD REVIEW

Enteric Pathogens

Salmonella, Shigella, Campylobacter, Yersinia and Vibrio — sort them by source, stool type, infectious dose and the complication each leaves behind.

FOCUS ON
Salmonella: flagella, H₂S ⁺, animal reservoir (poultry, eggs, reptiles), high infectious dose, ↓ by acid; S Typhi → typhoid (rose spots, constipation, relative bradycardia, gallbladder carriers). Shigella: nonmotile, H₂S −, humans only, tiny dose, invades M cells → dysentery. Campylobacter: comma/S-shaped, grows at 42 °C, poultry → Guillain-Barré, reactive arthritis. Yersinia enterocolitica: pork, milk → pseudoappendicitis. Vibrio cholerae: rice-water stools; V vulnificus: raw oysters + liver disease → sepsis.
ORGANISM
SOURCE · BIOLOGY
DISEASE
CLUE · COMPLICATIONS
Nontyphoidal Salmonella
Poultry, eggs, reptiles, pets; motile, H₂S ⁺; macrophage survival
Fever, diarrhea (may be bloody) 8–48 h after food
Antibiotics prolong carriage · osteomyelitis in sickle cell; bacteremia in elderly, HIV
Salmonella Typhi
Humans only; Vi capsule; invades Peyer patches → reticuloendothelial system
Typhoid: step-wise fever, abdominal pain, constipation then diarrhea, rose spots, relative bradycardia, hepatosplenomegaly
Gallbladder carriage · intestinal perforation · ceftriaxone, fluoroquinolone; vaccine for travelers
Shigella
Humans only; nonmotile, H₂S −; very low infectious dose (10–200); acid-stable; invades M cells, cell-to-cell spread; Shiga toxin (S dysenteriae)
Fever, cramps, bloody mucoid diarrhea (dysentery), tenesmus
Daycare outbreaks · reactive arthritis, HUS · azithromycin or ciprofloxacin
Campylobacter jejuni
Undercooked poultry, unpasteurized milk, puppies; comma-shaped, oxidase ⁺, grows at 42 °C
Fever, bloody diarrhea, abdominal pain (can mimic appendicitis)
Most common bacterial diarrhea in US · Guillain-Barré, reactive arthritis
Yersinia enterocolitica
Pork, unpasteurized milk, chitterlings; grows in the cold
Bloody diarrhea, mesenteric adenitis → pseudoappendicitis (RLQ pain)
Iron overload (deferoxamine) ↑ risk · reactive arthritis, erythema nodosum
Vibrio cholerae
Contaminated water; comma-shaped, oxidase ⁺; acid-sensitive; grows in alkaline TCBS
Massive rice-water diarrhea, dehydration, hypokalemia, acidosis
Oral rehydration therapy · vaccine
Vibrio parahaemolyticus
Raw seafood (Japan)
Watery or bloody diarrhea
—
Vibrio vulnificus
Raw oysters, seawater wounds
Fulminant sepsis, hemorrhagic bullae, necrotizing wound infection
Cirrhosis/hemochromatosis patients · doxycycline + ceftazidime
Reactive arthritis
HLA-B27; after Shigella, Salmonella, Campylobacter, Yersinia, Chlamydia
Conjunctivitis, urethritis, arthritis
Sterile joints

Mechanism — why infectious dose differs

Stomach acidKills most bacteria
→
Acid-sensitive: Vibrio, SalmonellaNeed high doses
Acid-resistant: Shigella~10 organisms enough

Antacids, PPIs and gastrectomy lower the dose needed — more Salmonella and C difficile.

Salmonella vs Shigella

FEATURE
SALMONELLA
SHIGELLA
Reservoir
Animals (Typhi: humans)
Humans only
Motility
Flagella
Nonmotile
H₂S
+
−
Infectious dose
High
Very low
Spread
Hematogenous (macrophages)
Cell-to-cell
Antibiotics
Avoid in mild gastroenteritis
Shorten illness

How it's tested

COMMON CONFUSIONYersinia pseudoappendicitis and Campylobacter both mimic appendicitis — Yersinia with pork/milk and iron overload, Campylobacter with poultry and later Guillain-Barré.
THE TRAPRose spots + relative bradycardia + constipation in a traveler = typhoid, even before diarrhea starts.
YOUR APPROACHFood/water source → stool (watery vs bloody) → fever → organism biology → post-infectious complication.

If you see… think…

Diarrhea after handling a pet turtle → Salmonella
Sickle cell patient with osteomyelitis → Salmonella
Rose spots, fever, relative bradycardia after travel → typhoid (S Typhi)
Chronic typhoid carrier site → gallbladder
Daycare outbreak of bloody diarrhea, tiny inoculum → Shigella
Ascending weakness 2 weeks after bloody diarrhea → Campylobacter → Guillain-Barré
RLQ pain mimicking appendicitis after chitterlings → Yersinia enterocolitica
Rice-water stools after drinking contaminated water → V cholerae
Cirrhotic with sepsis and bullae after raw oysters → V vulnificus
Conjunctivitis, urethritis, arthritis after dysentery → reactive arthritis (HLA-B27)