Molecular Biology of Two 
Enteropathogenic Bacteria 
Edmundo Calva, Ph.D. — International Research Scholar 
Dr. Calva is Associate Professor and Chairman of Molecular Biology at the Biotechnology Institute, 
National Autonomous University of Mexico (UNAM), Cuernavaca. He received his Ph.D. degree in 
molecular biology from the University of Wisconsin-Madison under Richard Burgess and did postdoctoral 
research at UNAM. He is currently President of the Mexican Biochemical Society. 
THE study of infectious diseases provides op- 
portunities to explore various basic biolog- 
ical phenomena. During an infection, entero- 
pathogenic bacteria participate in numerous 
biological events that may lead to intestinal ill- 
ness. Upon oral ingestion, the bacteria must un- 
dergo a series of interactions with the host that 
involve specific molecules, either on the surface 
or in the interior of the bacterial and host cells. 
For instance, bacteria adhere to epithelial gut 
cells and produce enterotoxins that result in diar- 
rhea. They can invade host tissue with the same 
effect. Invasion sometimes results in a systemic 
infection, in which the bacteria have means of 
protection against the immune system, seriously 
jeopardizing health. 
Current research questions in the area of bacte- 
rial pathogenesis address the molecular mecha- 
nisms of the bacteria-host interaction. Our knowl- 
edge of adherence, invasion, resistance to the 
immune system, and enterotoxin production, 
among other processes, is just being unveiled. We 
know only scant details of some of these pro- 
cesses in a few bacteria, and even then are not 
completely aware of the genetic variation within 
the species studied — variation that may result in 
different strains with varying capabilities for 
causing disease. Furthermore, some bacterial and 
parasite antigens share structures with stress pro- 
teins, like those expressed during heat shock. An 
open area of study involves defining the global 
genetic circuits that regulate virulence factors 
and determining whether these "regulons" share 
common features with those encountered in the 
response to stresses other than infection. 
Health biotechnology should benefit from the 
definition of bacterial antigens that have a role in 
infection or against which a specific immune re- 
sponse is mounted, thus permitting the develop- 
ment of better vaccines and diagnostic proce- 
dures as well as furthering our understanding of 
the structure and function of the immune system. 
Benefits should also emerge from the isolation 
and characterization of specific bacterial genes, 
together with knowledge about their distribution 
and polymorphism in bacterial populations. Such 
information should permit the rapid and specific 
detection of bacteria through nucleic acid ampli- 
fication procedures. These can not only be useful 
for monitoring infections in animals, plants, or 
humans, or the contamination of foodstuff, but 
should also be valuable in the definition of modes 
of transmission and environmental reservoirs for 
the bacteria. In this manner, molecular epidemi- 
ology will very likely shed light on yet other bio- 
logical phenomena. 
Salmonella typhi 
S. typhi is the causal agent of typhoid fever 
(TF) in humans, a disease estimated to afflict an- 
nually more than 1 2 million persons worldwide. 
TF is the result of a systemic infection, in which S. 
typhi can be isolated from blood cultures. As a 
gram-negative bacterium, 5. typhi has an outer 
membrane that surrounds an inner one and the 
cell wall. Thus exposed to the cell's exterior envi- 
ronment, outer membrane proteins (OMPs) have 
been shown to be important immunogens for 
protection against various bacterial infections in 
laboratory models. We have demonstrated the 
utility of OMPs for the rapid immunodiagnosis of 
TF in patients from different parts of the world, 
most of whom raise specific antibodies to OMP 
preparations. 
Our laboratory has reported the isolation and 
characterization of an S. typhi gene coding for 
OmpC, a major OMP. Amino acid sequence align- 
ment with other OMPs that form pores (porins) 
has allowed structure prediction in the porin su- 
perfamily. Using site-directed mutagenesis, we 
have constructed a gene that codes for a chimeric 
protein containing a foreign epitope inserted in a 
region predicted to be exposed on the cell sur- 
face. This epitope is a segment of a capsid protein 
from rotavirus, a causal agent of diarrhea, against 
which neutralizing antibodies are made. The chi- 
meric protein does indeed contain this epitope 
on a segment exposed at the bacterial cell sur- 
face, supporting the notion that the native region 
is located toward the exterior and indicating that 
OmpC could be used as a carrier of heterologous 
epitopes. This information on protein topology 
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