Chapter 9: The Enterics

Escherichia coli serves as the central organism, representing the predominant aerobic inhabitant of the normal colon yet capable of causing diverse infections ranging from uncomplicated urinary tract infections and neonatal meningitis to life-threatening sepsis and pneumonia, with specific pathogenic strains producing enterotoxins or shiga-like toxins responsible for various diarrheal syndromes. The discussion contrasts lactose-fermenting species such as Klebsiella pneumoniae, which presents particular risk in immunocompromised populations, and Enterobacter with non-fermenting organisms including Proteus mirabilis, distinguished by its characteristic swarming motility, and Serratia marcescens, recognized by distinctive red pigmentation. Particular emphasis is placed on highly virulent pathogens including Shigella, whose shiga toxin mediates the severe inflammatory response of dysentery, and Salmonella species, differentiated into typhoidal strains producing sustained bacteremia and chronic gallbladder carriage versus non-typhoidal strains causing acute gastroenteritis. The chapter extends to Yersinia enterocolitica, frequently mistaken clinically for appendicitis, and introduces the Vibrionaceae family including Vibrio cholerae and its potent enterotoxin causing severe osmotic diarrhea, Campylobacter jejuni implicated in hemorrhagic diarrheal illness, and Helicobacter pylori established as the primary etiologic agent of peptic ulcer disease. Finally, the chapter addresses anaerobic gram-negative rods, particularly Bacteroides fragilis and Fusobacterium species, which constitute the dominant components of intestinal flora and represent common causative organisms in intra-abdominal polymicrobial infections and abscess formation.