# ø¤º°* Medical Microbiology Winter 2006 Club*°º¤ø - page 29

okay, people! who is with me on this one? i am feeling a little nervous about all those slides! i'm looking forward to meeting everyone in this course! i will gather up any useful sites i find in... Read More

1. HI- Does anyone have a good website to learn serial dilution's? My book does not explain it well enough for me to wrap my head around a real question: You have 500,000 cells to start and you do a 10 (little 4) dilution in a step series of 1:10 dilutions, how many cells do you have? None of my answers match the choices...:< The book has no examples and lots of charts...with pictures of tubes but no math... :uhoh21:
2. Quote from Staric
HI- Does anyone have a good website to learn serial dilution's? My book does not explain it well enough for me to wrap my head around a real question: You have 500,000 cells to start and you do a 10 (little 4) dilution in a step series of 1:10 dilutions, how many cells do you have? None of my answers match the choices...:< The book has no examples and lots of charts...with pictures of tubes but no math... :uhoh21:
We haven't gotten to that part in our course yet but here is a formula that I found:

Number of colonies on plate X reciprocal of dilution of sample = number of bacteria/ml
For example: if 32 colonies are on a plate of 1/10,000 dilution, then the count is 32 X 10,000 = 320,000/ml in sample
Last edit by stpauligirl on Mar 10, '06
3. Quote from Staric
HI- Does anyone have a good website to learn serial dilution's? My book does not explain it well enough for me to wrap my head around a real question: You have 500,000 cells to start and you do a 10 (little 4) dilution in a step series of 1:10 dilutions, how many cells do you have? None of my answers match the choices...:< The book has no examples and lots of charts...with pictures of tubes but no math... :uhoh21:
I think you do it this way:
500,000 divided by 1:10 = 50,000 cells after the first dilution (first tube)
500,000 divided by 1:100 = 5000 cells after the second dilution (second tube)
500,000 divided by 1:1000 = 500 cells after the third dilution (third tube)
500,000 divided by 1;10,000 = 50 celss after the fourth dilution (fourth tube)
500,000 divided by 1:100,000 = 5 cells after the fifth dilution (fifth tube)

In serial dilutions, the original inoculum is diluted in a series of dilution tubes. Each succeeding dilllution tube will have only on-tenth the number of microbial cells as the preceding tube.
Last edit by stpauligirl on Mar 10, '06
4. That totally makes sense!!!!!!!!!!!!!!!!! Thank YOU!!!!
5. Quote from Staric
That totally makes sense!!!!!!!!!!!!!!!!! Thank YOU!!!!
I am so glad that I was able to help....I wasn't sure if I wrote it in a way so it would make sense. Good luck with it !
6. I hope someone can help me.

For our unknowns in lab, we have to write a lab report. I have no idea how to do this. I last took a biology class/lab about 25 years ago, and I don't remember ever having to write a lab report. Our lab TA gave us a flow chart to use in figuring out what our unknown is, but he didn't say a word about how to write a lab report. I think this is one of those things we're "supposed" to know from our pre-requisite classes.

Does anyone know of a good link to a sample lab report? Or can you give me some pointers on what I'm supposed to cover and how to format one?

I much appreciate any help!!
7. Quote from semisweetchick
I hope someone can help me.

For our unknowns in lab, we have to write a lab report. I have no idea how to do this. I last took a biology class/lab about 25 years ago, and I don't remember ever having to write a lab report. Our lab TA gave us a flow chart to use in figuring out what our unknown is, but he didn't say a word about how to write a lab report. I think this is one of those things we're "supposed" to know from our pre-requisite classes.

Does anyone know of a good link to a sample lab report? Or can you give me some pointers on what I'm supposed to cover and how to format one?

I much appreciate any help!!
We have to document all the biochemical tests that we perform on our unknown, date and results....I am wondering if you have recorded those and you could include those in your lab report. Also we will be interviewed about these tests (oral presentation of the unknown) and need to be able to answer questions about the uniqueness of these tests and how they came out for our specific unknown.... did you do a streak plate and record observations about your unknowns growth pattern, color etc. We have to report those findings, as well as growth on our initial slant. And don't forget your stains.....what are the results of your Gram..... - or +, what shape your specimen is (basillus or cocci) also report if you have an acid fast organism, if it is spore producing etc.
I hope this helped some and feel free to ask further questions.
Last edit by stpauligirl on Mar 13, '06
8. I'm still having difficulty with the whole Selective and Differential thing. Can anyone help out? Why is Mannitol Salt Agar both selective and differential? Any helpful sites? Thanks!

