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Questions and Answers
Large numbers of epithelial cells in feces indicate that the intestinal mucosa is scratched.
True
A large number of leukocytes in stool are typical of cholera.
False
Iodine staining method is preferred for detecting live motile trophozoites.
False
The saline wet mount method uses an isotonic solution to prevent osmotic lysis.
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Shigellosis typically shows a small number of pus cells per high power field.
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Iodine staining makes it easier to view the nuclei of some cyst forms of protozoans under the microscope.
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Saline wet mount is useful for detecting live motile trophozoites of protozoans like Entamoeba histolytica.
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Buffered methylene blue is used in the stool concentration technique.
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Peptone water is an example of simple media.
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Enriched media is used to grow non-fastidious bacteria.
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MacConkey agar is a type of selective media.
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Blood agar can be classified as a complex media.
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Selective media typically contain inhibitory agents that affect non-pathogenic bacteria but not pathogens.
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Microbial culture involves multiplying microorganisms by letting them reproduce in predetermined culture media under controlled conditions.
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The primary components provided in a culture medium for microbial growth are carbohydrates and amino acids.
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Culturing bacteria is the final step in studying their morphology and identification.
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Artificial culture media can approximate the pH of tissues and body fluids.
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One purpose of culturing bacteria is to estimate viable counts.
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Liquid media can be used to observe colony morphology.
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Solid media are used for the storage of cultures, pure culture isolations, and observing colony appearance.
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Agar concentrations of 0.2 - 0.5% in a medium make it semi-solid.
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Egg yolk and agar-agar are solidifying agents used to create solid media.
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Liquid media are not suitable for fermentation studies.
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Semi-solid media are used in fermentation studies and determining bacterial motility.
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Differential media can distinguish bacteria based on colony color.
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MacConkey agar is an example of an indicator media.
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Blood agar is made with a basal medium and 5-10% human blood.
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Beta hemolysis results in complete lysis of red blood cells, creating a clear area around colonies.
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Alpha hemolysis demonstrates partial lysis of red blood cells, resulting in a blueish discoloration around colonies.
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Gamma-hemolysis does not cause lysis of red blood cells.
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Normal stool pH is between 6 and 7.5.
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Benzidine powder is used to detect the presence of occult blood in stool.
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A stool pH of 8 is considered acidic.
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Mono sugar and di sugar levels in the stool of 10mg/g are within normal limits.
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Xylose Lysine Deoxycholate (XLD) Agar is recommended for isolating Salmonella and Shigella from fecal samples.
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MacConkey's Agar is used to inhibit gram-negative organisms.
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Toxins produced by bacteria can be ingested via food and cause diarrhea even if the bacteria themselves are not present in feces.
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Sorbitol MacConkey Agar is useful for screening for E. coli O157 because E. coli O157 ferments sorbitol.
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Study Notes
Microscopic Examination of Stool
- Microscopic analysis of stool specimens includes examining the sample for:
- Leukocytes
- Epithelial cells
- Blood
- Qualitative fat
- Parasites (eggs, larvae, cysts)
- A large number of leukocytes (> 50 cells per high power field) is typical of shigellosis
- A smaller number of pus cells (< 20 per high power field) is found in salmonellosis and invasive E. coli infections
- Few leukocytes (< 5 cells per high power field) are present in cholera, EPEC, and ETEC, and viral diarrhea
Epithelial Cells
- Normally, small to moderate numbers of epithelial cells are present in feces
- Large numbers of epithelial cells (or large amounts of mucus) indicate that the intestinal mucosa is scratched
Laboratory Diagnosis of Parasitic Infections
- The laboratory diagnosis of most parasitic infections is by demonstrating the ova of the parasite (stool ova and parasites test) in the infected person's stools
- Techniques used to examine the stool include:
- Saline wet mount examination
- Iodine preparation
- Buffered methylene blue stain
- Concentration techniques
Stool Examination (Saline Wet Mount and Iodine Staining) Method
- Gastrointestinal infestations (infections) by parasites are primarily diagnosed by detecting:
- Live motile trophozoites (protozoans)
- Cysts (inactive dormant stage of Protozoa)
- Eggs and larvae (in case of Helminths) in stool
- Saline wet mount is used for the detection of:
- Parasitic trophozoites and cysts of protozoa
- Eggs and larvae of helminths
- Iodine staining method is used for the detection of:
- Nuclei of some cyst forms of protozoans
Chemical Testing of Stool
- pH: Normal stool pH is 6-7.5
- pH is acidic in amoebic dysentery
- pH is alkaline in bacillary dysentery
- Occult Blood: Occult blood may be present in a variety of conditions, including malignancy of the gastrointestinal tract
- Reagent used is benzidine powder
- Reducing Factors: Mono sugar and di sugar levels in stool (6mg/g) indicate disturbance in enzymes that digest sugar (e.g. lactase, sucrase)
Stool Culturing
- Certain bacteria are normally found in feces (the "normal flora" of the bowel)
- The presence of pathological types of bacteria may produce diarrhea and other signs of systemic infection
- Bacteria produce diarrhea in three main ways:
- The organisms invade the intestinal wall, damaging tissue
- The organisms produce toxins within the intestine that alter gastrointestinal motility
- Toxins produced by bacteria are ingested (e.g. via foods) and produce diarrhea, although the organisms themselves are not detected in feces
Culture Media
- MacConkys Agar: Inhibits most of the gram-positive organisms between lactose fermenters and non-lactose fermenters
- Xylose Lysine Deoxycholate (XLD) Agar: Selective medium for the isolation of Salmonella and particularly Shigella from fecal samples
- Thiosulphate Citrate Bile Salt Sucrose (TCBS) Agar: Selective medium for the primary isolation of Cholerae
- Sorbitol MacConkys Agar: This medium is useful for screening 0157 E. coli
Culture Media Classification
- Based on consistency, culture media can be classified into:
- Liquid media
- Solid media
- Semi-solid media
Cultivation
- Microbial culture is a method of multiplying microorganisms by letting them reproduce in predetermined culture media under controlled laboratory conditions
- The artificial medium should provide nutrients and a pH approximating those of tissues and body fluids
Purpose of Culturing
- Isolation of bacteria
- Properties of bacteria (i.e. culturing bacteria is the initial step in studying its morphology and identification)
- Estimate viable counts
- To test for antibiotic sensitivity
- To create antigens for laboratory use
Differential and Indicator Media
- Certain media are designed to recognize different bacteria based on their colony color
- Examples: MacConkey agar, Thiosulphate Citrate
Blood Agar
- Blood agar is considered differential because it is used to distinguish pathogenic bacteria based on the effect of bacterial enzymes known as hemolysins, which lyse red blood cells
- Blood agar contains a basal medium and 5-10% sheep, horse, or rabbit blood
Types of Hemolysis
- There are three types of hemolysis:
- Beta hemolysis (β- hemolysis): Complete lysis or destruction of red blood cells
- Alpha hemolysis (α- hemolysis): Partial lysis of RBC, resulting in greenish discoloration
- Gamma-hemolysis (y-hemolysis): No lysis of RBCs
Microbiological Media
- Classifying Media:
- Based on the nutritional component:
- Simple (Basal) media
- Complex (Enriched) media
- Based on functional use or application:
- Basal media
- Enriched media
- Selective media
- Based on the nutritional component:
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Description
This quiz covers the microscopic analysis of stool specimens, including the identification of leukocytes, epithelial cells, blood, and parasites. It also touches on the characteristic findings in shigellosis and salmonellosis.
