Chapter 33: Enterobacteriaceae

Overview

The Enterobacteriaceae are a large and diverse family of gram-negative rods, members of which are both free-living and part of the indigenous flora of humans and animals; a few are adapted strictly to humans. The Enterobacteriaceae grow rapidly under aerobic or anaerobic conditions and are metabolically active. They are by far the most common cause of urinary tract infections (UTIs), and a limited number of species are also important etiologic agents of diarrhea. Spread to the bloodstream causes gram-negative endotoxin shock, a dreaded and often fatal complication. Historically, dying “of a fever” usually meant typhoid fever (Salmonella ser. Typhi), which because of its prolonged course and lack of localizing signs, caused unfortunates like Molly Malone to appear to be dying of fever alone. The term UTI encompasses a range of infections from simple cystitis involving the bladder to full-blown infection of the entire urinary tract, including the renal pelvis and kidney (pyelonephritis). The primary feature of cystitis is frequent urination, which often has a painful burning quality. In pyelonephritis, symptoms include fever, general malaise, and flank pain in addition to frequent urination. Cystitis is usually self-limiting, but infection of the upper urinary tract carries a risk of spread to the bloodstream. It is the leading cause of gram-negative sepsis and septic shock. Diarrhea is the universal finding with Escherichia coli strains that are able to cause intestinal disease; the nature of the diarrhea varies depending on the pathogenic mechanism. Enterotoxigenic and enteropathogenic strains produce a watery diarrhea, the enterohemorrhagic strains produce a bloody diarrhea, and the enteroinvasive strains may cause dysentery with blood and pus in the stool. The diarrhea is usually self-limiting after only 1 to 3 days. The enterohemorrhagic E coli are an exception, with life-threatening manifestations outside the gastrointestinal tract due to Shiga toxin production. Shigella is the classic cause of dysentery, which typically spreads person to person under poor sanitary conditions. The illness begins as a watery diarrhea but evolves into an intense colitis with fever and frequent small-volume stools that contain blood and pus. Despite the invasive properties of the causal organism, the infection usually does not spread outside the intestinal tract. Typhoid fever has a slow, insidious onset, and, if untreated, lasts for weeks. The primary symptom is a slowly rising fever often accompanied by abdominal pain but little else. It ends either by a gradual resolution or in death due to complications (eg, rupture of the intestine or spleen). Family members may note only the extended fever, although physicians may observe a subtle rash or feel an enlarged spleen. Diarrhea may occur once or twice during the course but is not a consistent feature.