22/08/2025
✨Gram Negative Bacilli 🦠🦠
" These common Gram Negative Bacilli have a distinctive appearance that allows for immediate preliminary identification " ☺️😊🥰
🌹🧫 Escherichia coli (E.coli)
🔸 Gram stain:
Gram negative bacilli (often short, plump rods).
🔸 Culture:
MacConkey Agar → Pink colonies (lactose fermenter).
EMB agar → Metallic green sheen.
🔸 Biochemical tests:
Indole positive.
Oxidase negative.
Catalase positive.
🔸 Clinical importance: UTI, septicemia, neonatal meningitis, gastroenteritis.
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🍁 Klebsiella pneumoniae
🔸 Gram stain: Gram negative bacilli with prominent capsule (capsule = halos around bacteria in India ink).
🔸 Culture:
MacConkey Agar :- Large, mucoid, pink colonies (strong lactose fermenter).
🔸 Biochemical tests:
Urease positive.
Indole negative.
🔸 Clinical importance: Pneumonia (currant jelly sputum), UTIs, septicemia, opportun
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💐🧫 Pseudomonas aeruginosa on
Blood Agar
Colonies large, flat, with irregular edges
Green-blue pigment (pyocyanin), sometimes yellow-green (pyoverdine)
May show slight beta-hemolysis occasionally
Rough texture or mucoid if the strain produces excess capsule
💐🧫 Pseudomonas aeruginosa on MacConkey Agar
Colorless colonies → non-lactose fermenter
Colonies usually round, smooth, sometimes with metallic sheen
💡 Notes:-
Blood Agar colonies have a distinctive odor: grape-like / earthy
Key lab features:
Oxidase positive (+ )
pigment ، characteristic odor
: Distinctive Test
Oxidase positive ✅
Catalase positive ✅
Characteristic odor: grape-like / earthy / incense-like
Motility:
Motile (polar flagella)
Clinical Significance:
Opportunistic pathogen → nosocomial infections (burns, wounds, UTI, pneumonia, especially in cystic fibrosis patients)
Highly antibiotic-resistant
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🧫 Proteus mirabilis 🦠🦠
Media / Appearance
Blood Agar → swarming pattern (colonies spread in wavy concentric rings)
MacConkey Agar → colorless colonies (non-lactose fermenter)
Distinctive Tests:
Urease positive ✅ (converts urea → ammonia)
H₂S positive (black precipitate on TSI/KIA)
Oxidase negative ❌
Pigment / Odor:
No distinctive pigment (colonies usually colorless)
Characteristic odor: strong, pungent, unforgettable (foul smell)
Motility:
Highly motile → swarming behavior
Clinical Significance:
Recurrent UTIs, kidney stones (struvite calculi)
Occasionally wound infections
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🧫 Enterobacter spp🦠🦠
Media / Appearance:
Blood Agar → large, moist, smooth colonies, sometimes mucoid (usually non-hemolytic / gamma hemolysis)
MacConkey Agar → mucoid pink colonies (lactose fermenter)
EMB Agar → pink colonies, no metallic sheen
Distinctive Tests:
Motility ✅ (peritrichous flagella)
Urease variable (depends on species)
Oxidase ❌
Indole ❌ (except E. cloacae may vary)
Citrate ✅ (can use citrate as sole carbon source)
Pigment / Odor:
Usually no distinctive pigment
Odor mild / not very strong
Motility:
Motile → peritrichous flagella
Clinical Significance:
Opportunistic pathogen → respiratory infections, UTIs, wounds, septicemia
Often hospital-acquired infections
Can be antibiotic-resistant
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🧫 Salmonella spp🦠🦠
Media / Appearance:
Blood Agar → Colonies round, smooth, moist, usually colorless to grey, typically non-hemolytic
MacConkey Agar → Colorless colonies (non-lactose fermenter)
XLD Agar → Red colonies with black center (H₂S positive)
Distinctive Tests (Biochemical):
Motility ✅
H₂S ✅ (produces black precipitate on TSI/KIA)
Urease ❌ (negative)
Oxidase ❌ (negative)
Indole ❌ (usually negative)
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🧫 Shigella spp🦠🦠
Media / Appearance:
Blood Agar → Colonies small, smooth, moist, usually colorless, non-hemolytic
MacConkey Agar → Colorless colonies (non-lactose fermenter)
XLD Agar → Red colonies, no black center (H₂S negative)
Distinctive Tests (Biochemical):
Motility ❌ (non-motile → key feature)
H₂S ❌ (negative)
Urease ❌ (negative)
Oxidase ❌ (negative)
Indole ✅ (depends on species: S. dysenteriae negative, S. flexneri usually
(negative, S. sonnei positive sometimes)
