The same molecule that keeps mighty trees standing also led to the first multicellular life forms – and can even be used to make sweet treats.
What is the most abundant large molecule on Earth? Perhaps a synthetic polymer? In fact, we are most likely to come across this molecule in the natural world – walking through a forest, for example. That’s because the molecule is cellulose – the substance produced by plants for structural support.
Earth’s plants produce at least 100 billion (1011) tonnes of cellulose each year – hundreds of times greater than the amount of plastics produced in the same time. As well as being hugely abundant, the cellulose produced by plants is extremely useful. You might be reading this article printed on a piece of cellulose-based paper, for example, and we dress in T-shirts and jeans made of cotton, another example of cellulose. Our furniture at home consists mostly of wood, which itself is mostly cellulose; some of us even live in wooden houses. Many people also use wood as an energy source to heat their homes, perhaps due to the timeless appeal of a real fire. And as a biofuel, wood is a key renewable energy resource.
Despite its giant molecular size, cellulose is a surprisingly simple molecule: it is built up solely from molecules of the sugar glucose. Sometimes several thousand glucose molecules make up a single macromolecule (giant molecule) of cellulose. Glucose itself is made by plants from carbon dioxide and sunlight, via the process of photosynthesis.
A cellulose macromolecule consists of a bundle of individual glucan chains. Each glucan chain is made up of cellobiose molecules, which consist of two linked glucose molecules (figure 1). The linear glucan chains associate with each other through hydrogen bonds, and the large number of these weak linkages gives cellulose its special properties. For example, they help cellulose to exclude water, and thus retain its structural properties even in wet conditions. In addition, they make the molecule resistant to chemical attack by acids and alkalis (Ross et al., 1991).
These useful properties mean that cellulose is found elsewhere in the living world, not just in plants. Some fungi have a cell wall made of cellulose (although in most fungi, this wall is made from chitin, another abundant macromolecule). Algae, some amoeba and even some invertebrate animals (mostly marine invertebrates known as tunicates) also produce cellulose. In sea squirts, for example, cellulose helps larvae to metamorphose into adults and is also part of the ‘tunic’, a kind of exoskeleton. In social amoeba, these single-celled life forms build up to become multicellular, mushroom-like organisms when nutrients are scarce, with cellulose used in the stalk and spores.
Perhaps surprisingly, some bacteria can also produce cellulose (Ross et al., 1991; Zogaj et al., 2001). Genome sequencing has revealed that this ability is present in a wide range of bacterial species, from the evolutionarily ancient thermophilic bacteria to those that associate with plants or colonise our gastrointestinal tract (Römling & Galperin, 2015). This last group includes two well-known gastrointestinal bugs, E. coli and salmonella (Escherichia coli, Salmonella typhimurium).
So why do bacteria need to produce cellulose? This molecule, which is so strongly associated with rigid structural properties in plants, helps bacteria to adapt to a surprisingly wide range of specialised environments. Cellulose helps bacteria that live in association with plants to attach to the plant’s surfaces – and in the case of pathogenic bacteria, to bind tightly to host cells and cause disease. Some bacteria living in saline springs (including thermophilic species and cyanobacteria) produce cellulose that seems to protect the bacterial cells from desiccation and other environmental threats such as ultraviolet light and even disinfectants.
Cyanobacteria, in fact, may well have been the origin of plants’ ability to produce cellulose. Through evolution, these bacteria became incorporated within plant cells as chloroplasts, bringing with them the genetic information needed to produce cellulose; these genes are now found in the genome of plants (Nobles et al., 2001). But how did this happen? The endosymbiotic theory suggests that, about one billion years ago, free-living photosynthetic cyanobacteria were captured by the ancestors of today’s algae, providing a tremendous advantage to the new combined organisms, which evolved and diversified to form the many photosynthetic species of plants and algae. This theory is supported by the fact that, although today’s chloroplasts have lost most of their original genes, the ancestor gene for cellulose synthase (the enzyme needed to make cellulose) has been transferred to the plant genome and still shows a striking similarity to its counterparts in modern cyanobacteria.
One of the main reasons that bacteria make cellulose is to produce ‘biofilms’ – cellulose films that form a matrix outside bacterial cells and help their multicellular communities to stick together. These bacterial communities were the earliest form of multicellular life on Earth, dating back to around 3.1 billion years ago. Today, there are many micro-organisms that show a similar multicellular lifestyle, which functions remarkably like tissue formation in higher organisms and is thought to make more efficient use of nutrients.
Cellulose films can also help bacteria to interact with higher organisms such as fungi, plants and animals. When highly pathogenic bacteria infect an organism, biofilms provide a mechanism for the bacteria to control the virulence (severity) of the infection. In chronic infections, however, when the bacteria deregulate their biofilm matrix, there is low degree of virulence, allowing the host and bacteria to continue to coexist (Pontes et al., 2015; Ahmad et al., 2016).
Bacterial cellulose has some very special features that make it different from plant cellulose, and these are only just beginning to be explored. Here are some of them: it is especially pure; it has a high surface area and water retention capacity; and it is a natural nanomaterial. Bacterial cellulose has considerable promise in terms of its economic value, and some bacterial cellulose products already being produced commercially. For example:
This ancient natural material could be well on its way to becoming a supermaterial of the future – revealing another way in which bacteria can help, rather than harm, us.