Citation: Smith, J.L., Liu, Y., Paoli, G. 2012. How does Listeria monocytogenes combat acid conditions?. Canadian Journal of Microbiology. 59:141-152. Technical Abstract: Listeria monocytogenes, a major foodborne pathogen, possesses a number of mechanisms which enable it to combat the challenges posed by acidic environments such as acidic foods and the acidity in the gastrointestinal tract. These mechanisms include the acid tolerance response, a two-component regulatory system (LisRK), the SOS response, sigma**B, changes in membrane fluidity, a proton pump (F0F1-ATPase), and enzymatic systems (glutamate decarboxylase and arginine deiminase). It is not clear if these mechanisms exert their protective effects separately or in concert but it is probable that these mechanisms overlap. Studies using mutants indicate that the glutamate decarboxylase system can protect L. monocytogenes when the organism is present in acidic juices, yogurt, salad dressing, mayonnaise, and modified CO2 atmospheres. It is probable that the glutamate pathway is a major mechanism in protecting L. monocytogenes during its passage through the acidic environment in the stomach. There is a need to study other potential acid resistance mechanisms of L. monocytogenes to determine their effectiveness in protecting the organism in acidic foods or during transit through the acid stomach.