BIOL 3150 – Microbiology for the Healthcare Professions
Midterm Study Guide Explanation
1. Contamination
Purpose of Disinfecting Surfaces
Disinfecting surfaces is important to:
Kill or reduce microorganisms on surfaces
Prevent cross-contamination between patients, samples, and healthcare workers
Reduce spread of infectious disease
Maintain a sterile working environment
Common disinfectants include alcohol, bleach, and hospital-grade disinfectants.
Why Hand Washing is Important
Hand washing:
Removes transient microorganisms
Prevents transmission of pathogens
Protects both healthcare workers and patients
Reduces healthcare-associated infections (HAIs)
Proper handwashing typically includes:
Wet hands
Apply soap
Scrub at least 20 seconds
Rinse
Dry with clean towel
2. Specimen Collection
Sterile Containers vs Nonsterile Containers
Sterile containers
Free from microorganisms
Used when contamination must be prevented
Examples:
Blood cultures
Urine cultures
Wound cultures
Nonsterile containers
Used when normal microbes are already present
Examples:
Stool samples
Sputum samples (sometimes)
Information Needed When Labeling a Specimen
Proper labeling prevents errors.
Required information usually includes:
Patient name
Patient ID number
Date and time of collection
Type/source of specimen
Collector’s initials
Physician or test ordered
Transport Media
Transport media keeps microorganisms alive during transport to the laboratory.
Purpose:
Prevents organisms from drying out
Prevents overgrowth of contaminants
Maintains viability of pathogens
Example: swabs placed in transport tubes.
Why Specimen Storage Requirements Matter
Improper storage can:
Kill pathogens
Allow contaminating organisms to grow
Give false results
Examples:
Some samples require refrigeration
Some must be processed immediately
Some must be kept at room temperature
When a Specimen Should Be Rejected
Common reasons:
Improper labeling
Leaking container
Contaminated sample
Wrong container
Sample stored improperly
Sample received too late
Insufficient quantity
3. Media
Nonselective Media
Supports growth of many different organisms.
Example:
Nutrient agar
Blood agar
Purpose:
Grow a wide range of bacteria.
Selective Media
Contains substances that inhibit some organisms but allow others to grow.
Example:
MacConkey agar (selects for Gram-negative bacteria)
Differential Media
Distinguishes organisms based on metabolic or biochemical characteristics.
Example:
MacConkey agar differentiates lactose fermenters vs non-fermenters
Colonies change color depending on reaction.
4. Staining
Simple Stain
Uses one dye to visualize bacteria.
Purpose:
Observe shape
Observe arrangement
Example stains:
Methylene blue
Crystal violet
Differential Stain
Uses multiple dyes to distinguish between types of bacteria.
Example:
Gram stain
Acid-fast stain
Gram Stain Steps and Purpose
1. Crystal Violet (Primary Stain)
Stains all cells purple
2. Iodine (Mordant)
Forms a complex with crystal violet
Helps dye stick to cell walls
3. Alcohol or Acetone (Decolorizer)
Removes stain from Gram-negative cells
Gram-positive cells retain stain
4. Safranin (Counterstain)
Stains Gram-negative cells pink/red
Gram Positive vs Gram Negative
Gram Positive Bacteria
Cell wall:
Thick peptidoglycan layer
Retains crystal violet
Color after stain:
Purple
Gram Negative Bacteria
Cell wall:
Thin peptidoglycan
Outer membrane present
Color after stain:
Pink/red
5. Streaking for Isolation (4 Quadrant Method)
Purpose
To separate individual bacterial cells so they grow into isolated colonies.
This allows identification of pure cultures.
Steps in Clinical Lab
Sterilize loop in flame
Obtain bacteria from sample
Streak first quadrant
Flame loop again
Drag bacteria from first quadrant into second
Repeat for quadrants 3 and 4
Incubate plate
Each quadrant dilutes the bacteria, producing isolated colonies.
6. Streaking for Quantification
Purpose
To estimate the number of bacteria in a sample.
Measured as:
CFU = Colony Forming Units
Each colony represents one viable bacterium (or group).
CFU Calculation Example
Formula:
C
F
U
/
m
L
=
Number of Colonies
Dilution
×
Volume plated
CFU/mL=
Dilution×Volume plated
Number of Colonies
Example:
50 colonies
0.1 mL plated
10⁻⁴ dilution
C
F
U
/
m
L
=
50
10
−
4
×
0.1
CFU/mL=
10
−4
×0.1
50
C
F
U
/
m
L
=
5
×
10
6
CFU/mL=5×10
6
7. Immunology
Antigen
An antigen is a substance that triggers an immune response.
Examples:
Bacteria
Viruses
Toxins
Foreign proteins
Antibody
An antibody is a protein produced by B cells that specifically binds to an antigen
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