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I just need to be pointed in the right direction so I can figure these answers out. Thanks in advance for anyone willing to help. :)




1. Culture media must provide many essential components in the artificial environment of a broth or solid medium. List the minimum items that must be provided for the successful growth of organisms and indicate what their role is for the organisms.


2. In addition to the items listed above, what other considerations must be considered once you have provided the above list and why?


3. Water from the tap is rarely ever used to make medium. Why?


4. How are selective and differential media different. How can the two be combined in a useful way?


5. After the addition of water to the powdered medium it must be sterilized in a timely fashion. Why?


6. What is the major difference between the preparation of a broth medium and one containing agar?


7. What are the major problems encountered when counting bacteria using a diluted sameple on a microscope slide?


8. Why do we use pipets to perform our dilution sequences rather than the inoculating loops? Does it make a difference in using pour tubes for our dilutions rather than dilution blanks? why?


As for what I know:


1. I know the items needed to have a successful growth but I'm not sure which ones can be left out.

2. I just don't understand this question.

3. I know that tap water contains contaminates but I thought that mediums had to be sterilized so should tap water matter?

4. I know how they are different but I have no idea about how they can be combined.

5. The powdered medium is agar. I think the answer is because it needs to be sterilized before it turns into a solid. Agar which is very time sensitive. Am I close on this one??

6. I'm still very new to the microbiology stuff and for some reason I thought all mediums contained agar so I'm not sure about this question. It just confused me.

7. I don't know what kind of problems could arise.

8. The only things I can think of is because dilution is very sensitive to amounts. The inoculating loops don't have a set amount of liquid that can be picked up. I don't know what a dilution blank is.


I understand this is not for answers and I want to point out that I am not looking for anyone to do this for me, but I am looking for help. I've tried. I've read and re-read my lab manuals and book but I am still having problems.

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What course are you taking? What textbook or lab manual are you using?


There are many courses that cover the analysis of water and wastewater. One such course is the Sacramento State (University of California) "Wastewater Treatment II" course. Another reference that describes in detail the methodologies about which you're asking is "Standard Methods for the Examination of Water and Wastewater". In the old days this reference was available only in book form, but now it can be accessed on-line (for a subscription fee, of course). You can find it here: http://standardmethods.org/



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3. Yes, media does have to be sterilised after it's made (usually by autoclave). However, it is best to start with the cleanest water possible (I use normal distilled water rather than the even purer but more expensive MilliQ).

Also, it's not just microorganism contaminants you need to worry about. Normal tap water contains all sorts of dissolved stuff (for example fluoride depending on where you live, and various minerals). Distilled/deinonized water removes lots of this as well.


5. The agar setting isn't really a problem (if it does set, it will melt again when it's sterilised by heating, and even if it didn't melt it would still be sterile after heating). The reason it must be autoclaved soon is so that stuff like fungus doesn't grow in the medium - you don't want fungal gloop in there even if it IS killed by subsequent heating.



8. I THINK you're right. I don't know why you would use an innoculating loop at all!

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1. There are certain elements that are needed by almost all organisms (think in terms of C and N sources, for instance. As well as of the composition of biomolecules found in all organisms, such as DNA). Certain elements are needed by almost all organisms, but there are (few) specialists that can do without (iron being an example).


2. The question is very open. I would interpret it in terms of additional needs (above the absolute minimum for all organisms) that certain bacteria might have.


3. As already mentioned it is the non-organic contamination that is undesirable, Ultrapure or double distilled water is used for microbiological applications. If you just have complex media (and rather simple lab bacteria) you may get away with simple distilled water.


4. What happens if you use them subsequently? What happens if you inoculate them (with your sample) at the same time? Think about known samples vs. unknown.


5. Agar is not the issue.


6. If you want to culture bacteria in a big batch bottle, would you put agar in?


7. Think about reproducibilty of the count as well as accuracy.


8. How did you do your dilutions?

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