lab pt 8

What are the results of mannitol fermentation on MSA?

Positive: yellow medium due to acid production. Negative: medium remains pink/red.

What is blood agar?

An enriched and differential medium containing red blood cells.

What does blood agar differentiate?

Bacteria based on their hemolysis patterns.

What are alpha, beta, and gamma hemolysis?

Alpha: partial hemolysis, greenish discoloration. Beta: complete hemolysis, clear zone. Gamma: no hemolysis.

What is the difference between enriched and enrichment media?

Enriched media contain extra nutrients for fastidious organisms; enrichment media favor multiplication of a particular organism in a mixed sample.

What is bacterial motility?

The ability of bacteria to move independently.

What structure commonly allows bacteria to move?

Flagella.

How can bacterial motility be determined?

Using a wet mount, hanging drop preparation, or semisolid motility agar.

What is a wet mount?

A preparation of living microorganisms suspended in liquid under a coverslip.

What is a hanging drop preparation?

A technique that suspends a drop of bacterial culture from a coverslip to observe living bacteria.

What is true motility?

Directional movement of bacteria using structures such as flagella.

What is Brownian motion?

Random jiggling caused by collisions with surrounding molecules, without net movement.

How is true motility distinguished from Brownian motion?

True motility involves directional movement across the field; Brownian motion is random vibration or jiggling.

How is the semisolid motility agar test performed?

Use a straight inoculating needle to stab the medium once and incubate.

What are positive and negative motility agar results?

Positive: diffuse growth spreading away from the stab line. Negative: growth remains along the stab line.

Why is semisolid agar used for motility testing?

It is soft enough to allow motile bacteria to move through it.

Why is water tested for fecal contamination?

To identify possible fecal contamination and potential exposure to disease-causing microorganisms.

What are coliforms?

Gram-negative, rod-shaped, facultatively anaerobic bacteria that ferment lactose with acid and gas production under standard testing conditions.

Where do most coliforms live?

In the intestinal tracts of humans and other warm-blooded animals.

What are examples of coliform bacteria?

Escherichia coli, Enterobacter, and Klebsiella.

Why are coliforms used as indicators of water contamination?

Their presence can indicate fecal contamination and possible exposure to enteric pathogens.

Are all coliforms exclusively found in feces?

No. Some occur naturally in soil, vegetation, and water.

What is the difference between total coliforms and fecal coliforms?

Total coliforms are a broader group; fecal coliforms are more closely associated with the intestinal tracts of warm-blooded animals.

Why is E. coli particularly useful as an indicator of fecal contamination?

It is strongly associated with the intestines and feces of warm-blooded animals.

Why not test water for every specific pathogen instead of testing for fecal contamination?

There are many possible pathogens, making individual testing expensive and time-consuming. Indicator organisms provide practical screening.

Does finding coliforms prove a specific pathogen is present?

No. It indicates possible contamination, not proof that a particular pathogen is present.

What does MPN stand for?

Most Probable Number.

What does MPN estimate?

The statistical estimate of viable microorganisms in a sample.

Why is the MPN method used?

To estimate bacterial concentrations when direct counting is difficult.

How is the MPN test generally performed?

Inoculate multiple broth tubes with different sample volumes or dilutions, incubate, and record positive and negative results.

What indicates a positive presumptive coliform tube?

Acid and gas production from lactose fermentation.

What is the purpose of a Durham tube in water testing?

To collect gas produced by lactose-fermenting bacteria.

How is the MPN value determined?

Compare the pattern of positive tubes across sample volumes or dilutions with an MPN statistical table.

What do higher and lower MPN values indicate?

Higher: greater estimated bacterial concentration. Lower: smaller estimated bacterial concentration.

What is the presumptive test in water analysis?

An initial screening test for possible coliforms, usually using lactose broth and observing acid and gas production.

What is the confirmed test?

A follow-up test used to verify the presence of coliform bacteria.

What is the completed test?

A final verification involving isolation and examination of suspected coliform bacteria.

What are the differences between sterilization, disinfection, and antisepsis?

Sterilization destroys all microbial life, including endospores; disinfection reduces or eliminates many pathogens on nonliving surfaces; antisepsis uses antimicrobial chemicals on living tissue.

What are the differences between selective and differential media, and bactericidal and bacteriostatic agents?

Selective media inhibit certain organisms; differential media distinguish organisms. Bactericidal agents kill bacteria; bacteriostatic agents inhibit their growth.

What are the differences between Gram-positive and Gram-negative bacteria, pure and mixed cultures, and fermentation and respiration?

Gram-positive: thick peptidoglycan, purple; Gram-negative: thin peptidoglycan, outer membrane, pink. Pure culture: one species or strain; mixed culture: multiple species. Fermentation: no electron transport chain; respiration: uses an electron transport chain.