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.
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| 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.
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| 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.
Sources
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)
