- Maybe the deadline is over but I was working on this post so I posted it anyway, peace and love-
The ocean water is an heterogeneous environment where microorganisms must navigate to locate resource and conditions suitable for their growth. Chemotaxis is a fundamental mechanism that allow planktonic microorganism to aggregate around phytoplankton cell, colloidal particle, plume of organic substrates and resources. In such environment specific secondary metabolite can influence the behaviour of motile marine bacteria and a good example can be the quorum sensing for intraspecific signalling or antibiotic compounds for interspecific competition.
It’s knew that Vibrio SWAT3-wild-type (SWAT3-wt) and the human phatogen Vibrio cholerae are both particle-attached bacteria and the first one express antibiosis. SWAT3-wt can produce andrimid (inhibitor of acetyl-CoA carboxylase, an essential enzyme) that instead SWAT3-111 (a mutant) cannot produce. The andrimid potently inhibits the growth of V. cholerae but there are few informations on the effect of such molecule in much real natural condition as sub-lethal concentration.
V. cholerae is a particle attached bacterium and the authors in this work try to understand if andrimid can have a role in antagonistic interations with the other Vibrio species. They develop a chemotaxis assay combining a microchannel and a diffusion disk (on an agar surface Fig.1) to quantify the swimming behaviour, speed and turning rate. They used microvideography and cell tracking methods to monitor the swimming pattern of V. cholerae when exposed to lethal concentration of andrimid (i.e. cell stop swimming) and when exposed to sub-lethal concentration, either of the pure compound or when produced by a colony of SWAT3-wt.
To have a control reference, the authors performed two assays, one (1) with no chemical stimuli (to test the effect of the microchannel) and another (2) with a growing colony of SWAT3-111 (to be sure that other secondary metabolites don’t give effects). For both controls the swimming behaviour of V. cholerae were not related to the position in the microchannel and distance from the growing colony. They measured swimming speeds of 52.6 and 53.2 μm/s for (1) and (2) respectively. Then exposing V. cholerea in two andrimid gradients (one from pure antibiotics and one from a growing colony of SWAT3-wt) they found similar values of motility between controls and in a zone called non-lethal zone (1-1.2 cm distant from the source). In contrast, in a nearer zone to the sourse (called sub-lethal) they found significantly altered swimming behavior. V. cholerae was faster of about 25% (67.4-65.6 μm/s) in comparison to the controls. Also was defined a lethal zone where the bacteria showed a slower swimming speed (35% less than controls). Another interesting results were the turning rate and the run length (30% more) both increased in the sub-lethal zone meaning that V. cholerae run away from the danger. Were also measured swimming trajectories and the result was significantly shifted way 180° direction, so again away from the source of andrimid.
This results gave more information on species-specific competition for resources since the two Vibrio species tested here are both reported to colonize particulate organic matter in planktonic environments. The authors indeed suggest a mechanism to explain the observed decrease attachement by V. cholerae to particle previously colonized by SWAT3. So this rise some questions in my mind: how abbundant is SWAT3 in nature? May it explain also the fluctuation of V. cholerae outbreak in particular location? Such interaction can take place near nutrient rich particles likely determine the winner in the competition for space and resources and ultimately the abundance of V. cholerae in environment. Obviously more molecular study on this field could give additional data on this chemotaxis related competition. Maybe monitoring the abundance of SWAT3 in natural samples can also give some more light on the difficult manage of harmful outbreak of disease related to V. cholerae.
Graff, J. R., Forschner-Dancause, S. R., Menden-Deuer, S., Long, R. A., & Rowley, D. C. (2013). Vibrio cholerae exploits sub-lethal concentrations of a competitor-produced antibiotic to avoid toxic interactions. Frontiers in microbiology, 4.