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Thread: Need Advice About Microbiology

  1. #1 Need Advice About Microbiology 
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    I've done an experiment in my college.

    I took one petri dish with agar inside it. I divided it into three areas, i.e. A, B and C. On A, I streaked my finger. Then I washed my hand with soap and streaked my finger on B. Last I washed my hand with alcohol and streaked my finger on C.

    I took another petri dish also with agar inside it. I used a sterile loop of wire to took the soap and streaked it.

    After a week, the colonies could be seen on the agar plates. A had many colonies, B had less colonies than A and C had none. So, I could take a conclusion that the soap wasn't sterile enough. But I didn't see any colonies in the last plate so that my hypothesis was wrong.

    Any idea why B contained colonies? I'm rubbish in biology as well.


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  3. #2 Re: Need Advice About Microbiology 
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    Quote Originally Posted by georginho_juventusygr
    I've done an experiment in my college.

    I took one petri dish with agar inside it. I divided it into three areas, i.e. A, B and C. On A, I streaked my finger. Then I washed my hand with soap and streaked my finger on B. Last I washed my hand with alcohol and streaked my finger on C.

    I took another petri dish also with agar inside it. I used a sterile loop of wire to took the soap and streaked it.

    After a week, the colonies could be seen on the agar plates. A had many colonies, B had less colonies than A and C had none. So, I could take a conclusion that the soap wasn't sterile enough. But I didn't see any colonies in the last plate so that my hypothesis was wrong.

    Any idea why B contained colonies? I'm rubbish in biology as well.
    It is practically impossible to wash all the bacteria off of your hands with soap, some will almost always remain. Soap really isn't an optimal place for bacteria to grow so I doubt there were bacteria in the soap. Also you have to take into consideration that your technique may be bad and you got contamination from the air. You never know what's floating around in a biology lab . They should do the experiment where you leave a petri dish uncovered for 3 hours, then incubate it.

    Most likely though is that you just didn't wash all the bacteria off of your hands, to do that you have to wash for like a good minute or two.

    (Also you can very easily pick up new bacteria removing the cover of the dish and touching things before you streaked.)


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  4. #3 Re: Need Advice About Microbiology 
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    Quote Originally Posted by i_feel_tiredsleepy
    Quote Originally Posted by georginho_juventusygr
    I've done an experiment in my college.

    I took one petri dish with agar inside it. I divided it into three areas, i.e. A, B and C. On A, I streaked my finger. Then I washed my hand with soap and streaked my finger on B. Last I washed my hand with alcohol and streaked my finger on C.

    I took another petri dish also with agar inside it. I used a sterile loop of wire to took the soap and streaked it.

    After a week, the colonies could be seen on the agar plates. A had many colonies, B had less colonies than A and C had none. So, I could take a conclusion that the soap wasn't sterile enough. But I didn't see any colonies in the last plate so that my hypothesis was wrong.

    Any idea why B contained colonies? I'm rubbish in biology as well.
    It is practically impossible to wash all the bacteria off of your hands with soap, some will almost always remain. Soap really isn't an optimal place for bacteria to grow so I doubt there were bacteria in the soap. Also you have to take into consideration that your technique may be bad and you got contamination from the air. You never know what's floating around in a biology lab . They should do the experiment where you leave a petri dish uncovered for 3 hours, then incubate it.
    But it should have also happened when I used alcohol. No?

    Most likely though is that you just didn't wash all the bacteria off of your hands, to do that you have to wash for like a good minute or two.

