“
ease of understanding, the organisms associated with the enteric tract are further subdivided into three groups: (1) pathogens both inside and outside the enteric tract, (2) pathogens inside the enteric tract, and (3) pathogens outside the enteric tract. As is true of any classification dealing with biologic entities, this one is not entirely precise. For example, Campylobacter causes enteric tract disease but frequently has an animal source.Nevertheless, despite some uncertainties, subdivision of the large number of gram-negative rods into these functional categories should be helpful to the reader. The organisms that are not readily Gram-stained fall into six major categories: Mycobacterium species, which are acid-fast rods; Mycoplasma species, which have no cell wall and so do not stain with Gram stain; Treponema and Leptospira species, which are spirochetes too thin to be seen when stained with Gram stain; and Chlamydia and Rickettsia species, which stain well with Giemsa stain or other special stains but poorly with Gram stain. Chlamydia and Rickettsia species are obligate intracellular parasites, whereas members of the other four genera are not. 105
”
”