ABSTRACT
Figure 180 Salmonella species Hektoen-Enteric agar culture showing non-lactose-fermenting green colonies with black centers, indicative of hydrogen sulfide production from ferric ammonium sulfate in medium. Colonies of Salmonella typhi (typhoid bacillus) may appear green without dark centers
Figure 179 Dysenteric stool characterized by gross blood and mucus. Stool specimen transferred from collection vessel into Petri dish
Shigella are non-motile bacilli and, with the exception of two biotypes of S. boydii, do not produce gas from glucose fermentation. Four species comprise the genus: S. dysenteriae, S. boydii, S. flexneri, and S. sonnei, the most common isolate in industrialized countries. Occasionally, in the setting of immunosuppression, Shigella, especially S. flexneri, may invade the bloodstream. Shigella species are hostbound to humans and monkeys, and less than 200 organisms are required to establish an infection, thereby facilitating person-to-person transmission via contaminated hands. Other venues include
contaminated water and food, especially in countries with poor sanitation. Shigellosis is characterized by watery or bloody diarrhea (dysenteric) with mucoid stools. Shigellae are enteroinvasive and can cause necrosis of the colonic epithelium.
