Infectious Disease View of Rnombinaut USA 
617 
doses (Ml*'), Smith rejx>rietl that none of the 
administered strains of E. coli |*cisisted in the in- 
testine for more than 10 days [19], On the other 
hand, Cooke et al. demonstrated regular coloniza- 
tion when large numbers of E. coli were feel to 
volunteers Similarly. Formal and Hornirk im- 
planted the US strain of F.. coli for as long as 
three months in some volunteers following a sin- 
gle feeding (20). 
I hc source of the F.. coli strain is related to its 
ability to implant in humans. Strains derived 
from animal sources were found by Smith to 
colonize jxrorlv in humans [19], Hetliaratchv con- 
firmed this result but noted that some coloniza- 
tion occurred even with strains of animal origin 
[ 21 ]- 
Characteristics of Pathogenic Microorganisms 
A critical constellation of virulence factors is re- 
t| tii reel by a microorganism in order to produce 
disease: (/) survival in the environment so that 
it can spread front animal to animal, ( 2 ) some 
mechanism for penetrating the skin or a muco- 
sal surface such as the bowel, genitourinary tract, 
or oropharynx. (3) multiplication within the 
host. (/) systemic spread within the host. (5) re- 
sistance to host defense mechanisms, and (h) pro- 
duction of toxin or some other mechanism to 
damage the host to cause those symptoms asso- 
ciated with “disease.” Freter has emphasized that 
the absence of any one of these characteristics 
will break the chain of events, rendering the mi- 
croorganism avirulent [10], Parenthetically, it 
should he noted that E. coli kl2 is intrinsicallv 
impaired in most, if not all. of these properties. 
The surface antigens of F.. coli arc important 
determinants in the virulence of the particular 
strain [22] 0rskov cl al. have noted that both the 
polysaccharide and the protein surface antigens 
play a role in pathogenicity [23, 21]. The O anti- 
gens rendet the organism resistant to phagocyto- 
sis and bacteriocidal forces. Specific O serotypes 
are associated with infantile diarrhea, while oth- 
ers have been related to enterotoxigenic diar- 
rhea. dysentery-like disease, or urinary tract in- 
fections [25], The presence of kl capsular anti- 
gen is highly correlated with neonatal meningi- 
tis and other systemic E. coli infections [24]. The 
fimbriae antigens or pili have been associated 
with adhesive and colonization properties [26, 
27 j. No single surface antigen determines patho- 
genicity, but organisms that are lacking these sur- 
face structures are less virulent. E. coli kl2 is 
defective in production of O antigen lipopoly- 
satcharitle and does not make capstdar anti- 
gens [21. 28], 
The prefix ”k" causes some ambiguity in this 
nomenclature [23]. The capsular polysaccha- 
ride antigens, such as kl. are designated by this 
letter, taken from the German word for capsule. 
The protein antigens involving the fimbriae sur- 
face structures are also termed "k." i.e., K88. To 
add to the confusion, the ”k' of E. coli kl2 is 
not related to eirher of these surface antigens, 
but is a historical appellation assigned to the 
classic strain used in genetic lesearch. (There are 
also wild-type strains of E. cult causing urinary 
tract infections which carry a "KI2" capstdar 
polysaccharide antigen: these strains are not re- 
lated to the traditional E. coli kl2 used in gen- 
etics experiments.) 
Epidc-minlugs of E.toli Infection 
The diseases caused by E. coli are divided into 
two broad groups, intestinal and extraintestinal 
infections In the United States, E. coli intesti- 
nal disease, generally manifesting itself by mild 
diarrhea, is relatively uncommon [29, 30], The 
organisms are transmitted by the fecal-oral route. 
Ferson-to-jKTson spread is rather unlikely, and 
for this reason secondary transmission of E. coli 
from the index case to another person is rarely 
observed. These epidemiologic characteristics are 
based on the requirement for a large oral inocu- 
lum of F.. coli to initiate disease, an inoculum 
estimated to be at least 10 fi -10 1 " organisms [29, 
31]. Under natural circumstances, only highly 
contaminated sources such as food and water can 
serve as vehii les of transmission. 
The extraintestinal diseases include urinary 
tract infections, septicemia, meningitis, pneumo- 
nia. and wound infections. The E. coli strains re- 
sponsible for these infections are different from 
those causing intestinal disease [2-4, 32, 33]. E. coli 
is a leading cause of community-acquired infec- 
tions that lead to hospitalization [29]. The uri- 
nary tract is the major site. In community- 
acquired infections of the urinary tract, approxi- 
U55] 