    (Also you can very easily pick up new bacteria removing the cover of the dish and touching things before you streaked.)
    And the petri dish was open before I streaked my finger.
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  5. #4  
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    Well then the most likely conclusion is just that you didn't wash all the bacteria off of your hands. Especially if it was that Tryptan Soy Agar (the dark yellowish one) almost anything can grow on that stuff, so it is easy to get some colonies from a few bacteria surviving on your hands. Also, depending on how much alcohol was on your finger you could have actually killed any bacteria from the air which got on the plate.
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  6. #5 Re: Need Advice About Microbiology 
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    Quote Originally Posted by georginho_juventusygr

    After a week, the colonies could be seen on the agar plates. A had many colonies, B had less colonies than A and C had none. So, I could take a conclusion that the soap wasn't sterile enough. But I didn't see any colonies in the last plate so that my hypothesis was wrong.
    B contained colonies because soap didn't remove all bacteria from your hand.


    then there are two possible explanantions depending on how you did your experiment:
    1. The last plate didn't contain colonies, because you either used a sterile piece of soap or a piece of soap that was reasonable sterile.

    2. or if you used the same piece of soap that you washed your hands with, the soap isn't a good living environment for the bacteria from your hand. bacteria were indeed brought into contact with the soap but found no foothold on the bacteria.
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    When I did this experiment, I had actually more bacteria on the equivalent of B than A. The explanation I got was that the soap could actually make the bacteria more mobile.
    ...Wait, what?
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    Quote Originally Posted by CircularlyPolarized
    When I did this experiment, I had actually more bacteria on the equivalent of B than A. The explanation I got was that the soap could actually make the bacteria more mobile.
    I don't quite understand why the motility of the bacteria would affect the amount of colonies o.O
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  9. #8  
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    each bacterium can create a new colony.
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  10. #9  
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    Quote Originally Posted by spuriousmonkey
    each bacterium can create a new colony.
    Yes and so? This shouldn't change the number of colonies you see. If you consider the number of bacteria usually on a finger, you should get a lawn with unwashed hands. With washed hands even if the bacteria were for some reason motile as a result of the soap, (which makes no sense) they will still settle down and form colonies where they land on the agar. Barely any skin bacteria are flagellated, and gliding motility is so slow that it takes several hours to move a single centimeter.
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  11. #10  
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    Bacteria don't really want to fall off their little perch with relish. If they are living in a suitable environment they probably rather stay.

    Soap is a detergent which could losen whatever hold they have on the skin or vice versa, whatever hold the skin has on bacteria.

    Therfore if soap increases motility more bacteria will move onto the agar plate, creating more colonies. Or something similar.
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  12. #11  
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    Quote Originally Posted by spuriousmonkey
    Bacteria don't really want to fall off their little perch with relish. If they are living in a suitable environment they probably rather stay.

    Soap is a detergent which could losen whatever hold they have on the skin or vice versa, whatever hold the skin has on bacteria.

    Therfore if soap increases motility more bacteria will move onto the agar plate, creating more colonies. Or something similar.
    You should be getting less colonies with washed hands, as a detergent soap desolves lipids, which does indeed cause the bacteria to dissociate from the hair follicles and their other holds, but if you are washing your hands properly they should also be getting washed away with water .
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    Quote Originally Posted by CircularlyPolarized
    When I did this experiment, I had actually more bacteria on the equivalent of B than A. The explanation I got was that the soap could actually make the bacteria more mobile.
    I did a similar experiment back in school, however half of the class used a bar of soap to wash their hands, and the other half used liquid soap (i.e from a pump bottle or dispenser). Those who used the bar of soap had far more bacteria in the equivalent of B than A, whilst those who used liquid soap had little or no bacteria at all in B.
    Those that used the bar of soap earliest had less bacteria in B than those who were last to use the [same] bar of soap to wash their hands with.

    The conclusion we made was that the bar of soap can become [greatly] contaminated by the bacteria on our hands through use, as some of this bacteria washes off onto the bar (and is not killed at least immediately), whereas for the liquid soap this is impossible because each individual participant extracted a sample of soap through the pump, and did not touch the source at all.

