Calcifying nanoparticles: one face of distinct entities?

Calcifying nanoparticles (CNPs, nanobacteria, nanobacteria-like particles) were discovered as cell culture contaminants by Kajander et al. more than 25 years ago, and the first results of their work were published some years later (Kajander et al., 1997; Kajander and Ciftcioglu, 1998). The nature of CNPs has been obscured so far. Possibly representing a new proposed class of living organisms or inorganic nanostructures, CNPs exist as coccoid, coccobacillar, or bacillary particles of 80–500 nm in diameter, consisting of a central cavity surrounded by the hydroxyapatite shell, and possessing an ability to grow and divide in culture medium forming biofilms. The phenomenon of CNPs raised intensive discussions amongst the scientific community; there are active debates on their nature and potential role in clinical medicine. According to a number of fundamental and clinical studies, CNPs are suspected to cause ectopic calcification-related diseases such as atherosclerosis, heart valve calcification, placental calcification, nephrolithiasis, cholecystolithiasis, type III chronic prostatitis/chronic pelvic pain syndrome, and testicular microlithiasis. Electron microscopy is considered to be a gold standard for the visualization of CNPs; researchers usually observe the colonies of CNPs after the culturing in DMEM or RPMI-1640 for 4–8 weeks. Other means used by multiple groups for the detection of CNPs are various serological methods, including ELISA, immunohistochemistry, immunofluorescence reaction, immunoblotting, and Ouchterlony immunodiffusion. Although PCR has been previously used to identify possible genome of CNPs, there are doubts on credibility of this method since primers could have been designed based not on putative genome sequences of CNPs but on the genome sequences of contaminating bacteria. The general features of CNPs are indicated in Table ​Table11.