Wednesday, 20 November 2013

Vibrio coralliilyticus Search Patterns across an Oxygen Gradient

Bacteria have to use chemotaxis in mobility search patterns for locating nutrient sources, arranging themselves in chemical gradients and to initiate pathogenesis. There are 4 known search patterns: run and reverse, run and tumble, straight swimming and the recently described 3-step run, reverse and flick technique. Previous studies show V. alginolyticus uses “3-step flick” to enter nutrient rich zones.

 Majority evidence suggests that bacteria utilise one search strategy per species, with marine bacteria most commonly utilising run and reverse.  However, in this paper V. coralliilyticus is shown to demonstrate the three search strategies of run and reverse, straight swimming and 3 step flick.

The 3 step flick is like a combination of run and tumble, and run and reverse tactics, allowing the re-examination of nutrient areas with reverse movement, but paired with a beneficial 90 degree flick, altering orientation of the microbe, increasing the probability of entering desirable environments.

V. coralliilyticus is a pathogen reliant on chemotaxis to invade organisms such as corals. This 3 flick technique is thought to allow V. coralliilyticus to navigate the differing mediums of corals (water, mucus and tissue cells). Turbulent waters in the marine environments cause constant flux of variables affecting V. coralliilyticus. Nutrient levels change diurnally and nocturnally while oxygen saturation in coral tissues flips to extreme highs (250%) and lows (<2%). Water around the coral stays at a constant, only layers of 1mm on the surface of the coral tissue changing with light and dark conditions.

The authors investigate how V. coralliilyticus’ search patterns change relative to fluctuating oxygen gradients.

An observation chamber was set up, where the centre of the slide was highly anoxic and the edges oxic. V. coralliilyticus was shown to exhibit more flick-search patterns in the oxic conditions, and more run and reverse techniques in anoxic conditions.
The chemotaxic search pattern behaviour of this bacterium changed over the oxic-anoxic interface, identified over a microscope transect.
When entering the anoxic environment there was a significant decrease in 3 step flick behaviour, though over all no preferences were found for search patterns in oxic and anoxic conditions.
This lead to the conclusion that V. Coralliitycus can use both search patterns in both anoxic and anoxic conditions while maintaining search pattern velocities over the oxic-anoxic intersect.

The relationship between oxygen saturation and a bacterium flick or non-flick response is still unknown. The authors discuss that the 3-flick mechanism could be either triggered oxygen saturation (as seen in Escherichia coli) – or perhaps due to bacteria monitoring changes in the electron transfer rate via redox sensors through sensory pathways (the sensory system can convert environmental signals directly into rotational changes of the flagella motor). Further research should investigate the actual mechanisms behind these actions.

One criticism of this paper is the short discussion that does not segregate the results to infer meaning to the reader. Leading to statements that could be construed as misleading such as the “3-step flick”. Is present in oxygenated environments and not in deoxygenated” – though this may infer to statistical differences in data there are still trends that should not be ignored. Having said this, it is still useful paper as it is one of the first to investigate a relationship between oxygen and 3-flick search patterns.

Further research could be focused on how temperature affects the “3-step flick” due to the temperature dependence of this micro-organism. Secondly it would be interesting to repeat this experiment with more extreme oxygen saturation levels/gradients, with perhaps more sophisticated methods in place. It may also be useful to investigate V. coralliiticus response to oxygen saturation in light and dark settings, to see whether any search pattern behaviours are influenced by this factor – perhaps shedding light on whether infection of coral is nocturnal or diurnal specific.  

Winn, K. M., Bourne, D. G., & Mitchell, J. G. (2013). Vibrio coralliilyticus Search Patterns across an Oxygen Gradient. PloS one, 8(7), e67975.
Adam Battishill & Caroline White

1 comment:

  1. What strikes me is how two-dimensional our understanding of bacterial motility patterns is, yet marine bacteria live in a three-dimensional world. For example, I doubt that bacteria ever truly move in straight lines; they are probably moving in 3D spirals. Have the motility patterns described only been observed in between glass slides?; Could these patterns be a product of limited motility because of experimental design?