So ninja particles are specially designed to target bacteria and kill them, but how? The first step is finding the offending bacterial cells in a sea of mammalian cells. This is where that key tenet of "opposites attract" takes hold. The surface of bacterial cells is more negatively charged than that of mammalian cells. In order to specifically be attracted to the bacterial cells, the ninja particles must have the opposite charge – positive. They garner this positive charge on their surface through a process called self-assembly. Each particle is made up many, many smaller strands of polymers. These polymers clump together, or self-assemble, to form little balls called micelles. Because of attractive interactions between different parts of the polymer chain, these micelles form naturally in water with the outside of the ball coated in a positive charge. And voila – the ball of positive charge is naturally attracted to the negatively charged microbe.
Once there, the ninja particle attaches itself to the bacterial cell. The positively charged portions of the particle that helped selectively find the bacterial cells also act as antibacterial agents, poking holes into the cell wall. This process, called membrane lysis, ruins the structure of the cell, causing the guts of the cell to start to ooze out, with no hope of recovery. This, in fact, is where the researchers came up with the name "ninja" for their particles. The kill method of perforating the cell wall with holes is similar to what might happen if the cell were attacked with a ninja star.
One of the best parts of this process is that bacteria are never given the chance to develop any resistance. Antibiotics work by selectively crippling certain parts of the cell's mechanism, keeping most of the structural features intact. The ninja particle method, in contrast, is very physically damaging to the cell, and the bacteria don't have the opportunity to potentially develop a resistance to the ninja particles [source: Nederberg et al].
The lifetime of the ninja particles can be fine-tuned so that they are able to kill the bacterial cells before being killed off themselves. Eventually, however, enzymes in the body start degrading the particles and they fall apart, with the resulting smaller bits getting excreted by the body [source: Hedrick].