In order to detect traces of harmful bacteria, the current method relies on harvesting the blood of the horseshoe crab. Amebocytes, which play a similar role to white blood cells in responding to pathogens, are extracted from the crab blood and used to produce a solution which reacts in the presence of bacteria. Although this method has been used for decades, it is having a devastating effect on the horseshoe crab population, and the birds which feed on them.

Now researchers based at Princeton University led by Assistant Professor Michael McAlpine, have managed to produce a bacterial sensor which is electronically readable [Mannoor et al., PNAS, (2010), doi:10.1073/pnas.1008768107]. The sensor exploits an antimicrobial peptide found on the skin of African clawed frogs. However, it is possible to synthesis the peptides in the laboratory, meaning there is no similar threat to the frog population.
Attempts have been made to use proteins (long chained amino acids) in the past. However, as well as being highly selective, the proteins do not survive for long outside of a well controlled body-like environment. The peptides (short chained amino acids) are much more robust, and are also sensitive to a much wider range of bacteria.
The device is effectively a micro-capacitor, with the antibacterial microbial peptides (AMPs) acting as the dielectric medium. The AMPs are immobilized on a gold microelectrode array, and respond in the presence of bacteria. The bacteria bind to the AMPs, and in doing so the dielectric permittivity changes. By measuring the change of impedance across the device, it is not only possible to detect the bacteria, but also differentiate between strains. The chip is reusable, and according to McAlpine it is possible to “just wash off the chip and use it again. We’ve used the same chip over and over”.
When tested on E. coli, the chip was found to have a surprisingly good minimum detection limit, of 1 bacterium per micro litre. However, in order to truly challenge current methods, the sensitivity needs to be increased. McAlpine says “if we can improve the sensitivity ten or a hundred fold, then we can be competitive”.
The team is currently working on reducing the chip down to the nanoscale, in order to improve the sensitivity. If successful, McAlpine is hopeful we can look forward to a “revolutionary change” in how bacterial detection is approached.

Stewart Bland