The primary virulence factor of K. pneumoniae is its carbohydrate capsule (Figure 1). The capsule allows the organism to resist phagocytosis, and hence death
in professional phagocytes. Encapsulated strains have been shown to be many fold more virulent than nonencapsulated Klebsiella in a variety of different animal models. There have been reports of several toxins produced by K. pneumoniae, including enterotoxins, by a number of strains isolated from patients with tropical sprue. However, none of these toxins could account for the extensive lung destruction seen in lobar pneumonia produced by K. pneumoniae. Probably the best candidate for this devastating pathological effect is the endotoxin produced by this organism. Indeed, a compound referred to as the extracellular toxic complex (ETC) has been described for K. pneumoniae. This material is composed of endotoxin, capsule and protein, and the endotoxin portion was shown to contain the toxic moiety. This material was lethal when injected intra-peritoneally into mice, and when it was introduced into the lungs of mice, the pathology seen after 24, 48 and 72 hours was similar to the damage caused by an active K. pneumoniae lobar pneumonia. All K. pneumoniae serotypes examined to date produce the ETC.
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