ENTERIC BACTERIAL INFECTIONS wide variety of micro-organisms commonly found in the gastrointestinal‘ tract, particularly the gram-negative, nonsporulating bacilli, have become increasingly important in clinical medicine. They are the principal organ: isms found in, infections of the abdominal viscera, peritoneum, and urinary tract, as well as being • frequent secondary ‘invaders of the respiratory tract, burned’ or traumatized skin, and sites of decreased host resistance and instrumentation. Currently, they are the most frequent cause of life-threatening bacteremia. Infections with these organisms will be considered together because of their common habitat in the gut and on mucous surfaces, the similarity of epidemiologic and pathogenic characteristics, and the common approach used in diagnosis, treatment, and prevention. Bacteriology.
The gastrointestinal flora is exceedingly complex, The large intestine contains about 1010 to 10″ organisms per gram of contents. Of these, 90 to 95 percent are obligate anaerobes. Most, common are the gram-negative bacilli, Bacteroides, and Fusobacterium, gram-positive bacilli including Bifidobacterium, Eu-bacterium, Corynebacterium ‘species, and a wide variety of anaerobic streptococci. Other anaerobes include the gram-positive spore-forming rods of the Clostridia species and gram-negative cocci, Veillonella. Lactobacilli and enterococci are also present. ENTERIC BACTERIAL INFECTIONS well-known aerobic gram-negative rods, which are members of the family Enterobacteriaceae account for only 5 to 10 percent of the total flora. These include the most common, E. coli, as well as the Klebsiella-Enterobacter group, Proteus, Providencia, Edwardsiella, Serratia, and, under pathologic conditions, Salmonella and Shigella.
Pseudomonas is entirely unrelated.
ENTERIC BACTERIAL INFECTIONS species, and is usually found in only small numbers in the bower. Various yeasts and related forms are also found in lesser numbers in the normal large intestine: Although the human gastrointestinal tract is usually considered to be colonized in the anatomic regions proximal to the cardia of the stomach and distal to the ileocecal valve, recent studies have demonstrated organisms in the jejunum and almost always in the ileum; Moderate changes in the diet do not affect the ratio of predominant bacteria in the feces, but antimicrobial therapy has a strong selective effect and is the single most important reason for the increasing emergence in human infection of these heretofore unusual organisms. All the micro-organisms of the gastrointestinal tract are potentially pathogenic under conditions of altered host resistance. The major diagnostic problem is to differentiate superficial colonization from actual tissue invasion. Endotoxins.
The gram-negative bacteria of the gastrointestinal tract produce disease by the invasion of tissue and by the release of a pharmacologically active lipopolysaCcharide from the cell wall, known as endotoxin. EndOtoxins from a wide variety of unrelated species behave quite similarly, regardless of the inherent pathogenicity of the micro-organism from which they are de-rived or their antigenic structure. In the intact micro-organism, they exist as corn-, plexes of lipid, polysaccharide, and protein. The biologic activity seems to be a property of the lipid and carbohydrate portions. When inoculated intravenously, the endotOXins cause fever, leukopenia, circulatory collapse; capillary hemorrhages, necrosis of tumors, and the Shwartzman phenomenon.
Noteworthy is the remarkable tolerance that develops after repeated injections of endotoxin. For example, the first intravenous injection in a man of as little as 0.01 ml. of typhoid vaccine will give rise to a violent response, with chill and high fever; yet after 10 to 14 daily injections of in-. creasing quantities, the subject can accept 25 ml. or more without symptoms and with only a slight rise in temperature. This state of tolerance is not obviously dependent on specific antibodies; it ex-tends to endotoxins of unrelated bacterial strains. The clinical-features of graM-negative bacteremia (vide infra)’resemble the reaction of laboratory animals or man to intravenous injection of purified endotoxin preparations, and may well represent a direct “pharmacologic” response to bacterial.
The phenomenon of endotoxin. tolerance may explain the remarkable tendency of the symptoms of pyelonephritiS to subside spontaneously. By contrast, endotoxin tolerance is not a feature of experimental typhoid fever; ‘but, rather, volunteers infected with this organism have been Shown’ to be hypersensitive to its effects. ElidotOxin tolerance is currently under intensive study because of the presumed important pathologic effect of this substance. Tolerance may be due to enhanced removal by the reticuloendothelial system, enzymatic -degradation, cellular desensitization, a common immunologic factor, or combinations of these.
Antibodies to the 0 or somatic antigens of E. coli have been most extensively studied. Very low titers are present in human newborns, presumably because they are mostly of the high molecular weight IgM variety, and do not readily pass the placenta. Human colostrum is rich in 0 antibodies, but this is not absorbed during breastfeeding. Colonization of the digestive tract, however, is soon accompanied by appearance of a wide variety of antibodies in serum, which contains virtually all the E. coli 0 antibodies by one year of age. Serologic response to the specific 0 antigens of the invading strain of E. coli can be demonstrated in acute pyelonephritis. It is likely that the great susceptibility of the new-born to overwhelming gram-negative bacterial sepsis is related to lack of maternal antibodies.