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

Abstract

Streptococcus pneumoniae is a devastating global pathogen. Prevalent in sub-Saharan Africa, pneumococcal serotype 1 is atypical in that it is rarely found as a nasopharyngeal coloniser, yet is described as one of the most common causes of invasive pneumococcal disease. Clonal sequence type (ST)-306 and ST615 are representative of the two major serotype 1 lineages A and C, respectively. Here we investigated the virulence properties and haemolytic activities of these 2 clonal types using in vivo mouse models and in vitro assays. A lethal dose of ST615 administered intranasally to mice led to the rapid onset of disease symptoms and resulted in 90% mortality. In contrast, mice exposed to the same infection dose of ST306 or a pneumolysin (Ply)-deficient ST615 failed to develop any disease symptoms. Interestingly, the 2 strains did not differ in their ability to bind the immune complement or to undergo neutrophil-mediated phagocytosis. Upon comparative genomic analysis, we found higher within-ST sequence diversity in ST615 compared with ST306 and determined that ZmpA, ZmpD proteins, and IgA protease, were uniquely found in ST615. Using cell fractionation and cell contact-dependent assay, we made the unexpected finding that ST615 harbours the expression of two haemolytic variants of Ply: a cell-wall restricted fully haemolytic Ply, and a cytosolic pool of Ply void of any detectable haemolytic activity. This is the first time such a phenomenon has been described. We discuss the biological significance of our observation in relation to the aptitude of the pneumococcus for sustaining its human reservoir.

Meningitis-associated pneumococcal serotype 8, ST 53, strain is hypervirulent in a rat model and has non-haemolytic pneumolysin which can be attenuated by liposomes

“…Sequencing of the pneumolysin gene confirmed that this strain harboured a non-haemolytic pneumolysin. Non-haemolytic pneumolysin has been found to be associated with meningitis outbreaks in serotype 1 and has already been reported in recent publications in serotype 8 strains (Kirkham et al, 2006;Jefferies et al, 2007;Panagiotou et al, 2020). Although the serotype 8, ST53, strain had no haemolytic activity, we found it produced large quantities of pneumolysin, as seen with serotype 1 strains with increased pathogenicity (Jacques et al, 2020).…”

Bacterial pore-forming toxin pneumolysin drives pathogenicity through shed toxin-loaded host extracellular vesicles

“…Zafar et al, showed that administration of recombinant PLY dose of 200 ng/day induced shedding of pneumococci and mucosal inflammation ( 35 ). Intranasal administration of PLY at 10 ng/g body weight (corresponding to 0.25 μg for 6-8 week mice) restored virulence of an avirulent ST306 pneumococcal clone expressing a non-cytolytic PLY variant ( 36 ). Based on the above studies, we tested two intranasal doses of 0.25 and 0.5 μg PLY, but we observed mortality at 0.5 μg dose.…”

Serotype 1 pneumococcus: epidemiology, genomics, and disease mechanisms

“…This contrasts with the depletion of pneumolysin in an ST615 clone which renders the isolate completely avirulent in murine models of pneumonia [97]. The pore-forming ability of pneumolysin was described as a critical virulence factor for pneumococci [98].…”