Bacterial diseases of the intestinal tract can be highly communicable and may spread in epidemic fashion. Their agents enter the
body through the mouth in contaminated food or water, or as a result of direct contacts with infected persons. Among the Enterobacteriaceae, the organisms of pathogenic significance belong to the genera Salmonella, Shigella, and Yersinia. Also certain Escherichia coli strains can produce disease by several mechanisms including invading tissue or producing toxins. Such strains are
referred to as enteroinvasive or enterotoxigenic, respectively.
The many species of Salmonella can be distinguished on the basis of their serological properties as well as their biochemical
activities. These organisms characteristically cause acute gastroenteritis when ingested, but some also can find their way
into other body tissues and cause systemic disease. Among these, the most important is Salmonella typhi, the agent of typhoid fever,
a serious systemic infection. The salmonellae are gram-negative bacilli that are usually motile. They usually do not ferment lactose
but display a variety of other fermentative and enzymatic activities.
Shigella species are the agents of bacillary dysentery. These organisms are gram negative and nonmotile. They usually
do not ferment lactose. In fermenting other carbohydrates they produce acid but not gas (with one exception). They can also be
identified to species by serological methods.
Yersinia enterocolitica is the cause of acute enterocolitis, primarily in children. Its symptoms may mimic those produced
by Salmonella, Shigella, or enteroinvasive E. coli. Occasionally, the symptoms are more suggestive of acute appendicitis. The organism
grows better at room temperature (25°C) than at 35°C; therefore, it may not be isolated unless the physician notifies the
laboratory that yersiniosis is suspected. In this case the isolation plates are incubated at both temperatures. Yersinia are gramnegative
bacilli that are motile at 25°C but not at 35°C. They ferment sucrose, but not lactose. Y. enterocolitica is urease positive.
Disease-producing E. coli were once thought to be associated only with epidemic diarrhea in babies, but they are now
known to be a common cause of “traveler’s diarrhea” (“turista”) and a variety of other gastrointestinal diseases. Some of these
strains may be distinguished from others by immunological typing of cell wall (O) and flagellar (H) antigens. In addition, an enzyme
immunoassay is available to detect E. coli toxin directly in stool specimens.
The pathogenic Enterobacteriaceae
are first isolated from clinical specimens by using highly selective media to suppress
the normal flora in feces and to allow the pathogens to grow. Many of these media contain lactose, with a pH indicator, to differentiate
the lactose-nonfermenting Salmonella, Shigella,
(colorless on these agars) from any lactose-fermenting normal
flora that may survive (pink or red colonies see colorplate 16 and 17
). EMB or MacConkey agar is commonly used, together
with two more highly selective media such as Hektoen enteric (HE) and bismuth sulfite (BiS) agars. In addition, an
“enrichment” broth containing suppressants for normal enteric flora may be inoculated. After an incubation period to allow enteric
pathogens to multiply, the enrichment broth is subcultured onto selective and differential agar plates to permit isolation of
the pathogen from among the suppressed normal flora. Subsequent identification procedures are based on the same types of biochemical
tests that you have studied, but may be more extensive to differentiate the enzymatic activities of enteric species that are
closely related to Salmonella or Shigella
Other bacterial pathogens are associated with intestinal disease. Campylobacter jejuni
, a curved, gram-negative bacillus,
may be the most common bacterial agent of diarrhea in children and young adults (see colorplate 6
). It has relatively strict growth
requirements and special procedures must be used to isolate it in the laboratory. Some vibrios, notably Vibrio cholerae
of cholera) and Vibrio parahaemolyticus
(of the family Vibrionaceae
), represent other examples of significant intestinal tract pathogens.
These organisms can also be isolated from cultures of fecal material and identified by their characteristic morphological and metabolic properties. Although the choice of isolation media and identification procedures must be varied according to the nature of
the organism being sought in a specimen, the principles are the same as those we are following here. You should read further about
infectious diseases acquired through the alimentary tract, including bacterial food poisonings, and be prepared to discuss the essential
features of their laboratory diagnosis, beginning with the collection of appropriate specimens.