RESPONSE OF BRUCELLA-SUIS-1330 AND BRUCELLA-CANIS-RM6 66 TO GROWTH ATACID PH AND INDUCTION OF AN ADAPTIVE ACID TOLERANCE RESPONSE/

Acid pH is an environmental stress often encountered by Brucella durin g both the ''environmental'' and the ''pathogenic'' stages of its life . We have investigated the behaviour of B. suis biovar 1 and B. canis in acid conditions. Growth at suboptimal pH was characterized by a dra matic reduction in growth yield due to an early onset of stationary ph ase. B. suis was more resistant to low pH than B. canis, which lysed a t pH 4.6. Viable counts measured after a 4-h acid shock at pH 3.2 show ed that the relative survival of B. suis was 1,000-fold greater than t hat of B. canis. An adaptive acid tolerance response (ATR) was induced in both species by culture at pH 5.8; however, while the acid-sensiti ve B. canis had more than a 2,000-fold increase in survival following acid shock at pH 3.2, the increase in survival of B. suis was only aro und 50-fold. The kinetics of the induction of ATR were followed: for B . suis, 1-2 h (1 generation) at pH 5.8 were required to induce acid to lerence (50-fold protection), and these levels remained constant over 24 h. B. canis became relatively acid-resistant after only 30-min expo sure to pH 5.8. Levels of acid tolerence continued to increase and wer e maximal at 24 h. Stationary phase pH 7.2 cultures of either species did not exhibit acid resistance, suggesting that like Salmonella, Bruc ella does not have an rpoS-controlled stationary phase acid resistance .