Problem:

In my lab we've observed a phenomenon in which a culture of E. coli is found to shift from normal rod growth to filamentous growth and then back to normal rod growth again several times over the course of 400 hours. This was shown by an OD600 graph which oscillates from high values (~5) to low (~0.5) (the culture was also viewed directly).

The shift to filamentous growth is usually associated with an SOS response in E. coli and this makes sense (the media is M9 supplemented with 0.2% casamino acid and 0.1mM thiamine hydrochloride), however we don't know why it switches back from filamentous to normal growth.

Required:

Question: Has anyone observed a similar phenomenon?

Does anyone have any suggestions why this might be happening? Please explain.