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I have inoculated six flasks of 200 ml potato dextrose broth, one of which does not show any turbidity and the medium is clear with a fungal disc in it.

All were inoculated at the same time and from the same pure fungus culture.

What might be the reason for this anomaly?

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Several explanations are conceivable. Knowing what type of fungus you are working with and how exactly you proceeded to inoculate the cultures could help narrowing down the possibilities.

Here are some possible explanations:

  • Since you inoculated six liquid cultures from the same stock culture, the process might have been too long. Let me explain. Most fungi are strictly aerobes and do not tolerate well even short lack of oxygen. So, as I can guess, while you were inoculating the dextrose broths one after the other, the stock culture just remained unaerated on your bench. Since it can take a few tens of minutes to prepare everything and to proceed to the inoculations, it is possible that while you were doing so, the physiological state of the fungi just kept deteriorating, to such a point that one of the last inoculations did not contain enough viable cells to initiate a growth in the broth.
  • You might have forgotten to shake the stock culture just before taking the sample used in order to inoculate the broth. So it is possible that all the biomass in the stock culture was sedimented and you just inoculated your broth with the supernatent.
  • Also, a classic mistake is to mix up some of your broths. You think that you have inoculated all six broths with the stock culture, but maybe you unintentionally skiped one, or maybe you inoculated the same broth twice.
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