hydrological conditions (rapid increase of the discharge in
rivers, tidal impact in coastal zones, etc.). In some cases,
accumulation in sediments led to the death of pathogens,
especially strict aerobic species that are dependent on oxygen. However, sediments are also a favored place for survival; indeed, microorganisms find in the sediments
conditions that fully or partly protect them against solar
radiation and predators and that are rich in nutrients.
15.7.2 The Abiotic Factors
15.7.2.1 Temperature
Temperature plays a very important role, and it has often
been suggested that low temperatures favor the survival of
pathogens in reducing their energy costs (slowed metabolic
activity). However, freezing helps maintain viruses but has a
lethal effect on most bacteria and protozoa or alters their
physiological state. The optimum temperature for growth of
pathogens whose primary habitat is the digestive tract of
humans and warm-blooded animals is close to 37
C. Their
ability to grow in the natural environment is greatly reduced
or grow with the exception of some ecosystems where more
appropriate conditions may be encountered.
However, human activities can occasionally and locally
change the temperature of aquatic ecosystems. It can be due to
the release of hot waters from cooling systems of nuclear
power plants which can promote the growth of some bacterial
pathogens and also the development of thermophilic amoeba
such as Naegleria fowleri (Pelandakis and Pernin 2002;
Behets et al. 2007). In addition, men have built artificial
ecosystems where conditions favoring the development of
certain human pathogenic species are met. Artificial
hydrosystems such as hot water distribution systems, air conditioning circuits, spas, cooling towers, etc., have become
preferred habitat for Legionella (Fields et al. 2002). In several
countries in Europe and Asia, it has been revealed that the
abundance and dispersion of Legionella pneumophila follow
a seasonal dynamic with peaks during the warmer months of
the year (Wery et al. 2008; Lin et al. 2009).
15.7.2.2 Light
Natural light plays a very important role in the selfpurification of the environment. If the very bactericidal
(maximum absorption at 256 nm DNA) ultraviolet radiation
UV-C (wavelength between 100 and 290 nm) do not reach
the earth’s surface and are used only in the treatment processes for disinfection of drinking water or wastewater, UVB (wavelength between 290 and 320 nm) and UV-A (wavelength between 320 and 400 nm) radiations have a bactericidal effect on many pathogens causing direct or indirect
cellular damages (Davies-Colley et al. 1997). Cellular
damages linked to UV-B and UV-A are rather indirect
damages to DNA by photooxidation and production of
reactive oxygen species that can damage DNA. Thus, solar
radiation plays a very important role in the mechanisms of
self-purification of the natural environment. Obviously, this
effect is much more important in the shallow ecosystems
with clear water (containing low SS concentration). This
self-purification process is used in waste stabilization ponds.
15.7.2.3 Nutrients
Most heterotrophic pathogens require high concentrations
of organic nutrients to multiply; they are copiotrophic
microorganisms, while autochthonous microorganisms are
oligotrophic* organisms adapted to poor environments.
This nutritional requirement makes them less competitive
with regard to the native oligotrophic microflora of natural
environments. However, some habitats such as sediments of
fluvial and coastal aquatic systems may have favorable
conditions in terms of energy, especially at low temperature
when metabolic activity is reduced.
15.7.2.4 Salinity
For a long time, it was thought that the salinity of the
seawater was sufficient to remove most pathogens.
However, it is now demonstrated that many pathogens are
very tolerant to salinity. In fact, the salinity of seawater is
much closer to the salinity encountered in the gastrointestinal tract than to the salinity of freshwater, which often
have a hypo-osmotic effect on pathogenic microorganisms.
Some protozoa such as Cryptosporidium survive very well in
coastal waters and can contaminate shellfish like oysters that
are most often eaten without cooking. Most bacteria of
enteric origin are halotolerant and can survive in brackish
and marine waters.
15.7.3 The Biotic Factors
Various biotic factors may play a role in the demographic
behavior of microorganisms in the environment. Most of
these factors are based on the interactions within
microorganisms or between microorganisms and higher
organisms. These are essentially predation, parasitism, and
competition. Pathogenic microorganisms are subject to predation by protozoan and viral lysis as it is the case for all
autochthonous bacteria.
Predation by protozoa (nano- and microzooplankton) is
recognized as the leading cause of mortality of autochthonous bacteria in natural aquatic environments. Heterotrophic
nanoflagellates (size between 2 and 20 μm) are the main
grazers of bacteria followed by ciliates. These organisms
also consume bacteria of enteric origin released into the
water, and many studies have shown that predation by
protozoa was the first biotic factor responsible for the
decrease in the abundance of fecal bacteria in aquatic
environments. Predation is particularly important when the
15 Environmental and Human Pathogenic Microorganisms
647
rivers, tidal impact in coastal zones, etc.). In some cases,
accumulation in sediments led to the death of pathogens,
especially strict aerobic species that are dependent on oxygen. However, sediments are also a favored place for survival; indeed, microorganisms find in the sediments
conditions that fully or partly protect them against solar
radiation and predators and that are rich in nutrients.
