Hypervirulent pneumococcal serotype 1 harbours two pneumolysin variants with differential haemolytic activity

Area of Science:

• Microbiology
• Pathogen Research
• Genomics

Background:

• Streptococcus pneumoniae causes significant global disease, with serotype 1 being a major contributor to invasive pneumococcal disease, particularly in sub-Saharan Africa.
• Pneumococcal serotype 1 strains ST306 (lineage A) and ST615 (lineage C) represent distinct major lineages.
• The atypical epidemiology of serotype 1, with low carriage but high invasive disease rates, necessitates investigation into virulence factors.

Purpose of the Study:

• To investigate the virulence properties and hemolytic activities of two major serotype 1 lineages, ST306 and ST615.
• To compare the genomic characteristics and protein expression profiles of ST306 and ST615.
• To elucidate the role of pneumolysin (Ply) variants in the pathogenesis of Streptococcus pneumoniae serotype 1.

Main Methods:

• In vivo mouse models of intranasal infection.
• In vitro hemolytic assays and complement binding assays.
• Neutrophil-mediated phagocytosis assays and comparative genomic analysis.
• Cell fractionation and cell contact-dependent assays to characterize pneumolysin localization and activity.

Main Results:

• Intranasal inoculation with ST615 resulted in rapid disease onset and 90% mortality in mice, whereas ST306 or a pneumolysin-deficient ST615 strain did not cause disease.
• ST615 exhibited higher within-strain sequence diversity and unique presence of ZmpA, ZmpD proteins, and IgA protease compared to ST306.
• ST615 unexpectedly expressed two hemolytic variants of pneumolysin: a cell-wall restricted active form and a cytosolic inactive form.

Conclusions:

• The differential virulence of ST615 and ST306 is linked to specific genetic factors and pneumolysin expression.
• ST615's unique dual pneumolysin expression strategy may contribute to its high invasive potential and ability to sustain a human reservoir.
• Understanding these virulence mechanisms is crucial for developing strategies against invasive pneumococcal disease caused by serotype 1 strains.