Molecular Pathogenicity Studies 
of Enteric Bacteria 
Gary K. Schoolnik, M.D. — Associate Investigator 
Dr. Schoolnik is also Associate Professor of Medicine and of Microbiology and Immunology at Stanford 
University School of Medicine. He received his M.D. degree from the University of Washington. He was an 
intern, resident, and chief resident in internal medicine at Massachusetts General Hospital, a fellow in 
infectious diseases with King Holmes and Thomas Buchanan at the University of Washington, and a 
postdoctoral fellow with Emil Gotschlich at the Rockefeller University. He founded the Division of 
Geographic Medicine at Stanford University and has established a research center for the study of 
infectious diseases in southern Mexico. 
BACTERIAL, viral, and parasitic infections of 
the gastrointestinal tract cause an estimated 
500 million illnesses and 5 million deaths each 
year among children living in the developing 
countries. The principal mission of our labora- 
tory is to discover how these infectious agents 
cause disease, how they are spread, and how this 
information can lead to new tactics for preven- 
tion and control. This effort has entailed work in 
two settings: molecular studies in our laboratory 
at Stanford University, and epidemiological in- 
vestigations at a field laboratory in southeastern 
Mexico, where infections of this kind are 
common. 
In the first setting, the unit of analysis is the 
organism itself. We try to determine how it at- 
taches to, invades, and damages human cells. This 
anthropomorphic orientation views the disease- 
causing capacity of an infectious agent as the cen- 
tral research issue. In the second setting, the unit 
of analysis is a household or a village. In this con- 
text we seek to understand how the organism is 
transmitted within the community, what its reser- 
voirs are, and how it manages to survive as a via- 
ble entity in the real world. 
This ecological orientation seeks to understand 
how the organism adapts to different environmen- 
tal habitats. Within the context of a Third World 
village, these habitats include contaminated food 
and well water, sewage, and the gastrointestinal 
tracts of people and animals. By using biochemi- 
cal and genetic tools to study infectious agents as 
they inhabit and move among these different envi- 
ronmental niches, we have begun to understand 
the underlying molecular mechanisms for this re- 
markable capacity. This in turn is beginning to 
lead to new strategies for the control of these dis- 
eases through vaccination and epidemiological 
interventions. Examples of work in progress are 
described below. 
Enteropathogenic Escherichia coli 
Enteropathogenic E. coli (EPEC) are a common 
cause of infantile diarrhea in Third World chil- 
dren. When biopsies of the small intestine are 
performed, colonies of EPEC are found attached 
to the underlying epithelia. It is evident that the 
bacteria interact not only with the host cells to 
which they are bound but also with each other. 
Beneath these adherent colonies, structural 
changes in the host cell also occur, indicating 
that EPEC have altered the absorptive surface of 
the intestinal cell. It is now clear that structural 
changes of this kind are directly responsible for 
the diarrheal syndrome. 
From these studies we have learned that EPEC's 
pathogenic strategy consists of at least three dis- 
tinct steps, which probably occur in the follow- 
ing order: the coalescence of individual bacteria 
into infectious units composed of several to 
hundreds of organisms (an event that probably 
occurs in the small intestine, soon after ingestion 
of the bacteria); attachment of these infectious 
units to the surface of intestinal epithelial cells; 
and, following close contact between the bacte- 
ria and the epithelial cells, the organism's partial 
penetration of the cell, in association with con- 
comitant rearrangement of its cytoskeleton. 
Each of these steps is performed by a distinct 
surface structure of the bacteria. First, the bacte- 
ria are organized into infectious units through the 
production of rope-like appendages that emanate 
from the organism's surface. Termed "bundle- 
forming pili," these appendages create a network 
within which the bacteria become enmeshed. 
Then the bacteria bind to human intestinal epithe- 
lial cells through the activity of rod-like filaments 
that project from the organism like spines from a 
porcupine. These filaments bind fibronectin mol- 
ecules that are located around the periphery of 
each intestinal epithelial cell. Finally, an outer 
membrane protein of the organism interacts with 
integrin-like molecules of the epithelial cell, an 
event that triggers changes in the architecture of 
the cell's cytoskeleton. 
The production of at least one of these surface 
structures is controlled by physiochemical sig- 
nals that operate in the intestinal lumen, where 
they serve as unique signatures of that habitat. 
Thus, when under the influence of these signals, 
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