Extracellular pH is an extrinsic factor that mediates chronological aging.

<p>(A) Changes in medium pH. Medium pH of YOM36 cells grown in SD or SC as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151894#pone.0151894.g001" target="_blank">Fig 1</a> were measured at the indicated times. (B), (C), (D) Chronological lifespan and cell growth after pH adjustment. Culture media of YOM36 cells at day 3 were recovered and pH of the SC culture and SD culture were adjusted to 2.2 or 3.0 with HCl or NaCl, respectively. The pH-adjusted used medium was recombined with the original, centrifuged cell pellet and then CFUs (B), cell density (C), and medium pH (D) of the culture were examined at the indicated times. Error bars represent the means ± SD (n = 3). (A) **<i>P</i><0.01; (B) <i>P</i><0.01 (SD vs SD⇑; SC vs SC⇓); (C) <i>P</i><0.01 (SD vs SD⇑; SD vs SC⇓); (D) *<i>P</i><0.05; **<i>P</i><0.01.