<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
