15.7.4.1 Special Case of Global Climate Change
Today, the seasonal variations in climate that affect the
survival or spatiotemporal dynamics of pathogens are combined to the phenomenon of global climate change. The
Intergovernmental Panel on Climate Change (IPCC http://
www.ipcc.ch/) gathered data sets from 1850 to the present
concerning changes in air temperature across the world and
has demonstrated in his 2007 report a significant increase of
0.76
C (+/À0.19
C) of the earth’s temperature, with an
acceleration of the increase since the 1980s (Delecluse
2008). This increase appears to induce a series of chain
reactions that may change humidity rates, precipitation
levels, exposure to sunlight, etc. The consequences of
climate change can be multiple, including increased sea
levels, a reduction in snowy areas, an increase in forest
fires, but also changes in the balance between living
organisms and changes in the distribution of some species.
Climate change could induce many species extinctions, and
today we can observe significant effects on amphibians.
These changes will also affect pathogens and vectors at the
origin or source of transmission to humans.
There are only few solid demonstrations of an effect of
climate change on infectious diseases. However, from 460 to
377 BC, Hippocrates described the climate as a determinant
of many diseases. The most frequently cited case of impact
that these changes could have on human diseases concerns
the transmission of pathogens via mosquitoes and cholera.
For example, epidemiological investigations on cases of
encephalitis caused by a flavivirus transmitted by ticks suggest a correlation between changes in spatial distribution of
these vectors, global warming for some Nordic countries
such as Sweden, and the impact of this disease (Lindgren
and Gustafson 2001). As another example, monitoring
programs of cholera in Bangladesh suggest a replacement
of the traditional strain by Vibrio cholerae by strain El Tor.
This replacement would be due to a change in the weather
by changing rainfall during the monsoon and, therefore,
affecting salinity (Koelle et al. 2005). El Tor strain would
be more competitive with low salinity, thereby increasing
the exposure of local populations to this strain.
In addition, it should be noted that the cumulative data
on the impact of cyclical climate phenomena such as ENSO
(El Nin ˜o Southern Oscillation – abnormally high ocean
temperatures in the eastern part of the South Pacific Ocean
causing heavy rainfall) on infectious diseases suggest highly
significant effects of global climate change on the incidence
of infectious diseases. To illustrate, a positive correlation
between ENSO and the annual incidence of visceral leishmaniasis has been observed in Brazil (Ready 2008). In addition, the
cholera epidemic affecting South America in 1991 would also
be linked to this phenomenon (Salazar-Lindo et al. 2008).
Global changes can thus significantly alter the ecology of
infectious agents, and current predictions likely underestimate their consequences.
15.8 Viable but Non-culturable (VNC)
Bacteria
In 1982, Xu and coworkers described viable bacteria in a
state of survival or dormancy making them non-culturable
by classical culture-based methods. These non-culturable
bacterial forms were obtained by exposing Escherichia coli
and Vibrio cholera cells resuspended in sterile seawater, at a
low temperature without nutrient supplements. Numbering
these viable but non-culturable bacteria by direct epifluorescence microscopy as those which are able to elongate in
the presence of yeast extract and nalidixic acid (Kogure et al.
1979) showed counts significantly higher than those obtained
from plate count agar or most probable number approaches.
These bacteria were able to persist for several days or weeks
in seawater while they were unable to form colonies on agar
plates (Box 15.4). This finding sparked a major interest in
the international community. Indeed, the health significance
of these viable but non-culturable (VNC) forms became of
concern, especially in the case of pathogenic bacteria that
could have been maintained in a viable state without being
detectable so far by the classical methodologies implemented
in health risk assessment.
Box 15.4: The Viable but Non-culturable (VNC) State
in Bacteria
Philippe Lebaron
The proportion of culturable bacteria in the
environment is usually very low and often less than
1 %. This percentage is often expressed in reference
to the total number of bacterial cells that can
be enumerated on the basis of direct quantification by
epifluorescence microscopy or flow cytometry
(cf. Chap. 17), after labeling of nucleic acids by a
fluorescent probe (Box Fig. 15.6). Three main reasons
can explain this “non-culturable state”:
– Many bacteria have specific needs, which are
frequently not satisfied by the formulation of
the available commercial culture media, to complete their metabolism and as a consequence to
grow. Moreover, in the environment, bacteria live
in multispecies communities including other
bacteria, macroorganisms, and plants and are often
organized in complex structure such as biofilms.
This multispecies organization allows many
interactions between the different members of
the community that are, sometimes, required for
growth of a specific member.
