Thanks for your card about a source for gradocol membranes. I had hoped we might be able to get n sot at closer hand, from
Luria, but his assistant at Urbana hasn't sent them yet. We'll get to it, I hope, before too long.
The whole problem is still very confusing. Zinder finds still that different factors are changed quite independently of each
other, so that this system bears no resemblance to a "sexual" process as in X-12. The ability to produce or respond
to the filterable agent appears to be quite general among typhimurium strains, although some respond more or less well to
penicillin or phage. The effect of penicillin seems to be mediated by the release of the donor's own lysogenic phages
which (possibly after a virus mutation, but I doubt it) can act back on their source. The agent goes through the sintered
Pyrex filters which so far has prevented anything that can regenerate bacteria directly from coming through. There seems to
be no doubt that phage- or penicillin treated Salmonella regularly permeate filters (e.g. 14-lb Mandler) which are quite effective
against untreated cultures. To a lesser extent this may also hold for K-12, but nothing interesting by way of genetic activity
has been found in such filtrates. The relationship between the Salmonella agent (FA) and L- (or as Norman Horowitz dubbed
them, "h.ll"-) forms is still unsettled. Zinder has not been able to cultivate anything (except persistent contaminants)
on serum agar from filtrates sterile on, say, nutrient agar. The sintered Pyrex filters do however pass granules that swell
into large bodies with rabbit anti-O serum, so these, at any rate, have not been separated from FA.
I do hope you were serious about two things you mentioned: your interest in cultivating L-forms from Salmonella, and the possibility
of a visit of more than a day-or-two-'s duration. With respect to the former, we may have been trying the impossible the
wrong way: one has explicitly claimed to be able to cultivate them from filtrates, barring the stabilized "L"s of
S. moniliformis, and a note from Nobel about having done this once with E. coli B - but which I have not yet been able to
confirm. Esther and I are just about finished with the indirect selection experiments, using replica plates (i.e. velvet)
to isolate drug and phage-resistant mutants from populations never in contact with the selective agent. Three or four enrichment
steps are needed, but the methal should be quite generally applicable. Did you see the newspaper accounts of Hinshelwood's
address to the BAAS?