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Microbiology ` microbiology—systems Microbiology ` microbiology—systems SEcTioN ii 179

Bugs causing diarrhea
Bloody diarrhea
Campylobacter Comma- or S-shaped organisms; growth at 42°C
E histolytica Protozoan; amebic dysentery; liver abscess
Enterohemorrhagic O157:H7; can cause HUS; makes Shiga-like toxin
E coli
Enteroinvasive E coli Invades colonic mucosa
Salmonella (non- Lactose ⊝; flagellar motility; has animal reservoir, especially poultry and eggs
typhoidal)
Shigella Lactose ⊝; very low ID 50 ; produces Shiga toxin; human reservoir only; bacillary dysentery

Y enterocolitica Day care outbreaks; pseudoappendicitis
Watery diarrhea
C difficile Pseudomembranous colitis; associated with antibiotics and PPIs; occasionally bloody diarrhea
C perfringens Also causes gas gangrene
Enterotoxigenic E coli Travelers’ diarrhea; produces heat-labile (LT) and heat-stable (ST) toxins
Protozoa Giardia, Cryptosporidium
V cholerae Comma-shaped organisms; rice-water diarrhea; often from infected seafood
Viruses Rotavirus, norovirus, enteric adenovirus

Common causes of pneumonia
NeoNates (< 4 WK) cHilDreN (4 WK–18 yr) aDUlts (18–40 yr) aDUlts (40–65 yr) elDerly
Group B streptococci Viruses (RSV) Mycoplasma S pneumoniae S pneumoniae
E coli Mycoplasma C pneumoniae H influenzae Influenza virus
C trachomatis S pneumoniae Anaerobes Anaerobes
(infants–3 yr) Viruses (eg, influenza) Viruses H influenzae
C pneumoniae Mycoplasma Gram ⊝ rods
(school-aged
children)
S pneumoniae
Runts May Cough
Chunky Sputum
Special groups
Alcoholic Klebsiella, anaerobes usually due to aspiration (eg, Peptostreptococcus, Fusobacterium, Prevotella,
Bacteroides)
IV drug users S pneumoniae, S aureus
Aspiration Anaerobes
Atypical Mycoplasma, Chlamydophila, Legionella, viruses (RSV, CMV, influenza, adenovirus)
Cystic fibrosis Pseudomonas, S aureus, S pneumoniae, Burkholderia cepacia
Immunocompromised S aureus, enteric gram ⊝ rods, fungi, viruses, P jirovecii (with HIV)
Nosocomial (hospital S aureus, Pseudomonas, other enteric gram ⊝ rods
acquired)
Postviral S pneumoniae, S aureus, H influenzae

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