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Lab Chapters 7,8,9,24,25,26

Why will gram-positive cells more than 24 hours old stain gram-negative?

The cell wall can't retain primary stain

Can iodine be added before the primary stain in a Gram stain?

No because iodine causes the crystal violet to chemically bond to the peptidoglycan cell walls
Chemical bonding prevents the crystal violet from washing away by helping it adhere

List the steps of the Gram-stain procedure in order (omit washings), and fill in the color of gram-positive cells and gram-negative cells

Chemical gram-positive gram-negative
Crystal violet purple purple
iodine (mordant) purple purple
alcohol (ethanol) violet clear
safranin violet red

Which step can be omitted without affecting determination of the Gram reaction?


Suppose you performed a Gram stain on a sample from a pure culture of bacteria and observed a field of red and purple cocci. Adjacent cells were not always the same color. What do you conclude?

The culture is old

Suppose you are viewing a Gram-stained field of red rods and purple cocci through the microscope. What do you conclude?

Mixed culture

Considering you can't identify bacteria from a Gram stain, why might a physician perform a Gram stain on a sample before prescribing an antibiotic?

To determine its sensitivity to the antibiotic correlates with cell wall type

If you performed a Gram stain on human cells, what would happen?

Primary stain would be removed easily because human cells don't have cell walls

What are the large blue-stained areas on the sputum slide?

Fragmented WBCs

What is the decolorizing agent in the Gram stain?


What is the decolorizing agent in the acid-fast stain?


What diseases are diagnosed using the acid-fast procedure?

Tuberculosis and Leprosy

What is phenol (carbolic acid), and what is its usual application


How might the acid-fast characteristic of Mycobacterium enhance the organism's ability to cause disease?

Cells are not digested by phagocytes
Cell walls contain lipids (mycolic acid) that resist drying and environmental stress so bacteria can survive harsh conditions

Clinical specimens suspected of containing Mycobacterium are digested with sodium hydroxide (NaOH) for 30 minutes prior to staining. Why is this technique used? Why isn't this technique used for staining other bacteria?

Digestion removes unwanted bacteria,sputum, and human cells.
Not used for staining because it kills other bacteria

The acid-fast stain is used to detect Cryptosporidium protozoa in fecal samples. Which of the following would you expect to be a major component of their cell walls: carbohydrates, lipids, or proteins?


What are the Gram reactions of Clostridium and Bacillus?

Bacillus = gram-positive
Clostridium = gram-positive (usually)
gram-negative (some)

How might a capsule contribute to pathogenicity?

Interferring with phagocytosis
Capsule provides virulence to disease fighting cells

How might a flagella contribute to pathogenicity?


Of what advantage to Clostridium is an endospore?

Endospores remain dormant until conditions improve
They can survive harsh environments

You can see endospores by simple staining. Why not use this technique?

They look white or transparent

How would an endospore stain of Mycobacterium appear?


What type of culture medium would increase the size of a bacterial capsule?

Bacterial capsules are composed of polysaccharides. Give them disaccharides

Describe the microscopic appearance of encapsulated Streptococcus if stained with safranin and nigrosin

Bacteria (within capsule) - Safranin red
Capsule (outer layer of bacteria - Clear
Background - Dark (nigrosin)

In the Dorner endospore stain, a smear covered with carbolfuhsin is steamed, then decolorized with acid-alcohol and counterstained with nigrosin. Describe the microscopic appearance after this procedure.

Endospore - Red
Bacteria - Colorless
Background - Dark/Black

How could the procedures used in this experiment be altered to measure bacteriostatic effects?


Why isn't one antimicrobial agent equally effective against all three bacteria?

Each antimicrobial agent inhibits growth differently by:
Cell wall structure

Is the disk-diffusion technique measuring bacteriostatic or bactericidal activity?

Can't say without subculture

In which growth phase is an organism most sensitive to an antimicrobial agent?

Log phase

Why is disk-diffusion technique not a perfect indication of how the drug will perform in vivo? What other factors are considered before using the antimicrobial agent in vivo?

Variable such as serum, body pH, ionic content, O2 level.
How drug is metabolized and excreted and the side effect of the drug

What effects would the presence of tetracycline in the body have on penicillin therapy?

Tetracycline is bacteriostatic (stops growth)
Penicillin needs growing bacteria in order to kill it. So penicillin is useless with tetracycline

Disk-diffusion test against bacterium
Which drug should be used to treat an infection caused by this bacterium?

Largest zone of inhibition

Broth dilution: What is the minimum bactericidal concentration of each antibiotic?

A = 1:80 B = 1:150

Broth dilution: What is the minimum bacteriostatic concentration?

A = 1:190 B = 1:160

Which antibiotic is more effective against Staphylococcus aureus?


Which antibiotic is more effective against Salmonella enterica?


What is a surgeon trying to accomplish with a 10 minute scrub with a brush followed by an antiseptic?

Scrubbing is an attempt to dislodge as many microbes as possible
Antiseptic is used to kill as many microbes as possible

How do normal microbiota and transient microbiota differ?

Normal = permanent microorganisms
Transient = present only for days or weeks

If most of the normal microbiota and transient microbiota aren't harmful, then why must hands be scrubbed before surgery?

Because externally they may not be harmful but internally they can cause infection

Aerobic bacteria were isolated from 25 soap products. Data are expressed as percentage of soap products contaminated. What conclusions can you draw from these data?

Bar soap has highest % of bacteria
Pump soaps prevent bacteria from getting into the soap

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