Team:Brasil-SP/Results/Sporulation

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Sporulation and Germination

Bacillus subtilis is a Gram-positive bacterium, considered a model organism which has been studied over the last 50 years. In this study we provided the microbiological bases to the right working of our device. In addition, we summarized the information we got so other teams can use our page as a basis to their work.

Who is it?

When we began to outline our project, we first decided to use Escherichia coli as a chassis. However, looking at Bacillus subtilis more closely we concluded it would be a much better choice to what we aimed.

More than a thousand years ago humans started working with B. subtilis. Japaneses discovered that fermented soybeans could be delicious at breakfast, and for centuries Bacillus subtilis has been used to produce natto¹. Nowadays, Bacillus subtilis is also used as a biological factory, to produce enzymes and fine biochemicals. It’s also an interesting host for heterologous protein expression. These Gram positive bacteria can produce and secrete large amounts of protein to the cell culture medium. In addition, the genre Bacillus is noticeable by its high capacity of adsorbing and uptaking exogenous DNA². This is one of the main reasons it has been extensively used in applied and fundamental research in the past decades.

As a model organism, there is available a lot of detailed data about it. Bacillus subtilis was the first Gram-positive bacteria to have its entire genome sequenced and annotated (strain 168)³. Following this project, all essential genes were identified⁴. Currently, large-scale data such as transcriptome⁵, proteome⁶, secretome⁷ and metabolome⁸ are available.

Furthermore, its use is promoted mainly due to the non-pathogenic nature of this organism. It has been awarded the GRAS (Generally Recognized as Safe) status by the US Food and Drug Administration - FDA. In the opposite, Bacillus subtilis is close enough to other clinically relevant Gram-positive pathogens to be used as an organism relevant to research new targets to antimicrobials and anti-infectives².

Bacillus subtilis can easily uptake exogenous DNA, secrete a lot of proteins in the medium, has been used through centuries and is a safe chassis. Can it be better?

Yes, it can.

We haven’t mentioned before what made us choose Bacillus subtilis as our favorite “pet”-chassis and why you should use it too: the endospores.

In a suitable growth medium, cells double in length and then divide centrally to produce two identical daughter cells. However, bacteria's life can be stressful and complex, needing to face a lot of adversities and we can use this in our favor. In this situation, like starvation, many bacteria trigger a cellular response involving many genes. In Bacillus subtilis this process entails the activity of more than 500 genes and takes approximately ten hours.⁹

The sporulation process starts with an asymmetric division. The biggest part of the cell (called mother) engulfs the smallest (called prespore). When this process is completed, the membrane which surrounds the cytoplasm of the prespore (now called the forespore) gets a very amorphous appearance, probably due to the absence of peptidoglycan layer to define the cell shape. Next, a modified form of cell wall, known as cortex, is synthesized between the forespore membranes, providing it with an oblong shape. Simultaneously, the protein spore coat begins to be deposited on the outside surface of the forespore. The lysis of a mother cell releases the mature spore. The spore is a resistant structure, conceived to resist to environmental adversities like heat, radiation and toxic chemicals, remaining dormant until exposed to mild conditions when it gets germinated. As a commercial product, it’s perfect. Storing and switching the cell on are equally easy, bringing to an open-and-shut product.¹⁰

The primary question is: exposing our cells to a stressful medium will lead them all to form spores? Certainly no. Sporulation is the ultimate bacteria's decision. And when they finally decide to do it, there are always some of them who betray the movement.

Many of other strategies are tried before sporulation. These include secretion of antibiotics and other chemical weapons to kill other microbes that are in the same niche, the activation of cellular motility by flagella to search for new sources of nutrients and the secretion of hydrolytic enzymes to remove extracellular polysaccharides and proteins. Furthermore, before sporulation starts the cells check chromosome integrity and the state of chromosomal replication, so if the process begins the bacterium is sure that it will be finished.⁹

Once situation is extreme, cells must choose between three ways: sporulation, competence or cannibalism. Most cells make the commitment to sporulation. However, the genetic material released by cell lysis isn’t wasted by the colony. Some cells opt for a different state of competence, triggered by ComK exceeding a certain threshold level. If a cell chooses this way, it can uptake exogenous DNA resulting from lysis and uses it to repair his own DNA or sometimes new genetic information. In the case of Bacillus subtilis approximately 10% of cells choose this path and after approximately twenty hours they switch back to the vegetative state. On the other hand, it has been showed that some bacteria further along sporulation to produce and secrete antibacterial factors that block sibling cells from sporulating. This makes them to lysis. Such cannibalistic cells feed on the nutrients released by the lysed cells. So, in the end, they prevent their own progression toward sporulation by satiating their feeding needs.⁹

But it's even more important to understand how spore's germination occurs. In our device, we provide nutrients to the bacteria. These nutrients act as germinants. The spores can recognize them as receptor proteins encoded by the gerA family of operons, which includes gerA, gerB and gerK. We can induce germination providing alanine (Ala) or a mixture of asparagine (Asn), glucose, fructose and potassium ions (AGFK) for example. The first one activate the alanine receptor GerA and the second, the asparagine receptor GerB. The interaction between nutrient germinants and receptors triggers the replace of spore core's huge depot of dipicolinic acid and cations by water. This effect triggers the hydrolysis of the spore's peptidoglycan cortex by one of two redundant enzymes. The completion of cortex hydrolysis and subsequent germ cell wall expansion allows spore core hydration. These results in resumption of spore metabolism and macromolecular synthesis.¹¹ Understanding this process we can choose when and how our product will start to work.

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