Scarce Evidence of Yogurt Lactic Acid Bacteria in Human Feces after Daily Yogurt Consumption by Healthy Volunteers
Rosa del Campo, Daniel Bravo, Rafael Cantón, Patricia Ruiz-Garbajosa, Raimundo García-Albiach, Alejandra Montesi-Libois, Francisco-Javier Yuste, Victor Abraira, and Fernando Baquero
Applied and Environmental Microbiology, January 2005, p. 547-549, Vol. 71, No. 1
Following the same theme as the last post, I'm discussing this article about probiotic yogurt bacteria that has a much less rosy take on the impact of the probiotics themselves. It also is a very differently run study.
First, the study is a paragon of design. A huge number of volunteers 114, with all sorts of medical tests prior to the experiment. Pasteurized and unpasteurized yogurts done double blind, and, after a 2 week 'no yogurt' period, reversed in the same patients. A control group with no yogurt. Admittedly, there were only three samples per patient - one to start and one after each time period - but there is also a much simpler question being asked: can we detect the yogurt bacteria in the feces?
This is less important then it might sound, given that the lactobacilli are probably most important in the small intestine, and most will be digested in the large instestine should they wash out of the small intestine and die. However, in the case of a successful colonization, there would be expected to be some shedding (after all, how would lactobacilli get from person to person, otherwise?) and they are looking for 'detectable presence' rather than expecting overwhelming numbers.
They used all sorts of molecular and culture means to detect the bacteria. Most of these detection methods failed. PCR detected the bacteria not a single time, despite being able to detect them in a mixture of yogurt and feces 100,000:1 (I said this was good research, not fun research). Concentrations down to 10^3/gm feces would have been detected via culture - they were not detected either. This is 100-1000 times less than the typical E. coli concentration, and easily 1 part in 10^9 of the total population. As for DNA hybridization, the most sensitive method, it detected the DNA in less than 10 patients for each round. The pasteurized yogurt contributed 1 of the positive signals for each type of lactic acid bacteria under study, the others contributed about 7 or so - many times more, but not a good percentage of the 114 volunteers, given their daily consumption of the yogurt.
In short, if the bacteria are colonizing the patients, they are doing so in a way that does not result in stable fecal shedding at normally detectable levels. It is considered probable that the DNA hybridization was due to the DNA already present in the yogurt, not the reproduction of the bacteria.
The authors wisely do say they don't know whether the bacteria reproduce in the small intestine. However, they have made their point, that these "probiotic bacteria" are not replicating extensively. If the yogurt is beneficial, it is not because it is acting like an inoculum to the large intestine, certainly, and probably not to the small intestine either. They speculate it might be a 'prebiotic' - giving nutrients to existing bacteria.
The results would possibly be quite different in an antibiotic treated host. However, this study punches a hole or two in common hypotheses about the healthful effects of yogurt and how they inoculate the host. It was a great study, but must have been quite a bit of work for all those negative PCR products. Good job!
Rosa del Campo, Daniel Bravo, Rafael Cantón, Patricia Ruiz-Garbajosa, Raimundo García-Albiach, Alejandra Montesi-Libois, Francisco-Javier Yuste, Victor Abraira, and Fernando Baquero
Applied and Environmental Microbiology, January 2005, p. 547-549, Vol. 71, No. 1
Following the same theme as the last post, I'm discussing this article about probiotic yogurt bacteria that has a much less rosy take on the impact of the probiotics themselves. It also is a very differently run study.
First, the study is a paragon of design. A huge number of volunteers 114, with all sorts of medical tests prior to the experiment. Pasteurized and unpasteurized yogurts done double blind, and, after a 2 week 'no yogurt' period, reversed in the same patients. A control group with no yogurt. Admittedly, there were only three samples per patient - one to start and one after each time period - but there is also a much simpler question being asked: can we detect the yogurt bacteria in the feces?
This is less important then it might sound, given that the lactobacilli are probably most important in the small intestine, and most will be digested in the large instestine should they wash out of the small intestine and die. However, in the case of a successful colonization, there would be expected to be some shedding (after all, how would lactobacilli get from person to person, otherwise?) and they are looking for 'detectable presence' rather than expecting overwhelming numbers.
They used all sorts of molecular and culture means to detect the bacteria. Most of these detection methods failed. PCR detected the bacteria not a single time, despite being able to detect them in a mixture of yogurt and feces 100,000:1 (I said this was good research, not fun research). Concentrations down to 10^3/gm feces would have been detected via culture - they were not detected either. This is 100-1000 times less than the typical E. coli concentration, and easily 1 part in 10^9 of the total population. As for DNA hybridization, the most sensitive method, it detected the DNA in less than 10 patients for each round. The pasteurized yogurt contributed 1 of the positive signals for each type of lactic acid bacteria under study, the others contributed about 7 or so - many times more, but not a good percentage of the 114 volunteers, given their daily consumption of the yogurt.
In short, if the bacteria are colonizing the patients, they are doing so in a way that does not result in stable fecal shedding at normally detectable levels. It is considered probable that the DNA hybridization was due to the DNA already present in the yogurt, not the reproduction of the bacteria.
The authors wisely do say they don't know whether the bacteria reproduce in the small intestine. However, they have made their point, that these "probiotic bacteria" are not replicating extensively. If the yogurt is beneficial, it is not because it is acting like an inoculum to the large intestine, certainly, and probably not to the small intestine either. They speculate it might be a 'prebiotic' - giving nutrients to existing bacteria.
The results would possibly be quite different in an antibiotic treated host. However, this study punches a hole or two in common hypotheses about the healthful effects of yogurt and how they inoculate the host. It was a great study, but must have been quite a bit of work for all those negative PCR products. Good job!
posted by BenK at 7:28 PM
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