Lab diagnosis of diarrhoea involves collecting stool or rectal swab samples and transporting them to the lab within 2 hours using Cary Blair medium. Samples are analyzed through microscopy, culture, biochemical tests and serotyping to identify bacteria like E. coli, Salmonella, Staphylococcus aureus, and Vibrio that can cause diarrhoea based on their characteristics like colony morphology, gram staining, motility, and biochemical reactions. Epidemiological typing through bacteriophage is also done to trace the source of infection for some bacteria.

1. Lab diagnosis of DIARRHOEA
Made by –
Prashant Arya
Roll no. - 74

2. Definition of Diarrhoea
Diarrhoea is defined as an increase in the
frequency, fluidity or volume of bowel
movements, relative to the usual habits of an
individual.
Passage of three or more motions a day can
be taken as diarrhoea

4. Sample collection
I. Stool
II. Rectal swab

5. Transport
Specimens after collection should reach
the laboratary with minimum delay(2
hours).
The medium used here is CARY BLAIR’S
MEDIUM.

6. History
- History of travel.
- History of antibiotics.
- Duration of diarrhoea.
- History of passage of blood.
- History of passage of mucous.
- History of rice water stools.
- History of food intake.
- History of fever

7. Sample processing-
- Routine/Microscopy
-Gross- colour, consistency, blood, mucus,
parasite, pH, occult blood
Microscopy- Wet mount- saline & iodine.
- To look for pus cells/RBC.
- To look for ova/cyst/trophozoite.
- To look for darting motility (vibrio).

8. Culture
 Pre-enrichment
-Blood agar
-MacConkey agar
-Deoxycholate agar (DCA)/Xylose lysine
deoxycholate (XLD)
-Thiosulphate citrate-bile-salts agar (TCBS)(if
there is history of rice water stool)

9. Enrichment media
- Selenite F broth
- Alkaline peptone water ( if there is
history of rice water stool )

10.  Post-enrichment media
- MacConkey agar
- DCA/XLD
- TCBS (if there is history of rice water stool)

11.  Colony characteristics :-
1. Staphylococcus aureus –
Blood agar- 2-4mm in
diameter,circular,raised,opaque
Nutrient agar -golden yellow pigment.
2. E.coli – On MacConkey media, colonies are pink
due to lactose fermentation.
3. Salmonella – On MacConkey agar &DCA, colonies
are colourless due to non-lactose fermentation
(NLF).
4. Vibrio – On MacConkey agar , colonies are pale
TCBS shows yellow colonies.

12.  Gram-staining
- Staph. aureus-Gram positive cocci arranged in
clusters- 1um in diametre , non-sporing.
- E. coli-Gram negative bacillus, 1-3um*0.4-0.7um
, non-capsulated , non-sporing ,
- Salmonella-Gram negative bacillus –, 1-
3um*0.5um , non-sporing , non-capsulated.
- Vibrio-Gram negative curved/ comma shaped
rod , non-sporing , non-capsulated , 1.5um*0.2-
0.4um in size.

13.  Motility
MOTILE – Salmonella
, E.coli , vibrio
(darting motility).
NON-MOTILE –
Staphylococcus
aureus, Shigella

14.  Biochemical reactions
Indole Urease Citrate VP MR
E.Coli + - - - +
Salmonella - - +/- - +
Staphylococcus aureus-
1. Catalase test – positive
2. Coagulase test (slide & tube) – positive
3. Mannitol fermentation – acid production without gas.
Vibrio
1. Catalase – positive
2. Oxidase – positive
3. Indole – positive
4. Reduces nitrates to nitrites
5. Voges–proskauer(VP) – positive

15. Serotyping
Epidemiological typing
 Bacteriophage typing - It is done for
epidemiological purposes to trace the
source infection.
 It is done in Staphylococcus aureus and
Salmonella.