Microbiology Lab Quiz 11 Practice

MSA Plate Purpose

Selective for salt-tolerant organisms; differential for mannitol fermentation.

What does selective do?

Only bacteria that tolerate high salt can grow.

What does differential do?

Shows whether bacteria ferment mannitol.

What does it mean when there is yellow around colonies?

mannitol fermented; fermentation makes acid

Is there fermentation in S. aureus?

yes

Is there fermentation in S. epidermidis?

no

What is pigment?

color produced by cells

What is the purpose of coagulase test?

Distinguish S. aureus from other Staphylococcus spp.

Positive coagulase result

Clumping visible on slide.

What is the purpose of catalase test?

Distinguish Staphylococcus spp. (positive) from Streptococcus spp. (negative).

Positive catalase result

Bubbles after adding H₂O₂

Upper Respiratory Tract Includes

Nose, throat, middle ear, eustachian tubes

Lower Respiratory Tract Includes

Trachea, bronchial tubes, alveoli

Normal throat microbiota includes

Staphylococcus, Streptococcus, Neisseria, Haemophilus

Why infection rates are low despite pathogenic bacteria presence?

Microbial antagonism

Microbial Antagonism

Normal microbiota suppress others by competition or inhibitory substances.

Strep identification based on

Hemolysis + Lancefield grouping

Lancefield Grouping

Groups catalase-, coagulase- Streptococcus by carbohydrate antigens on cell wall (A-O)

Hemolysis

RBC lysis/destruction

Alpha Hemolysis

Partial hemolysis → green/cloudy zone.

Beta Hemolysis

Complete hemolysis → clear zone.

Gamma Hemolysis

No hemolysis → no change.

Streptococcus pneumoniae Hemolysis Type

Alpha

Differentiation of S. pneumoniae

Optochin sensitivity + bile solubility

Optochin Sensitivity Result

ZOI ≥ 15 mm → sensitive (S. pneumoniae).

Bile Solubility Result

Colonies disappear / cloudy liquid becomes clear.

Streptococcus pyogenes Lancefield Group

Group A

Streptococcus pyogenes Hemolysis Type

Beta

Differentiation of S. pyogenes

Bacitracin sensitivity

Blood Agar Components

Nutrient agar + 5% defibrinated sheep's blood + 0.5% NaCl.

Why NaCl Added to Blood Agar?

Prevents spontaneous hemolysis.

Why Catalase Test Not Done on Blood Agar?

Blood contains catalase → false positives

Normal Mouth Microbiota

Primarily Streptococcus spp.

S. mutans & S. sanguinis Capsule

Form dextran from sucrose → adherence to teeth.

S. salivarius Capsule

Forms levan → colonizes tongue.

Dental Plaque

Mass of bacteria, dextran, debris on teeth.

Dental Caries Cause

Acid from fermentation → enamel erosion.

Why Sucrose Promotes Plaque

Sucrose → dextran → adherence → plaque.

Sugarless Gum Sweeteners (mannitol/sorbitol)

Cannot be converted to dextran.

MSB Agar Purpose

Selects for S. mutans.

Why Crystal Violet in MSB Agar?

Inhibits Staphylococcus spp.

Why Bacitracin in MSB Agar?

Inhibits most oral bacteria except S. mutans.

S. mutans Colony Appearance

Drop of polysaccharide on colony.

Purpose of Sucrose Blood Agar

Encourages capsule formation + demonstrates hemolysis.

Why Chew Gum Before Saliva Collection?

To stimulate saliva production.

Why Use 1 µL Calibrated Loop?

To determine CFU/mL accurately.

S. salivarius Colony Appearance

Large, raised, mucoid.

S. sanguinis Colony Appearance

Glistening colony w/ indentation in agar; non-movable.

Colony forming unit (CFU)

one cell --> one colony