How can you account for the observation that a mass of bacterial
growth shows pigmentation while the individual cells have transparent
cytoplasm?

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Request for Question Clarification by blazius-ga on 16 Sep 2002 00:00 PDT

By "a mass of bacterial growth", do you include the growth medium and
possible excreta from the bacteria, or would you only consider the
bacteria cells?

There are at least two ways to approach your question:

1)  If you are referring to the fact that a pellet (or "mass") of
bacterial cells after centrifugation is visible, while the cells
individually are not, aceresearcher's 2nd answer is quite correct -
it's not that the individual cells are colourless, it's just that
there isn't much substance to their coloured bits and so, individually
there isn't enough colour to perceive with the naked eye (until you
pack all the cells together).  A common technique for estimating the
quantity of bacteria in a culture is to use a spectrophotometer (a
device that shines a light through a sample and can detect small
differences between the amount of light put it versus the amount which
comes out, thus giving you the amount absorbed by the sample) to
measure the small amount of colour that the bacteria do have.  A nice
site illustrating this:
http://www-micro.msb.le.ac.uk/LabWork/bact/bact1.htm

2)  Some bacteria only express certain proteins when there are enough
other bacteria around - this is an effect called "quorum sensing" and
results in large colonies of bacteria acting in concert to change
their behavior when a certain growth density is achieved.  Pigment
proteins may be among those proteins only expressed when a certain
density is achieved.  This effect may be multiplied by the effect
above, such that large quantities of bacteria expressing pigment due
to their density become quite visible upon pelleting.
A couple of sites which discuss quorom sensing and pigment expression:
Quorum sensing: http://www.wisc.edu/McNair/pdfs/sum01/QQpaper.pdf

Paper on bacterial diversity, search page for quorum sensing for
section on luciferase (luminescent pigment) production:
http://www.fiu.edu/~biology/mcb3023/lectures/Diversity5.htm

fairly technical paper on pigment and quorum sensing:
http://papa.indstate.edu/amcbt/volume_26/v26-4p3-13.pdf

I hope this answers your question.

synarchy

Rather than engage in web research for this one, I asked my husband,
who is a microscope nerd (pathologist). He had a couple of
suggestions:

It is possible that the bacteria is reacting with the medium, or with
a component of air (oxygen, nitrogen, carbon dioxide, etc), and the
pigmentation is a by-product of the reaction. So researching the
behaviors of the bacteria (if the strain is known to you) might answer
your question.

He also pointed out that, while an individual bacterium's cytoplasm
may **appear** to be colorless, it would only have to contain a
miniscule (not visible to the human eye) amount of color for a colony
of thousands or millions of bacteria to show a pronounced color.

Hope this helps! :)

There are many ways to describe or quatify a "mass of bacterial
growth"

The main thing is that an answer is not useful unless it is qualified
with how the measurements were made.

dry weight is often used to describe an amount of cells. First you
weigh a filter (several times to get an estimate of error). Cells from
a volume of culture are then collected by filtration and the filter
and cells dried untill the weight doesn't decrease any futher. You can
then weigh the filter and cells (several times) and subtract the
original mass os the filter.

Hope this helps
James