It is intended to prevent contamination with the use of a food. As a candidate food, attention is given to an acidic fluid food which comprises an antibacterial fluid food having been acidified by adding a fermented milk product as a protein source. The antibacterial function of the acidic fluid food is compared with the antibacterial functions of the fermented milk product employed as the starting material and a neutral fluid food. Although it has been already known that fermented milk such as yogurt has an antibacterial function, the above-described acidic fluid food unexpectedly shows a stronger antibacterial function than the fermented milk product employed as the starting material on gram-positive bacteria such as MRSA which are important from the viewpoint of public health.
