Microbiology Lab Final Flashcards
What is the spherical shape of bacteria
cocci
What is the rod shaped bacteria
bacilli
What color will gram positive be under the microscope
purple
What color will gram negative be under the microscope
pink
What are the steps and purpose for each part of the gram stain
1.) Crystal violet: primary stain
2.) Grams Iodine: mordant(intensifies)
3.) Alcohol: decolorizer
4.) Safranin: counterstain
What are four sources of error for gram staining
1.) Picking up colony with hot needle or heat fixing too long
2.) Decolorizing for too short or long
3.) Leaving stain on for too long
4.) Smear too thick/too much bacteria
What are the steps for a catalase test
1.) Place one or two colonies on a slide with inoculating loop
2.) Drop 1 drop of hydrogen peroxide over it
3.) Observe for bubbling
What is the interpretation of results for the catalase test
positive: bubbling
negative: no bubbling
What are three sources of error for catalase test
1.) reversing the order of procedure can give false positive reactions if you put hydrogen peroxide on first
2.) Cultures more than 24 hrs old may give false negative reactions
3.) Improper storage of hydrogen peroxide can give false negative
What is the interpretation of results for the mannitol salt agar
- positive for mannitol fermentation: yellow colonies
- negative for mannitol fermentation: red colonies
What is a source of error for the mannitol salt agar
heavy growth can cause mannnitol fermenting organism to revert to peptone use, giving false negative
Why do some organisms change color during mannitol salt agar test
Because the presence of mannitol in the medium allows organisms that ferment it to turn color
What is the interpretation of results for the bile esculin agar
- positive: blackening of agar plate
- negative: no blackening of agar plate
What are the two sources of error for the bile esculin agar
1.) If large inoculum is used, S. viridian can give a false positive reaction
2.) H2S producing bacteria may show a blackening of the agar
Why does organisms turn color in the bile esculin agar
Organisms that are positive can split the esculin and release glucose into the medium, the ferric citrate allows color change to black with the ability to hydrolyze esculin
What is the interpretation of results for the Bacitracin test
- positive: area of inhibition immediately surrounding the bacitracin disc
- negative: growth up to the bacitracin disc
What are the two sources of error for the bacitracin test
1.) Only beta-hemolytic strep should be used, others may give false positive
2.) heavy inoculum may cause colonies to grow to the disc producing false positive results
What is the interpretation of results for the optochin test
- positive: area of inhibition greater than 14 mm in diameter immediately surrounding the optochin disc
- negative: no inhibition or area of inhibition that is less than 14 mm
What is the source of error for the optochin test
1.) Only alpha-hemolytic strep should be used, others may give false positive
List the steps for the latex agglutination test
1.) Drop 1 drop of the staph latex reagent into a circle
2.) Use sterile loop to transfer two colonies into the circle and spread in liquid to cover entire circle
3.) gently rock card from side to side and observe agglutination for up to 20 seconds
What is the interpretation of results for the staph latex agglutination test
- positive: agglutination within 20 sec
- negative: no agglutination in 20 sec
What is a source of error for the staph agglutination test
1.) False negative or false positive results can occur if inadequate amounts of culture or reagent are used
How do you perform the oxidase test step by step
1.) Place two drops of oxidase reagent on the filter paper
2.) The colony to be tested is picked up with a wooden stick and smeared over moist area
What is the interpretation of results for the oxidase test
- positive: intense deep purple hue within 5-10 seconds
- negative: absence of coloration or coloration later than 20 secs
