Single species can enhance or inhibit their own growth via changing the pH.

<p>The curves show bacterial density over time, and the color shows the pH. (a) <i>C</i>. <i>ammoniagenes</i> increases the pH and also prefers these higher pH values, leading to a minimal viable cell density required for survival. Increasing the buffer concentration from 10 mM (−buffer) to 100 mM (+buffer) phosphate makes it more difficult for <i>C</i>. <i>ammoniagenes</i> to alkalize the environment and therefore increases the minimal viable cell density. (b) <i>P</i>. <i>veronii</i> also increases the pH yet prefers low pH values. Indeed, <i>P</i>. <i>veronii</i> populations can change the environment so drastically that it causes the population to go extinct. Adding buffer tempers the pH change and thus allows for the survival of <i>P</i>. <i>veronii</i>. An Allee effect can also be found in <i>L</i>. <i>plantarum</i> and ecological suicide in <i>S</i>. <i>marcescens</i> (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s008" target="_blank">S8 Fig</a>). Note that buffering often just slightly affects the final pH values (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s002" target="_blank">S2 Fig</a>) but saves the population by delaying the pH change (as shown in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s004" target="_blank">S4 Fig</a> and discussed in more detail in [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.ref035" target="_blank">35</a>]). Linlog scale is used for the y-axis. The data for this figure can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s022" target="_blank">S1 Data</a>. Ca, <i>Corynebacterium ammoniagenes</i>; CFU, colony-forming unit; Pv, <i>Pseudomonas veronii</i>.</p

Modifying the environment drives interactions between microbes.

<p>Four different interaction types can be found depending on how the environmental changes act on the organisms themselves and each other. (a) <i>L</i>. <i>plantarum</i> and <i>C</i>. <i>ammoniagenes</i> produce bistability. (b) <i>S</i>. <i>marcescens</i> and <i>L</i>. <i>plantarum</i> show successive growth. (c) <i>P</i>. <i>veronii</i> commits extended suicide on <i>L</i>. <i>plantarum</i>. (d) <i>S</i>. <i>marcescens</i> can stabilize <i>P</i>. <i>veronii</i> when the medium is sufficiently buffered. For a more detailed description of the different interactions cases, see the main text. The media composition and protocols are slightly different for the different cases. See <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#sec004" target="_blank">Materials and methods</a> for details. We use the words bistability, successive growth, extended suicide, and stabilization merely to characterize the interactions outcomes and not any “intentions” of the bacteria. Linlog scale is used for the y-axis. The bacteria were diluted every 24 h into fresh media with a dilution factor of 1/100x (a and b) or 1/10x (c and d). The data for this figure can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s022" target="_blank">S1 Data</a>. Ca, <i>Corynebacterium ammoniagenes</i>; CFU, colony-forming unit; Lp, <i>Lactobacillus plantarum</i>; Pv, <i>Pseudomonas veronii</i>; Sm, <i>Serratia marcescens</i>.</p

Modifying and reacting to the environmental pH can drive bacterial interactions

<div><p>Microbes usually exist in communities consisting of myriad different but interacting species. These interactions are typically mediated through environmental modifications; microbes change the environment by taking up resources and excreting metabolites, which affects the growth of both themselves and also other microbes. We show here that the way microbes modify their environment and react to it sets the interactions within single-species populations and also between different species. A very common environmental modification is a change of the environmental pH. We find experimentally that these pH changes create feedback loops that can determine the fate of bacterial populations; they can either facilitate or inhibit growth, and in extreme cases will cause extinction of the bacterial population. Understanding how single species change the pH and react to these changes allowed us to estimate their pairwise interaction outcomes. Those interactions lead to a set of generic interaction motifs—bistability, successive growth, extended suicide, and stabilization—that may be independent of which environmental parameter is modified and thus may reoccur in different microbial systems.</p></div

Bacteria modify the environment and react to it.

<p>(a) A collection of soil bacteria grown in a medium that contains urea and glucose can lower or increase the pH (initially set to pH 7, dashed line). The soil the microbes were isolated from has a buffer capacity similar to the experimental medium (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s002" target="_blank">S2 Fig</a>). Also, growing the soil bacteria in Luria-Bertani medium causes pH changes (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s002" target="_blank">S2 Fig</a>). (b) By changing the environment, bacteria influence themselves but also other microbes in the community. (c) <i>Lactobacillus plantarum</i> and <i>Pseudomonas veronii</i> prefer acidic, <i>Corynebacterium ammoniagenes</i> prefers alkaline, and <i>Serratia marcescens</i> has a slight preference towards alkaline environments. Fold growth in 24 h is shown. The bacteria were grown on buffered medium with low nutrients to minimize pH change during growth (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#sec004" target="_blank">Materials and methods</a> and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s002" target="_blank">S2 Fig</a>). (d) Starting at pH 7, <i>L</i>. <i>plantarum</i> and <i>S</i>. <i>marcescens</i> decrease and <i>C</i>. <i>ammoniagenes</i> and <i>P</i>. <i>veronii</i> increase the pH. Only little buffering, 10 g/L glucose and 8 g/L urea as substrates were used in (d). (e) Microbes can increase or decrease the pH (blue environment is alkaline, and red environment is acidic) and thus produce a more or less suitable environment for themselves. Blue bacteria prefer and/or tolerate alkaline and red acidic conditions. The soil bacteria in (a) were isolated from local soil, whereas the 4 species in (c) to (e) were obtained from a strain library (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#sec004" target="_blank">Materials and methods</a> for details). The data for this figure can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s022" target="_blank">S1 Data</a>. Ca, <i>Corynebacterium ammoniagenes</i>; Lp, <i>Lactobacillus plantarum</i>; Pv, <i>Pseudomonas veronii</i>; Sm, <i>Serratia marcescens</i>.</p