Changes in growth, rRNA content, and cell morphology of Listeria monocytogenes induced by CO2 up- and downshift

Cell morphology, rRNA content, and growth were examined for Listeria monocytogenes LO28 and EGD, respectively, grown in brain–heart infusion (BHI) and on slices of sausage at 10 °C in 100% CO2, 100% N2, and air. In CO2, filamentous cells were formed by both strains on sausage slices and by L. monocytogenes EGD in BHI. Filamentation was not induced by anaerobiosis only. Fluorescent in situ rRNA hybridization (FISH) of cells grown in BHI showed that the L. monocytogenes EGD filaments consisted of chains of individual slightly elongated cells. The rods formed by L. monocytogenes LO28 had the same size in air and CO2. Septation and cell division were induced in the filaments after a CO2 downshift (i.e., exposure to air). In BHI, the number of colony forming units increased rapidly when L. monocytogenes EGD grown in CO2 was exposed to air whereas the number of L. monocytogenes LO28 remained almost unchanged. On sausage slices, the number of colony forming units also increased rapidly for both strains in response to CO2 downshift. Large variations in rRNA content of individual cells were observed in the tested scenarios. The results demonstrate the risk of underestimating the number of infectious units under circumstances where filamentation may occur. Furthermore, the study illustrates the lack of residual inhibitory effect of CO2 in this type of products after opening.