Author
 |
JARVIS, GRAEME - CORNELL UNIVERSITY |
|
Russell, James |
|
Submitted to: Current Microbiology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/7/2001 Publication Date: N/A Citation: N/A Interpretive Summary: The human gastric stomach is a barrier to many food borne pathogens, but E. coli becomes highly acid resistant if it is grown under appropriate conditions. Other workers indicated that E. coli O157:H7 was always highly acid resistant, but our work showed that a freshly isolated E. coli and E. coli O157:H7 responded to both amino acids and acetate. If the amino acid supply was high, both strains became highly acid resistant and the respons to acetate was negligible. However, if E. coli was grown in minimal media that would more closely simulate the GI tract, acid resistance was decreased 1000 to 10,000 fold and acetate stimulated survival. These results support the idea that cattle diets can be manipulated to alter the acid resistance of E. coli. If the acid resistance of E. coli can be decreased food safety could improved. Technical Abstract: After a recently isolated Escherichia coli strain (3TF4) was grown anaerobically in a basal medium containing maltose (2 mg/ml) and low concentration of yeast extract (0.25 mg/ml), only 0.01% of cells survived an acute acid shock that mimicked the human gastric stomach (pH 2.0, 1 h). Growing cultures were more sensitive to acid shock (pH 2.0, 1 h) than stationary phase cells, but this difference was more dramatic if Trypticas had been added to the growth medium (10 and 500-fold, respectively). Cell survival decreased if the duration of the acid shock was prolonged, and much less killing was observed at pH values greater than 3.0. Cultures were provided with sodium acetate (50 mM, pH 7.0) were more acid resistant than those not receiving sodium acetate, but this effect was greater if the cells had reached stationary phase (100 versus 1000-fold, respectively). Cultures that had been grown aerobically in Luria broth were already so acid resistant (pH 2.0, 1 h survivals greater than 50%) that they did not response to sodium acetate (50 mM, pH 7.0). Anaerobic cultures that were refrigerated (5C) stopped growing, but they had approximately the same degree of acid-resistance as those that were immediately subjected to aid shock (pH 2.0, 1 h). E. coli 3TF4 and O157:H7 had similar patterns of induction for extreme acid resistance, and this observation contradicts the idea that O157:H7 is always highly acid-resistant. Because bacterial competition in GI tract is intense, it is unlikely that the E. coli would ever have an abundance of amino acids, but diet-dependent changes in volatile fatty acid concentration could affect the extreme acid resistance of E. coli. |
