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While studying bacteria that break down oil spills, a chance discovery by Mel Rosenberg changed the way we understand bad breath. Since then, the halitosis guru has invented a highly successful mouthwash, written children鈥檚 stories about halitosis and studied how different cultures tackle bad breath. During talk of teeth, tongues and tonsils, he told Richard Fisher how to keep every breath fresh.

Why spend your career sniffing bad breath?

It鈥檚 an intellectual gold mine 鈥� albeit a smelly one 鈥� and it鈥檚 a problem that almost everyone suffers from or worries about. Furthermore, you can learn almost everything you want to know about bacteria from the mouth. At least 600 different bacterial species live there. For them, it鈥檚 like living in a tropical rainforest. Most have adapted exclusively to the oral cavity. Understanding bacteria is essential for understanding life.

What causes the smell?

Bacteria feed on food debris, dead cells and mucus by clipping off sugar linkages on glycoproteins, which exposes polypeptides. Next, other types of bacteria break down these exposed polypeptides into their building blocks, which break down further into individual amino acids, and eventually foul-smelling gases. These include volatile sulphur compounds, such as hydrogen sulphide and methyl mercaptan, and nitrogen-containing gases such as skatole and indole, which smell like faeces. Other nitrogenous gases that may be involved include cadaverine and putrescine, whose odours are related to corpses and decay. Dozens of gases are probably involved.

鈥淭he gases include skatole and indole, which smell like faeces鈥�

Where are the bacteria most prolific?

The most common source of the smell is the back of the tongue. We think that a mucus secretion from your nose, called post-nasal drip, rolls down the back of the throat, where some of it sticks to the tongue for hours or even days. Bad breath can also come from poor oral hygiene and dental problems, the nasal passages, the tonsils, and hundreds of medical conditions.

Did you start your career studying bad breath?

My PhD was focused on finding out how bacteria that degrade oil after an ocean spill stick to the tiny oil droplets, which they break down to derive energy.

Later, in the early 1980s, I joined the fledgling dental faculty at Tel Aviv University and embarked upon a project with my dentist friend Ervin Weiss to find out whether oral bacteria would also stick to oils. At the time, almost nobody in academia was studying bad breath. And anyway, researchers tended to be industry based and so were not publishing their methods or data.

You discovered that bacteria鈥檚 fondness for oil could make for a lucrative breath freshener. How?

Bacteria need to stick to surfaces in the mouth and to one another, to avoid being washed down the throat to the gut where few survive. We developed the notion of 鈥渂eating adhesion with adhesion鈥�. We figured that if we could also get the bacteria to stick to oil droplets that a person could spit out, we might have a commercial product. Our first idea was an oily toothpaste, but it turned out that this had already been patented, though not successfully produced. So we turned to mouthwash.

The resulting product, called Dentyl pH, is now marketed by the US company Blistex. It is now one of the top two mouthwash brands in the UK and we plan to market it globally.

Why are we so sensitive about our breath?

The mouth is one of the exposed sexy parts of your body, and the sense of smell is probably our most intimate. Lots of people worry unduly about their breath. We call it halitophobia. Up to one-quarter of the people who consult specialists like me have little or no bad breath yet believe that they do. We鈥檙e talking about almost 2 million people in the US and about half a million in the UK. They are often disappointed, even irritated, when an expert tells them they don鈥檛 smell.

My colleagues and I did an experiment where we asked subjects to lick their wrists and score the resulting smell. We then compared their self-estimations with ratings from our odour judges. People scored themselves as nearly twice as smelly as the odour judges did. The study suggested that people have preconceptions about how bad their breath is, which dramatically affects their objectivity.

Do you encounter any other misconceptions?

Many people think that bad breath comes from the stomach, but that is extremely rare. Mouth sprays are another myth 鈥� most only mask the odours for a few minutes. Some oral health companies have taken a somewhat cynical approach: that it鈥檚 not about what works, but about the perception of what works. If a mouthwash stings or foams, don鈥檛 necessarily be convinced that this helps. The burning sensation, for example, is your own cells in distress, not the bacteria.

How do people from different cultures battle bad breath?

Brazilians chew cinnamon bark. Iraqis chew cloves. In Singapore, they chew the peel of giant guavas. In east Asia, they prefer anise seeds. Some of these products have useful antibacterial activity. I have never found a culture where bad breath is not considered a problem. People everywhere seem to worry about it.

Will there be a permanent cure?

Not in my lifetime. Our mouths will always have sufficient moisture and organic substrates to provide the conditions for a dense, smelly bacterial jungle. But even if it were possible, you wouldn鈥檛 want to nuke all the bacteria in your mouth as they keep other invaders at bay, such as yeast. A fungal infection in your mouth is much harder to deal with.

What are your top tips for fragrant breath?

First ask a trusted family member to what extent you suffer from the problem: it may not be as bad as you think. Gargle mouthwash with your tongue sticking out, because it allows the mouthwash to reach the back of your tongue. Don鈥檛 use mouthwash straight after brushing because toothpaste contains foaming soap, which takes some of the mouthwash鈥檚 effective ingredients out of commission by binding to them.

The best time is just before bed so the mouthwash is active all night. When you sleep, bacteria produce more odours because there is minimal saliva flow to wash them away. Beyond that, clean your tongue, visit the dentist twice a year, avoid coffee and alcohol, eat a good breakfast with rough foods, and remember to floss once a day.