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Why is my negative control showing results?

About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.

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Why is my negative control showing results?

Postby ilovebiology77 » Wed Feb 29, 2012 4:11 pm

Hi there, so I used MRS agar to grow lactic acid bacteria. As a negative control, I used a neutral pH, 7 as suggested by members on this forum.
I expected no growth of lactic acid bacteria. However, I observed there was still quite a considerable amount of growth. I'm rather confused as this is well out of the range at which the LAB grows. Any explanations for these results? I was thinking maybe some kind of mutations of the LAB, causing them to adapt to pH change?

Many thanks!
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Postby JorgeLobo » Fri Mar 02, 2012 1:37 pm

How did you determine the growth was attributable to LAC? And you negative control was meant to show growth.
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Postby xenopus » Sun Mar 04, 2012 6:52 pm

This may be a problem of too low pH of your negative control. Try with pH 9,6 as a negative control. Acidic pH is optimal for LAB's what does not mean they cannot grow at medium pH.
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Re: Why is my negative control showing results?

Postby JorgeLobo » Sun Mar 04, 2012 8:05 pm

The negative control is one that fails to establish the conditions LAC is know to bring to yogurt - preservation. This preservation is thought to be primarily a product of low - acidid (4-5) - pH, not one that fails to include viable LAC.
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