Cell aggregation and aerobic respiration facilitate survival of Zymomonas mobilis ZM4 in an aerobic minimal medium

Zymomonas mobilis produces ethanol from glucose near the theoretical maximum yield, making it a potential alternative to yeast for industrial ethanol production. A potentially useful industrial feature is the ability to form multicellular aggregates called flocs, which can settle quickly and exhibit higher resistance to harmful chemicals. While spontaneous floc-forming Z. mobilis mutants have been described, little is known about the natural conditions that induce Z. mobilis floc formation and the genetic factors involved. Here we found that wild-type Z. mobilis forms flocs in response to aerobic growth conditions but only in a minimal medium. We identified a cellulose synthase gene cluster and a single diguanylate cyclase that are essential for both floc formation and survival in an aerobic minimal medium. We also found that NADH dehydrogenase 2, a key component of the aerobic respiratory chain, is important for survival in an aerobic minimal medium, providing a physiological role for this enzyme which has previously been found to be disadvantageous in aerobic rich media. Supplementation of the minimal medium with vitamins also promoted survival but did not inhibit floc formation.