Fluorescent nanodiamonds are widely used as abrasives, optical or magnetic labels, in drug delivery or nanoscale sensing. They are considered very biocompatible in mammalian cells. However, in bacteria the situation looks different and results are highly controversial. This article presents a short review of the published literature and a systematic experimental study of different strains, nanoparticle sizes and surface chemistries. Most notably, particle aggregation behaviour and bacterial clumping are taken into consideration to explain reduced colony counts, which can be wrongly interpreted as a bactericidal effect. The experiments show no mechanism can be linked to a specific material property, but prove that aggregation and bacteriostatic effect of nanodiamond attachment play a significant role in the reported results.

Keywords