MadSci Network: Microbiology |
Hi Linda,
It's true, methylene blue can be used to detect the presence of oxygen.
There is actually a page about the principle used, it's at texas
university.
However, there are several things that might go wrong in your experiment.
I
don't know the exact protocol one should follow, cause I never did this
experiment. Below I'll summarize some bottle-necks..
The most common bacteria to grow in milk are lacto-bacteria. These
bacteria produce yoghurt, cheese and other diary products, but also make
milk turn 'bad'.
These bacteria however don't need to use oxygen to grow, they produce
lactic acid. Nevertheless, they can survive in an oxygen containing liquid
like milk.
Also, other bacteria do consume oxygen, but in milk fresh from the carton,
there should be no bacteria, or when they are present, not in amounts to
be
detected with methylene blue. There should be quite a lot of bacteria to
consume the oxygen and care should be taken to prevent oxygen from
re-entering the liquid (so, no stirring, shaking, mixing, but use a bottle
which can be capped and fill it completely before capping).
Of course the concentration of methylene blue you use is improtant, if
a small amount is used, changes in color can be detected faster. However,
I
don't know which concentrations to use ....
Something that does change in milk when lactobacteria grow is the acidity
(pH). In a lot of fermentation processes the pH is a measure for the
microbiological activity. pH can be detected with indicator paper, but
there are also several pH indicator-solutions available. Usually sterile
milk will have a pH of about 6-8, but when lactobacteria start to grow it
will drop significantly. If you find a nice indicator on your 'lab shelf'
that will change colors somewhere in this range, it might work.
If you really want to have quick growth, you could see if you could find
yoghurt of which the lactobacteria aren't killed (not pasteurized or
sterilized) and add a small amount to fresh milk with the indicator, store
the 'culture' for one or a few days at a place at room-temperature of a
bit
hotter (30C) and mix once in a while. Bacteria should start
growing... (WARNING: DON'T EAT THE CULTURE, OTHER HARMFULL BACTERIA
COULD BE GROWING IN HERE TOO!)
One of the easiest ways to detect bacteria is using a 'rich' (ie
containing
enough nutrients) clear solution and see the bacteria make the solution
more and more turbid...
Good luck with the experiments..
Rolf Marteijn
Wageningen Agricultural University
dept. Foodtechnology and Nutrional Sciences
division of Bioprocesengineering
Animal cell culture workinggroup
Try the links in the MadSci Library for more information on Microbiology.