A mousetrap for bacteria

ANTISEPTICS have saved countless lives, but they are most effective when the bacteria they are attacking are individual cells in suspension. Once bacteria have attached themselves to solid surfaces and formed films, they are far harder to eradicate with standard disinfectants. Bacterial contamination of medical devices is a particular problem, as those devices are then used on people whose immune systems may be in less than tip-top condition. Surgical instruments can be heat-sterilised or treated with ultraviolet light, but that is not appropriate for everything. The result is that infections arising from bacteria attached to surfaces in clinics and hospitals are reckoned to cause up to 1.4m deaths per year.

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Can anyone smell cheese?

In an attempt to develop a better method of disinfection, a team led by David Whitten of the University of New Mexico and Kirk Schanze of the University of Florida set out to design the equivalent of a mousetrap for bacteria. The device they have come up with is a hollow capsule five microns (millionths of a metre) across. It is made of alternating layers of two electrically conducting polymers, one positively charged and one negatively charged, that have an unusual property: when they are exposed to light they turn nearby oxygen molecules into “singlet oxygen”, a particularly reactive form of the element that is highly toxic to any bacteria inside the capsule.

To test this idea, the researchers ran a series of experiments in which they exposed their newly built microcapsules to Pseudomonas aeruginosa, a lethal bacterium commonly found in hospitals, and also to Cobetia marina, a bacterium that frequently adheres to ships and marine equipment, causing fouling. They found that in both cases the microcapsules attracted and captured nearby bacteria. After one hour of exposure to light, they reported in a recent issue of Applied Materials & Interfaces, the capsules killed more than 95% of the bacteria used in the study.

What is killing the bacteria is clear: it is the singlet oxygen. What is luring them into the microcapsules, though, is not well understood. Dr Whitten and Dr Schanze have, however, observed that the capsules extrude thin, flimsy fibrils of the two polymers. These fibrils seem to attract bacteria and then guide them into the killing chamber. The researchers speculate that the positive electric charge on one of the polymers may be the bait, since many bacteria are negatively charged and would thus be attracted electrostatically to the polymer in question. Alternatively, because both bacteria and polymers are repelled by water they may be pushed together by this joint repulsion.

However it works, the upshot is what Dr Whitten describes as a micro-sized “roach motel” (“Bacteria check in, but they don't check out”). He suggests that a thin coating of the capsules could be applied to surfaces. If the idea can be scaled up, it may prove a useful weapon in the fights against hospital-caused infection and marine-fouling alike.