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My E. coli cultures used to take 13.5 hours to reach stationary phase. I have not changed any of the parameters like temperature, shaking speed, media formulation/components or inoculation volume. However, the cultures now take 15.5 hours to reach stationary phase. What could be causing this issue? Is there any chance there is a phage/fungal contamination in the lab?

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    $\begingroup$ Fungal and phage contamination could both be checked by plating the culture. If you see plaques or multiple colony morphologies, you have your answer $\endgroup$
    – acvill
    Apr 27, 2022 at 16:30
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    $\begingroup$ If you resolve your question experimentally @JohnFlamour, please be sure to answer your own question here so that others can learn from your experience! $\endgroup$ Apr 27, 2022 at 19:45
  • $\begingroup$ Are they sequential, you may have a mutation. $\endgroup$
    – John
    Apr 30, 2022 at 15:51
  • $\begingroup$ Several questions that might help diagnose the problem. How are you standardizing your inoculum? Using overnights or colony suspensions can be less consistent than back diluting from log-phase growth, leading to variable lag times. Are you starting from the same freezer stock for each experimental replicate, or just using subcultures? Subcultures can accumulate mutations. Has the maximum growth rate changed as well, or just the time to reach stationary? If it's contamination, I would expect the max growth rate to change (mutations from sub culturing could also cause this). $\endgroup$
    – MikeyC
    Oct 11, 2022 at 15:53

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Have you considered the culture that you are using as inoculum is old and less active, resulting in a longer lag phase?

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