The Subtle Interplay of Nanoparticles and Bacteria

Gold nanoparticles cluster on bacteria and can even kill them, but the clustering patterns and lethality depend on the size of the nanoparticles and the type of bacteria, researchers have found (J. Am. Chem. Soc., DOI: 10.1021/ja301167y).

Understanding how nanoparticles interact with the membranes of bacteria is crucial for developing new antibacterial agents, says Vincent Rotello of the University of Massachusetts, Amherst. A benefit of a potential nanoparticle antimicrobial is that the bugs should not be able to develop resistance to a method that physically damages cell membranes, Rotello says: “There’s nothing the bacteria can do if you blow it apart.”

Other scientists have already shown that gold nanoparticles with a positively charged coating can disrupt bacterial membranes. To investigate the effect, Rotello and his colleagues coated 6-nm gold nanoparticles with a positively charged, water-repelling molecule. Using transmission electron microscopy and ultraviolet-visible spectroscopy, they observed the effect of the particles on cultures of two bacteria, Escherichia coli and Bacillus subtilis.

The nanoparticles did not harm the bacteria. At first, the particles moved around on the cells’ surfaces, but they started to aggregate within a few minutes. Different grouping patterns formed on the two bacteria because of the difference in their membranes’ lipid and protein composition, the researchers think. E. coli cells harbored a large number of small clusters spread evenly over their surface, while the aggregates on B. subtilis were larger and fewer.

When the team repeated the experiment with 2-nm particles, the effect was drastically different: B. subtilis cells immediately ruptured, while E. coli withstood the attack but protrusions formed on its cell membranes.

Rotello thinks that the water-repelling molecule on the nanoparticle surface digs into the membrane’s lipid layer. Clusters of nanoparticles force the lipids apart, changing the membrane’s curvature and creating the protrusions, which eventually pinch off and rupture the membrane. The 2-nm particles are simply a better size than the 6-nm ones to deform the cell membrane, Rotello thinks. And because B. subtilis has only one cell membrane, to E. coli’s two, Rotello says, it is more susceptible to the nanoparticles.