Influence of the Polysaccharide Capsule on the Bactericidal Activity of Indolicidin on Streptococcus pneumoniae

Abstract

Streptococcus pneumoniae is a pathogen responsible for high morbidity and mortality worldwide. The polysaccharide capsule confers protection against phagocytosis and influences many aspects of pneumococcal pathogenesis. The capsular polysaccharides (CPS) are highly immunogenic and exhibit great structural variability, with more than 100 serotypes described so far. Antimicrobial peptides (AMPs) are an important part of the innate defense mechanisms against many pathogens. Indolicidin is a cationic AMP produced by bovine neutrophils, with bactericidal effects against several bacteria. CPS has been shown to interfere with the ability of AMPs to kill pneumococci, but the effects of capsule variability on susceptibility to indolicidin have not been explored. The present work determined the effects of capsule on resistance to indolicidin in vitro. Using a bactericidal plate assay, we observed that different pneumococcal serotypes exhibited variable resistance to indolicidin, which correlated with the capsule net charge. Interestingly, the effect of capsule expression on resistance to indolicidin was dependent on the serotype; bacteria with lower zeta potential were more resistant to indolicidin when capsule was present, while those with less negative surface charge were more resistant in the absence of capsule. The addition of purified CPS partially rescued the bacteria from the bactericidal effects of indolicidin, while the addition of anticapsular antibodies accentuated the peptide’s bactericidal action, suggesting a possible new protective mechanism induced by polysaccharide-based pneumococcal vaccines.

Pneumococcal surface protein A (PspA) prevents killing of Streptococcus pneumoniae by indolicidin

“…Previous studies have shown that indolicidin and synthetic peptides derived from it can reduce pneumococcal viability in vitro 30 , while pneumococcal capsular polysaccharides may interfere with indolicidin activity 31,32 . However, the role of PspA on pneumococcal interactions with indolicidin has not been explored.…”

“…Pneumococcal susceptibility to indocilidin was determined using an in vitro bactericidal assay adapted by our group 32 . The wild-type and mutant bacteria were grown as described, pelleted and washed twice in sterile PBS by centrifugation at 8,600 x g for 5 min, resuspended in the initial volume of sterile PBS, and incubated with increasing concentrations of indolicidin (ANASPEC), ranging from 7.5 to 60 µg/ml, for one hour at 37 ºC.…”

Recent advances in the therapeutic potential of cathelicidins

“…In an assay mimicking a bacterial membrane, treatment with a bovine cathelicidin, BMAP-28 disrupted the packing of the bilayer, resulting in thinning of the membrane and pore formation (Agadi et al, 2022). This mechanism is shared among other members of the cathelicidin family with alpha-helical structures, such as LL-37, Dermcidin, BMAP-27, indolicidin and PMAP-36 (Agadi et al, 2022;Waz et al, 2022). Conversely, in pore-less models, the CAMPs do not transverse the membrane but instead cover it, like a carpet.…”

Baseline Pneumococcal IgG Levels and Response to 23-Valent Pneumococcal Polysaccharide Vaccine among Adults from Beijing, China

“…Streptococcus pneumoniae ( S. pneumoniae ) is the most common opportunistic pathogen that colonizes the human nasopharynx and is responsible for a wide range of infections, including otitis media, community-acquired pneumonia, septicemia and meningitis. Its capsular polysaccharide is one of the most important virulence factors that can induce protective antibodies [ 1 ]. The composition of capsular polysaccharide divides S. pneumoniae into multiple serotypes, and more than 100 serotypes have been identified so far [ 2 , 3 ].…”