Microbiology Lab Techniques Quiz

Microbial Sample Sequence

• Collect sample
• Perform Gram stain
• Inoculate culture media
• Incubate for 18-24 hours at 37°C
• Check for growth

Bacteriological Media

• Agar: A semisolid medium
• Broth: A liquid medium
• Plate: A flat, round container with agar
• Tube: A screw-top container, potentially containing agar or broth
• Slant: A tube of agar that has solidified on a slant

Selective Media

• Media containing compounds inhibiting the growth of specific organisms, thereby selecting for particular bacteria.
• MacConkey II Agar (Mac): Selects gram-negative bacteria, inhibits gram-positive growth.
• Mannitol Salt Agar: Selects for halotolerant organisms, like Staphylococcus spp., inhibits growth of other bacteria, with a red indicator that turns yellow if mannitol is fermented.
• Bismuth Sulfite Agar: Selects gram-negative bacteria, inhibits growth of other bacteria.
• Eosin Methylene Blue Agar: Selects gram-negative bacteria, inhibits growth of other bacteria, differentiates lactose fermenters from non-lactose fermenters.
• Columbia Colistin-Nalidixic Acid Agar (CNA): Selects gram-negative bacteria, inhibits gram-positive growth, often used with 5% sheep blood.

Differential Media

• Media that distinguish between different types of bacteria based on biochemical reactions.
• MacConkey II Agar (Mac): Differentiates between lactose fermenters (purple colonies) and non-lactose fermenters, inhibits gram-positive bacteria.
• Salmonella-Shigella Agar (SS): Differentiates between lactose-fermenting bacteria (pink colonies) and bacteria that aren't, identifies bacteria that produce hydrogen sulfide (black precipitate).
• Mannitol Salt Agar: Differentiates between organisms capable of fermenting mannitol and those that cannot (yellow colonies in cases of mannitol fermentation).

Hemolysis

• The process of red blood cell (RBC) destruction, releasing their contents.
• Alpha: Incomplete hemolysis, narrow greenish zone.
• Beta: Complete hemolysis, clear zone around bacterial colony.
• Gamma: No hemolysis.
• Delta: Double zone of hemolysis.

Other Important Media

• Eosin Methylene Blue (EMB): Differentiates between lactose fermenters and non-lactose fermenters.
• Phenylethyl Alcohol (PEA) Agar: Selects for Gram-positive bacteria. Often used to isolate Staphylococcus species.

Nutritive Media

• Growth media for various bacterial and fungal species.
• Blood Agar: Supports the growth of many bacteria, sometimes used to differentiate bacteria based on the type of hemolysis.
• Trypticase Soy Agar (TSA): A general-purpose medium for growth of many bacteria and some fungi.

Mueller-Hinton Agar (MH)

• A general-purpose medium, often used in antibiotic sensitivity testing (Kirby-Bauer test).
• Standardized results for antibiotic susceptibility testing.
• Used with blood for fastidious organisms

Gram-Negative Bacteria and Fermentation

• Escherichia coli (E. coli): A lactose fermenting bacterium.
• Salmonella: A non-lactose fermenter that produces hydrogen sulfide.
• Shigella: A non-lactose fermenter and doesn't produce hydrogen sulfide.

Other Important Tests

• Catalase Test: Detects the presence of catalase, which bubbles when hydrogen peroxide is added to a positive sample. Examples: Staphylococcus spp are positive. Streptococci spp are generally negative.
• Coagulase Test: Identifies Staphylococcus species based on their ability to produce coagulase (clotting plasma).
• Kirby-Bauer Test: Measures antibiotic susceptibility through zone of inhibition. Requires control organisms.
• Optochin Test: Used to differentiate Streptococcus pneumoniae and other Streptococcus species.
• Oxidase Test: Detects the presence of cytochrome oxidase.
• CAMP Test: Differentiation test for Streptococcus agalactiae.
• California Mastitis Test (CMT): Cowside test used to detect mastitis in cows.
• Bile Esculin Test: Identifies bacteria that hydrolyze Esculin.
• Simmons Citrate Test: Detects citrate utilization by bacteria, important for identifying certain species, like Klebsiella spp.
• Triple Sugar Iron Agar (TSIA): Distinguishes bacteria based on their ability to ferment different sugars.
• Sodium chloride (0.85%): A sterile solution used for diluting Gram-negative bacteria for API 20E testing.