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Would you expect an obligate anaerobe to grow ona slant incubated aerobically? Why?
An obligate aerobe is unable to grow aerobicallybecause oxygen inhibits its growth.
Which kind of organism would you expect to growboth in the agar deep and on the slant?
A facultative organism could grow in both theagar deep and slant.
Which kind of organisms can grow aerobically onslants?
Both the facultative and the aerobe could growon the slants.
Which of the three organisms you inoculatedcould grow throughout the agar deep and on top?
The facultative organism is the only on whichcould grow throughout the deep.
Why did you boil the agar deeps longer than ittook to melt the agar?
The agar deep was boiled 10 minutes to drive outthe oxygen.
What is the most expedient method forsterilizing a heat-sensitive liquid that contain a spore-forming bacterium?
Membrane filtration followed by dilution platingof the filtrate. It should be void of spore forming bacteria.
What are three advantages of using metal capsrather than cotton for test tube closures? Are there any disadvantages?
Advantages: easier to fit on test tube, can bereused; Disadvantages: more apt to fall off, more apt to become contaminatedwith long term storage (screw caps are best for this purpose).
How would you sterilize a heat-sensitive growthmedium containing thermoduric bacteria?
By membrane filtration.
Why can’t you use UV to sterilizemicrobiological media, e.g., agar or broth?
UV can’t sterilize media because it cannotpenetrate plastic or glass, or the surface of the agar.
How does UV cause mutations?
UV forms thymine dimers causing the cell toinsert an incorrect base, leading to mutation.
Give a possible reason some organisms in thesoil (or meat) were able to grow after exposure to UV but not others?
Some organisms are naturally more resistant toradiation because they may have more efficient repair mechanisms. Resistantorganisms in soil and a few in group meant are also likely to be sporeformer.
Frequently organism isolated from theenvironment are pigmented, while organisms isolated from the intestine or otherprotected places are not. Can you provide an explanation for this?
Organisms in the environment are exposed to theUV in sunlight and pigmentation may protect them. Organisms in the intestinesof animals are not exposed to UV and therefore do not need to use resources toform pigments.
Mutations can lead to cancer in animals? Explainwhy persons living in the southern half of the US have a higher incidence ofskin cancer than those in the northern half.
People living closer to the equator are exposedto more UV and therefore suffer more mutation in their skin cells.
What relationship did you find, if any, betweenthe Gram-staining reaction of a microorganisms and its susceptibility toantiseptics and disinfectants?·
Gram+ organisms are less susceptible than Gram-organisms to antiseptic and disinfectants.
Name some other factors affecting the size ofthe zone of growth inhibition that were not included in your modified Kirby-Bauer test.
Growth stage of test organisms, rate of diffusionof the antimicrobial test agent, microorganisms demonstrating marked strain tostrain variability in growth rate cannot be tested, some antibiotics should notbe tested on media supplemented with blood.
To what general groups of organic compounds doeshexachlorophene belong? What are the advantages and disadvantages of usinghexachlorophene for surgical scrub-downs?
Hexachlorophene is a phenolic compound:2,2’-methylenebis [3,4,6-trichlorophenol]. It demonstrates excellent inhibitoryactivity against Gram+ bacteria. It is used chiefly as a germicidal soap.Because of potential neurotoxicity in humans, the FDA has limited its use.Another disadvantage is limited activity against Gram- bacteria.
How could normal skin flora be helpful to thehost?
Normal flora prevents pathogenic organisms fromcolonizing the skin because the niche is already filled.
Why did you wipe your forehead with ethanolbefore sampling it?
Alcohol disinfects the surface of the skin andorganisms on the surface are killed. The normal flora immediately emerges tothe surface.
How can you immediately distinguish Staphlococcus from Propoionibacterium in a Gram stain?
Staphylococcusis a Gram+ coccus andPropionivacterium Gram+ (diptheroids) or rods.
Why does Staphylococcusprobably cause more contamination than Propionibacterium,even though most people have higher numbers of the latter?
Propionibacteriumare obligate anaerobes, and most plates are incubed only for a day or twoaerobically.
What is the predominant organism in your throatflora? Did you observe any beta-hemolytic streptococci in your throat culture?
Alpha hemolytic Strepococci are usually the most predominant organism observed.Most students have a few Beta hemolytic colonies, but their presence isimportant only if there are high numbers. E.coli and Staphylococcus can also produceBeta hemolysis, but their presence is usually not significant.
What is the difference between alpha- andbeta-bemolysis?
a.Alpha hemolysis partially lyses red blood cells and beta hemolysis completelylyses them.
b.Alpha hemolysis produces a partial clearing of blood around the colony. Betahemolysis produces a complete clearing around the colony.
It is very important to treat strep throatbecause of the serious disease that might follow: rheumatic fever andglomerular nephritis. It is, however, important not to treat a sore throat withantibiotics if the cause is viral.
Name one genus of Gram- cocci.
If a student had a cold and sore throat cause bya virus, how would the virus appear on the blood agar plate?
Since virus can grow only in tissue culturecells, there would be no indication of virus on the plate and only normalbacterial flora would be seen.
Can you determine whether an organism canferment a sugar if it does not grow in the broth? Explain.
If the organism has no grown in the broth thereis now way to know what its fermentation capabilities are. Many organisms cangrow in the base broth without carbohydrates, but if they can ferment a sugarthat is available, they will.
How can an organism have a positive test foracid from glucose in a fermentation tube, but have a negative methyl red test,which is also a glucose fermentation test?
The pH indicator in the fermentation tube willturn red even if only a slight amount of acid is produced. Methyl red will turnred only if a large amount of acid is produced as in the mixed acid pathway.
Were there any organisms that did not fermentany sugars? If yes, which organisms?
Obligate aerobes do not ferment. Among theseorganisms Pseudomonas does notferment any sugar.
When comparing a lactose fermentation tube witha MacConkey plate, what additional information does a fermentation tube andagar plate give?
A fermentation tube can also indicate whetherthe organism not only produces acid, but produces has. MacConkey platesindicate that the organism is Gram- as well as fermenting lactose.
Why are Group D streptococci not included in thepolyvalent positive control?
Group D streptococci do not have a groupspecific antigen in their cell wall. The latex slide agglutination test isbased on detection of a group specific antigen.
What are some reasons for including positive andnegative controls?
Positive and neative streptococcus controls helpassure that the latex agglutination test results are accurate.
What advantages are there of the latex slidetest over the capillary precipitin test for Lancefield grouping of pathogenicstreptococci?
The latex slide agglutination test is easier tovisualize than a capillary precipitin test. You must have the proper ratio ofantibodies to antigen particles in order to form the precipitin line. Thecapillary precipitin test requires a considerably longer time to perform.
Streptococcuspneumonia bacteria often posses cell wall surface antigens that react withLancefield Group C antiserum. In this event, hoe would you determine if thepositive agglutination result is due to a Group C Streptococcus or to S.pneumonia?
Streptococcuspneumonia can be differentiated morphologically from other streptococci in that it is the only streptococcus that does not form chains when grown in a broth medium. When grown onan agar medium It is the only streptococcusthat forms lancet shaped encapsulateddiplococcic.
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