This article collates published information regarding the in vitro antibacterial activity of both metal and carbon nanoparticles. The aims are to establish a consensus regarding modes of antibacterial activity, and to evaluate the applicability of current knowledge to prediction of likely effects of nanoparticles upon important microbial processes in environmental exposures. The majority of studies suggest that nanoparticles cause disruption to bacterial membranes, probably by production of reactive oxygen species. Contact between the nanoparticle and bacterial membrane appears necessary for this activity to be manifested. Interfacial forces such as electrostatic interactions are probably important in this respect. However, the toxicity of free metal ions originating from the nanoparticles cannot be discounted. Passage of nanoparticles across intact membranes appears to be unlikely, although accumulation within the cytoplasm, probably after membrane disruption, is often observed. To date, published studies have not been designed to mimic natural systems and therefore provide poor understanding of the likely consequences of intentional or unintentional environmental release. The limited studies currently available fail to identify any significant effects at the microbial level of nanoparticles in more complex systems.