Hypervirulence

Hypervirulence is a term used in microbiology to describe strains of a pathogen that exhibit a markedly greater capacity to cause disease than reference strains of the same species. It denotes an elevated virulence phenotype that can arise from additional virulence determinants, often carried on mobile genetic elements, regulatory changes, or combinations of both. Hypervirulent strains may cause severe disease in otherwise healthy hosts and can display expanded tissue tropism, higher bacterial burdens, or increased transmission potential.

Mechanisms and markers commonly associated with hypervirulence include enhanced capsule production and a mucoid phenotype, robust iron acquisition systems such as siderophores, production of toxins or invasins, and immune evasion strategies. Many hypervirulent strains harbor virulence plasmids or pathogenicity islands carrying genes that promote these traits. In laboratory and clinical settings, hypervirulence is studied through genomic analyses, phenotypic assays, and animal models, with clinical presentation often featuring rapid progression and more severe outcomes.

Notable examples include Klebsiella pneumoniae, where hypervirulent strains (hvKp) are associated with a hypermucoviscous phenotype and the spread of virulence plasmids; hvKp can cause community-acquired liver abscesses and endophthalmitis. Other documented hypervirulent lineages include certain strains of Clostridioides difficile (such as BI/NAP1/027) linked to increased toxin production and outbreaks. References to hypervirulence also appear for some Staphylococcus aureus and Streptococcus species, though definitions and criteria vary across organisms.

Implications for public health and clinical practice include the need for genomic surveillance, monitoring for convergence of hypervirulence with antimicrobial resistance, and vigilance in infection control. Diagnosis and risk assessment rely on a combination of genomic and phenotypic data, with the term serving as a descriptive label for pathogens with unusually high disease-causing potential.