Tuesday, 14 May 2013

Dental Flossing - A Microbiologist's Perspective on Dental Hygiene

I hate flossing! I’d put it up there with leg waxing and eyebrow plucking on my list of hated personal hygiene chores. Recently, I decided to look into what all the fuss was about, in order to try and scare myself into flossing my teeth and gums more regularly, and maybe even building the habit into my daily routine.
Image provided by mconnors (www.morguefile.com/creative/mconnors)

The mouth is a pretty complex environment. It’s got soft parts, hard parts, moisture, fluctuating food and oxygen levels and there’s a constant movement of the contents in one direction or another. About a milliliter of saliva is produced every minute, most of which is swallowed quickly. Throughout the day, as we eat and drink, the surfaces of our mouths get covered in chewed up food, and mixed around with saliva which contains digestive enzymes. So, what’s actually going on in the mouth, from a microbiology perspective?

Firstly, let’s get to grips with the mouth itself. We all know about teeth and gums, but there’s more going on behind your lips than just that. There’s also the inside of the cheeks, the tongue and the roof of the mouth (or the palate) to think about.

All of these different surfaces represent different types of habitats for Bacteria. For example, gums are soft and fleshy with a strong blood supply. This means they’re easy for Bacteria to cling to, but there are white blood cells nearby to attack Bacteria. Teeth, on the other hand, are very hard and slippery – difficult for microbes to get hold of. The skin cells on the inside of cheeks are constantly being rubbed off as we talk and chew, so the Bacteria living there have to work hard to cling to their home. It’s hard to believe, but the surfaces of the cheeks and tongue are completely replaced every 48 hours! There are thousands of tiny nooks and crannies (or in science terms: niches) for Bacteria in the mouth to occupy, and the conditions in these niches can vary dramatically, with differences in moisture and oxygen levels, availability of nutrients and competition with neighbouring Bacteria. The tongue, for example, is covered in millions of taste buds, which give it a rough, bumpy texture – creating a huge surface area that can be covered in microbes. Like skin and lungs, the mouth is constantly being invaded by microbes; however, the mouth is a unique habitat because of the hard teeth, which don’t shed their outer layers on a regular basis.
Image provided by Alvimann (www.morguefile.com/creative/alvimann)
The mouth of a healthy person contains a huge number of Bacteria: in fact, a normal mouth contains between 200 and 500 different species of Bacteria at any one time!! It has long been known that mechanical removal of these Bacteria is the most effective method of cleaning the teeth, but once you’ve brushed your teeth, there’s an almost immediate re-colonisation of the surface. This colonisation happens in a cycle, each time you brush your teeth.

Seconds after you put your toothbrush down and wipe away the toothpaste moustache, a layer called the ‘dental pellicle’ forms over the enamel surface of your teeth. This layer contains lots of glycoproteins, which select for the colonisation of some Bacteria, and inhibit the colonisation of others. The squeaky clean feeling you get after you’ve given your teeth a really good scrub is a result of this smooth pellicle layer.

So, who arrives first? Some of the early colonisers of clean teeth are Bacteria from the Streptococcus genus, particularly Streptococcus sanguis. These are relatives of the pathogen that causes ‘strep-throat’. Other Strep’ species are found all over the different surfaces of the body. Streptococci breakdown lactose in milk, producing lactic acid, so are vital workhorses in the cheese industry. What’s interesting is that Streptococcus sanguis colonisation of the teeth is encouraged by the pellicle, but Streptococcus salivarius is inhibited (despite what the name might suggest)! These Bacteria bind by interacting with the glycoproteins in the pellicle using receptors on their surface. Some Bacteria have tail-like projections from their surfaces that can be used to swim through saliva and interact with the pellicle to stick to teeth.

