Microbiology Lab Exam Study Guide Rated A+

Microbiology Lab Exam Study Guide Rated A+ What color do Gram-Positive bacteria stain? Purple What color do Gram-Negative bacteria stain? Pink What are the 4 steps of a gram reaction? Primary stain crystal violet, mordant iodine, decolorizer alcohol-acetone, and counterstain safranin. What are some ways you could mess up a gram stain? And how will you know you messed up? a. Not heat fixing your slide - clear slide, b. over decolorizing - slide has both pink and purple even though the culture was pure, so gram positive bacteria will stain pink and purple c. Under decolorizing - slide has both pink and purple even though the culture was pure, so the gram negative bacteria will stain purple and pink d. Overheating your slide or not letting your smear completely dry - bacteria will burn off leaving your slide clear e. Forgetting to counterstain - you will have gram positive bacteria that appear purple and gram negative bacteria will appear colorless Look over cell shape and arrangement (i.e. cocci and strepto = Streptococcus) Look over colony morphology characteristics Which grows better on MSA (Mannitol Salt Agar) staphylococcus or enterococcus? Why? Staphylococcus bacteria grows better on MSA because it has a high salt content that makes it very difficult for enterococcus bacteria to grow on it. Also some staphylococcus species, like staphylococcus aureus, are able to ferment the mannitol in the agar and change the pH of the agar as evidenced by a color change from pink to yellow What is in BEA agar that makes it selective? The Bile salts in the BEA are what make it selective What is in BEA agar that makes it Differential The Esculin Azide in the agar is what makes it differential How can you tell if an enterococcus species is growing on a BEA plate? The media will turn dark brown Why do we use EMB agar? It allows us to selectively culture gram negative bacteria and differentiate based on lactose fermentation. Why is E. coli green on EMB? Because it is fermenting the lactose in the agar and is stained by the combined eosin Y and methylene blue dyes. What species of bacteria does a catalase test differentiate between? Between staphylococcus/micrococcus and streptococcus/enterococcus bacteria Which species is catalase negative? Streptococcus/enterococcus species What does a positive oxidase test look like? A positive oxidase test will show as a blue color change to the bacteria on the swab. What is another term for beta hemolysis? Complete hemolysis What is another term for alpha hemolysis? Incomplete hemolysis Match the following plates with the correct Hemolysis When washing your hands, what are the most commonly missed areas? Cracks of the hands, around the nails beds, in between the fingers, on the side of the pinky, around any jewelry, in any scars, cuts, or abrasions How long should you wash your hands for? 20 seconds True or false? It is better to use hand sanitizer instead of washing your hands. False. ALWAYS WASH YOUR HANDS!!! What are the different modes of transmission? Give an example of each. Contact transmission: direct contact transmission involves person to person contact like shaking hands; while indirect contact transmission involves and intermediate inanimate object or fomite, like a door handle. Vehicle transmission: occurs through a medium like water, food, soil or the air. Vector transmission: mechanical vectors carry a pathogen without being infected themselves, like a house fly; while biological vectors are infected by the pathogen they are spreading, like mosquitos. What is Snyder's media used for? What does it mean when the media turns yellow? Snyder's media is used to test the susceptibility of dental carries. If the media turns yellow within 72 hours that patient is deemed susceptible to dental carries. Why can B. subtilis survive pasteruization while S. aureus cannot? B. subtilis produces endospores What does a possitive Strep A test look like? And what does a negative strep test look like? A positive strep test shows as both a blue and red line showing up on the strep strip. A negative strep test shows up as a single red line on the strep strip. What are the three terms used to describe growth in a broth tube? Turbidity is the term used to characterize the visible growth within a broth culture. Surface growth is the growth at the meniscus of the broth culture. Sediment is the term used to describe the presence of precipitate, usually dead or non-motile cells, at the bottom of a broth culture tube. If an autoclave is working properly what color will the BTsure vial be after a 15min autoclave cycle? Purple How do you use and Enteropluri tube? Read the positive agar windows and circle the corresponding numbers. Add up each grouping of numbers to obtain a 5 digit biocode. Look up the biocode and find your unknown bacteria. What is the difference between a pure culture and a mixed culture? A pure culture contains a single bacteria and will present itself as a distinct colony morphology throughout a streak plate. A mixed culture is characterized by 2 or more bacterial organisms and is presented on a streak plate as 2 or more dissimilar colony morphologies. What are some reasons you would see a completely dark field when looking through a micorscope? The power is off or the machine is not plugged in, the objective is not clicked into place, or the bulb is burned out. Why do we use immersion oil? Immersion oil increases the resolution, by decreasing the light refraction, making the image appear sharper.