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Evaluation

can any1 suggest any limitations and how it could effect my results for the following practical:
Growing e.coli and m.luteus in test tubes containing liquid broth. Then using a colorimeter to measure growth after they had grown for 48hrs.

Reply 1

MadNatSci

mss

The first thing that springs to mind is that you have two different species of bacteria and they might have different characteristics; this might affect your colorimeter readings. For example the colorimeter might suggest that there are different numbers of bacteria when in fact they are the same, because the bacteria are different sizes or, I don't know, colours(!), or something, so making your broth look different. An improvement might be to count the bacteria more directly: you could take a sample from each broth at a specific time, serially dilute it (so that your bacteria don't completely cover the plate) and plate the species on agar jelly, then count the colonies. Assuming that each colony was formed (or perhaps founded!) by one bacterium, you can then multiply back and so work out how many bacteria were in the broth at the time you took the sample.

It's difficult to know about any other limitations without knowing your whole method; what exactly did you do? Were you checking what nutrients the bacteria required? How well did you control the other variables? (such as temperature, pH, etc) Another factor controlling growth might be the population density of the bacteria themselves; when you get too many bacteria for the nutrient content of the broth, they'll stop growing. All worth bearing in mind!