Although carbon dioxide (CO 2 ) is known to inhibit growth of most bacteria, very little is known about the cellular response. The food-borne pathogen Listeria monocytogenes is characterized by its ability to grow in high CO 2 concentrations at refrigeration temperatures. We examined the listerial responses of different strains to growth in air, 100% N 2 , and 100% CO 2 . The CO 2 -induced changes in membrane lipid fatty acid composition and expression of selected genes were strain dependent. The acid-tolerant L. monocytogenes LO28 responded in the same manner to CO 2 as to other anaerobic, slightly acidic environments (100% N 2 , pH 5.7). An increase in the expression of the genes encoding glutamate decarboxylase (essential for survival in strong acid) as well as an increased amount of branched-chain fatty acids in the membrane was observed in both atmospheres. In contrast, the acid-sensitive L. monocytogenes strain EGD responded differently to CO 2 and N 2 at the same pH. In a separate experiment with L. monocytogenes 412, an increased isocitrate dehydrogenase activity level was observed for cells grown in CO 2 -containing atmospheres. Together, our findings demonstrate that the CO 2 -response is a partly strain-dependent complex mechanism. The possible links between the CO 2 -dependent changes in isocitrate dehydrogenase activity, glutamate metabolism and branched fatty acid biosynthesis are discussed.Modified atmosphere packaging (MAP) of foods is used to improve shelf life and safety by inhibition of spoilage organisms and pathogens. The gases used in MAP are most often combinations of oxygen (O 2 ), nitrogen (N 2 ), and carbon dioxide (CO 2 ). In most cases, the bacteriostatic effect is obtained by a combination of decreased O 2 and increased CO 2 concentrations (12). CO 2 is colorless, odorless, noncombustible, and bacteriostatic (21). It is both water and lipid soluble (12), and it does not leave toxic residues on MAP-stored foods (19).In food preservation MAP is frequently used for fresh muscle foods and ready-to-use vegetables stored and displayed at chill temperatures (12). The solubility of CO 2 increases at low temperatures (29), and it has recently been demonstrated that bacterial growth inhibition is proportional to the concentration of CO 2 dissolved in the water phase of a food matrix (8). The solubility of CO 2 is increased at neutral pH, high water activity, high buffer capacity, and high headspace-to-sample ratio (29).Listeria monocytogenes is a food-borne pathogen that can cause the severe human infection listeriosis (2). Cases of listeriosis caused by the consumption of contaminated meat or meat products have been reported since the 1990s (13), and several national prevention programs have been initiated (15). The organism is able to grow in the presence of CO 2 at low temperatures (2). CO 2 concentrations above 70% (N 2 balance gas) are required to inhibit growth of L. monocytogenes (11,17,20,27,43) at chill temperatures (Ͻ7°C), but even at such high CO 2 concentrations, the presence of ju...

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