Phase-contrast microscopy helps reveal MRSA colony behaviors in time-lapse video

Scientists at the University of Nottingham and the University of Sheffield (both in England) used phase-contrast microscopy to observe methicillin-resistant Staphylococcus aureus (MRSA), a well-known superbug thought to have been a static or non-motile organism. The technique enabled the researchers to discover that MRSA shows signs of active motility.

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Methicillin-resistant Staphylococcus aureus is responsible for several infections in humans ranging from superficial to life-threatening, which are difficult to treat because of antibiotic resistance. Now, the scientists have shown that S. aureus, a spherical bacterium with no propulsive tail or appendages, may be capable of "active" motility and moving independently. Motility is central to bacterial behaviors including biofilm formation, virulence, and host colonization, so the discovery could have implications for future clinical treatments.

“Our research has focused on observing the formation of dendrites—branch-like structures that emerge from the central colony of bacteria,” says Dr. Steve Diggle from the University of Nottingham’s School of Life Sciences. “Using high-powered microscopy, we saw that the bacteria can spread across the surface of an agar plate in structures that we have called ‘comets’. These advance outwards and precede the formation of dendrites. We have observed and photographed the comets ‘seeding’ cells behind them, without losing mass, which then grow into observable dendrites. After eight hours of colony growth, the comet heads are the main source of movement. Cells in the tail follow the comet heads for a while while bacteria further away no longer move. Our time-lapse video shows the whole remarkable process.”

The researchers found the comet heads are composed of aggregates of S. aureus cells held together by a matrix of slime, and display no observable pili or flagella (propulsive tails or appendages). They also observed that under certain conditions, comets can also etch the agar, leaving behind tracks. The moving S. aureus colonies are also capable of avoiding other colonies. The study also showed that the addition of fluid is not able to effectively move comet heads, but can easily move the cells in the comet ‘tails’.

S. aureus motility on agar plate.

The research team believes these newly observed and time-lapse photographed behaviors are consistent with active motility, and most closely resemble gliding motility. The revelations could inform research into new ways to tackle S. aureus infection, as the underlying motility mechanism(s) of the bacteria could be a target for future vaccines and inhibitory pharmaceutical compounds.

The researchers conclude that if S. aureus has true motility as indicated by this work, it would be the first example of a Gram-positive bacteria with a typical Gram-positive cell wall moving without flagella or pili. This begs the question of whether other Gram-positive organisms may also be motile in a similar way and opens the way for further research in the area.

Full details of the work appear in the journal Scientific Reports; for more information, please visit http://dx.doi.org/10.1038/srep17698.

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