EVERY TIME Christopher Wilcox meets a stranger in a casual social setting and is asked what he does for a living, he replies that he is the keeper of the bacteria.

Christopher Wilcox gets a lot of funny looks from people.

Actually, Wilcox and the two varieties of bacteria he cares for are the essential characters in the complex process of making yogurt, a business that is growing by leaps and bounds. The bacteria live at the Dannon yogurt plant in Long Island City N.Y. Wilcox says he "supervises and checks the microscopes to make sure the two organisms are there and healthy." And for Wilcos, 27, "Every day is an interesting experience on the job."

There was a time when yogurt, like black olives and matinis, was considered "an acquired taste" in this country. Health food enthusiasts raved about it and made fantastic claims for its curative powers, but they were about the only people who found charm in the tangy, custard-like milk product that had been a staple for centuries on tables in Turkey, Arabia, India and Russia.

For years in the United States, only healthy food storesa nd the ethnic stores that catered to Eastern European immigrants carried it. Then the health fad hit. Now then stuff is found everywhere, from the neighborhood bodega to the supermarket.

Bill Camacho, Dannon's advertising manager, says that "figures for the etire market have quadrupled in the past 10 years." Dannon, New York's largest supplier, expects to sell 250 million containers in the city alone this year. The yogurt boom, according to Camacho, can be traced to a growing tendency of Americans to snack and eat on the run and to pay more attention to food labels because of worry about harmful agents in food. The best yogurt contains only yogurt and sometimes fruit is mixed in. While yogurt may not be the cure-all that the health food faddists first claimed it to be, it has a lot going for it. It is a soured milk with some of the butterfat removed and some milk protein added. Because it is partially fat-reduced milk and because the bacteria break down some of the milk sugar, or lactose, yogurt is easier to digest than milk.

Yogurt originated in the Mideast a long, long time ago.Nobody knows just when. It is the Turkish word for the fermented thick curdled milk of cows and goats. That milk becomes yogurt when the bacteria in it do away with 80 percent of the lactose by turning it into lactic acid.

In ancient times, milk was left to stand and ferment on its own and soured and thickened as the bacteria did their work. Sometimes the yogurt would come out all right -- sometimes it wouldn't. Then yogurt-makers discovered that they could inject a small amount of culture from a successful batch into the next batch and get nearly the same results. Today all yogurt makers use that technique.

The two organisms that Wilcox looks after -- streptococcus thermophykus, which appear as a dot under the microscope, and lactobacillus bulgaricas, which appears as a slash -- must be maintained at a 50-50 ratio. Wilcox calls the two organisms the "odd couple" of the dairy industry.

"One sets the stage for the other," he explains. The two bacteria work together to produce acids that give yogurt its distinct flavor. They reproduce every 20 minutes by breaking in half. A container of yogurt holds millions of the organisms. Wilcox says he looks for a clean sharp, subtle taste. "My own interpretation of it is a nutty almond flavor," he adds. "There also is a certain viscosity and body one looks for."

While the interplay between the bacteria is a complex biological and chemcial procedure, Wilcox maintains that "there also is an art and a mystique about them." The ratio between the organisms periodically goes from the ideal balance of 50-50 to 45-55. When that happens, it takes Wilcox a week or two to correct the situation. And meanwhile, Dannon goes right on making yogurt from other cultures that have the proper balance.

To maintain the balance, Wilcox might increase or lower the temperature on the holding tanks where the cultures live. The ideal temperature is 100 degrees. He might make changes in incubation time, change the percentage of milk solids, or make other slight modifications.

Wilcox, who was a food-science major at Cornell University, says he becomes attached to certain individual cultures that react better and thrive under his care. Once a culture has been successfully cultivated, it is then added to cow's milk, which is stored in warm rooms for a three-hour incubation period, after which the yogurt is ready for chilling and shipping to stores.

Frozen yogurt has gelatin added and is whipped so that air is incorporated, Wilcox explains, which makes it easier to work through the self-serve dispensers. A little sugar is also added, depending on the flavor, something that upsets yogurt purists. Danon first added a preserve, strawberry, to the product in 1947. Since then, other fruits have been added; but strawberry remains the most popular flavor.

The two bacteria Wilcox spends so much time with were first isolated by Ilya Metchnikoff, a Russian bacteriologist doing research at the Pasteur Institute in Paris. It was Metchnikoff, a Nobel Prize winner for medicine in 1908, who first theorized that yogurt was a prime factor in the longevity of certain Bulgarians who ate it. The theory remains just that -- a theory -- but Dannon cautiously uses it in its television commercial featuring a 89-year-old Russian Georgian who eats yogurt to make his 114-year-old mother happy.

Although it started with the Turks, it is fast becoming as American as apple pie. There is still room for growth, though. The United States per capita consumption of yogurt is only one-tenth that in Europe. CAPTION: Illustration, no caption, By Susan Davis for The Washington Post