The specific activities of esterases and certain other molecular prope
rties including immunospecificity indicate that the electrophoretic va
riations of these enzymes in bacterial populations are the result of a
llelic variations at specific gene loci. The esterase polymorphism of
Enterobacteriaceae and some other species isolated from man or animals
demonstrates that esterases can distinguish between bacteria at the s
pecies or subspecies level, both by their biochemical properties and b
y their electrophoretic differences. The esterase data complement DNA
hybridization studies and agree with ribosomal DNA polymorphism, espec
ially for delineating a phylogenetically distinct group of highly path
ogenic strains in Escherichia coli. A two-dimensional electrophoretic
profile obtained by establishing a direct correspondence between homol
ogous esterase bands resolved by independent runs of isoelectric focus
ing and standard electrophoresis considerably improves the detection o
f allelic variations, whereas protein titration curves (electrophoresi
s in pH gradient) can be used to demonstrate the real electrophoretic
homogeneity of allozymes or evalue their molecular relationship in ter
ms of apparent amino acid substitutions. This overview establishes tha
t esterases, by their significant electrophoretic polymorphism, are re
liable molecular markers for systematics and epidemiology, and are sui
table enzyme systems for studying population genetics and phylogeny.