Bacterial growth in the cytosol: lessons from Listeria

Abstract

We suggest that the significance of studying the capacity of the cytosol to support bacterial growth is not to debate whether cytosolic pathogens have adapted to grow efficiently in this niche, but to understand the molecular barriers to the evolution of cytosolic pathogens. If a bacterial species can access the cytosol and replicate to some extent, selection for more rapid growth can occur. By contrast, if access to the cytosol does not provide at least a minimally permissive environment, it is hard to envision how cytosolic pathogens might have evolved. Thus, the ability of the host cytosol to support some bacterial growth would probably be a prerequisite for the evolution of an intracytosolic pathogen.Vazquez-Boland appears to be essentially in agreement with our conclusion that only specifically adapted bacteria can grow efficiently in the cytosol [1xThe host cytosol: front-line or home front?. O'Riordan, M. and Portnoy, D.A. Trends Microbiol. 2002; 10: 361–364Abstract | Full Text | Full Text PDF | PubMed | Scopus (31)See all References[1]. Control of the Listeria monocytogenes hpt gene by PrfA, a transcriptional regulator of L. monocytogenes virulence factors, is a good example of a specific bacterial adaptation to the intracellular environment. Nevertheless, a mutation in hpt results in only a 50% reduction in the intracellular replication rate, and bacterial numbers still increase significantly [2xHpt, a bacterial homolog of the microsomal glucose-6-phosphate translocase, mediates rapid intracellular proliferation in Listeria. Chico-Calero, I. et al. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 431–436Crossref | PubMed | Scopus (129)See all References[2]. This suggests that in the absence of such a specific adaptation, the cytosol can still support significant bacterial growth, consistent with the original observation that Bacillus subtilis could grow in the host cytosol [3xBacillus subtilis expressing a haemolysin gene from Listeria monocytogenes can grow in mammalian cells. Bielecki, J. et al. Nature. 1990; 345: 175–176Crossref | PubMed | Scopus (231)See all References[3]. Furthermore, we observe that the optimal intracellular doubling time of L. monocytogenes in J774 macrophages (approximately 45 minutes) is very similar to that of bacteria grown in rich broth (brain–heart infusion) (M. O'Riordan and D.A. Portnoy, unpublished). This does not support the idea that the host cytosol is poor in available nutrients.Goebel and colleagues used a novel approach to address this question, microinjecting bacteria into the host cytosol to determine its replication potential. They concluded that non-adapted bacterial species could only grow in the cytosolic environment if the host cell was undergoing apoptosis [4xMicroinjection and growth of bacteria in the cytosol of mammalian host cells. Goetz, M. et al. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 12221–12226Crossref | PubMed | Scopus (85)See all References[4]. This phenomenon was proffered as an explanation for the results of multiple studies in which non-adapted bacteria did grow in the cytosol of the host cell (reviewed in [1xThe host cytosol: front-line or home front?. O'Riordan, M. and Portnoy, D.A. Trends Microbiol. 2002; 10: 361–364Abstract | Full Text | Full Text PDF | PubMed | Scopus (31)See all References[1]). It is not apparent that non-adapted bacteria, such as B. subtilis, engineered to access the cytosol by the expression of listeriolysin O (LLO), would cause host cell apoptosis. In the case of L. monocytogenes, only one cell line, derived from dendritic cells, has been shown to undergo apoptosis upon infection or upon treatment with LLO [5xApoptosis of mouse dendritic cells is triggered by listeriolysin, the major virulence determinant of Listeria monocytogenes. Guzman, C.A. et al. Mol. Microbiol. 1996; 20: 119–126Crossref | PubMed | Scopus (141)See all References[5]. The epithelial and macrophage cell lines used in the microinjection study have not been shown to be susceptible to apoptosis by treatment with LLO or by L. monocytogenes infection. Indeed, macrophages do not undergo apoptotic death when infected by wild-type L. monocytogenes expressing LLO [6xThe mechanism of cell death in Listeria monocytogenes-infected murine macrophages is distinct from apoptosis. Barsig, J. and Kaufmann, S.H. Infect. Immun. 1997; 65: 4075–4081PubMedSee all References[6]. In addition, unlike wild-type Salmonella typhimurium, which normally replicate in a vacuole, an S. typhimurium sifA mutant can grow in the cytosol of epithelial cells [7xDisruption of the Salmonella-containing vacuole leads to increased replication of Salmonella enterica serovar Typhimurium in the cytosol of epithelial cells. Brumell, J.H. et al. Infect. Immun. 2002; 70: 3264–3270Crossref | PubMed | Scopus (83)See all References[7]; in this case, no LLO is present. Therefore, we maintain that non-adapted bacteria are able to grow in the intact host cytosol, and that the apparent contradictions between published studies result from differences in technical approach and interpretation, in addition to the host cell type.