Hello all, I am interested in performing a carbon utilization test for several L. monocytogenes strains. I have encountered various protocols and I'm unsure which would work best. Some use phenol red, others use bromothymol blue, etc. Is there a difference or do you generally just use whichever is convenient/on hand? Does it differ depending on which carbon sources you are testing for? Thank you.
Choice of pH indicator depends upon the factor, that different indicators provides changes as per to their limits. For example, if you are using Bromocresol Purple, which is purple at around neutral pH and turns yellow near pH 5. Now the question is if your bacterial stain does not produce enough acid to go near that pH, this indicator will not work and even though you bacterium is producing acid (from the carbohydrate) you will not get the result. For this you have to use phenol red (6.8 to 8.4; where minor changes can be monitored).
Additionally, although we use very low concentration of these indicators but these may have some inhibitory activities on over growth and development of the culture that you dealing with. So a prior analysis should be undertaken to check which indicator works well for your culture. In my experiance, Phenol red, bromothymol blue, bromocresol purple works wells and does not interfere with growth. The overall concept is for a general microbiology. For Listeria, these will not have any inhibitory effect.
Since, ability of the strain (in the question), to produce acid from different carbohydrates varies, answer is yes for last part of your question.
Hope it helps.
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