A Whiff of Tax­on­omy – Zy­momonas

by Elio

Zy­momonas is a Gram-neg­a­tive al­phapro­teobac­te­r­ial genus with a sin­gle known species, Zy­momonas mo­bilis that promise much but do not quite de­liver, at least not yet. It is a fac­ul­ta­tive anaer­obe. No­table is that it car­ries out ethanol fer­men­ta­tion at a level 2.5× greater than yeast. Part of the rea­son for this re­mark­able skill is that Z. mo­bilis is con­sid­er­ably more re­sis­tant to ethanol (up to 16% v/v) than yeast. How­ever (and a big 'how­ever' it is), it has lim­ited in­dus­trial use for sev­eral rea­sons: It can only use sim­ple sug­ars, such as glu­cose, fruc­tose, and su­crose, and not the com­mer­cially cheaper ones de­rived from lig­no­cel­lu­lose, such as xy­lose and ara­bi­nose. In ad­di­tion, Z. mo­bilis is highly sen­si­tive to plen­ti­ful in­hibitors such as acetic acid and phe­nols. Not sur­pris­ingly, ex­ten­sive at­tempts have been made to bio­engi­neer strains with­out these draw­backs (see, for ex­am­ple, here and here). This seems to be an on­go­ing en­deavor whose end is not yet in sight.

Fig­ure 1. Elec­tron mi­cro­graph of Zymo­monas mo­bilis cells. Source. Frontis­piece: Ethanol pro­duc­tion by Z. mob­ilis. Mod­ified from Source

More about Zy­momonas. It was first found in cer­tain al­co­holic bev­er­ages such as pulque (in Mex­ico) and palm wines (in Africa). Fur­ther­more, it can spoil beers and ciders. It makes ethanol by a dif­fer­ent path­way (Ent­ner-Doudo­roff) than yeast (Em­b­den-Mey­er­hof-Par­nas), pro­duc­ing half the ATP and thus re­sult­ing in a greater ethanol yield. Be­ing a fac­ul­ta­tive anaer­obe, it does not re­quire ex­pen­sive aer­a­tion in its fer­men­ta­tion. It gets even bet­ter. Zy­momonas is ca­pa­ble of fix­ing ni­tro­gen, thus by­pass­ing the ex­pense of hav­ing to pro­vide a ni­tro­gen source to its fer­men­ta­tions.

One more Zy­momonas dis­tinc­tive­ness is that its mem­brane con­tain hopanoids, those steroid-like mol­e­cules that may oth­er­wise be the most abun­dant nat­ural prod­ucts on earth and among the most durable ones (they are found in 3.7 bil­lion-year-old shales. See a piece here by Paula We­lander). Zy­momonas is a cham­pion hopanoid maker, sug­gest­ing that these lipids pro­tect it from ethanol tox­i­c­ity, a fact es­tab­lished by ge­netic ex­per­i­ments.

The au­thors of one re­view con­clude: "Z. mo­bilis can serve as a model for bio­fuel and bio­chem­i­cal pro­duc­tion and the knowl­edge gained from model strain stud­ies can be ex­tended to­wards the de­vel­op­ment of ad­di­tional bio­cat­a­lysts." A bac­terium well worth know­ing.