The colonising Bacteria multiply to big numbers, with 250,000-630,000 Bacteria per mm2 of tooth surface within the first four hours of colonisation! Although that sounds huge, these Bacteria actually grow relatively slowly, only dividing once every 4-6 hours on average. Microbiologists in general would scoff at this and tell you about the famous E. coli which has a doubling time of about 20 minutes under laboratory conditions. The reason these dental Bacteria multiply so slowly is probably because there’s (surprisingly) very little food available to them. The textured areas of the teeth used to chew are more likely to retain food, so of course these areas have higher numbers of Bacteria than the smooth sides of teeth.

Once a layer of these pioneer species has formed and grown, then another wave of invaders arrives. Instead of sticking to the pellicle, these ‘bridging species’ bind to the pioneer species to get a foothold on your teeth. This is not a random event; the guys who were first in the queue get to be very selective about who else can join the party. For example Fusobacterium nucleatum binds directly to Streptococci and the other Bacteria already on the teeth. It’s worth pointing out, at this stage, that the conditions on the surface of teeth are very different depending on whether you’re above or below the gum line. Above the gum line there’s lots of air and saliva, below the gum line there’s much less oxygen available. This means that the Bacteria found at these different sites are different, because they’re adapted to different conditions.

Bridging species start to grow and multiple, once they’ve made direct connections to the pioneer microbes on the teeth. As the numbers of microbial cells increase the original layer is cut off from the oxygen and food. The Bacteria produce and release all kinds of chemicals: to communicate with each other, to kill their competing neighbours and to secure their place in the growing biofilm – the plaque on your teeth. This plaque contains lots of waste products from the Bacteria, including acid from the breakdown of sugars, like the lactic acid produced by Streptococci. This is plaque acid, which erodes the hard, enamel surface of the teeth. After these bridging species have made a nice thick layer of cells and cell material, the final invaders start their assault. Species like Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia band together in a complex and bind to the Bacteria in the plaque biofilm, especially below the gum line. These three are known to be oral pathogens, they cause the gums to become inflamed and to shrink away from the teeth. This exposes more of the tooth’s surface, and allows the colonisation process to happen all over again.
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So, back to flossing; as I mentioned earlier, the best way to remove plaque from the teeth is by mechanical removal –mouthwash isn’t as effective at getting those slimey bugs off compared with a good scrub using a toothbrush and toothpaste. Although the cyclical colonisation of the teeth is completely natural and healthly, we all want to avoid gum disease, and to do that we need to wipe the slate clean and force the process to start all over again, before the likes of P. gingivalis have the opportunity to bed down in the gums and cause chaos. Brushing doesn’t clean between the teeth, and the surfaces which can’t be reached by brushes add up to a surprisingly big area, containing millions of Bacteria. Instead we need to use alternative, mechanical methods to clean out the bridging and climax species from those hard-to-reach areas on a regular basis. This is why flossing (or using those cute, inter-dental brushes) is so important. We can rely on the fact that the hundreds of Bacterial species that make up dental plaque take at least 24-48 hours to settle and grow into the worst form of harmful plaque, so we’ve got that window of opportunity each day to get rid of them. Once the layer of plaque is removed, saliva contains minerals to re-harden the tooth enamel, after plaque acid has gone.

So, although it’s hateful, I’m determined to make a much better effort to floss everyday. Knowing the good it does, maybe you should too.


Human Oral Microbial Ecology and Dental Caries and Periodontal Diseases (W.F. Liljemark and C. Bloomquist, 1996)

The efficacy of interdental brushes on plaque and parameters of periodontal inflammation: a systematic review(DE Slot, CE Dörfer, GA Van der Weijden, 2008)

Defining the healthy "core microbiome" of oral microbial communities (Egija Zaura, Bart JF Keijser, Susan M Huse and Wim Crielaard, 2009)

Role of the Oral Microflora in Health (Philip D. Marsh, 200)
Dental plaque formation (Burton Rosan and Richard J Lamont, 2000)
Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia: the ‘red complex’, a prototype polybacterial pathogenic consortium in periodontitis (Stanley C. Holt, Jeffrey L. Ebersole, 2005)

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