Collective dynamics of <i>Escherichia coli</i> growth under near-lethal acid stress

Many neutralophilic bacteria, including <i>Escherichia coli</i>, can withstand acidic conditions due to the action of several protective mechanisms. While the survival of <i>E. coli</i> under growth-inhibitory extreme acid stress is well understood, less is known about the physiology and ecology of <i>E. coli</i> under severe but growth-permissive acidity. Here, we observed that growth of <i>E. coli</i> MG1655 in a rich medium with a pH value of 4.1-4.4 exhibits a characteristic multiphasic growth pattern, which consists of an initial exponential elongation, up to 10 times the normal cell length, in the absence of cell division, followed by growth arrest and subsequent resumption of exponential growth. The duration of the growth arrest phase was strongly dependent on the pH of the medium, but also on the initial cell density of the culture, suggesting the collective nature of this phenomenon. Cell density-dependent multiphasic growth at the near-lethal pH, including the initial increase in cell volume associated with either elongation or widening of the cell body, was also observed for all tested natural <i>E. coli</i> isolates and for <i>Salmonella enterica</i> serovar Typhimurium. A transient increase in volume apparently enables bacteria to induce acid resistance systems, primarily the lysine-dependent Cad system, to subsequently modify the pH of the medium in the density-dependent manner. Consistent with the collective recovery, even a minor fraction of acid-tolerant cells could cross-protect acid-sensitive cells, enabling them to resume growth in coculture. Thus, collective dynamics plays an important role in bacterial growth under near-lethal acid stress and has potentially important ecological consequences.IMPORTANCESince many <i>E. coli</i> and other Enterobacteriaceae isolates are gastrointestinal pathogens for humans, it is important to understand their growth under acidic conditions imposed by the host physiology in the stomach, colon, and inside macrophages. Here we show that <i>E. coli</i> growth under near-lethal acidic conditions is a collective phenomenon that critically depends on cell density and on acid-resistance systems and is favored by changes in cell morphology as a response to high acidity. This collective behavior enables cross-protection between bacteria in microbial communities, and it might have implications for pathogen proliferation in the acidic environment of the human gastrointestinal tract and possibly also for their interactions with immune cells.