Mycobacterium avium resists exposure to the acidic conditions of the stomach

Organisms of the Mycobacterium avium complex are common pathogens in immuno suppressed patients such as individuals with AIDS. There is evidence that i n AIDS patients, the main route for M. avium infection is the gastrointesti nal tract. The stomach is a formidable barrier to pathogens and the ability to resist exposure to pH lower than 3 has been shown to be a virulence det erminant of enteric pathogens. Incubation of three clinical isolates of M. avium under acidic pH revealed resistance of M. avium grown both to the exp onential and stationary phase at pH 2.2 for 2 h. Inhibition of protein synt hesis had no effect on the acid tolerance. When the duration of the incubat ion at pH 2.2 was extended to 24 h, bacteria grown to the stationary phase had a significantly greater tolerance to acid than exponential phase bacter ia. M. avium incubated with acid in the presence of water was significantly more resistant to pH 2.2 than M. avium in the presence of buffer. Pre-adap tation in water prior to exposure to acidic conditions was also associated with increased resistance to pH 2.2. Isoosmolarity of Hank's balanced salt solution appears to be responsible for the impaired resistance to acid betw een 2 and 24 h of incubation. These findings indicate that M. avium is natu rally tolerant to pH < 3 and that pre-adaptation under conditions similar t o the conditions where M. avium is found in the environment results in incr eased acid resistance. (C) 2000 Federation of European Microbiological Soci eties. Published by Elsevier Science B.V. All rights reserved.