Microbiology Exercise 14

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Why is it important to record the presence of growth?

To make sure bacteria culture is present


incubate the tubes at 35 C for 48 hrs. or longer. Why is the time of incubation important.

allows time for fermentation


What happens when the organism uses peptones in the agar?

It produces ammonia which raises the PH and turns the agar red. This is done aerobically so only the top portion will be red.


Why would you not analyze results too early?

The organism needs time to ferment the glucose otherwise you could get a false postive and the tube would be all yellow.


Why not analyze the results too late?

If all the sugar get use up when all the peptones are used up it would turn back to red giving you a false negative.


a bacterium that doesnt ferment glucose will not ferment lactose.

agree, if a bacterium cannot ferment glucose which is a simple sugar, it cannot ferment lactose either because lactose is a complex


color of uninoculated medium

red (neutral)


E.coli and Enterbacter aerogenes are small gram-negative rods. How can you differentiate them?

E.coli produces large amounts of acid from pyruvic acid (pH < 5.5)

En. aerogenes makes acetoin (pH > 6.0 neutral)


Why are fermentation tubes evaluated at 24 and 48 hrs?

fermentation occurs 1-24 hrs

fermentation that results in acid production will turn the indicator yellow (pH of 6.8 or below)

prolong incubation periods greater then 24 hrs, bacteria will begin growing oxidatively on the peptone after using up the carbohydrate supplied

causing neutralization of the indicator and turning it red because of ammonia production.


What would happen if an organism used up all the carbohydrates in a fermentation tube.

the organism would oxidase peptone and produce ammonia resulting in turning the indicator from yellow(production of acid) to red (neutral)


if an organism metabolizes glucose aerobically, what result will occur in the fermentation tubes?

gas will be trapped in the inverted or Durham tube. Bubbles will appear


MRVP test

is a differential media, used to distinquish organisms that produce large amounts of acid from glucose and organisms that produce the neutral product acetoin

large amount of acid from glucose + methyl red = red (MR positive; pH is below 4.4)

if neutral products are produced + methyl red = yellow (pH above 6.0)

production of acetoin is detected by the addition of potassium hydroxide and a-naphthol = upper part of the medium will turn red

negative V-P test = light brown


Fermentation Tubes

Simmon's Citrate Agar Slant (SC)

is a selective media, allows the growth of certain type of organisms, while inhibiting the growth of other organisms.

For example, organisms that have the ability to utilize a given sugar are screened easily by making that particular sugar the only carbon source in the medium for the growth of the microorganism





is the disk-diffusion technique measuring bacteriostatic or bacterialcidal activity? Explain

Bacteriostatic, until we can perform a subculture to determine if it is bacteriocidal.


In which growth phase is an organism most sensitive to an antibiotic?

During the log phase when the cells are dividing exponentially


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

Other factors contribute to performance in vivo. These include, variations in PH, patient nutrition, side effect, other medical conditions, additional drug interactions.


what effect woiuld presence of tetracycline in the body have on penicillin therapy?

tetracycline would inhibit effectiveness of PCN (no reproduction) PCN needs bacteria to grow to inhibit cell wall synthesis, but tetracycline is a bacteriostatic


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

Each antimicrobial agent inhibits growth differently by:
Cell wall structure


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




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




Which antibiotic is more effective against Staphylococcus aureus?



Which antibiotic is more effective against Salmonella enterica?



After you have observed a gram-pos coccus, what is the additional info you need before perfoming a coagulase test?

if its a salt tolerant organism or mannitol-fermenting organism


why is mannitol salt agar used as a selective medium for normal skin microbiota?

It is both a selective and differential medium. The salt concentration doesn't let some bacteria grow. The mannitol component is used for differential analysis for microorganisms that ferment mannitol


list three characteristics of Staphylococcus aureus


salt tolerant

produce coagulase


list the three factors that protects the skin from infection

dry layer of epidermis


salt;hypertonic environment


what is coagulase? how does it relate to pathogenicity?

coaguluase is an enzyme that coagulates (clots) the fibrin in blood

cell bound coagulase could provide an antigenic disguise if it clotted fibrin on the cell surface

could make the bacterial cells resistant to phagocytes or tissue bactericides or even drugs which might be unable to diffuse to their bacterial target


assume that you isolated S. aureus from your skin. How would you determine whether it is penicillan-resistant?

disk-diffusion method





subculturing to a blood agar plate and placing bacitracin and optochin disks on the area subcultured will help determine whether pathogenic streptococci is present. Explain

S. pneumoniae cannot be differentiated from other alpha-hemolytic streptococci on blood agar.

optochin inhibition is used to identify this pathogen; inhibition zone greater then 15mm or larger indicates optochin sensitivity

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