4 
A third session on the ecology and epidemiology of vector-host systems was 
chaired by S. Falkow (University of Washington). This workshop emerged, in part, 
from expressed fears that microorganisms containing cloned fragments of foreign DHA 
may potentially pose a threat to health or disrupt, the normal ecological chain in 
some manner. Consequently, this session was devoted to a review of currently avail- 
able information on the ecology and epidemiology of E. coli and related bacterial 
species since it was recognized that E. coli K-12 would be the prokaryotic host most 
commonly employed in the cloning of DNA molecules in the immediate future, 
F. 0rskov (Escherichia Reference Center, Copenhagen) reviewed the state of E. coli 
serotyping and what has been learned about the distribution- of E . coli types in 
health and disease. Only certain E. coli types are generally recognized as good 
colonizers of the human gut and such strains come from a handful of the 160 well 
defined 0 (lipopolysaccharide) antigen types and invariably possess K (acidic poly- 
saccharide capsule) antigens. Some serotypes apparently have become disseminated 
worldwide and possibly represent the proliferation of a bacterial clone because of, 
as yet unknown, selective pressures. In contrast, E. coli K-12 has no detectable 0 
or K antigens and is considered to be rough. This may account, at least in part, 
for its demonstrable poor ability to colonize the human or animal gut. However, 
R. Freter (University of Michigan) pointed out that we still remain largely igno- 
rant of the factors which control intestinal E. coli populations. Freter also 
noted that while adherence to the mucosal surface of the small intestine is impor- 
tant in .the pathogenesis of E. coli diarrheal disease, the ’normal* long-lasting 
symbiotic relationship between a mammalian host and bacterium is established in the 
cecum and colon. It is in these locations that factors come into play to determine 
whether an E. coli strain passing through the intestine will become successfully 
implanted or whether it will be quickly eliminated in the feces. The factors con- 
trolling implantation include competition for substrates, inhibitors and the 
physiological state of the organism when it reaches the large bowel. For example, 
ingested E. coli previously grown under usual laboratory conditions fare poorly 
while cells of the same strain 'pre-adapted* in Eh, pH, etc., often colonize well. 
Freter has developed a continuous flow culture model which may be useful in study- 
ing the mechanisms of implantation. Falkow reviewed the pathogenicity of E. coli. 
E. coli causes diarrheal disease either by direct invasion of the bowel epithelium 
or by elaboration of enterotoxin(s) . While invasive E. coli appear to owe their 
pathogenicity to a constellation of at least five unlinked chromosomal gene clus- 
ters, toxigenic E. coli species generally owe their pathogenicity to the possession 
of two species, Ent and K. The introduction of Ent and K plasmids may be sufficient 
to convert a normal wild-type E. coli into a strain now capable of causing overt 
clinical disease. However, the introduction of these plasmids into E. coli K-12 
sublines had no discemable effect on their ability to cause disease, although the 
K-12 strains could now better colonize calves. Despite the observation that E. coll 
K-12 did not appear to offer a significant hazard as a potential enteric pathogen 
even when it possessed well-defined determinants of pathogenicity it was emphasized 
by 0rskov, Freter and Falkow that E. coli K-12 strains carrying recombinant DNA 
molecules could still act as effective genetic donors in vivo and still posed a 
significant problem requiring control. E. Geldreich (U.S. Environmental Protection 
Agency, Cincinnati, Ohio) discussed the possible outcomes of the release of E. coli 
containing recombinant DNA molecules into the aquatic environment and concluded that 
total reliance cannot be placed on sewage treatment and the natural self-purifica- 
tion capacity of receiving waters to limit potential hazards. While these are 
realistic barriers to the dissemination of E. coli and associated fecal organisms 
via the water route, they are not infallible because of technological limitations, 
improper operational practices and system overloading. Finally, M. Starr (University 
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