You can see his problem -- who would want to breathe into the face of the world's top halitosis researcher? Fortunately, Rosenberg is well out of sniffing range. He's based in Israel, at Tel Aviv University, where he's trying to find out why some parts of the human body are so smelly, and what can be done about it.
Even if you don't have bad breath, your armpits probably smell from time to time, and so do your feet. And though you may not own up to it, everybody passes gas. Check out the average bathroom and you'll see the amount of time, effort and money that goes into eliminating bodily odors.
But our methods can be crude. Aside from washing regularly, often all we do is daub ourselves with fragrances. "Masking," or covering up bad smells with nice ones, works OK, but these days there are more sophisticated strategies to tackle odors at their source, thanks to people like Rosenberg and his fellow microbiologists.
"There are probably a few dozen compounds responsible for bad smells," says Rosenberg. "The smells are very different but they have a common origin." That source is bacteria.
The human body is teeming with microbes. By some estimates, we have 100 trillion living on us and in us. As far as the microbes are concerned, we're one gigantic buffet. Their diet ranges from the compounds in saliva and sweat to the snowstorm of dead skin cells we shed daily; from the food particles lodged in our mouths to the conveyor-belt banquet that is our digestive system.
All this bacterial feasting has the potential to create bad smells. Bacteria break down large molecules such as proteins, fats and carbohydrates into smaller ones to release energy. Where oxygen is available, the final breakdown products can be odorless carbon dioxide and water. But where oxygen is in short supply, such as in the gut or any nooks and crannies, bacteria belch out a range of organic molecules, some of which stink to high heaven.
Especially malodorous are the breakdown products of proteins. These include the nitrogen-containing molecules called amines, which tend to smell of ammonia, rotting meat and corpses: "very stinky," says Rosenberg. Proteins also contain sulphur, which can generate hydrogen sulphide, the rotten egg gas.
Bacteria can be divided into two families, known for historical reasons as Gram-positive and Gram-negative. In bad breath research, the mainstream view is that Gram-negatives are the bad guys. If these bacteria are incubated in a dish of saliva they produce a "horrendous odor," Rosenberg says. A saliva soup of Gram-positive bacteria, on the other hand, smells much nicer, so they are seen as the good guys.
Rosenberg's group, however, has turned up evidence that Gram-positives may not be so squeaky clean after all. In 2002, a Ph.D. student named Nir Sterer showed that smelly mouths tend to have high levels of the enzyme beta-galactosidase, which is made mainly by Gram-positive bacteria (Journal of Dental Research).
This was puzzling because beta-galactosidase breaks down sugars, and it is proteins that make the worst smells. Rosenberg's team has an answer. Most of the proteins in saliva are glycoproteins -- protein molecules with a sugar coating. His idea is that the more Gram-positive bacteria there are in your mouth breaking down the sugars, the more the underlying proteins are exposed to Gram-negatives.
The group is now developing a test kit for beta-galactosidase, called "Okay To Kiss." If you want to check yourself for dragon breath, you spit on the device -- preferably out of sight of the intended object of you attentions. "If it turns blue, you're not OK to kiss," Rosenberg says.
Rosenberg reckons the device will go on sale in 18 months, which would cement his position as the world's number one halitosis entrepreneur. He's already responsible for a flavor used in toothpaste, and a patent tongue scraper ("minimizes gagging"), to name just a few of his inventions.
He also turns out to be the brains behind Dentyl pH, a lurid two-tone mouthwash on sale in Europe, Australia and the U.S. (where it's marketed as Blistex). Rosenberg discovered that bacteria, with their fatty coats, are best removed by an oily solution combined with a watery one. He came up with the idea of a two-phase solution that had to be shaken before use. The rest is mouthwash history.
Can you tell if you have halitosis by huffing into your hand? Not really, Rosenberg says. That's probably why an estimated 1 in 5 of us are regular sufferers.
"People wander around with bad breath, even dentists. You go to a bad breath conference and some of the people there have bad breath," he notes.
A smell perhaps even more feared than bad breath is B.O. While the sweat glands on most of our body secrete a watery liquid, the armpits are one of the few places that also have glands with a more milky secretion, containing lipids and proteins. Whether or not these compounds have any function is disputed. To the bacteria on our skin, though, their role is clear: food. Microbes such as Staphylococcus epidermidis feast on them, releasing volatile compounds whose odors lab testers variously describe as "rancid," "cheesy" and "oniony."
PLUG YOUR SWEAT GLANDS
Since the turn of the 20th century the key weapon in the fight against B.O. has been aluminium. Antiperspirants contain various aluminum salts that, when applied to a human armpit, form compounds that physically plug sweat glands.
