Urinary tract infection

The Enterobacteriaceae are the most important etiology of both community-acquired and nosocomial urinary tract infection, with Escherichia coli the most frequently encountered pathogen. The success of E. coli, in part, may be attributed to the adaptation of specialized adherence fimbriae which facilitate attachment to normal uroepithelium.

The symptoms of urinary tract infection are common to all uropathogens but may assist in defining the location and extent of infection. In women the three principal manifestations are urethritis, characterized primarily by dysuria; cystitis, characterized by frequency, urgency, dysuria, suprapubic tenderness with or without fever; or pyelonephritis, characterized by nausea and vomiting, fever, chills, flank pain, and costovertebral angle tenderness. In the male urinary tract additional structures may be involved as seen in epididymitis and acute or chronic prostatitis. In the elderly or the patient with an indwelling urinary catheter or a spinal cord injury many of the signs and symptoms may be absent and the lines separating infection from asymptomatic bacteruria may be blurred. Presenting features rarely distinguish the infecting agent with the exception of urinary tract infection with alkaline urine (pH8), often in the presence of staghorn renal calculi, which is strongly suggestive of Proteus species.

An important corollary of urinary tract infection is asymptomatic bacteriuria – the presence of a uropathogen in significant quantity, usually defined as >10⁵ organisms/mL, with or without pyuria in the absence of symptoms. Although asymptomatic bacteriuria infrequently progresses to symptomatic infection, treatment is only indicated in pregnancy (done for the increased risk for pyelonephritis), young children, or prior to instrumentation of the urinary tract, as with insertion of a Foley catheter. Bacteriuria in the presence of symptoms defines urinary tract infection. Although >10⁵ organisms/mL is often used as the cutoff for significant bacteriuria in defining a urinary tract infection, for a properly obtained sample yielding a likely pathogen in the presence of appropriate symptoms, >10⁴ organisms/mL is a more sensitive measure. For samples obtained by suprapubic tap or other means of percutaneous drainage a lower threshold of >10³ organisms/mL may be appropriate.

Gastrointestinal and intrabdominal infection

Although many of the Enterobacteriaceae are part of the normal gut flora this group is the principal cause of intra-abdominal infections through a variety of mechanisms. Several strains of E. coli are associated with diarrheal disease of the small and large intestine (Table 134.2). Most, if not all, of those infections are self-limited illnesses although the severity varies. Enterotoxigenic E. coli (ETEC) is the most common cause of travelers’ diarrhea and enjoys a long list of colorful synonyms such as Montezuma’s revenge. Disease is the result of cholera-like toxins that result in excess secretion of water by the small bowel into the gut lumen, overwhelming the capacity of the colon to resorb water. For most of the enteropathogenic E. coli diagnostic testing is not routinely available although specific toxins or genes may be detected by PCR or DNA probes. The important exception is enterohemorrhagic E. coli (EHEC) and if suspected the stool should be cultured on sorbital-MacConkey agar to identify the O157:H7 strain most commonly associated with this disease. EHEC typically manifests as acute bloody diarrhea accompanied by abdominal pain and cramping. Fever is notably absent. Most commonly associated with undercooked ground meat, outbreaks have been associated with unpasteurized apple cider and other uncooked or unpasteurized products. Hemolytic-uremic syndrome, characterized by acute renal failure and thrombocytopenia, complicates 7% to 15% of cases of EHEC with the greater proportion of cases occurring in the very young and the elderly.

Obstruction of diverticuli or the appendix may be part of the mechanism behind diverticulitis and appendicitis, both polymicrobial infections in which the Enterobacteriaceae have an active role. Gut flora may, through perforation of a viscus, form an abscess anywhere within the abdominal or pelvic cavities or result in peritonitis. Introduction of these organisms into the biliary system, especially in the content of obstruction, may result in cholecystitis. Liver abscess may result from extension of an infectious process in the biliary system or may be seeded from the gut via the portal venous system. These infections may be characterized by fever and abdominal pain which may or may not be localizing. Leukocytosis with a left shift may be an important clue to the diagnosis. In cholecystitis and cholangitis the total bilirubin is usually elevated if the common bile duct is involved but is only elevated 25% of the time if not obstructed. Alkaline phosphatase is elevated in two-thirds of liver abscess, usually with little to no elevation of bilirubin or aminotransferases. Recently, specific Klebsiella serotypes have been implicated as the etiology of liver abscess with metastatic infection to the eye and central nervous system; this syndrome affects predominantly diabetic patients and is well described in East and Southeast Asia, though recent reports identify it as an emerging global threat.

Infection of the pancreas or a pancreatic pseudocyst may be extremely troublesome to diagnose as the signs and symptoms of abdominal pain, nausea, vomiting, fever, and leukocytosis may all be manifestations of acute pancreatitis. Infection should be suspected with the appearance of a new febrile episode, especially after manipulation of the pancreas as during endoscopic retrograde cholangiopancreatography (ERCP).

Finally, the Enterobacteriaceae are the primary etiologic agent of spontaneous bacterial

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