The Enterobacteriaceae (Enteric Bacilli)

The human intestinal tract is inhabited from birth by a variety of microorganisms acquired, at first, from the mother. Later, organisms are carried in with food and water or introduced by hands and other objects placed in the mouth. Once inside, many cannot survive the acid conditions encountered in the stomach or the activity of digestive enzymes in the upper part of the intestinal tract. The small intestine and lower bowel, however, offer appropriate conditions for survival and multiplication of many microorganisms, primarily anaerobic species, that live there without harming their host.

When feces are cultured on bacteriologic media, it becomes apparent that most facultatively anaerobic bacterial species normally inhabiting the intestinal tract are gram-negative, nonsporing bacilli with some culture characteristics in common. This group of organisms is known as “enteric bacilli,” or, in taxonomic terms, the family Enterobacteriaceae. However, some of the bacterial species that are classified within this group are important agents of intestinal disease. These usually are acquired through ingestion and are referred to as “enteric pathogens.”The anaerobic organisms play little role in enteric disease and are not recovered in routine fecal cultures because they require special techniques for isolation.

One enteric organism that normally inhabits the intestinal tract, Klebsiella pneumoniae, is also sometimes associated with pneumonia. It is a gram-negative, nonmotile bacillus (see colorplate 5) that can cause infection when it finds an opportunity to invade the lungs or other soft tissue and the bloodstream. Like the pneumococcus, pathogenic strains of K. pneumoniae possess a slimy, protective capsule that is larger and more pronounced than most bacterial capsules (see colorplate 12).
In the experiments of this exercise we shall first study some of the cultural characteristics of those enteric bacilli that normally inhabit the bowel, and then apply this knowledge to understanding the methods used for isolating and identifying the important enteric pathogens.

The gram-negative enteric bacilli are not fastidious organisms. They grow rapidly and well under aerobic conditions on most nutrient media. The use of selective and differential culture media plays a large role in their isolation and identification. Their response to suppressive agents incorporated in culture media and their specific use of carbohydrate or protein components in the media provide the key to sorting and identifying them.