🚀 go-pugleaf

Dear John and Kristina,
  You are quite right that any revertant capable of using
lactose to grow into a colony in 48 hours should be blue
on X-gal.  Therefore the logical possibilities are:

1.  The cells on the lactose plate were growing on
something else.  How big were the colonies (compared
to on glucose, for example).  How pure was your lactose?
Did you autoclave it -- it breaks down to glucose and
galactose.
2.  The colonies that you tested on Xgal were not the
same colony that you streaked on lactose.  You say you
streaked a single colony on lactose, but did you use a
different one for the Xgal?  Did you test the colonies
grown on lactose on Xgal?  Maybe the one colony was
a rare revertant, but the rest on the cells are Lac-.
Try picking a few white colonies from Xgal onto lactose.
3.  Your apparent Lac+ colony was a contaminant.  What
color was it (coli is nearly pure white on minimum
medium)?  (guess what coli smells like--
but not strongly)
Cheers,
pat

Patricia L. Foster
Boston University School of Medicine
Boston, MA USA
pfoster@bu.edu