Microbiology Lab Exam

What is the purpose of Antibiotic Sensitivity Testing?

To measure the effectiveness of antimicrobials against pathogenic organisms.

What is the Kirby-Bauer test also known as?

The disk diffusion test.

How are antibiotic impregnated disks used in the Kirby-Bauer test?

They are placed onto plates with bacterial lawn growth and incubated to allow for diffusion.

What indicates that an organism is susceptible to an antimicrobial in the Kirby-Bauer test?

A clear zone of inhibition around the disk.

What does the term 'Zone of Inhibition' refer to?

The area around the disk where bacteria cannot grow, indicating sensitivity.

What is the Minimum Inhibitory Concentration (MIC)?

The lowest concentration of an antimicrobial that inhibits the growth of a microorganism.

What is the relationship between distance from the disk and antibiotic concentration?

There is a logarithmic decrease in antibiotic concentration as the distance from the disk increases.

What is measured to determine antibiotic sensitivity in the Kirby-Bauer test?

The diameter of the zone of inhibition in millimeters.

What is the specified amount of Erythromycin in the antimicrobial disks?

15 mcg (µg).

What is the mechanism of action for Penicillin?

It inhibits the formation of peptidoglycan cross-links in Gram-positive bacteria.

What type of bacteria does Neomycin primarily target?

Gram negatives and a few Gram positives.

What is the action of Tetracycline?

It blocks tRNA from attaching to mRNA, preventing protein synthesis.

What type of infections is Staphylococcus aureus associated with?

Nososomial infections.

What is a common treatment for Methicillin-resistant Staphylococcus aureus (MRSA)?

Vancomycin.

What type of pathogen is Pseudomonas aeruginosa?

An opportunistic Gram-negative rod.

What is the first step in conducting Antibiotic Sensitivity Testing?

Obtain an NA plate and mark three areas on the agar side.

What should be done after plating the organisms in Antibiotic Sensitivity Testing?

Incubate plates at the appropriate temperature.

What is the role of the antibiotic sensitivity sheet in the testing process?

It is used to measure the zones of inhibition and determine susceptibility.

Mannitol Salt Agar (MSA)

Selective AND differential media that favors Staphylococcus growth and can distinguish between pathogenic and nonpathogenic strains.

Selective media

Grows a specific type of microbe.

Differential media

Able to distinguish two organisms on one plate.

Mannitol

A carbohydrate present in MSA.

7.5% NaCl

Provides a selective factor to MSA as Staphylococci can survive in this level of salinity.

pH indicator phenol red

Turns yellow below pH 6.8, red at pH 7.4 to 8.4, and pink at pH 8.4 and above.

Pathogenic species (S. aureus)

Ferments mannitol, producing acid that drops pH and turns media yellow.

Nonpathogenic species of Staphylococcus

Will grow on MSA but will not produce a color change.

MacConkey Agar

Selective and differential media that contains lactose, bile salts, neutral red, and crystal violet.

Bile salts and crystal violet

Inhibit growth of Gram+ organisms in MacConkey Agar.

Enterococcus faecalis

A Gram+ lactose fermenter that can cause nosocomial infections and has high antibiotic resistance.

Neutral red

pH indicator that is colorless above pH 6.8 and red at pH less than 6.8.

Acid accumulation upon lactose fermentation

Turns the dye red in MacConkey Agar.

MacConkey Agar Use

For isolation and differentiation of members of Enterobacteriaceae.

Lactose fermenters

Organisms that can ferment lactose, leading to acid production.

E. coli

Gram Negative organism found inside intestines

S. epidermidis

Gram positive organism found on the skin and mucous membranes

S. aureus

Gram positive organism found on the skin and in the nose

E. faecalis

Gram positive bacteria found in the gut

P. aeruginosa

Gram negative organism found in water, plants, and soil

What are the minimum standards for lab practices due to hazards presented by microbes?

The US Government developed a set of Biosafety Levels (BSLs) providing guidelines for handling different microbes in a lab setting.

What should you do with bags and textbooks when you arrive at the lab?

Put them in the front of the room.

What personal protective equipment should be worn in the lab?

Lab gloves and protective eyewear.

What is the proper procedure for handling culture plates?

Always place the lid back on culture plates when they are not in use.

What should you do if there is a culture spill?

Cover spills with paper towels soaked in 10% bleach solution and inform the professor or TA.

What is the purpose of an autoclave?

To sterilize materials using steam under pressure.

What is the maximum fill level for containers to be autoclaved?

No container should be more than half full to prevent boiling over.

What is the temperature and duration for autoclaving materials?

Held at 121°C for 15 minutes, with a total cycle time of about 45 minutes.

What is the first step in proper hand washing technique?

Remove watch or bracelets.

How long should you scrub your hands while washing them?

At least 20 seconds.

What should be done with gloves after handling microbes?

Dispose of gloves in a proper biohazard container.

What is the definition of aseptic technique?

A set of specific practices performed under controlled conditions to minimize contamination by pathogens.

What should you do with unknown chemicals or stain runoff?

Consult the TA for proper disposal.

What should be done with broken glass in the lab?

Dispose of it in the designated broken glass container.

What should you do with disposable gowns in the lab?

Wear them when preferred and dispose of them properly.

What is prohibited in the lab regarding food and drink?

It is prohibited to bring food or drink into the lab.

What should be done to the working area before starting lab work?

Wipe the working area with a 10% bleach solution.

What should you do with towels used for drying hands?

Discard them in the regular trash.

What is the objective of the lab assignment related to hand washing?

To learn proper hand washing technique.

What should you review before the first lab quiz?

Review the PPT and notes from the lab.

Why is staining necessary in microbiology?

Staining is needed to visualize cells.

What are the common shapes of microbes identified through simple staining?

Cocci, rods, spirals, or pleomorphic.

How does cell arrangement affect microbial identification?

Arrangement is determined by the number of planes and whether cells separate post-division.

What arrangement do cocci exhibit?

Cocci exhibit the most arrangement variety.

What is a diplococcus?

A diplococcus is formed when 2 daughter cells remain attached after division.

What is the arrangement of streptococcus?

Streptococcus forms from continuous division in the same plane.

What is the appearance of staphylococcus?

Staphylococcus appears as grape-like clusters due to irregular division.

What stain is used for cocci from a nasal swab?

Crystal violet stain.

What is the causative agent of syphilis?

Treponema pallidum, which is a spirochete.

What is the shape of Vibrio cholerae?

Curved rods.

What is the significance of pleomorphic bacteria?

Pleomorphic bacteria can change shape and may be opportunistic pathogens.

What are basic stains composed of?

Basic stains consist of a solvent and a colored molecule (chromagen).

What is a chromophore?

A chromophore is the part of the chromagen that gives it color.

What is the role of heat-fixation in staining?

Heat-fixation kills bacteria, adheres them to the slide, and coagulates cytoplasmic proteins for visibility.

How long should a basic stain be left on a bacterial smear?

The stain should be left for one minute.