(continued)
15 Environmental and Human Pathogenic Microorganisms
649
Today, the seasonal variations in climate that affect the
survival or spatiotemporal dynamics of pathogens are combined to the phenomenon of global climate change. The
Intergovernmental Panel on Climate Change (IPCC http://
www.ipcc.ch/) gathered data sets from 1850 to the present
concerning changes in air temperature across the world and
has demonstrated in his 2007 report a significant increase of
0.76
C (+/À0.19
C) of the earth’s temperature, with an
acceleration of the increase since the 1980s (Delecluse
2008). This increase appears to induce a series of chain
reactions that may change humidity rates, precipitation
levels, exposure to sunlight, etc. The consequences of
climate change can be multiple, including increased sea
levels, a reduction in snowy areas, an increase in forest
fires, but also changes in the balance between living
organisms and changes in the distribution of some species.
Climate change could induce many species extinctions, and
today we can observe significant effects on amphibians.
These changes will also affect pathogens and vectors at the
origin or source of transmission to humans.
There are only few solid demonstrations of an effect of
climate change on infectious diseases. However, from 460 to
377 BC, Hippocrates described the climate as a determinant
of many diseases. The most frequently cited case of impact
that these changes could have on human diseases concerns
the transmission of pathogens via mosquitoes and cholera.
For example, epidemiological investigations on cases of
encephalitis caused by a flavivirus transmitted by ticks suggest a correlation between changes in spatial distribution of
these vectors, global warming for some Nordic countries
such as Sweden, and the impact of this disease (Lindgren
and Gustafson 2001). As another example, monitoring
programs of cholera in Bangladesh suggest a replacement
of the traditional strain by Vibrio cholerae by strain El Tor.
This replacement would be due to a change in the weather
by changing rainfall during the monsoon and, therefore,
affecting salinity (Koelle et al. 2005). El Tor strain would
be more competitive with low salinity, thereby increasing
the exposure of local populations to this strain.
In addition, it should be noted that the cumulative data
on the impact of cyclical climate phenomena such as ENSO
(El Nin ˜o Southern Oscillation – abnormally high ocean
temperatures in the eastern part of the South Pacific Ocean
causing heavy rainfall) on infectious diseases suggest highly
significant effects of global climate change on the incidence
of infectious diseases. To illustrate, a positive correlation
between ENSO and the annual incidence of visceral leishmaniasis has been observed in Brazil (Ready 2008). In addition, the
cholera epidemic affecting South America in 1991 would also
be linked to this phenomenon (Salazar-Lindo et al. 2008).
Global changes can thus significantly alter the ecology of
infectious agents, and current predictions likely underestimate their consequences.
15.8 Viable but Non-culturable (VNC)
Bacteria
In 1982, Xu and coworkers described viable bacteria in a
state of survival or dormancy making them non-culturable
by classical culture-based methods. These non-culturable
bacterial forms were obtained by exposing Escherichia coli
and Vibrio cholera cells resuspended in sterile seawater, at a
low temperature without nutrient supplements. Numbering
these viable but non-culturable bacteria by direct epifluorescence microscopy as those which are able to elongate in
the presence of yeast extract and nalidixic acid (Kogure et al.
1979) showed counts significantly higher than those obtained
from plate count agar or most probable number approaches.
These bacteria were able to persist for several days or weeks
in seawater while they were unable to form colonies on agar
plates (Box 15.4). This finding sparked a major interest in
the international community. Indeed, the health significance
of these viable but non-culturable (VNC) forms became of
concern, especially in the case of pathogenic bacteria that
could have been maintained in a viable state without being
detectable so far by the classical methodologies implemented
in health risk assessment.
Box 15.4: The Viable but Non-culturable (VNC) State
in Bacteria
Philippe Lebaron
The proportion of culturable bacteria in the
environment is usually very low and often less than
1 %. This percentage is often expressed in reference
to the total number of bacterial cells that can
be enumerated on the basis of direct quantification by
epifluorescence microscopy or flow cytometry
(cf. Chap. 17), after labeling of nucleic acids by a
fluorescent probe (Box Fig. 15.6). Three main reasons
can explain this “non-culturable state”:
– Many bacteria have specific needs, which are
frequently not satisfied by the formulation of
the available commercial culture media, to complete their metabolism and as a consequence to
grow. Moreover, in the environment, bacteria live
in multispecies communities including other
bacteria, macroorganisms, and plants and are often
organized in complex structure such as biofilms.
This multispecies organization allows many
interactions between the different members of
the community that are, sometimes, required for
growth of a specific member.
(continued)
15 Environmental and Human Pathogenic Microorganisms
649