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The other day, one of my commenters stated that “a well-informed sixth-grader should be able to distinguish between MRSA and E. coli“.

Here’s a nutrient agar plate with some of the bacteria that we isolated from a local creek last fall. We identified our bacteria by sequencing the 16S ribosomal DNA, but for various reasons, that I won’t go into here, we don’t know which sequences belong to which colonies.

i-baa7f5842895316e8e0aa50f7b4ae8c8-creek_bacteria.jpg
Figure 1. Bacteria isolated from a local creek.



I guess we need some well-informed sixth graders. ;-)

Comments

  1. #1 Steve
    January 1, 2009

    Using colonial morphology (ie looking at the bugs on an agar plate) you can with experience have an educated guess to the genus level, for some organism groups – Staphs, Streps, Klebsiella’s, Bacillus etc. Coliforms, well you just lump em into coliforms. You also need to know the source of the sample and your growth media. Quite a few variables to take into account.

    Bacterial identification is not difficult, but for wild samples like the above requires more than a 6th grade education.

    Anyway that plate looks like a load of Staphs and some coliforms, need a proper look at the plate tho ;)

  2. #2 Sandra Porter
    January 1, 2009

    That’s exactly the point. If you don’t know how to identify your bacteria by morphology or some other characteristic (like growth on differential media), and you don’t know what you’re working with, it’s not going to be simple.

    BTW – we didn’t find any E. coli or Staph in our 16S rRNA sequences, but we only sequenced a few colonies from each plate. Most of our sequences came from Pseudomonas, Erwinia, and Chryseobacterim.

  3. #3 Faz
    January 1, 2009

    This is a near impossible problem. For a start, using colony morphologies to conclusively identify colonies is a difficult science. When doing this, I generally take interesting colonies and subculture them onto separate plates, and then let them grow out for a few days. The ways colonies age can tell you something extra about them, some colonies smell different than other. But that information comes with experience, and other tests need to be performed to confirm this, such as gram staining, serotyping and other such things.

    I can understand which someone would immediately assume that this plate has e.coli and staph on it. If it was growing in my incubator, that would be my first assumption, as I am used to working with isolates from human tissue.
    People often forget that e.coli and staph don’t survive very long without their human hosts, so you wouldn’t expect to find them in a creekbed (unless it was near to a sewage outlet). and if they are present they are in a weakened state, and need to be specially treated in order for them to recover from their stressed state. (unless they are clostridium)

    More likely to be present would be a corynebacterium of some sort (on blood agar, they look quite similar to staph. I’ve never grown them on another medium, so i don’t know what they should look like here) or a vibrio species(never grown these, so I have no idea what they would look like).

    So the well informed sixth grader will not only need to know how culture conditions affect bacterial growth, they would need a gram-staining kit, an oxidase kit and catalase kits.

    And it is surprising how similar staph and coliform colonies can be when grown on some media.

  4. #4 Sandra Porter
    January 1, 2009

    Faz – that’s why I like DNA sequencing. :-)

  5. #5 Steve
    January 2, 2009

    Its quite suprising just how much ID you can do with gram staining, oxidase, catalase, an anaerobic Jar and a good reference book.

    Another point to touch on, in that to identify MRSA you need to do antibiotic testing (ie Methicillin) as well as bacterial identification, and thats a whole ballgame in itself!

  6. #6 TomJoe
    January 2, 2009

    Its quite suprising just how much ID you can do with gram staining, oxidase, catalase, an anaerobic Jar and a good reference book.

    Indeed. Though all of that is a bit above your typical 6th grade area of expertise. Now, no doubt you could probably train them to become proficient in these tests and make a tentative identification, however … if you were in a hospital and the doctors thought you had a multiple antibiotic resistant Staph infection and your life depended on the results … would you want a 6th grader performing the tests, or a trained professional?

  7. #7 Hannah
    April 15, 2012

    Can someone please help me identify some bacteria on an agar plate. Just a general educated guess will be good enough. i am in grade 12 doing an experiement on bacterial colonies on hands and i have a few different colours. i was just wondering if you can have a look and see what you think they could be?

  8. #8 Sandra Porter
    April 15, 2012

    Hi Hannah,

    Send me a photo, digitalbio at gmail dot come

    This might not be enough information, but I can take a look.

  9. #9 Sandra Porter
    April 15, 2012

    Hi Hannah,

    Send me a photo, digitalbio at gmail dot come

    This might not be enough information, but I can take a look.

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