Sorting the good from the bad bacteria

There are good bacteria and bad bacteria – some coexisting in the same species – and researchers at MIT have developed a new microfluidic technique which can quickly distinguish bacteria within the same strain.

The new microfluidic chip is etched with tiny channels resembling an elongated hourglass with a pinched midsection. Researchers injected bacteria through one end and watched how cells travelled from one end to the other via dielectrophoresis. Depending on the cells’ phenotype, they either passed through the narrow section, or trapped at the opening.

A bacterium’s reaction to voltage serves as a fingerprint for its phenotype, which in turn is a clue to how virulent that bacterium can be.

“Even within one species, we can distinguish different physical characteristics resulting from individual genes, which can cause different levels of disease,” said Cullen Buie, the Mitsui Career Development Assistant Professor of Mechanical Engineering. “This could help doctors prescribe medication and understand quickly the health of a patient and where disease is going.

Buie says the surface of a cell can tell you a lot about its activity, or its potential danger. Pseudomonas aeruginosa poses a risk to those with weakened immune systems; in cystic fibrosis patients, it can turn pathogenic, forming thick mucus layers in the lungs, aided by pili on the surface.

“For bacteria that cause disease, there’s a lot happening on the outer surface,” Buie said. “They basically change their outer surface, and that can be an indicator of how dangerous they might be.”

Researchers fed ordinary strains of P. aeruginisa and mutant strains missing certain genes to make biofilms through each channel on the chip, applying a low voltage. The resulting electric field was more intense at the narrowest part of each channel, creating a dielectrophoretic force that repels certain cells depending on their phenotype.

They found that the device was able to distinguish benign cells from those better able to form biofilms. The researchers next hope to test bacteria from bodily fluids, not pure cultures.

The work – published in PLOS ONE – could speed up monitoring of cystic fibrosis and other diseases. The team say the device may one day be incorporated into a portable, disposable system for clinical use, providing fast results without lengthy lab work.