Interaction of free living protozoa with human pathogenic bacteria

• Bacteria live in harsh environments, characterized by a constant competition for nutrients and the menace of bacterivorous predators such as protozoa and nematodes.

• In the course of their evolution, some bacteria developed sophisticated defence mechanisms, including the formation of biofilms, the capacity to avoid lysosomal killing and to replicate intracellularly within protozoa.

Source- Environmental predators as models for bacterial pathogenesis by Hubert Hilbi, Stefan S. Weber, Curdin Ragaz, Yves Nyfeler and Simon Urwyler

Why protozoa?Consumption by protozoa is considered to be a major source of

bacterial mortality in environment.

Bacterial pathogens have evolved survival mechanisms, especially the ability to subvert being killed within phagolysosomes. They are able to survive the complex and often inhospitable habitat and, ultimately, create idiosyncratic niches inside their host cells where they are able to flourish.

Thus protozoa have been termed as ‘biological gymnasia’ whereby intraprotozoan bacterial pathogens train for their encounter with the more evolved mammalian cells.

Source- From protozoa to mammalian cells: a new paradigm in the life cycle of intracellular bacterial pathogens by Omar S. Harb, Lian-Yong Gao and Yousef Abu Kwaik

Bacterial Species Protozoan Host

Legionella spp. Acanthamoeba spp.

Vibrio cholerae Acanthamoeba and Naegleria spp

Chlamydia spp. Acanthamoeba spp.

Salmonella spp. Acanthamoeba rhysodes

Pseudomonas aeruginosa Acanthamoeba spp. And Dictostelium

Francisella tularensis Tetrahymena pyriformis

Mycobacterium marinuma Dictostelium discoideum

Replication of L. pneumophila within the host cell

Killing of the host cell

Initial Interaction between L. pneumophila and the host cell

Source- Invasion of Protozoa by Legionella pneumophila and Its Role in Bacterial Ecology and Pathogenesis by Yousef Abu Kwaik, Lian-Yong Gao, Barabara J. Stone, Chandrasekar Venkatraman, and Omar S. Harb

Receptor on the cell surface

Cytoskeleton proteins

Attachment by bacterial ligand (pilus)

Unknown

Lysosome

intracellular multiplication/ defective organelle transport

Legionella-containing vacuole (LCV)

Vesicular trafficking

ER- derived replicative vacuole

Source- Environmental predators as models for bacterial pathogenesis by Hubert Hilbi, Stefan S. Weber, Curdin Ragaz, Yves Nyfeler and Simon Urwyler

Source- From protozoa to mammalian cells: a new paradigm in the life cycle of intracellular bacterial pathogens by Omar S. Harb, Lian-Yong Gao and Yousef Abu Kwaik

Role of protozoa in Legionnaires’ Disease

Following intracellular replication within protozoa, L. pneumophila exhibit a dramatic increase in resistance to harsh conditions, including high temperature and acidity, which may facilitate bacterial survival in the environment.

Intracellular L. pneumophila within protozoa are more resistant to chemical disinfection and biocides than in vitro-grown bacteria.

Protozoa have been shown to release vesicles of respirable size that contain numerous L. pneumophila organisms, the vesicles are resistant to freeze-thawing and sonication. Following their release from the protozoan host, the bacteria exhibit a dramatically enhanced ability to infect mammalian cells.

Advantages of studying the interactions between pathogenic bacteria and its protozoan host

The virulence mechanisms of pathogenic bacteria such as Legionella, Mycobacterium, Pseudomonas or Vibrio were found to be not only relevant for the interactions of the bacteria with protozoa but also with mammalian hosts including humans. Thus, non-mammalian model hosts provide valuable insight into the pathogenesis of environmental bacteria.

A better understanding of these interactions might contribute to the development of novel therapeutic compounds to combat recognized and emerging infectious agents.

Further characterization of the functions of various genes involved in intracellular infection and of their roles in alteration of endocytic trafficking of the bacteria will help microbiologists to understand the manipulations of host cell processes by a proficient intracellular pathogen.

Understanding the functions of these loci and their roles in blocking maturation of the phagosome will allow both microbiologists and cell biologists to exploit them as tools to study endocytic trafficking and vesicular fusion.

References

From protozoa to mammalian cells: a new paradigm in the life cycle of intracellular bacterial pathogens (Omar S. Harb, Lian-Yong Gao and Yousef Abu Kwaik)

Environmental predators as models for bacterial pathogenesis (Hubert Hilbi, Stefan S. Weber, Curdin Ragaz, Yves Nyfeler and Simon Urwyler)

Invasion of Protozoa by Legionella pneumophila and Its Role in Bacterial Ecology and Pathogenesis (Yousef Abu Kwaik, Lian-Yong Gao, Barbara J. Stone, Chandrasekar Venkataraman and Omar S. Harb)

Bacterial–protozoa interactions; an update on the role these phenomena play towards human illness (William J. Snelling, John E. Moore, James P. McKenna , Donna M. Lecky, James S.G. Dooley)