More than a mouthful

Bacterial populations in the mouth

There are good guys and bad guys in the mouth, and understanding how they cause or prevent disease will lead to healthier teeth and gums.

The human mouth is home to around 500 different types of bacteria. This sounds alarming but in fact most of them are probably harmless - even positively good for us. "The oral microflora is generally a good thing," says Professor Howard Jenkinson at the University of Bristol. "Without it we are more open to danger, since less harmful bacteria in the mouth stop the more harmful bacteria colonising. We see that from the effects of broad-spectrum antibiotics, which can destroy the protective components of oral microflora and lead to Candida (yeast) and other infections. It’s a balance, as in all of life."

And as in all of life, this fragile balance can tip either way. Bacteria, which form a biofilm over teeth, can invade the enamel of the tooth, initiating tooth decay. "While dental caries is not a life-threatening disease, it's still a major problem in particular groups of children, often in the developing world," says Professor Jenkinson.

"There’s also a new population of elderly people with a high incidence of caries, because people keep their teeth longer nowadays and then roots become exposed as the enamel is worn away."

Moreover, although oral bacteria escape into the bloodstream every time people brush their teeth or floss, usually without causing damage, things can sometimes go wrong. More virulent strains can colonise, for example, leading to an abscess, or even septicaemia. "Or if you have a heart lesion, oral bacteria that get into the bloodstream can promote formation of vegetation (a mixture of fibrin, platelets and bacteria) on the heart valve, leading to infective endocarditis. It is very dangerous, debilitating and can be fatal," says Professor Jenkinson.

Oral streptococci - of which more than 30 species are now recognised - make up a high proportion of the oral microflora and cause a variety of diseases. Some colonise the mucosal surfaces of the mouth. Others, including Streptococcus mutans, a major cause of tooth decay, live mainly on teeth. It is therefore important to find out which streptococci cause disease in humans - and the mechanisms by which they do so.

Toolkits

"We’re trying to find the major culprits," says Professor Jenkinson. "Bacteria have specific proteins - or tools - which direct or enable them to cause specific diseases or infections. Each Streptococcus has a slightly different toolkit, since each protein does a slightly different thing. Some help the Streptococcus adhere to the tooth, others enable them to penetrate dentin or bind to other bacteria, and others help them bind to the epithelial cells (the ‘soft’ tissue of gums and cheeks). Some streptococci have enough tools just to survive but not penetrate the teeth (dentin) and the epithelial cells (inside cheek, gums and tongue). Others have the full complement of tools in their toolbox, enabling them to penetrate human tissues. Other tools or mechanisms enable them to cause disease."

One of these ‘tools’ is a surface antigen, known as SAI/II, which is the route by which Streptococcus binds to the teeth. It does so by binding particular salivary proteins on the tooth surface, SAG (salivary agglutinin glycoprotein) and PRP (proline-rich proteins).

With Wellcome Trust support, a consortium of three laboratories at the University of Bristol, Guy’s Hospital, London, and Umea University, Sweden - headed by Professor Jenkinson, Professor Charles Kelly and Dr Nicklas Strömberg, respectively - is investigating the precise ways in which SAI/II binds to purified SAG, PRP and cultured epithelial cells. A better understanding of these mechanisms would suggest ways to counter binding and prevent infection.

Professor Jenkinson’s group added the gene of this antigen to Lactococcus, which does not normally bind to teeth. Equipped with SAI/II, however, the modified Lactococcus bound to collagen and penetrated dentin. "This was proof of the principle," explains Professor Jenkinson. "It proved that this is the protein that binds the bacteria to the tooth. Once we know this, we can try and understand more about the SAI/II family, and think about ways of blocking their function."

Professor Kelly, in collaboration with Professor Tom Lehner and Dr Julian Ma at Guy’s Hospital, came up with a way of doing that in 1998. "We synthesised a peptide, p1025, that recognises the receptors on Streptococcus mutans, and we put that on people’s teeth," says Professor Kelly. "The compound competed with the bacterial receptors for the human proteins and prevented the bacteria from binding to the teeth. It’s like having a broken-off key stuffed in a keyhole, so the real bacterial tooth adhesion key won’t fit in the lock. We are now trying to identify further bacterial binding sites and block them."

Although the sequences of five SAI/II proteins from different species of streptococci are now known, the host receptors and modes of binding are poorly understood. Using genetic and biochemical approaches, the consortium aims to map the interaction between SAI/II and the host receptors, and gain greater understanding of the molecular basis of those interactions.

"It seems the same bacterial protein (SAI/II) can be adapted to bind to different human tissues (teeth or epithelial cells) in different streptococci," says Professor Kelly. "We now want to clarify the specific differences between the binding sites of these different adhesins and the human tissue they bind to. Professor Jenkinson has shown that Streptococcus gordonii does not cause tooth decay, but it uses SAI/II to bind to epithelial cells, for example, and modulates the function of these cells, initiating other forms of disease. Streptococcus mutans, on the other hand, binds to teeth, and causes decay."

As well as variation between bacteria, it also appears that variations in human proteins - namely the SAG and PRP salivary receptors - impact on the composition of the oral biofilm. This suggests that human genetic susceptibility is one of the strongest factors determining whether oral bacteria cause disease.

"We’ve been able to look in more detail at the human proteins and what they’re binding to," says Professor Kelly, "largely thanks to Nicklas Strömberg who has exceptional methods and resources for purifying human salivary proteins. Previously the preparations of SAG and PRP that people have used have been a rather complex mix, making it difficult to draw any clear conclusions. It’s nice to have well-defined receptor molecules to look at."

The consortium hopes that understanding the enormous variety that exists in both bacterial and human proteins will eventually lead to the design of more precise and sophisticated interventions. "We want to get away from killing bacteria with broad-spectrum antibiotics," says Professor Jenkinson. "Instead we want to target individual cells and biological systems, interfere in a gentle way, tip the balance, give it a nudge. This will help nurture a healthy bacterial flora - and leave the good bacteria to keep doing their good work in our mouths."

Teeth gone by

Before ether started being used for surgical operations in the 1850s, dental caries could be a life-threatening condition because - not surprisingly - people were reluctant to have their teeth pulled out without anaesthetic. If bacteria penetrated the dentin and caused an abscess, which was inaccessible to treatment without extracting the tooth, the infection often spread through the bloodstream and the patient died from septicaemia.

Tooth decay may not always be life threatening but it certainly doesn’t look appealing - and that seems to have been the consensus for thousands of years. Dental aesthetics can be traced back to the ancient Egyptians, who would plug the hole in the gum vacated by a rotten tooth with gold, and use a wire brace to attach a false tooth made of ivory to the gold ‘root’.

More than a mouthful

Bacterial populations in the mouth

Toolkits

See also

Further reading