Vibrio coralliilyticus Antibiotic Production in Natural-type Habitats: Evidence of Competitive Advantage?

Vibrio coralliilyticus is a putative tropical-coral pathogen with genetic variation over populations in different oceanic regions.  The different physiological characteristics (e.g. pathogenicity; metabolic activity) of V. coralliilyticus subspecies/ecotypes may potentially provide a competitive advantage and facilitate exploitation of distinct niches.  Wietz et al (2011) investigated the colonisation, growth and biosynthesis of V. coralliilyticus strain S2052, isolated from sediment in the tropical Indian Ocean; S2052 had demonstrated antibacterial activity via production of andrimid, a wide spectrum antibiotic that functions via interference with fatty acid synthesis. 

Wietz et al. (2011) do not include a method section, although some method rational is discussed:  Vibrios are abundant in chitinous microenvironments (e.g. crustacean exoskeletons), and chitin is the most abundant biopolymer in marine environment; chitinolytic activity is a core function of the Vibrionaceae (thought to control several genetic and physiological characteristics) and may be a reason for their ubiquity.  Vibrios also occur on epiphytic macroalgae, and are believed to degrade algal-derived carbohydrates.  Yet investigations into bacterial physiology often use substrates that do not reflect natural conditions, resulting in a shifted phenotype and metabolic state unlike that observed in the organism’s natural environment (e.g. excess nutrients can increase synthesis of storage compounds and suppress antibiotic production).   Therefore, Wietz et al. (2011) investigated andrimid production by S2052 in chitinous and/or algal conditions; they also explored antibiosis and biosynthetic temperature optima among V. coralliilyticus strains. 

Wietz et al. (2011) reported that in the presence of chitin, V. coralliilyticus focuses on andrimid production, and cease producing most other secondary metabolites, including energy storage molecules such as polyhyrdroxybutyrates.  Examination of growth kinetics revealed distinct differences between cultures grown on laboratory media and those grown with chitin: in laboratory media, andrimid was produced after only 10 hours, with a peak production of approximately 10 pmol cell^-1; cultures grown with chitin produced andrimid much later, after 18 hours, but with a peak production approximately double that of laboratory media grown cultures, at 20 pmol cell^-1.

Wietz et al. (2011) also cultured S2052 with axenic Artemia (brine shrimp).  In this live chitin model system, cell densities reached approximately 10⁸ cells ml^-1, and andrimid was produced (confirmed by LC-MS from ethyl acetate extracts using the selective ion trace).  S2052 attachment to chitinous Artemia exuviae was demonstrated via light microscopy; the authors cite this result as revealing the bacterium’s preference for a natural chitin source, yet unfortunately what it was preferred against is not evident.  Similarly, distinct colonisation of an Artemia exoskeleton as the ‘sole source’ of organic substrate is shown (via SEM microscopy), but the identity and affect of any additional substrate is unclear.  The effect of aqueous macroalgal extracts as a sole nutrient source for S2052 were also investigated: the type and concentration of extract appeared to influence andrimid production (e.g. Laminaria saccharina extract elicited andrimid production at concentrations of 25%, 50% and 100%; Fucus vesiculolus extract elicited andrimid production at a 50% concentration only), suggesting that antibiotic biosynthesis is influenced by substrate type or concentration.

Finaly, Wietz et al (2011) compared secondary metabolite profiles, and maximum temperature optima of several different V. coralliilyticus strains.  Andrimid production was recorded in S2052 only; known antibacterial activity of strain LMG20984 was therefore attributed to a different compound.  Maximum biosynthetic capacity was recorded in S2052 at 25⁰C, 5⁰C lower than observed in strains LMG20984 and LMG10953.  The authors suggest that this reflects the temperature-dependent pathogenicity of LMG20984, and that the lower optimum temperature of S2052 makes it less pathogenic.  This appears contradictory, as a high metabolic rate at lower temperatures may make S2052 more virulent than LMG20984, at lower temperatures.

Wietz et al. (2011) suggest that the focus on andrimid production by S2052 in chitinous conditions implies antagonism by V. coralliilyticus in its natural habitat, potentially providing a selective advantage against competing bacteria in surface colonization.  Unfortunately, support for this proposition is not clear due to the sparsity and ambiguity of the results presented.  Variations in antibiosis and biosynthetic temperature optima were recorded; the authors suggest that closely related V. coralliilyticus strains respond differently to environmental variation and therefore occupy different niches: a plausible idea, which does not seem to the be directly addressed by this investigation.   The authors also suggest that bacterial taxonomy may benefit from the inclusion of secondary metabolite analysis, as very closely related strains have distinct metabolite profiles, thus potentially increasing accuracy of identification and ease of distinction.  This proposal appears to have some merit, as identification via traditional 16S rDNA sequencing is thought to be inadequate for characterization of Vibrio ecotypes.

Reviewed by Joanna Warwick and Michiel Merkx for seminar 3.

Wietz, M., Månsson, M. and Gram, L. (2011) Chitin stimulates production of the antibiotic andrimid in a Vibrio coralliilyticus strain Environmental Microbiology Reports 3, 559–564. Available at  http://doi.wiley.com/10.1111/j.1758-2229.2011.00259.x