Why is my negative control showing results?

[ilovebiology77]

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!

[JorgeLobo]

How did you determine the growth was attributable to LAC? And you negative control was meant to show growth.

[xenopus]

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.

[JorgeLobo]

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.