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Howdy all! Why yes, I did post again…

Today’s topic spawns off of a discussion I had with a friend a while back about agar plates, as they had never really heard the basics of what a petri plate is and why it is so crucial to a modern microbiologist’s work (despite being “low tech”).  As such, lets take a brief run through of agar plates.

Of course, when I was in grade-school, this is what I thought of when I heard Petri plate.

Example of agar agar in food stuffs.

So, first lets hit the history of agar plates in true Alton Brown style.  Agar, or if you’re being picky “agar-agar”, is a substance isolated from red algae, aka: seaweed.  This polysaccharide (a polymer of galactose, I believe) provides stability to the algae’s cellular walls and just happens to make a very firm structure.  As such, we humans found the texture and structure of this gelatinous seaweed appealing and most asian cultures have adapted it to suit their fancy.  (Americans, on the other hand, are a bit more slow on the uptake.  Weird textures freak us out it seems!).  Agar-agar is also used in jellies, jams, soups and other various food-stuffs that need to be thickened or solidified, as it produces a smooth texture and has a melting point that is easy to reach in the kitchen without burning. (85C is the common temp we lab rats give).

Sciency-food-fact! Often, myself and my peers refer to agar plates as “similar to Jell-O”. (In fact, in lieu of a proper plate, I demonstrated an experiment once using Jell-O as a growth substrate!).  Unfortunately, this is a bit of a misnomer…Jell-O also contains a giggly food thickener, like agar-agar, called gelatin. However, it is a collagen polymer derived from animal tissue, not a sugar polymer from plants.  The collagen is a much more loose mesh, molecularly, and does not give as firm of a structure as the plant cell polymer does.  (Play with your upper ear for a good example of collagen).  A plant polymer, like agar-agar, affords more stability and more rigidity to the cells. (Try bending a leaf, it doesn’t work quite so well as your ear…) Therefore, if you were to compare a treat solidified by agar-agar to one solidified by gelatin, the agar-agar product would be more solid and have a thicker, smoother texture than that of the gelatin product.  And now you know!

Anyway, back to science.

A lab tech in Robert Koch’s lab, clearly enjoying her jelly tea, realizes that this agar-agar stuff would be a great solidification substance to grow bacteria on.  Thus, agar plates were born.

MacConkey agar. The growth in pink indicates lactose utilization, while the yellow does not.

We use agar-agar to solidify damn near all sorts of media into small, compact little circles that allow us to more easily quantify and isolate the microscopic lifeforms we work with.  From LB and minimal medias to MacConkey and blood agar, we use this white powder for damn near everything.  Its relatively simple, toss an amount into the liquid media (i.e. 15g for 1L of LB in most labs), autoclave it up, let it cool and pour. Simple as that.

Agar has numerous practical uses.  We can identify enviromental isolates (or at least start to) using several selective medias in agar.  For example, MacConkey agar grows only Gram negative bacteria and will change color depending on the isolate’s ability to ferment lactose.  Blood agar plates demonstrate whether a species can lyse bloodcells (either beta or alpha) and often help to differentiate between Strep and Staph isolates.  Hektoen enteric agar is used to identify organisms from, as the name states, the Enterobacteriaceae family.  I’ve used HEA before to identify a Shigella species in particular, but it is often used for any fecal MO’s (like Salmonella).

A fine example of a quadrant streak that shows the isolation of several distinct colonies, as indicated by color.

We also modify agar in order to isolate a pure culture: a colony in which the cells came from ONE lone parent organism via the quadrant or enviromental streak.  We can also identify microorganisms by their resistance to an antibiotic impregnated into the media.  For example, alot of plasmids that we use commercially have an antibiotic resistance cassette placed within the DNA.  This provides us researchers an easy form of identification of clones who had the correct plasmid shoved into their membranes as they will grow on, say, LB with Amp.

Of course, this technique and the other plates I’ve mentioned will have to wait for another day.  This post is getting long and my intentions were only to give a brief explanation of agar and why we use it!  Clearly I got carried away!  So stay tuned for my next post and perhaps the next in my series of HTWADYB!




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