Well it is plausible since tryptone is a trypsin digest of casein (milk protein). However, I would think this is one of those things that people assume but not really test as the problem is mostly solvable by adding glucose to your LB (if catabolite repression is intact). However, keep in mind that there is no absolute suppression of the promoters and you might get leaky expression just because of biology.

Here is the link to the original article you refer to:

https://bitesizebio.com/10262/shut-off-background-lac-expression-in-lb/

Dear Tri

to my knowledge no kactose is present in in formulation whixh is quite simple (yeast extract, peptone and salt). is it possible that in yeast extract and peptones are presebt some tracas of lactose but i dont think those are in significative amount

(https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2168924/)

there are alternative versions of lb designed to promote auto induction where lactose is supplemented.

i think that the absence of glucose, which act a repressor of lac can Cavour dome leasing in lb and i suggest to you to see:

ProteoCool n°5 (E.coli high cell density media overview) and

ProteoCool n°7 (High cell density E.coli protein expression using EnpressoB media)

available on my blog: ProteoCool ( https://proteocool.blogspot.com) where there are reported 2 interesting media:

enpresso which is based on continuos feed of glucose released from starci which is very well repressed and YGR which do not contain lactose but is poorly repressed and suitable to autoinduction.

ciao

Manuele

Absolutely it can be present in the tryptone. We recently had a new batch of tryptone and there was so much lactose present we didn't need to add IPTG (this was with a clone in pET9a which does not have the lac operator, pET24a was fine i.e. no auto induction and IPTG was essential for expression). We are an NMR group so we know it's lactose! Articles by Studier F.W. are a good read on this subject, and try this website for more information on media: https://www.bnl.gov/biosciences/staff/Studier.php

Thank Frederick W Muskett Manuele Martinelli Hanna Alalam

addition of 1% glucose can prevent leaky expression from certain vectors...

http://wolfson.huji.ac.il/expression/procedures/bacterial/Glucose%20supression.pdf