I have plasmid expressed a lytic protein in bl21 cells and did a growth kinetics and post induction there is reduction in turbidity which continued for three hours but post five the culture started growing back why does this happen because reduction in turbidity mean that the cells are lysed and killed if so how are they growing back?
If E. coli BL21 cells are regrowing after lysis in a lytic protein expression system, it could be due to incomplete lysis, weak induction, mutations making some cells resistant, protein misfolding, or the presence of dormant persister cells. To fix this, you can optimize induction conditions, use a stronger lysis system, check for mutations, and confirm proper protein expression.
The cells growing after the induction period are probably cells that either have a mutation in the lytic gene, or have lost the plasmid entirely. Therefore they are able to grow. Initially they likely represent a very tiny proportion of all the cells, but once the the "normal" cells die off then only these mutant or plasmid cells can grow. Loss of plasmid is especially common if you are using ampicillin selection.
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