15.7.2 The Abiotic Factors
15.7.2.1 Temperature
Temperature plays a very important role, and it has often
been suggested that low temperatures favor the survival of
pathogens in reducing their energy costs (slowed metabolic
activity). However, freezing helps maintain viruses but has a
lethal effect on most bacteria and protozoa or alters their
physiological state. The optimum temperature for growth of
pathogens whose primary habitat is the digestive tract of
humans and warm-blooded animals is close to 37
C. Their
ability to grow in the natural environment is greatly reduced
or grow with the exception of some ecosystems where more
appropriate conditions may be encountered.
However, human activities can occasionally and locally
change the temperature of aquatic ecosystems. It can be due to
the release of hot waters from cooling systems of nuclear
power plants which can promote the growth of some bacterial
pathogens and also the development of thermophilic amoeba
such as Naegleria fowleri (Pelandakis and Pernin 2002;
Behets et al. 2007). In addition, men have built artificial
ecosystems where conditions favoring the development of
certain human pathogenic species are met. Artificial
hydrosystems such as hot water distribution systems, air conditioning circuits, spas, cooling towers, etc., have become
preferred habitat for Legionella (Fields et al. 2002). In several
countries in Europe and Asia, it has been revealed that the
abundance and dispersion of Legionella pneumophila follow
a seasonal dynamic with peaks during the warmer months of
the year (Wery et al. 2008; Lin et al. 2009).
15.7.2.2 Light
Natural light plays a very important role in the selfpurification of the environment. If the very bactericidal
(maximum absorption at 256 nm DNA) ultraviolet radiation
UV-C (wavelength between 100 and 290 nm) do not reach
the earth’s surface and are used only in the treatment processes for disinfection of drinking water or wastewater, UVB (wavelength between 290 and 320 nm) and UV-A (wavelength between 320 and 400 nm) radiations have a bactericidal effect on many pathogens causing direct or indirect
cellular damages (Davies-Colley et al. 1997). Cellular
damages linked to UV-B and UV-A are rather indirect
damages to DNA by photooxidation and production of
reactive oxygen species that can damage DNA. Thus, solar
radiation plays a very important role in the mechanisms of
self-purification of the natural environment. Obviously, this
effect is much more important in the shallow ecosystems
with clear water (containing low SS concentration). This
self-purification process is used in waste stabilization ponds.
15.7.2.3 Nutrients
Most heterotrophic pathogens require high concentrations
of organic nutrients to multiply; they are copiotrophic
microorganisms, while autochthonous microorganisms are
oligotrophic* organisms adapted to poor environments.
This nutritional requirement makes them less competitive
with regard to the native oligotrophic microflora of natural
environments. However, some habitats such as sediments of
fluvial and coastal aquatic systems may have favorable
conditions in terms of energy, especially at low temperature
when metabolic activity is reduced.
15.7.2.4 Salinity
For a long time, it was thought that the salinity of the
seawater was sufficient to remove most pathogens.
However, it is now demonstrated that many pathogens are
very tolerant to salinity. In fact, the salinity of seawater is
much closer to the salinity encountered in the gastrointestinal tract than to the salinity of freshwater, which often
have a hypo-osmotic effect on pathogenic microorganisms.
Some protozoa such as Cryptosporidium survive very well in
coastal waters and can contaminate shellfish like oysters that
are most often eaten without cooking. Most bacteria of
enteric origin are halotolerant and can survive in brackish
and marine waters.
15.7.3 The Biotic Factors
Various biotic factors may play a role in the demographic
behavior of microorganisms in the environment. Most of
these factors are based on the interactions within
microorganisms or between microorganisms and higher
organisms. These are essentially predation, parasitism, and
competition. Pathogenic microorganisms are subject to predation by protozoan and viral lysis as it is the case for all
autochthonous bacteria.
Predation by protozoa (nano- and microzooplankton) is
recognized as the leading cause of mortality of autochthonous bacteria in natural aquatic environments. Heterotrophic
nanoflagellates (size between 2 and 20 μm) are the main
grazers of bacteria followed by ciliates. These organisms
also consume bacteria of enteric origin released into the
water, and many studies have shown that predation by
protozoa was the first biotic factor responsible for the
decrease in the abundance of fecal bacteria in aquatic
environments. Predation is particularly important when the
15 Environmental and Human Pathogenic Microorganisms
647