SUBJECT: Agar -- what it is, where it came from
DATE: 4/95
Janice Glime asked about agar- I'll try to be the first on the block with
the correct answer.
Agar is a compound in its own right. It is a polysaccharide found in the
cell walls of some red algae and is unusual in containing sulfated galactose
monomers. It requires nothing but extraction and purification to become
agar, but is sometimes chemically modified into agarose for special applications.
Agar added to media simply gels them into a convenient solid form.
Laminaria has various oddball polysaccharides like laminarin (a storage
polysaccharide) and alginic acid (from cell walls). They are chemically
different from agar and, to my knowledge, not widely used.
Gelatin is not a polysaccharide at all but a mixture of peptides derived
from the structural protein collagen. Collagen is the major component of
connective tissue, which is why gelatin is derived from animal by-products.
As a protein gelatin has considerably different chemical properties from
agar.
Hope this helps.
John Dickerman
Northern Illinois University
T80JWD1@wpo.cso.niu.edu
Thanks to John Dickerman for a very useful summary of the chemistry of
agar. The reason it is so useful in microbiology is that there are
practically no bacteria that can digest it. The early microbiologists trying
to make solid media started with gelatin, but a great many bacteria love
to digest that, and then you come back next morning to find a dish of
cloudy liquid instead of nice colonies.
Jean's suggestions for Kitchen Microbiology are neat, in fact I helped
my son do a project along these lines a few years ago when he was in
junior high. The only thing I would caution is that there are quite a
few bacteria that are not killed completely by boiling, so it is
advisable to sterilize the medium in a pressure cooker. 15 minutes
amounts to the standard autoclaving treatment.
There are some bacteria still that are not even killed in a pressure
cooker, but they are fairly rare. If you want to find them, the
standard place to look is in dried hay. What survive heating are
actually the endospores. I haven't tried this yet, but I should
sometime.
While I am on my history kick, it is interesting that even after
Pasteur's famous "swan neck flask" experiment supposedly put paid
to
spontaneous generation, the idea was not completely accepted, because
when there were endospores in the broth, they grew like crazy after
being boiled. The heating actually induces them to germinate. It was
Tyndall who figured out that you have to cook endospores twice to kill
them, the first time germinates them so they lose their resistance, the
second time they die. We are all familiar with Pasteurization, but who
remembers Tyndallization?
There is much to be learned from cloudy water.
----------------------------------------------------------------------
Walter Ogston ogston@hobbes.kzoo.edu
Department of Biology Phone: (616)337-7010
Kalamazoo College Fax: (616)337-7251
Kalamazoo, MI 49006-3295
I don't know where the recipe came from, but I've used something called
kitchen agar for teacher workshops. It does fine for exercises such as
exposing an agar plate to various sources of bacteria and fungi. Here's
the
recipe:
Mix 1 boullion cube (I used beef, don't know if it makes a difference) in
100
ml water. Add 10 ml of this to 40 ml water. Add 2 pkg Knox gelatin and heat.
Pour into petri plates. A substitute for petri plates is those little glass
dishes you put under furniture casters.
Jean Dickey
This recipie produces an enriched gelatin material which various
microorganisms will grow on similarly to an enriched agar medium. The
problem with using gelatin as the base is that many microorganisms release
proteases which will digest the gelatin (it's protein-based), causing it
to
liquify. Agar is preferentially used because it is a carbohydrate
derivative and thus unaffected by proteinases. It will retain its gelled
characteristic while gelatin turns to soup.
Sincerely,
Roger Christianson 503-488-0223 (home)
Department of Biology 503-552-6747 (office)
Southern Oregon State College 503-552-6415 (fax)
1250 Siskiyou Boulevard rchristi@max.sosc.osshe.edu
Ashland, OR 97520
Thought I would add my own trivia to this thread. The word agar comes
from the Malay word "agar-agar" meaning jelly. It was in 1881
that
Robert Koch showed the value of agar in culturing bacteria, and
imported the stuff from Japan which had a monopoloy on the agar trade
until 1940...
Graham R. Kent
Dept. of Biology
Smith College
Northampton, MA 01063
gkent@smith.smith.edu
There may be just a bit more to the story of the beginning of
the use of agar in Robert Koch's lab. I vaguely recall hearing
in a microbiology course long ago that agar was actually
suggested to Prof. Koch as a potential medium component by a
woman working in his lab. Does anyone have any additional
information about this historical tidbit? If it is accurate,
it is not surprising that a woman's contribution was minimized,
especially in that era.
Leland Johnson
johnson@inst.augie.edu
It has been my impression that agar was suggested by Frau Hesse, the wife
of a co-worker of Koch's. I think in today's terminology we would
regard Hesse as a post-doc with Koch. Hesse apparently discussed his
work with his wife, who was not a scientist, but did know her way
around the kitchen, and she suggested that they try agar-agar. At
least this is the story I tell my students.
Crellin Pauling
There is an article in ASM News Vol 58, #8, p 425-428, 1992, by
Wolfgang Hesse telling about his family and their contributions to Bacti.
Evidently Lina Hesse, tech and illustrator for her husband Walther
Hesse suggested agar-agar when asked asked why her jellies and puddings
stayed
solid in the hot summers in Dresden. Walther told Koch about it. Do check
the article if you want more information - it is a nice one. Lina Hesse
was
an American - Fanny Angelina Eilshemius from NY.
Jan Phelps
UWC-Baraboo/Sauk County
Baraboo, Wi 53913
jphelps@uwcmail.uwc.edu
The position of Lina Hesse is interesting in relation to our current
perceptions of the roles of women in society. As a remote connection,
Jonathan Harwood, in his book "Styles of Scientific Thought: the German
Genetics Community 1900-1933" documents a number of women who made
significant contributions to research but were very often not paid for
their work, and when a lab was closed they were essentially turned out
on the street. Is "postdoc" an accurate description of this kind
of
position, or is the current postdoctoral position uncomfortably close to
this phenomenon from the "bad old days"?
----------------------------------------------------------------------
Walter Ogston ogston@hobbes.kzoo.edu
Department of Biology Phone: (616)337-7010
Kalamazoo College Fax: (616)337-7251
Kalamazoo, MI 49006-3295