    This also means that bars of soap are not reliable. Did you use a bar of soap in your experiments, CircularlyPolarized or georginho_juventusygr?
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    Good explanation, obviously multiple uses of the same bar of soap would result in it getting covered with bacteria. Over time these bacteria will die on the soap for the most part but immediate use of the soap produces more bacteria on your hands .
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    Quote Originally Posted by elhermoso
    Quote Originally Posted by CircularlyPolarized
    When I did this experiment, I had actually more bacteria on the equivalent of B than A. The explanation I got was that the soap could actually make the bacteria more mobile.
    I did a similar experiment back in school, however half of the class used a bar of soap to wash their hands, and the other half used liquid soap (i.e from a pump bottle or dispenser). Those who used the bar of soap had far more bacteria in the equivalent of B than A, whilst those who used liquid soap had little or no bacteria at all in B.
    Those that used the bar of soap earliest had less bacteria in B than those who were last to use the [same] bar of soap to wash their hands with.

    The conclusion we made was that the bar of soap can become [greatly] contaminated by the bacteria on our hands through use, as some of this bacteria washes off onto the bar (and is not killed at least immediately), whereas for the liquid soap this is impossible because each individual participant extracted a sample of soap through the pump, and did not touch the source at all.

    This also means that bars of soap are not reliable. Did you use a bar of soap in your experiments, CircularlyPolarized or georginho_juventusygr?
    I used liquid soap. What I still don't understand is the statement that the soap isn't good environment for the bacteria to grow. Why? Is this because of the basic environment of the soap? (I consider that the soap is basic. I'm maybe wrong though.)

    But taking a note of what spuriousmonkey said that the soap didn't remove all bacteria from my hand, I ask myself why. Since the soap isn't good environment for the bacteria to grow, surely they should have died when I washed my hand.

    And what's up with the alcohol so that it can kill the bacteria?

    Maybe the result of my class is useful.

    Code:
    Zone                |     Mean of total of bacteries
    -----------------------------------------------------
    A (no action)       |     16
    B (soap)            |     61
    C (alcohol)         |     3
    D (soap)            |     101
    E (alcohol)         |     2
    F (soap)            |     78
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  16. #15  
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    Bacteria don't die that easily. Soap is not a disinfectant (bacteria-killer). It is a detergent. It is basically a cleaning agent. Removing dirt/stuff from your hands.
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  17. #16  
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    Soap isn't bactericidal, unless it contains added disinfectants, however it is a horrible environment for bacteria to grow. They will essentially starve to death over time on a piece of soap.
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  18. #17  
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    Quote Originally Posted by elhermoso
    Quote Originally Posted by CircularlyPolarized
    When I did this experiment, I had actually more bacteria on the equivalent of B than A. The explanation I got was that the soap could actually make the bacteria more mobile.
    I did a similar experiment back in school, however half of the class used a bar of soap to wash their hands, and the other half used liquid soap (i.e from a pump bottle or dispenser). Those who used the bar of soap had far more bacteria in the equivalent of B than A, whilst those who used liquid soap had little or no bacteria at all in B.
    Those that used the bar of soap earliest had less bacteria in B than those who were last to use the [same] bar of soap to wash their hands with.

    The conclusion we made was that the bar of soap can become [greatly] contaminated by the bacteria on our hands through use, as some of this bacteria washes off onto the bar (and is not killed at least immediately), whereas for the liquid soap this is impossible because each individual participant extracted a sample of soap through the pump, and did not touch the source at all.

    This also means that bars of soap are not reliable. Did you use a bar of soap in your experiments, CircularlyPolarized or georginho_juventusygr?

    We used liquid soap.

    As for how hands washed with soap caused more colonies than unwashed hands, I might have used the wrong words. I did not mean that the soap made the bacteria more motile, only that it loosened their grip to the surface.
    ...Wait, what?
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  19. #18  
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    Quote Originally Posted by CircularlyPolarized
    We used liquid soap.

    As for how hands washed with soap caused more colonies than unwashed hands, I might have used the wrong words. I did not mean that the soap made the bacteria more motile, only that it loosened their grip to the surface.
    Then you simply did not wash your hands long enough, that is why physicians say to do it for a full minute. Also occasionally soaps can be colonized by bacteria and give misleading results, some bacilli are capable of living in soap.

    If hand washing is done properly, then loosened bacteria should be washed away with the soap.
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  20. #19  
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    Indeed, I am a slob :P
    ...Wait, what?
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