Prey attraction in a generalist microbial predator, Dictyostelium discoideum
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Abstract
Generalist predators often live in environments that vary in the type and abundance of prey species. The prey species in turn vary in their susceptibility and suitability to predation. How do generalists navigate this variation in prey abundance and profitability and seek out their preferred prey? We investigated this in the soil protist Dictyostelium discoideum , a generalist predator of many species of bacteria. Despite their generalist diet, amoebas experience considerable variation in growth rate on their prey bacteria. We tested if amoebas innately prefer the more profitable prey bacteria. We found that the levels of chemoattraction of naïve amoebas towards prey bacteria increased with increasing growth rate of the amoebas on those bacteria. This suggests that D. discoideum amoebas have an innate prey preference that is adaptive. Next, we tested how experience with prey bacteria affects chemoattraction in amoebas. Given the number of bacterial species in soil, learning from experience should be advantageous. However, we found no evidence for learning; instead amoeba preference in learning experiments was partially explained by innate attractiveness of the prey bacteria. Our results suggest that generalist amoebas are innately attracted to the more profitable prey bacteria and this innate attraction cannot be overridden by recent experience.
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The bacteria for the experiments were grown on LB (Fisher Scientific) or SM/5 plates (2 g glucose (Fisher Scientific), 2 g BactoPeptone (Oxoid), 2 g yeast extract (Oxoid), 0.2 g MgSO4 (Fisher Scientific), 1.9 g KH2PO4 (Sigma-Aldrich), 1 g K2HPO4 (Fisher Scientific), 15g agar (Sigma-Aldrich)), and resuspended to a concentration of OD600 5 in KK2 buffer.
It would be helpful to include a column in Table 1 that shows the media each bacteria was grown on to see if the chemicals present in the growth media correlate with the chemotaxis observed
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However, when amoebas were familiarized to Variovorax boronicumulans (Vb), they preferred all the novel bacteria over the familiar bacterium (Vb) (Estimated marginal means = -0.18, 95% CI (-0.32, -0.04), p = 0.01).
This is very interesting! Especially given their high growth rate when fed on Vb in Fig 2! Very cool paper and really neat work! Thank you for sharing it!
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For some bacterial species where the front was hard to see, we collected amoebas from the entire plate after ∼ 36 - 40 hours. We suspended the amoebas in KK2 buffer and centrifuged them at 300g, 10 °C for 3 minutes. We washed the amoeba pellet three more times to remove any bacteria.
How did you confirm axenic cultures at this point? Did they go through filtration as well as the centrifugation?
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o account for day-specific effects (Figure S1), we scaled all amoeba movement estimated on a given day by dividing it by net amoeba movement towards Klebsiella pneumoniae (averaged across 3 replicates) on that day.
Wow! The fourth day of experiments had wildly different effects! Do you have any idea why things were so different on this day?
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We found no significant relationship between these two measures suggesting that these patterns are not driven by chemokinesis (Figure S4: MCMCglmm, Fixed effect posterior mean = 0.04, 95% CI (-0.28, 0.10), pMCMC = 0.24).
This calculation was taking all data points into account, yes? But its possible that they are performing some chemokinesis in certain environments (specifically Ar), right? Just as they seem be chemostatic in some environments (Aa, Cr, Mm, Ba, Nc, Ra). It would be interesting to the breakdown by species
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We used ImageJ version 1.53 (Schneider et al. 2012) to estimate number of amoebas that moved out of the amoeba spot towards each of the two potential attractants.
The depiction in Figure 1 is extremely helpful in understanding the assay! As a reader with no experience, it would also be helpful to see an example raw image. Reading the paper i was wondering if the ameoba crawl up or down as frequently as they crawl towards the KK2 or if the KK2 has its own chemo-attraction. Seeing the raw image could clear this up a bit
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(2 g glucose (Fisher Scientific), 2 g BactoPeptone (Oxoid), 2 g yeast extract (Oxoid), 0.2 g MgSO4 (Fisher Scientific), 1.9 g KH2PO4 (Sigma-Aldrich), 1 g K2HPO4 (Fisher Scientific), 15g agar (Sigma-Aldrich))
I'm guessing these measurements are per Liter of medium, correct?
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