Sunday, 9 December 2007

Case 5 follow up

WFH's case of wound infection.

Revised list of suspected microorganisms:
1. S. aureus
2. S. pyrogenes
3. Clostridium perfringens
4. Clostridium botulinum
5. Pseudomonas aeruginosa
6. S. faecalis (enterococcus)
7.Proteus mirabilis


Key characteristics

Lab diagnosis

S. aureus

Produce coagulase enzyme

Coagulase positive (agglutination)

Produce catalase

Catalase positive [Frothing when react with H2O2 ]

Ferments mannitol

Yellow colonies on mannitol salt agar

Lyses rbc

B haemolysis on BA

B lactamase

B lactam resistant. Sensitivity to naficin, methicillin, vancomycin

Gram positive (purple) cocci, “grape-like”

S. pyogenes

Lyses rbc

B haemolysis on BA

Does not produce catalase

Catalase negative

Bacitracin sensitive

Hippurate and cAMP negative

Gram positive cocci in chains

C. perfringens

Double zone of haemolysis

B haemolysis on BA

Alpha lecithinase

Lecithinase positive on egg yolk agar

Gram positive, large pink rods

Proteolysis on CM (meat turn black)

C. botulinum


Swarming effect on agar

Lipase positive

Lecithinase and protease neg

Meat at the bottom of CM remain brown

Gram positive, oval and subterminal spores, bacilli

P. aeruginosa

Non fermentative

Pink colonies on macconkey agar

Oxidase positive

Pyocyanin pigment

Blue on pseudomonas agar P

Pyoverdin pigment

Fluorescein on Pseudomonas agar F

Utilizes citrate

Citrate + (green to blue on citrate agar slant)


Unable to hydrolyse and deaminate tryptophan

Indole -

Does not utilize carbohydrates


S. faecalis

Non haemolytic

Gamma haemolysis on BA

Tolerant to 6.5% NaCl

Growth on mannitol salt agar

Resistant to bile

Hydrolyses esculin

Blackening of esculin agar

Does not produce B lactamase

B lactam sensitive

Gram positive cocci in chains

P. mirabilis

Unable to metabolise lactose

Pink colonies on MAC

Distinct ordor

Utilises urea and citrate

Citrate +

H2S production

Black precipitate on TSI

Unable to hydrolyse and deaminate tryptophan

Indole -

Nitrogen reductase -

Mixed acid fermentation

Methyl red +

Unable to ferment glucose

Voges- Proskauer -

Produce catalase

Catalase +

Oxidase -

Phenylalanine deaminase +

Yeng Ting
Tg 02

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