This document discusses the interaction between pathogenic bacteria and protozoa. It notes that some bacteria have evolved defenses against protozoan predators, like forming biofilms or replicating intracellularly. Protozoa are considered a major source of bacterial mortality but also a "biological gym" where pathogens can train for mammalian hosts. Several examples of bacterial-protozoan interactions are provided, and it is noted that studying these interactions provides insights into bacterial pathogenesis in humans.

1. Interaction of free living protozoa with
human pathogenic bacteria

2. • 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.

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

4. 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.

5. 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

6. 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

7. Initial Interaction between L. pneumophila and the host
cell
Replication of L. pneumophila within the host cell
Killing of the host cell

8. Attachment by bacterial ligand (pilus)
Receptor on the
cell surface
Cytoskeleton proteins
Unknown
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

9. Legionella-containing
vacuole (LCV) intracellular multiplication/
defective organelle transport
Lysosome
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

10. 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

11. 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.

12. 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.

13. 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)