Listeria monocytogenes can grow in macrophages without the aid of proteins induced by environmental stresses.

Listeria monocytogenes is a facultative intracellular pathogen which is able to survive and grow within phagocytic cells. Some facultative intracellular bacteria have been shown to respond to the hostile environment within phagocytic cells by producing a set of stress proteins. Since L. monocytogenes has a mechanism for intracellular survival that is distinct from those of other bacteria, we studied the phenotypic response of the bacterium to phagocytosis by macrophages. After phagocytosis of L. monocytogenes EGD by J774-1 macrophage cells, the microorganism rapidly increased in numbers about 20-fold during an incubation period of 5 h. In this phase of phagocytosis, the selective induction of 32 proteins was observed by two-dimensional gel electrophoresis. The responses to the environmental stresses of heat and hydrogen peroxide were also studied, and it was found that 14 heat shock proteins and 13 oxidative stress proteins were induced. Five of the induced proteins were common to both heat and oxidative stresses. By amino acid sequencing analysis, homologs of DnaK and GroEL were confirmed among the heat shock proteins. A comparison of the autoradiograms of the two-dimensional gels revealed that none of these stress proteins were among the proteins induced by L. monocytogenes within the macrophages. This behavior is entirely different from that shown by other facultative intracellular pathogens. Stress proteins known to be induced by environmental stresses were absent in intracellularly grown L. monocytogenes in the present study. This absence could be due to the mechanism by which the microorganisms rapidly escape from this stressful environment at a very early phase of phagocytosis.