Molecular Biology of Two Enteropathogenic Bacteria 
can be useful in the design of multivalent vac- 
cines against several infectious agents. 
We have used the S. typhi ompC gene to probe 
the genetic variability of the salmonella genus. 
In this manner we have been able to establish 
some phylogenetic relationships among several 
species. 
We have also determined that expression of 5. 
typhi OmpC is influenced differently by medium 
osmolarity than its well-studied counterpart in 
Escherichia coli. Nevertheless, expression of 5. 
typhi ompC is also dependent on the E. coli 
OmpR transcriptional activator. Apparently the 
two bacteria differ in the mechanisms of gene ex- 
pression in response to osmotic stress, although 
common effectors appear to be shared. We are 
presently studying this phenomenon in detail. 
In addition, we have isolated other S. typhi 
OMP genes, namely ompF and phoE, and are 
characterizing these to gain a better understand- 
ing of structure-function relations in porins and 
their genes. 
Campylobacter jejuni 
C. jejuni is one of the major causal agents of 
diarrhea throughout the world. Molecular biol- 
ogy of this organism has developed slowly, 
mostly because of the difficulty of maintaining 
stably cloned C. jejuni DNA segments in E. coli. 
Consequently, only a handful of C. jejuni genes 
have been isolated. The mechanisms underlying 
this DNA instability are not understood, and re- 
search in this area might well yield valuable 
results. 
The isolation of C. jejuni, producing a cholera- 
like enterotoxin, has been associated with clini- 
cal symptoms of a watery secretory type of diar- 
rhea. We have shown that the C. jejuni 
chromosome contains sequences similar to the 
enterotoxin genes of Vibrio cholerae (CT entero- 
toxin) and E. coli (LT, heat-labile enterotoxin). 
In cloning and characterizing these sequences, 
we have explored different host cells and differ- 
ent-size C. jejuni DNA fragments in order to find a 
successful cloning procedure. In the process, we 
have isolated two cryptic fragments, one highly 
specific for C. jejuni and C. coli (another 
diarrhea-producing Campylobacter) and the 
other revealing genetic variability between the 
two bacteria. We are testing the usefulness of 
both DNA probes in field epidemiology and char- 
acterizing them at the nucleotide level. 
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