Microbial Highlight: Zymomonas mobilis adding biofuel to fire

Today, Kenny Flynn and Steffen Poltak are introducing Microbial Highlights, a new FTDM series diving into the deep chasm of currently known microbial diversity. Largely due to difficulties with culturing, many microbial species remain undiscovered, but progress is being made! Recent discoveries of never-before-seen ultra-small bacteria and an unusual strain in NASA’s clean room facilities are opening entirely new venues for microbial discovery and research.

Zymomonas mobilis looking to the future of biofuel by Steffen Poltak. Morphologically accurate (minus the eyeball), Zymomonas mobilis sports polar flagella, one on each end, for movement. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Ruining apple cider since Ancient Rome

The consumption of cider, a fermented alcoholic beverage made from apple juice, is on rise in the United States; American breweries generated 32 million gallons in 2013, triple the production seen in 2011 according to Beer Institute.

Despite this recent resurgence in popularity, this alcoholic beverage has been a popular choice since the Romans perfected the technique of apple juice fermentation as early as 55 BC.

One major innovation the Romans made to the process was to start burning sulfur candles inside fermentation vessels before use. They found this added step actually helped to keep resulting cider fresh and prevented spoilage. How do brewers know if a batch of cider has spoiled? The odor is an obvious sign; spoiled cider gains a strong vinegar odor that is less than desirable.

The spoilage of wine, cider and other fermented products is actually caused by contaminating bacteria and yeast strains. The resulting vinegar smell is actually caused by the build-up of acetic acid, a common by-product of these contaminating culprits not produced by the desirable wine and beer grade yeast. By burning sulfur candles inside their fermentation vessels, the Romans were actually sterilizing, or killing the pesky microbes that residing on, their equipment by exposing them to sulfur dioxide (SO2) gas. They did this all without any knowledge of microbes or the antimicrobial properties of SO2, which is still used by brewers today.

However, preliminary sterilization did not solve everything for brewers. Ciders, especially sweet varieties with low acidity, remained susceptible to something referred to as “cider sickness.” This is a special kind of spoilage accompanied by an unusual smell of lemon and bananas, a ‘thinning’ of the hardy texture through the loss of tannins, a naturally bitter compound found in a variety of plants including apples, and the appearance of a milky haze.

The hostile environment of fermented alcoholic beverages

Luckily for brewers, contaminating bacteria have an especially difficult time surviving the fermentation process. While brewer’s yeast convert apple sugars into ethanol (alcohol), they consume necessary oxygen, reduce the pH and increase ethanol concentrations beyond the limits most bacteria can tolerate. Given these challenges, most bacteria spoil a batch during the early stages before the yeast can create such an environment.

Isolated from spoiled cider in 1912, Zymomonas mobilis is a unique bacterium because it behaves very similarly to brewer’s yeast: without oxygen it is capable of converting simple sugars into ethanol, tolerating concentrations as high as 8% (most ciders only go as high 8.5%) thanks to hopanoids present in its cell membrane. To make matters worse, Z. mobilis is also resistant to SO2, explaining why “cider sickness” has remained a problem since ancient Roman times.

The fermentation process of Zymomonas mobilis differs from yeast with the production of acetaldehyde. While responsible for the unusual odor by itself, this compound also binds to the tannin released by from apples, altering the desirable taste, body and appearance cider consumers enjoy.

Cider fermentation performed by a brewer's yeast and contaminating Zymomonas mobilis is remarkably similar. Apple sugar still becomes ethanol resulting in an alcoholic beverage but production of acetaldehyde by Zymomonas mobilis has unfortunate side effects on a cider's taste, texture and appearance. Created by Kenneth Flynn and Gabrielle Dowell and licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

An alternative microbe for ethanol production

Jumping ahead to today, researchers are now trying to harness Z. mobilis’ unusual characteristics to produce biofuel (in this application bad taste is luckily not a variable). Bacteria are good candidates for mass production of desirable compounds like biofuel. They are hardy, easy to work with, and they grow quickly. Despite these advantages, genetic engineers typically need to introduce entire metabolic pathways to enable bacteria to convert sugar into ethanol. Similar to how genetically engineering pigs to fly is laughably difficult; making such drastic changes to an organism’s abilities is nearly impossible.

Combined with recent advances in genetic engineering and Z. mobilis’ natural ability to generate ethanol, researchers are already making big strides. With rates of sugar consumption and ethanol yield 2.5 times higher than current yeast options, “cider sickness” may just be the solution Biotech has been seeking.

Steffen Poltak, Ph.D. is bringing attention to the microbial world by combining his training as a microbiologist and talent as an artist.

Want more? Stay hungry for more Microbial Highlights and check out the rest of his breathtaking work, including the non-microbial, on his website: