Before we get started:

Every system needs a fail-safe, and ours is no exception. While designing our previous components, it occurred to us that we need a contingency plan in case our Control and Cleanse sections failed, or were overwhelmed. At this point, our patient would probably require external help, in the form of a dentist check-up.

This problem is, however, more complex than you might think. For example, many people are unaware of the extent of their tooth decay until they start to feel severe pain – at which point it is too late for preventive measures! Many people also live in areas far from dental coverage, and a visit to the dentist becomes a significant investment in time and expense.

Most importantly, for people needing Long-Term-Care, or disabled persons, who are a significant target user of HOPE (see [Project Overview] for more information), timely dental checkups become even more of a challenge because it is up to the caretaker to discover and report the decay.

For this reason, we have decided to develop a system to detect tooth decay as it is happening, and to send out a warning message to HOPE’s user.

So how did we do it?

Right now you probably know the main microbial target of our project, S. mutans. However, aside from S. mutans, there exists numerous bacteria that also play a role in tooth decay. One of them is a closely related species in the same genus: Streptococcus sobrinus.

So what makes S. sobrinus so special? For starters, it is a very good indicator for tooth decay – even more so than S. mutans, its main cause! Also, like S. mutans, it is present in almost everyone’s mouth, meaning it is safe to use and safe to insert. Lastly, a peculiar trait for S. sobrinus is that its population rises sharply right before tooth decay is about to occur. Therefore, if we inserted a modified S. sobrinus, and allowed it to manufacture a distinct signaling product, we can detect when tooth decay is about to happen by the sudden flood of that product.

But what will that product be? A fluorescent protein is very visible, but natural low levels of S. sobrinus in your mouth would mean a perpetual colored glow on your teeth, which can be embarrassing and unsightly. Something that’s more pleasant, just as effective, but not quite as conspicuous is a smell, and specifically the smell of banana, caused by isoamyl acetate.

Isoamyl acetate is produced from many possible pathways, but a convenient three-step mechanism uses L-Leucine as the raw material, and involves Saccharomyces cerevisiae, otherwise known as the Baker’s yeast or the Brewer’s yeast. In this case, we stand on the shoulders of past teams, as the [2006 MIT iGEM team] has already biobricked the three enzymes. Because of this, we simply constructed the three enzymes together into a full circuit.

Aside from that, we added two terminators in front of the main enzymes, in order to act as a threshold (Please see [Control: Target] for more information on the use of terminators as thresholds). The significance of this addition is that it prevents a constitutive expression of the banana odor, and thus makes it less detectable and annoying to users. Only when tooth decay is about to happen and the amount of S. sobrinus multiplies hugely do users start to detect the characteristic banana odor.

Putting it to the test!

Our result