Absence of an early detectable increase in heat-shock protein synthesis by Listeria monocytogenes within mouse mononuclear phagocytes.

We investigated the behaviour of Listeria monocytogenes during the early phase of its in vitro phagocytosis by mouse resident peritoneal macrophages, and compared behaviour and modifications in protein synthesis occurring in a virulent and a non-virulent strain of L. monocytogenes. As previously shown, these two strains have differential responses to stress and heat shock in vitro. At between 1 and 3 h of phagocytosis, there is a general decrease in protein synthesis in Listeria. Synthesis of the major DnaK and GroEL heat-shock proteins also decreases. Synthesis of only a limited set of bacterial proteins is conserved or even increased during this early phase of phagocytosis. Similar modifications in protein synthesis are also observed in bacteria which have had only transient contact with macrophages without being phagocytosed. The discrimination, by the macrophage, between non-virulent and virulent L. monocytogenes occurs less than 30 min after initiation of phagocytosis: avirulent Listeria are totally degraded inside the resident peritoneal macrophages, whereas a significant fraction of virulent Listeria remain undamaged and alive. The distinct behaviour of the two strains of Listeria thus appears to be independent of a drastic change in bacterial protein biosynthesis.