Bacterial acid tolerance is a complex subject. Tolerance is often growth phase dependent, and mutations which increase or decrease tolerance occur with ease in some species. Therefore rigidly specifying a pH which defines tolerance is probably not possible. The attached paper may be helpful.

Dear Aparna,

First, bacteria that are able to colonise the colon must travel thru a GI system whose pH may be as low as 2. Bacteria that do not readily colonise the colon do not survive this journey. Secondly, for most bacteria, a huge change in the pH of the medium (mostly toward the acid side) has little effect on the Proton Motive Force (PMF) (change about 10% from that at neutral pH) unless the bacterium is challenged with a noxious agent. However, for many colon colonizing bacteria, the PMF at pH of 5 or less is not important for efflux pump activity. At pH above that used by ATP synthase for hydrolysis of ATP (pH above 6.5) all PMF transport processes are dependent upon metabolic energy. ATP synthase is therefore the key enzyme. The inability of your bacteria (do not know its species/strain) is obviously one that does not colonise the colon. Is it an environmental species? in any event I suggest you read the attached ms for more information.

Here is the suggested reference

Chapter Structure, Genetic Regulation, Physiology and Function of th...

Dear Aparna,

Acid tolerance is a property which helps the bacterium to survive acid shock. In case of acid tolerance, the bacteria when present in mild acidic conditions say pH 4.5-5.5 it develops tolerance towards highly acidic conditions which can be below pH 3. For example the canned food items or various juices are stored under a mild acid concentration if those items are contaminated with some bacterial agents say salmonella so the bacteria develops a tolerance property against pH as it is present in mild acid condition for a long time. Hence, when this bacteria goes to the stomach due to consumption of the contaminated food product it will be able to survive the effect of acidic conditions and may cause infection.