Several chemical methods have been established that induce
bacterial cell transformation. In a classic experiment, Mandel
and Higa (1) demonstrated that treatment of Escherichia coli
with CaCl2 made the cells susceptible to uptake of bacteriophage
DNA. Subsequently, it was shown that this technique
could be used to transform E. coli with bacterial chromosomal
and plasmid DNAs (2, 3). Since these initial studies, a
number of factors have been elucidated that produced an
increase in transformation efficiency. Such factors include
prolonged incubation of bacteria with CaCl2 (4), addition of
multiple cations into the transformation mixture (5) and
treatment of bacteria with dimethyl sulfoxide (DMSO),
hexaminecobalt, and dithiothreitol in the presence of both
monovalent and divalent cations (6). The latter modification
produced yields of >108 transformants per ,ug of DNA.
Polyethylene glycol (PEG) has also been shown to mediate
plasmid DNA uptake by protoplasts of a number of bacterial
strains, but PEG-mediated transformation of E. coli by
pBR322 DNA averaged 106 transformants per ,ug of DNA
(7), a value 2 orders of magnitude lower than that obtainable
by other methods (6).