Selective and Differential Media Quiz

Hemolysins

• Hemolysins are substances produced by bacteria that can lyse red blood cells.
• In the Blood agar test, their presence indicates the capability of bacteria to cause hemolysis.

Alpha Hemolysis

• Alpha hemolysis describes partial lysis of red blood cells, resulting in a greenish discoloration surrounding colonies.
• Streptococcus pneumoniae is a common example associated with alpha hemolysis.

Beta Hemolysis

• Beta hemolysis indicates complete lysis of red blood cells and hemoglobin, leading to a clear zone around colonies.
• Common examples include Streptococcus pyogenes and Staphylococcus aureus.

Gamma Hemolysis

• Gamma hemolysis refers to no hemolysis observed on blood agar, indicating that the bacteria do not lyse red blood cells.

pH Indicator

• Phenol red is a pH indicator used in Mannitol Salt Agar (MSA) that turns yellow in low pH conditions, indicating fermentation.

Staphylococcus (spp.)

• The MSA test contains 7.5% NaCl, selectively favoring the growth of staphylococci while inhibiting other bacteria.

Differential Media

• MSA includes mannitol for differentiating between Staphylococcus aureus (manitol fermenter) and Staphylococcus epidermidis (non-fermenter).

Mannitol Salt Agar (MSA)

• MSA is utilized for isolating and identifying staphylococci based on their ability to ferment mannitol.

S. aureus Fermenting Mannitol

• Bright yellow growth on MSA signifies a positive result for mannitol fermentation, indicating the presence of S. aureus.

Non-S. aureus Staphylococcus

• Pink growth on MSA signifies that the bacteria do not ferment mannitol, indicating other staphylococci species.

Gram Positive Selection

• Phenylethyl Alcohol Agar (PEA) is selective for Gram-positive bacteria by inhibiting Gram-negative bacteria.

Isolation

• PEA is employed to isolate staphylococci and streptococci from mixed cultures.

Gram Negative Selection

• MacConkey Agar is designed to select for Gram-negative bacteria.

Inhibition by Bile Salts

• In MacConkey Agar, bile salts and crystal violet inhibit the growth of Gram-positive organisms.

Lactose Fermenters

• MacConkey Agar differentiates lactose fermenters, which produce acid as a byproduct, from non-fermenters.

Type of Media

• Eosin Methylene Blue (EMB) Agar serves as both selective and differential media.

Selection of Gram Negative Bacteria

• EMB is specifically selective for Gram-negative bacteria.

Enteric Bacteria Differentiation

• The EMB test differentiates enteric bacteria, primarily those found in the intestinal tract.

Inhibition of Gram Positive Bacteria

• Eosin Y and methylene blue in EMB inhibit the growth of Gram-positive bacteria.

EMB Test

• EMB test is conducted to isolate and differentiate Gram-negative intestinal pathogens.

Inhibition in Hektoen Enteric Agar

• Bile salts in Hektoen Enteric Agar inhibit Gram-positive bacteria.

Differentiation for Enterics

• Hektoen Enteric Agar differentiates for enterics, particularly Salmonella and Shigella.

Mixed Acid Fermentation

• The Methyl Red (MR) test is conducted to detect mixed acid fermentation.

Neutral Fermentation Products

• The Voges-Proskauer (VP) test evaluates neutral fermentation products.

Positive MR Test Color

• A positive result in the MR test yields a red color, indicating acid production.

Selective and Differential Nature of MSA

• MSA's selectivity stems from high salt concentration; its differential nature arises from mannitol fermentation and phenol red indicator.

Examples of Differential Media

• Common types include MacConkey Agar, Eosin Methylene Blue Agar, and Mannitol Salt Agar.

MacConkey Agar Description

• MacConkey Agar functions as both selective for Gram-negative bacteria and differential for lactose fermenters.

Red or Pink Colonies in MacConkey Agar

• Red or pink colonies appear when organisms like E. coli produce acid, lowering the pH below 6.8.

Visual MacConkey Agar Results

• Pink indicates a positive result for lactose fermentation.

EMB Agar Description

• Eosin Methylene Blue Agar is selective and differential, primarily targeting Gram-negative intestinal pathogens.

Role of Eosin and Methylene Blue

• These dyes inhibit most Gram-positive bacteria while coloring colonies based on acid production from lactose fermentation.

EMB Plate Result Reasoning

• Green metallic sheen indicates high acid production; pink shows small acid production, while colorless signifies no fermentation.

Lactose Fermenters Examples

• Common lactose fermenters include Escherichia coli, Enterobacter, and Klebsiella.