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Acid Stability of Probiotic Microorganisms

In vitro unbuffered growth at pH 2.0 for 2 hours contact time

Individual pure culture samples of probiotic microorganisms contained in Protexin human health products were tested for stability under acidic conditions to mimic the extreme fasting pH of the human stomach.

In order to produce beneficial effects within the gastrointestinal (GI) tract, probiotic microorganisms must have the capacity to survive and metabolise in the gut. They must therefore be resistant to GI levels of acid.

Probiotic formulations also need to contain large numbers of viable organisms (highly concentrated) which, on ingestion, survive the rigorous onslaught of the mammalian upper gut in order to deliver their bacterial content to the small intestine. One of the primary barriers to the passage of bacteria is the acidity of the stomach. The pH of the stomach varies throughout the day under the influence of the buffering action which food or liquid may have on the stomach. However, the fasting pH of the human gut is around pH 3.0.

The amount of time for food to pass through the stomach also varies greatly from a few minutes to an hour or more. The food itself will have some neutralising effect on the pH of the stomach and hence a pH of 3.0 is probably the lowest to which the bacteria will be subjected. In addition the food may also have a physically protective role to play.

The conditions of our laboratory tests will therefore probably be the most vigorous conditions to which the bacteria will be subjected i.e., a pH 2.0 for 2 hours and the results obtained need to be analysed accordingly. It should also be noted that the laboratory conditions (in vitro tests) designed for these tests, whilst attempting to mimic the conditions within the stomach are, in fact, only a simplistic view of what is, in fact, a very complex situation.


When held at a pH 2.0 for a 2 hour period there is no significant loss in viability / concentration of any of the bacterial strains. The contact time is extreme – a two hour contact time period without any buffering effect of e.g. food or water, before plating out.

Total viable bacterial counts do not reduce in viability / concentration after contact with acid of pH 2.0 for 2 hours. This means that a high concentration of the probiotic microorganisms within Bio-Kult survive, which could reach the small intestine and establish themselves as part of the normal microflora. Laboratory tests are not necessarily a reflection of in vivo conditions although the experiments were designed to mimic the situation as closely as possible.


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