The microbes of yogurt:

You've all heard of the health benefits of "Acidophilus", but what exactly is it? To explore this matter the place to start is the side of a yogurt container - which for one brand has the following organisms listed:

L. acidophilus
B. bifidus
S. thermophilus
L. bulgaricus

Why would L. acidophilus and the other organisms in yogurt promote human health?

According to Bergey's Manual of Systematic Bacteriology (Vol. 2; 1986), L. acidophilus has been "isolated from the intestinal tract of humans and animals, human mouth and vagina." It therefore seems likely that yogurt may aid in the recolonization of the gut after antibiotic treatment or in recovery from vaginal yeast infections. Basically this organism can help ward off infections by lowering the pH of mucosal surfaces and occupying space and using up nutrients that pathogens would need in order to cause infections (see also note on "probiotics" below).

More information about the scientific inquiry into the health benefits of yogurt can be obtained at danone and related sites on the web.

Of the species listed above Lactobacillus bulgaricus is no longer recognized as a species, but was combined into another species and is now called L. delbrueckii subsp. bulgaricus. This organism has only been isolated from yogurt and cheeses and therefore likely would not colonize our guts or other body parts.

S. thermophilus is so named because it can grow at temperatures as high as 52 degrees C and can survive some pasteurization processes. It also will not grow at less than 20 degrees C (room temp. is about 22 degrees C). It is found naturally in milk products of all sorts and it is closely related to S. salivarius which according to Bergey's Manual (1986) is a common inhabitant of human and animal mouths, "being associated particularly with the tongue and saliva, and in feces."

Most of the above information was verified in:

Balows, A., H.G. Truper, M. Dworkin, W. Harder and K.-H. Schleifer (eds.). 1992. The prokaryotes, vol. 2. Springer-Verlag, New York.
which can be found in the Scientific Reference Section of Norlin Library (along with Bergey's Manual of Systematic Bacteriology ).

Several species of Bifidobacterium (a high G+C, Coryneform genus) are commonly isolated from human feces and therefore are probably normal inhabitants of human intestines. B. bifidum is usually associated with infants, but Bergey's Manual (1986) doesn't mention it being found in milk products. B. bifidum is an anaerobic bacterium that ferments some amino sugars (e.g. N-acetylglucosamine) and the sugar lactulose (a disaccharide consisting of fructose and galactose) found in breast milk and therefore B. bifidum is one of the initial colonists of the intestines of human babies. According to Rasic and Kurmann (1983), B. bifidum is often added to yogurt and other cultured milk products to promote good health. These bacteria have been used to treat the side effects of antibiotic therapy and to detoxify the intestines of patients with chronic liver disease (Rasic and Kurmann, 1983).

Rasic, J. Lj. and J.A. Kurmann. 1983. Bifidobacteria and their role. Birkhauser Verlag, Basel.

Yogurt recipe or The Ecology of Yogurt Making orStreptococcus thermophilus and Lactobacillus delbruekii subsp. bulgaricus vs. assorted critters that might spoil milk in a bad (stinky and perhaps toxic) way.

1. Scald milk (= heat to simmering) and then cool to 48 deg. C. This step kills most of the organisms in the milk.

2. Inoculate with yogurt that contains active cultures

3. Incubate at 48 deg. C (= temp. optimum for S. thermophilus). This allows S. thermophilus to grow and produce lactic acid.

4. Cool when yogurt looks chunky. This encourages L. delbrueckii (temp. opt. = 37 deg. C) to grow producing more lactic acid and aromatic compounds that contribute to the yogurt flavor.

Remember that Lactobacilli are more acid tolerant than Streptococci and therefore the above steps enhance a natural successional process.

Probiotics.

Probiotics is a new word for the practice of inoculating animals with various microorganisms in order to ward off pathogenic organisms. There are many commercial preparations now available to treat farm animals. Most of these preparations contain lactobacilli and streptococci (some even contain bifidobacteria) and are added as supplements to animal feeds.
There is also a new treatment out for chickens. Basically baby chicks are sprayed with a mixture of benefical bacteria (isolated from the guts of healthy adult chickens) that prevent the later establishment of pathogenic microbes in the chickens. Please read this brief article from "Science News" for more information.

Return to Microbiology lec. 7.