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.

The Klebsiella pneumoniae complex comprises seven K. pneumoniae- related species, including K. variicola . K. variicola is a versatile bacterium capable of colonizing different hosts such as plants, humans, insects and animals. Currently, K. variicola is gaining recognition as a cause of several human infections; nevertheless, its virulence profile is not fully characterized. The clinical significance of K. variicola infection is hidden by imprecise detection methods that underestimate its real prevalence; however, several methods have been developed to correctly identify this species. Recent studies of carbapenemase-producing and colistin-resistant strains demonstrate a potential reservoir of multidrug-resistant genes. This finding presents an imminent scenario for spreading antimicrobial resistant genes among close relatives and, more concerningly, in clinical and environmental settings. Since K. variicola was identified as a novel bacterial species, different research groups have contributed findings elucidating this pathogen; however, important details about its epidemiology, pathogenesis and ecology are still missing. This review highlights the most significant aspects of K. variicola , discussing its different phenotypes, mechanisms of resistance, and virulence traits, as well as the types of infections associated with this pathogen.

Klebsiella variicola is considered an emerging pathogen in humans and has been described in different environments. K. variicola belongs to Klebsiella pneumoniae complex, which has expanded the taxonomic classification and hindered epidemiological and evolutionary studies. The present work describes the molecular epidemiology of K. variicola based on MultiLocus Sequence Typing (MLST) developed for this purpose. In total, 226 genomes obtained from public data bases and 28 isolates were evaluated, which were mainly obtained from humans, followed by plants, various animals, the environment and insects. A total 166 distinct sequence types (STs) were identified, with 39 STs comprising at least two isolates. The molecular epidemiology of K. variicola showed a global distribution for some STs was observed, and in some cases, isolates obtained from different sources belong to the same ST. Several examples of isolates corresponding to kingdom-crossing bacteria from plants to humans were identified, establishing this as a possible route of transmission. goeBURST analysis identified Clonal Complex 1 (CC1) as the clone with the greatest distribution. Whole-genome sequencing of K. variicola isolates revealed extended-spectrum β-lactamase- and carbapenemase-producing strains with an increase in pathogenicity. MLST of K. variicola is a strong molecular epidemiological tool that allows following the evolution of this bacterial species obtained from different environments.

Fecal colonization by ESBL-EC is associated with increased risk of BSI by this strain, longer hospital stay, and higher related costs.

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