Hypervirulence and hypermucoviscosity: Two different but complementary Klebsiella spp. phenotypes?

Abstract

Since the hypermucoviscous variants of Klebsiella pneumoniae were first reported, many cases of primary liver abscesses and other invasive infections caused by this pathogen have been described worldwide. Hypermucoviscosity is a phenotypic feature characterized by the formation of a viscous filament ≥5 mm when a bacterial colony is stretched by a bacteriological loop; this is the so-called positive string test. Hypermucoviscosity appears to be associated with this unusual and aggressive type of infection, and therefore, the causal strains are considered hypervirulent. Since these first reports, the terms hypermucoviscosity and hypervirulence have often been used synonymously. However, new evidence has suggested that hypermucoviscosity and hypervirulence are 2 different phenotypes that should not be used synonymously. Moreover, it is important to establish that a negative string test is insufficient in determining whether a strain is or is not hypervirulent. On the other hand, hypervirulence- and hypermucoviscosity-associated genes must be identified, considering that these phenotypes correspond to 2 different phenomena, regardless of whether they can act in synergy under certain circumstances. Therefore, it is essential to quickly identify the genetic determinants behind the hypervirulent phenotype to develop effective methodologies that can diagnose in a prompt and effective way these hypervirulent variants of K. pneumoniae.

Keywords: Klebsiella pneumoniae; hypermucoviscosity; hypervirulence; invasive infections; regulator of mucoid phenotype; serotypes.

Figure 1.

String test. A hypermucoviscous phenotype is seen when a viscous filament ≥5 mm is produced after stretching a K. pneumoniae colony with a loop on an agar plate (positive string test).

Figure 2.

Venn diagram showing differences and similarities between hypermucoviscous and hypervirulent phenotypes. New evidence suggests that hypermucoviscosity and hypervirulence are 2 different phenomena. The green region shows the main characteristics of the hypermucoviscous phenotype that differ from those of the hypervirulent phenotype. The intersection highlights the elements that have been strongly associated with both phenotypes.