Clinical Micro Lab

Clinical microbiology

Focus: collecting and analyzing samples from patient to identify treatment

Microbiology research

Focus: collect and analyze samples to learn about the organism and develop new treatments for pathogens

Selective media

Suppress growth of unwanted media

Example: bile salts suppress Gram + bacteria & selects Gram - bacteria

Differential media

Distinguish visually between bacteria species, appear as different colors

Usually based on metabolism byproduct

Enriched media

Favor growth of target bacteria w/ added nutrients

Example: animal blood added to favor growth of pathogens

Mannitol Salt Agar (MSA)

Differential media: bacteria ferments mannitol producing acid, cause media to yellow

Selective media: high salt concentration prevents growth of most bacteria except Staphylococcus aureus if yellow

Blood agar (BAP)

Animal blood and nutrient base

Contain red blood cells

Both enriched and differential media

Enriched blood agar

Allows pathogens to grow that need animal blood

Differential media blood agar

Distinguishes microbes that lyse red blood cells (hemolysis)

Beta hemolysis

Complete breakdown of red blood cells

Zone of clearing around bacterial colonies

Oxidizing Fe

Example: Staphylococcus areus and Staphylococcus pyogenes

Alpha hemolysis

Partial breakdown of red blood cells

Greenish discoloration around bacterial colonies

Cell membrane still intact

Not functional

Example: Streptococcus pneumoniae

Gamma hemolysis

No breakdown of blood cells

No change visible

Example: Staphylococcus epidermis

Chocolate agar

Lysed red blood cells

Enriched media: grow bacteria that need nutrients in blood but can’t lyse red blood cells themselves

McConkey agar (MAC)

Selective medium: selects for Gram -

Bile salts and crystal violet inhibit Gram +

Bile salts disrupt cell membrane

Crystal violet bad for living cells

Differential medium: lactose fermenting vs non lactose fermenting grows as different colors

Changes pH of agar

Pink = more acidic = lactose fermenting

Steps of isolation & identification

1. Streak plate from a clinical sample

2. Isolate colony

3. Gram stain to start ID (shape, arrangement, gram reaction, size)

4. Metabolic test to complete ID (BAP, McConkey, MSA)

5. Antibiotic sensitivity tests (informs treatment options)