In recent years, among microorganisms, useful microorganisms which reach the intestinal tract in a viable state and exhibit effective health effects on the host have attracted attention. These useful microorganisms include certain bacteria of the genus Lactobacillus or the genus Bifidobacterium. Hitherto, there have been reported a number of health effects of bacteria of the genus Lactobacillus that include, in addition to the regulation of intestinal functions such as the improvement of constipation or diarrhea, an action to reduce the risk of breast cancer development (Patent Literature 1) and an interleukin-12 production-inducing ability (Patent Literature 2), and a number of health effects of bacteria of the genus Bifidobacterium that include a cholesterol absorption inhibitory action (Patent Literature 3) and an elastase activity inhibitory action (Patent Literature 4).
These useful microorganisms are required to reach the inside of the digestive tract in a viable state in order to effectively act in the digestive tract and to exert the health effects. However, there are a variety of growth inhibitory environments or growth inhibitors such as the temperature, the pH, oxygen, the osmotic pressure, and an acid until the useful microorganisms reach the inside of the digestive tract. Hence, it is important whether or not these microorganisms are resistant to the growth inhibitory environments or growth inhibitors upon utilizing these microorganisms. In particular, an acid is a major factor to determine whether or not these bacteria can reach the digestive tract. Hence, it can be said that acid resistance is one of the advantageous properties for useful microorganisms.
Among the useful microorganisms, in particular, bacteria of the genus Bifidobacterium are obligate anaerobes and vulnerable to oxygen, low pH and high acidity and have a number of difficulties in handling such as the proliferation at the time of manufacture or the survivability at the time of storage in a fermented food or beverage. In order to obtain the health effects of bacteria of the genus Bifidobacterium, it is considered that as many bacteria as possible are needed to reach the intestine in a viable state, and in particular, it is an important factor to increase the survivability of the bacteria in a food or beverage, namely, the arrival rate thereof to the intestine after eating or drinking. Hence, it can be said that bacteria of the genus Bifidobacterium having enhanced acid resistance and an increased arrival rate to the intestine are greatly required.
As useful microorganisms having acid resistance, Lactobacillus casei YIT9029 of the genus Lactobacillus (Patent Literature 5), Bifidobacterium breve YIT12272 of the genus Bifidobacterium (Patent Literature 6) and the like are known. These useful microorganisms having acid resistance are available as a number of commercial products in the form of various kinds of fermented milk products or viable pharmaceutical preparations. In particular, a fermented milk food or beverage has excellent palatability, and thus is easily taken or drunk continuously and suitable for administration of these useful microorganisms.
On the other hand, it is also required to weaken acid resistance of microorganisms in some cases, and thus it is significantly important to regulate acid resistance of a microorganism in a desired manner.
As an acid resistance regulating mechanism of microorganisms, pumping of protons to the outside of the bacterial cell by the ATP-dependent proton pump is mentioned. However, the ATP-dependent mechanism is presumed not to function as intracellular ATP of the microorganisms is depleted when the microorganisms are stored at a low temperature, and thus it is desirable to regulate acid resistance of microorganisms by utilizing an acid resistance regulating mechanism which functions even in a low temperature state as well.
The fadD gene is a gene that is confirmed to be present in some microorganisms and assumed to encode a long-chain-fatty-acid CoA ligase, an enzyme to convert a fatty acid to acyl-CoA.
However, the relationship between the fadD gene and acid resistance has not been known so far.