:smackingf
9. We're starting our unknown project this morning in lab. Should be interesting with lots of chaos. I hope my lab partner dropped over spring break.
10. Stpauligirl, thanks for your help with my question about the lab report. We don't start our unknowns until a week from Thursday, so I'll keep everything you said in mind.

Do we track the information in our notebooks and then have to prepare a typed report to turn in? (I realize that is more of a question for my instructor -- duh!) What I mean is, is this the common way of doing things?

We do not have to do an oral report, as far as I know. Thank goodness! It's not that I am afraid of public speaking, it's just that I don't want the hassle!
11. Quote from Kathyz
I'm still having difficulty with the whole Selective and Differential thing. Can anyone help out? Why is Mannitol Salt Agar both selective and differential? Any helpful sites? Thanks!

:smackingf
Kathyz, maybe this will help...

"Selective" relates to categorizing a bacterium, overall. Let's say you have a group of bacteria called "marbles." Within this large group are many different types of marbles: some are blue, some are orange, and some are orange with green swirls. A selective test will only allow SOME of these marbles to pass to the next round of choosing. Let's say our selective test is a funnel with a particular size hole. The orange marbles are smaller in diameter than the blue ones, so only the orange marbles (solid and swirled) pass through the funnel.

In this case, our test has "selected" the orange marbles. The blue ones are exlcluded.

"Differential" relates to categorizing bacteria within their subset. So, after our selective test, we're left with a subset of two types of marbles: solid orange and swirled. We want to separate the swirled ones from the solid ones, so we use a differential test. Let's say our differential test is a medium that can somehow make the swirled marbles glow in the dark. We put all the marbles onto this medium. After the test, the medium still contains ALL of the orange marbles, but only the swirled ones glow in the dark, which allows us to identify them.

Specific to MSA, it is selective because it categorizes bacteria into salt-tolerant and non-salt-tolerant subsets. In other words, bacteria such as streptococcus, which cannot tolerate a salty environment, will not grow on an MSA plate. Staph, on the other hand, CAN tolerate a salty environment, so it will grow on an MSA plate. Hence, MSA "selects for" salt tolerance.

However, "salt tolerant" encompasses a group of many different strains of staph bacteria (the "subset"). The "differential" aspect of MSA categorizes these subsets based on their ability to perform mannitol fermentation. So, you might put three strains (subsets) of staph on an MSA plate. All will grow, but only the one that performs mannitol fermentation will change the pH of the medium, thus giving a positive reaction.

I hope this helps and doesn't confuse you even more!
12. Quote from moonischasingme1
We're starting our unknown project this morning in lab. Should be interesting with lots of chaos. I hope my lab partner dropped over spring break.

My lab partner is a complete airhead. I have to spend so much time just going over what the instructor said TWO MINUTES AGO ("Did he say we each take a plate, or do we only get one plate per group?") that I get precious little time to actually DO THE EXPERIMENTS!

Ah, well. Before we know it, it will be over. Good luck on your unknown!
13. Quote from semisweetchick
Stpauligirl, thanks for your help with my question about the lab report. We don't start our unknowns until a week from Thursday, so I'll keep everything you said in mind.

Do we track the information in our notebooks and then have to prepare a typed report to turn in? (I realize that is more of a question for my instructor -- duh!) What I mean is, is this the common way of doing things?

We do not have to do an oral report, as far as I know. Thank goodness! It's not that I am afraid of public speaking, it's just that I don't want the hassle!
Our teacher gave us a report sheet where we will record everything that she wants us to report about for our unknown. the sheet has a list of tests that we need to do. I am surprised that your teacher isn't issuing a report sheet for you guys. I can send you a blank if you like, just think how much you would impress your teacher!!!! :spin:
Maybe she waits until you actually start to give you guys more direction.....it seems messed up to me.
Definitely keep track of everything you do with your unknown in a notebook. Even if you dont think something is significant.
Good luck and wait and see what happens when you get started, I am here for you if you need me.