Antibacterial mechanism of gold nanoparticles on Streptococcus pneumoniae.

Streptococcus pneumoniae is a causal agent of otitis media, pneumonia, meningitis and severe cases of septicemia. This human pathogen infects elderly people and children with a high mortality rate of approximately one million deaths per year worldwide. Antibiotic-resistance of S. pneumoniae strains is an increasingly serious health problem; therefore, new therapies capable of combating pneumococcal infections are indispensable. The application of gold nanoparticles has emerged as an option in the control of bacterial infections; however, the mechanism responsible for bacterial cell lysis remains unclear. Specifically, it has been observed that gold nanoparticles are capable of crossing different structures of the S. pneumoniae cells, reaching the cytosol where inclusion bodies of gold nanoparticles are noticed. In this work, a novel process for the separation of such inclusion bodies that allowed the analysis of the biomolecules such as carbohydrates, lipids and proteins associated with the gold nanoparticles was developed. Then, it was possible to separate and identify proteins associated with the gold nanoparticles, which were suggested as possible candidates that facilitate the interaction and entry of gold nanoparticles into S. pneumoniae cells.

Inclusion bodies and pH lowering: as an effect of gold nanoparticles in Streptococcus pneumoniae.

Streptococcus pneumoniae is a human pathogen whose principal virulence factor is its capsule. This structure allows the bacterium to evade the human immune system. Treatment of infections caused by this bacterium is based on antibiotics; however, the emergence of antibiotic-resistant strains makes this task increasingly difficult. Therefore, it is necessary to investigate new therapies, such as those based on gold nanoparticles, for which unfortunately the mechanisms involved have not yet been investigated. As far as we know, this study is the first that attempts to explain how gold nanoparticles destroy the bacterium Streptococcus pneumoniae. We found that the mean particle size was an important issue, and that the effect on the bacterium was dose-dependent. Cellular growth was inhibited by the presence of the nanoparticles, as was cell viability. The pH of the bacterial growth media was acidified, but interestingly the reactive species were not affected. A transmission electron microscopy analysis revealed the presence of inclusion bodies of gold nanoparticles within the bacterium. We present the first findings that attempt to explain how gold nanoparticles lyse Gram-positive bacteria.