microbiology
Medical Micro Lab Exam 2 Study Guide (Exercises 7–14)
This guide keeps everything from your study guide but organizes it so it’s easier to actually memorize for your exam.
I. Laboratory Techniques & General Microbiology
Bacterial Identification Tests
Catalase Test
Purpose:
Detects if bacteria produce the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen.
Positive Result:
Bubbles form
Negative Result:
No bubbles
Used to Differentiate:
Staphylococcus = Catalase positive
Streptococcus = Catalase negative
Memory Trick:
Staph = Staff = Active = bubbles
Slide Coagulase Test
Purpose:
Detects the enzyme coagulase, which causes plasma to clot.
Positive Result:
Clumping/agglutination
Negative Result:
No clumping
Important:
Staphylococcus aureus = positive
Staphylococcus epidermidis = negative
Biochemical Tests
Urea Test
Purpose:
Detects urease enzyme that breaks urea into ammonia
Positive:
Pink
Negative:
Yellow/orange
Example Positive:
Proteus
Citrate Test
Purpose:
Shows if bacteria can use citrate as sole carbon source
Positive:
Blue
Negative:
Green
Indole Test
Purpose:
Detects breakdown of tryptophan into indole
Positive:
Red ring after Kovac’s reagent
Negative:
Yellow ring
Example Positive:
E. coli
Methyl Red Test
Purpose:
Detects strong acid production from glucose fermentation
Positive:
Red
Negative:
Yellow
Sugar Tubes
Purpose:
Shows carbohydrate fermentation
Positive:
Yellow = acid produced
Gas Production:
Bubble in Durham tube
Oxidase Test
Purpose:
Detects cytochrome oxidase enzyme
Positive:
Purple/dark blue
Negative:
No color change
Example Positive:
Pseudomonas
TSI Agar (Triple Sugar Iron)
Purpose:
Tests:
Glucose
Lactose
Sucrose
Gas
H2S production
Reactions
K/A
Red slant / Yellow butt
Only glucose fermented
A/A
Yellow slant / Yellow butt
Glucose + lactose and/or sucrose fermented
K/K
Red slant / Red butt
No sugar fermented
H2S Production
Black precipitate
Antibiotic Sensitivity
Mueller Hinton Agar
Used for antibiotic sensitivity testing
Interpretation:
Measure zone of inhibition
Compare with chart:
Sensitive
Intermediate
Resistant
MIC (Minimum Inhibitory Concentration)
Definition:
Lowest antibiotic concentration that stops visible growth
Example:
First clear broth tube
That = MIC
Environmental & Atmospheric Conditions
Definitions
Aerobic
Needs oxygen
Anaerobic
No oxygen
Microaerophilic
Needs small amount of oxygen
Facultative
Can live with or without oxygen
Capnophilic
Needs extra CO2
N. gonorrhoeae Growth
Needs:
GC agar / Chocolate agar
CO2 rich environment
Warm temp
Important:
Very fragile organism
II. Clinical Applications & Pathogens
Urinary Tract Pathogens
Common:
E. coli
Proteus
Klebsiella
Enterococcus
Staph saprophyticus
Know selective media appearance
Urine Colony Count Calculation
Given:
1 colony = 1000 bacteria/ml
Formula:
Colonies × 1000 = bacteria/ml
Example:
150 colonies = 150,000 bacteria/ml
Urine Dipstick
Know:
Protein
Glucose
Blood
Nitrite
Leukocytes
pH
Ketones
Used for quick urinalysis
Staphylococci
Mannitol Salt Agar (MSA)
Selective For:
Staphylococcus
Differential For:
Mannitol fermentation
S. aureus
Result:
Yellow media
Why:
Ferments mannitol
DNase:
Positive
Coagulase:
Positive
S. epidermidis
Result:
Pink/red media
Why:
Does NOT ferment mannitol
DNase:
Negative
Coagulase:
Negative
Clostridium
Clostridium tetani
Disease:
Tetanus
Characteristics:
Anaerobic
Gram positive rod
Terminal spore
Symptoms:
Lockjaw, muscle spasms
Clostridium perfringens
Disease:
Gas gangrene
Characteristics:
Anaerobic
Gram positive rod
Double zone hemolysis
Meningitis & Specific Pathogens
N. gonorrhoeae
Gram stain:
Gram negative diplococci
Found:
Inside neutrophils
Disease:
Gonorrhea
Treponema pallidum
Seen With:
Dark field microscopy
Disease:
Syphilis
Syphilis Diagnosis
Early Stage:
Dark field microscopy
Later Stage:
Serology (VDRL, RPR, FTA-ABS)
Major Meningitis Pathogens
Neisseria meningitidis
Gram negative diplococci
Capsule
Ferments maltose
Streptococcus pneumoniae
Gram positive lancet diplococci
Alpha hemolytic
Optochin sensitive
Haemophilus influenzae
Small gram negative coccobacillus
Requires X and V factors
Chocolate agar
CSF Changes
Type | Protein | Glucose | WBC |
Bacterial | High | Low | High neutrophils |
Viral | Slight high | Normal | High lymphocytes |
Fungal | High | Low | High lymphocytes |
MEMORIZE THIS TABLE
III. Hematology & Blood Analysis
Hematocrit
Definition
Percentage of blood made of RBCs
Increased In:
Burns
Dehydration
Decreased In:
Aplastic anemia
Hemolytic anemia
CBC Normal Values
Hemoglobin
Female:
12–16 g/dL
Male:
14–18 g/dL
Hematocrit
Female:
37–47%
Male:
42–52%
White Blood Cells
Neutrophils
Function:
Fight bacteria
Increased In:
Bacterial infections
Lymphocytes
Function:
Fight viruses
Increased In:
Viral infections
Eosinophils
Function:
Parasites + allergies
Increased In:
Parasites + allergic reactions
Monocytes
Function:
Phagocytosis
Become macrophages
Basophils
Function:
Histamine release
Allergic reactions
IV. Vocabulary You MUST Know
Microbiology
Diplococci
Pairs of round bacteria
Agglutination
Visible clumping
Calibrated
Accurately measured
Enterobacteriaceae
Family of gram negative rods
Acid-base indicator
Chemical showing pH change
Pharmacology
Bactericidal
Kills bacteria
Bacteriostatic
Stops bacterial growth
Sensitive
Antibiotic works
Resistant
Antibiotic does not work
Intermediate
May work
Broad spectrum
Works against many bacteria
Blood Terms
Septicemia
Bacteria in blood
Blood culture
Test for bloodstream infection
Erythrocyte
Red blood cell
Leukocyte
White blood cell
Phagocytosis
Cells engulf bacteria
Differential
Percentage of each WBC type
MUST MEMORIZE FIRST (Highest Yield)
TOP PRIORITY:
Catalase vs Coagulase
TSI reactions
MSA results
MIC
CSF table
WBC functions
N. gonorrhoeae / T. pallidum
Hematocrit changes
UTI colony count calculation
Antibiotic sensitivity zones
This is your highest test material.