Colonization, Infection, and the Accessory Genome of Klebsiella pneumoniae

<i>Klebsiella pneumoniae</i> is a Gram-negative pathogen that has a large accessory genome of plasmids and chromosomal gene loci. This accessory genome divides <i>K. pneumoniae</i> strains into opportunistic, hypervirulent, and multidrug-resistant groups and separates <i>K. pneumoniae</i> from two closely related species, <i>Klebsiella variicola</i> and <i>Klebsiella quasipneumoniae</i>. Some strains of <i>K. pneumoniae</i> act as opportunistic pathogens, infecting critically ill and immunocompromised patients. These <i>K. pneumoniae</i> are a common cause of health-care associated infections including pneumonia, urinary tract infections (UTIs), and bloodstream infections. <i>K. variicola</i> and <i>K. quasipneumoniae</i> are often clinically indistinguishable from opportunistic <i>K. pneumoniae</i>. Other strains of <i>K. pneumoniae</i> are hypervirulent, infecting healthy people in community settings and causing severe infections including pyogenic liver abscess, endophthalmitis, and meningitis. A third group of <i>K. pneumoniae</i> encode carbapenemases, making them highly antibiotic-resistant. These strains act as opportunists but are exceedingly difficult to treat. All of these groups of <i>K. pneumoniae</i> and related species can colonize the gastrointestinal tract, and the accessory genome may determine if a colonizing strain remains asymptomatic or progresses to cause disease. This review will explore the associations between colonization and infection with opportunistic, antibiotic-resistant, and hypervirulent <i>K. pneumoniae</i> strains and the role of the accessory genome in distinguishing these groups and related species. As <i>K. pneumoniae</i> infections become progressively more difficult to treat in the face of antibiotic resistance and hypervirulent strains, an increased understanding of the epidemiology and pathogenesis of these bacteria is vital.