Don't you hate it when an insect flies into your mouth? You probably engage in a fair amount of unseemly spitting; or maybe you gingerly extract it, then inspect the soggy interloper on the end of your fingertip, before unceremoniously flicking it away. You might well rinse your mouth out: after all, you don't know where it's been, do you?

We're quite sensitive about other members of the animal kingdom intruding where they shouldn't, but it might surprise you to know that there are a number of microscopic creatures that happily live out their entire lives in our oral orifice. Most of them we would classify as bacteria, but this Entry is about a creature of another sort. Scientists call it Entamoeba gingivalis; we know it better as the tooth amoeba.

Bag of Jelly

Maybe you drew a picture of an amoeba at school? Typically it's rendered as the shape of a fried egg: an amorphous, single-cellular blob with a round nucleus somewhere inside. In reality, it could well look like this, but it wouldn't stay like it for long. It's called an amoeba because it constantly changes its shape; 19th Century zoologists chose its name from the Greek word amoibe, meaning 'change'.

Effectively, it's a wobbly bag of jelly. The bag is a very clever cell membrane; it allows water molecules to pass through, but exercises strict border control over most other things. The jelly, or if you want to get technical, the cytoplasm, is a translucent goo, not unlike the consistency of wallpaper paste. Suspended within this jelly is the cell's nucleus, which contains all the genetic material – the brains of the operation if you like, insofar as a one-celled organism could have brains.

Entamoeba gingivalis is very small – approximately 40 of them would fit end-to-end on a pinhead¹ – and not only are they too small to see with the naked eye, they're largely transparent, too.

Feeling Peckish?

One thing we should be thankful for is that we don't have to experience at first hand their table manners. We'll describe an amoeba dinner for one, but bear in mind as you read, that if this were being described on a TV popular science show, then it would be accompanied by some other-worldly sucking noises (not to mention the sci-fi music soundtrack).

A bag of jelly it may be, but don't be fooled; this beast is a predator that rules its domain, mercilessly hunting down its prey of rogue food particles, discarded human cells or bacteria caught in the wrong place at the wrong time. Now this is the clever bit: the amoeba, having caught its snack, realises that, being short of a few cells, it has no knife and fork, nor indeed hands to hold cutlery. Even if it did, then it would have difficulty locating its mouth, as it doesn't have one of those either. This is where the cell membrane earns its keep. It extends itself a couple of protruding feet, known as pseudopodia², which envelop themselves around the tasty morsel, then engulf it totally, pinching off this little food-filled bubble, or vesicle, from the cell's surface, such that it floats around within the cytoplasmic jelly. Now, the area of cell membrane which now surrounds this internal vesicle is coated with all the necessary proteins to digest the meal and absorb the nutrients the amoeba needs. Any unwanted waste can be discarded by manoeuvring the vesicle back to the external membrane and opening it out once again.

Making Babies

We're not born with tooth amoebas, but catch them at some point in our lives, usually after oral contact with another person or a slobbery pet³. Once we've caught them, however, they quickly set up home and start a family.

One very neat trick that amoebas, and indeed all single cellular organisms, can do is to reproduce by just dividing themselves into two. It's nothing short of a miracle that we polycellular creatures can reproduce at all, when you consider the lengths we go to, not to mention the very exact alignment of amino acids, in order not only to select a mate, but also to buy them chocolates, and then save up for a wedding, before 'exchanging genetic information'. Then we (well, some of us) have to gestate a foetus, and deliver another polycellular creature, which we then have to financially support for 20-odd years. No, the amoeba knows which side its bread is buttered.

Meet the Family

The tooth amoeba, not surprisingly, hangs around near your teeth⁴, but it has also been found colonising the uterus⁵, where it lurks around intrauterine devices (IUDs). It also has a number of cousins which live in your large intestine:

• Entamoeba coli, not to be confused with the harmful bacterium Escherichia coli or E coli, is one of a number of harmless amoebas inhabiting your gut. Others include Iodamoeba buetschlii and Entamoeba hartmanni.

• Entamoeba histolytica is one amoeba you don't want to catch. It attacks the wall of the intestine, causing ulcers and amoebic dysentry. It also attacks the liver, where it's responsible for amoebic hepatitis. There are a few others which cause mild diarrhoea, including Endolimax nana.

• Dientamoeba fragilis is not actually an amoeba, but an amoeba-flagellate - it once had little whips with which it would propel itself around, but these evolved away at some point in its past. This one's another large-intestine-dweller which can cause all sorts of gastro-intestinal symptoms, from diarrhoea to flatulence.

It's not only amoebas; there are other protozoans which you may come across, including one very similar family, the trichomonads. These are typically pear-shaped, but they will also change their form as amoebas do. They all have one or more of those whip-like flagella.

• Pentatrichomonas (or Trichomonas) hominis tends to be around when Entamoeba histolytica is infecting your gut. It may be transmitted in a similar way – through infected food, water and insects – but T hominis is not thought to cause disease.

• Trichomonas vaginalis on the other hand is an evil beast; you might not expect anything less of a creature that lives out its entire life in your genito-urinary tract. It causes a nasty little disease of the same name, but you need to get pretty up close and personal to catch it. If you really want to read about all the disgusting symptoms, then the BBC Health website has something bordering on 'too much information'.

• Trichomonas tenax (or Trichomonas buccalis) is another oral dweller. It flagellates itself through your saliva like a cross-channel swimmer, and, like the tooth amoeba, enjoys lurking around your tooth surfaces and gum crevices.

When Amoebas Attack

Most of us have tooth amoebas, but we shouldn't worry - in small numbers they're generally placid, and indeed may be doing us a service in feasting on those unwanted food particles; at the very least, we seem to live in harmony with them. That's not to say that everything is always perfect, though. The population can multiply, and in rare situations tooth amoebas will attack gum tissue and even bone. Scientific studies have shown that they are more likely to occur in people with poor dental hygiene, and in those with tooth and gum disease. Your dentist should be able to advise on all your oral health problems, and should they find that Entamoeba gingivalis is feasting on something it shouldn't, then a course of antibiotics may be the perfect way to say 'adios amoebas'.