GRAM-NEGATIVE ENTERIC BACTERIA
Gram-negative bacilli (GNB) are normal components of the human colonic flora and/or a number of environmental habitats and can colonize mucosal and skin surfaces, especially in pts in long-term care facilities and hospital settings. GNB cause a wide variety of infections involving diverse anatomic sites in both healthy and compromised hosts; extraintestinal infections due to Escherichia coli and, to a lesser degree, Klebsiella species are most common. Isolation of GNB from any sterile site almost always implies infection, whereas isolation from nonsterile sites requires clinical correlation. Early appropriate antimicrobial therapy improves outcomes. Given worldwide increases in multidrug-resistant GNB (e.g., due to extended-spectrum β-lactamases [ESBLs] and AmpC β-lactamases), combination empirical antimicrobial therapy pending susceptibility results may be appropriate for critically ill pts.
EXTRAINTESTINAL PATHOGENIC E. COLI (EXPEC)
In contrast to intestinal pathogenic E. coli (see below), ExPEC strains are often found in the intestinal flora of healthy individuals but cause disease only when they enter a normally sterile extraintestinal site (e.g., the urinary tract, peritoneal cavity, or lungs). Most ExPEC strains have virulence factor profiles distinct from those of other commensal strains and from those of pathogenic strains that cause intestinal infections.
The clinical presentation depends in large part on the site of the body infected by ExPEC.
UTI: The urinary tract is the site most frequently infected by ExPEC; see Chap. 144 for more details. E. coli causes 80–90% of ~6–8 million episodes of acute uncomplicated UTI in premenopausal women.
Abdominal and pelvic infection: The abdomen and pelvis represent the second most common site of infection by ExPEC, which may be isolated in the setting of a polymicrobial infection; see Chap. 81 for more details. Syndromes include peritonitis, intraabdominal abscesses, and cholangitis.
Pneumonia: ExPEC is generally the third or fourth most commonly isolated GNB in hospital-acquired pneumonia and can be a common cause of pneumonia in pts residing in long-term-care facilities; see Chap. 132 for more details.
Meningitis: E. coli is one of the two leading causes of neonatal meningitis (the other being group B Streptococcus). Strains with the K1 capsular serotype are generally involved.
Cellulitis/musculoskeletal infection: E. coli often contributes to infection of decubitus ulcers and diabetic lower-extremity ulcers, cellulitis, and burn-site or surgical-site infections. Hematogenously acquired osteomyelitis, particularly vertebral, is more commonly caused by E. coli than is generally appreciated. See Chap. 84 for more details.
Bacteremia: E. coli is one of the two most common blood isolates of clinical significance. E. coli bacteremia can arise from primary infection at any site, but originates most commonly from the urinary tract (50–67% of episodes) and next most commonly from the abdomen (25% of episodes). E. coli bacteremia is typically associated with sepsis. Endovascular infections are rare but have been described.