Most of the research into new ways to tackle B.O. is done by secretive private companies, says Karl Laden, a biochemist based in Haifa, Israel, who consults for the cosmetic industry and when it comes to B.O. has, quite literally, written the book ("Antiperspirants and Deodorants," published by CRC Press). The research tends to focus on combining aluminum salts with starches that release fragrances, or on better compounds for inhibiting bacteria. However, one new product from Procter and Gamble contains a doughnut-shaped molecule called beta-cyclodextrin, which is designed to trap malodorous molecules inside the ring.
Sweat is also the problem in the foot department. The soles of the feet have watery rather than milky sweat, but they make an awful lot of it. Even watery sweat contains chemicals that bacteria can feed on, mainly urea and glucose. Then there are all those dead skin cells floating around that bacteria find so appetizing.
The key reason feet can get so smelly is that they are marinaded in their own perspiration. Moisture builds up inside socks and shoes, neutralizing the skin's normally acidic condition and allowing bacteria to proliferate.
"You give them a lovely environment to grow in if you keep them in a damp, more neutral pH," says Michelle Cullen, a podiatrist at the University of Salford, UK.
Cullen has seen some cheesy feet in her time. When bacteria get really out of control, they start eating into the soles of the feet causing a condition known as pitted keratolysis. "That really is malodorous," she winces. That requires antibiotic treatment, but most of us can stick to Cullen's top tips for fresh-smelling feet: change shoes regularly, wear mixed cotton and polyester socks to wick sweat away, and use aluminium-based antiperspirants. "If you address the sweating, then you prevent the problem," she says.
Still, even feet with pitted keratolysis are no match for the smelliest part of the human body: the bowel. "It's a fact that bacteria make things in the gut that don't smell very nice," says Glenn Gibson, a food microbiologist at the University of Reading, UK.
Gibson reveals that his lab regularly creates models of the human colon, the lower part of the intestine where all the interesting bacterial action is. Food goes in at the top, bacteria bubble away in the middle and faces come out at the end.
This I have to see. So I invite myself over. It is sunny on the day of my visit and the Reading campus seems too nice a setting for a poo-making machine. I locate Gibson's lab in the Department of Food and Nutritional Sciences. Before he set up shop, he tells me, the air was filled with the aromas of cheese or freshly baked bread. "Now it smells of excrement," says Gibson. "The whole department complains about it."
Disappointingly, the lab smells only slightly musty on the day of my visit, as they have just one model up and running. You need several to get a good stench, says Gibson. Some days they have about 20 going at once. With that thought, I am led to the model.
As poo machines go, it's a magnificent creation. A vessel on the top shelf holds a mixture of semi-digested nutrients, which trickles down through three chambers representing the three segments of the large bowel -- the ascending colon, transverse colon and descending colon. The fifth vessel, which sits on the floor, is a jar of brown muck.
The model is for testing the effect of different ingredients on the output of the colon. Gibson's group feeds them to the model and analyses the output, including the gases. He is one of the world's leading fart scientists, and can talk at length on the subject.
"The population can be divided into two groups," he says, "smelly or inflammable." It all depends on which type of bacteria predominate in the bowel. People with sulphate-reducing bacteria are the smellies. They produce hydrogen sulphide -- more so if their diet is high in sulphurous foods such as eggs, white bread and red wine. In the inflammable camp are people with a preponderance of methane-making bacteria. Methane is odourless but it allows people to, as Gibson puts it, "light their own flatus."
There is a serious side to this research. Too much hydrogen sulphide may be linked to ulcerative colitis, a rare but nasty bowel disorder. Gibson's group is trying to develop ways to damp down sulphate-reducing bacteria, looking at various probiotics -- the gut's "good" bacteria -- and prebiotics, types of fiber that encourage good bacteria to grow. One approach is to try to switch patients over from making hydrogen sulphide to methane. As well as helping their colitis it might give them a great party trick.
Gibson's team is also investigating whether probiotics and prebiotics can boost the good bacteria in the gut to prevent invasion by the ones that cause food poisoning. He is working with Clasado, the manufacturer of a new prebiotic called B-GOS, or bifidobacteria galacto-oligosaccharide. In September, Gibson's group showed that people who took one sachet a day starting a week before they went on holiday were less prone to travelers' diarrhea. They got the trots at a rate of 23 percent, compared with 38 percent for those taking a placebo (European Journal of Clinical Nutrition).
As Gibson says, "there are not many people willing to spend their lives poking around in shit" but it is fortunate that some are -- and that they are also prepared to delve into armpits, mouths, feet and other smelly places. Despite the humanitarian benefits of their work, though, these scientists still face difficulties at parties.
"My wife always says tell them you do something different," Gibson says. "It's a funny area to be in."