EXÁMENES II – SANEAMIENTO E HIGIENE
1) MICROBIAL TOXINS
The nature of toxins
Most of our knowledge of microbial toxins has come from work on pathogenic bacteria, for the
search for bacterial toxins began shortly after the discovery of the role bacteria had as
ethiological agents for human disease. By 1890 the toxins of two important human pathogens,
Corynebacterium difteriae and Clostridium tetani had been discovered, and a sterile filtrate
which had been prepared from the fully grown culture had been observed to cause death when
injected into experimental animals. What autopsies revealed was that these animals showed the
characteristic lesions associated with the specific natural infection. The toxic substances -which
proved to be heat-labile and are now known to be proteins- were termed exotoxins, for they were
present in the medium, although not associated with the bacterial cells.
A number of other pathogenic bacteria have been subsequently shown by comparable methods to
produce exotoxins that have specific effects, yet filtrates which had been prepared from cultures
of many important pathogens failed to show toxicity. This led to the examination of the bacterial
cells themselves (which had been killed by heat) as possible toxic agents. Not only did such
experiments show that the cells of nearly all Gram-negative pathogenic bacteria are intrinsically
toxic but heat-killed cells of many non-pathogenic Gram-negative bacteria were shown to have
similar effects. The heat-stable toxins with which the cells of Gram-negative bacteria were
associated came to be known as endotoxins. Many years of intensive study were required to
reveal their nature and cellular origin; it is now known that endotoxins are lipopolysaccharideprotein complexes, which derive from the outer layers of the cell walls of Gram-negative
bacteria.
The examination of the cells and culture filtrates of pathogenic bacteria which had been grown in
vitro led to the recognition of a number of microbial products that damage the host, yet many
important bacterial pathogens remained to be studied, specially the causative agents of anthrax
and plague, for which this approach had failed to reveal any significant toxic product.
The toxins of both organisms were later found to be complexes of two or more substances, each
of which was non-toxic by itself but which together acted sinergistically to produce a toxic effect.
Such knowledge allowed the assay systems for the toxins to be refined to such an extent that -in
each case- it became possible to establish the production of toxin by cultures of bacteria in vitro.
1) Traduzca este texto
2) Sintetice brevemente los puntos principales de los que habla el texto.
3) Responda estas preguntas:
a)
Cómo fue que se descubrió la existencia de las toxinas de Corynebacterium difteriae y Clostridium
tetani?
b) Qué significa el término “exotoxins”?
c) Todas las bacterias producen “exotoxins”? Explicar.
d) Qué características tienen las “endotoxins”?
e) Cómo actúan las toxinas de “anthrax” y “plague”?
4) Marque con (X) las oraciones que expresan información contenida en el texto.
a)
b)
c)
d)
e)
Algunas toxinas son estables al calor, pero otras no.
Las endotoxinas y las exotoxinas tienen diferente composición.
Los clostridios forman esporos que pueden sobrevivir por largo tiempo.
Los agentes causantes de antrax y peste tuvieron que ser estudiados de una forma diferente al resto.
Los métodos de aislamiento de toxinas son extremadamente complejos y lentos.
2) ENVIRONMENTAL BENEFITS OF NOx CONTROL IN NORTHWESTERN
EUROPE
Five environmental problems have been
identified, which should see significant
improvement in the United Kingdom, if
action is taken within the United Nations
Economic Comission for Europe (UN
ECE) Second NOx protocol to control
NOx emissions. These problems are:
acidification and eutrophication of soil
ecosystems; episodic peak ozone and
human health; ozone damage to crops and
forest trees; wintertime NOx pollution
episodes and summertime fine particulate
formation and human health. There are a
range of environmental and health
thresholds that can be used as a guide to
setting NOx emission reduction targets for
these problems within the proposed
Second NOx Protocol. It is recognized
however, that achieving some of these
targets will require actions beyond those
regarded as feasible even if their
desirability is acknowledged. These targets
can at least provide bench marks against
which agreed reductions can be assessed,
within the context of a mullet-pollutant
mullet-effect framework to the proposed
Second NOx Protocol.
Introduction
It has been understood for a
considerable time that NOx emissions
from motor vehicles and power stations
cause urban and regional scale
photochemical smog and, when
deposited, cause acidification and
eutrophication effects in various
ecosystems (1). In Europe in 1988, this
understanding led the member states of
the
United
Nations
Economic
Commission for Europe to agree the
Sofia protocol to the International
Convention
on
Long
Range
Transboundary Air Pollution (2). The
main provision of the Sofia protocol
aims to freeze NOx emissions at their
1987 levels from the year 1994 onwards
(3). There is a case for taking much
firmer steps in Europe to decrease NOx
emissions over the next few decades
within the framework of the proposed
Second NOx Protocol.
Over much of northwestern Europe,
nitrogen dioxide (NO2) now dominates
over sulfur dioxide (SO2) as the acidic
gas in highest concentration on a ppb
basis (4). The nitrate content of
suspended particulates has risen steadily
in absolute terms and relative to sulfate
aerosol in southern England since 1954
(5). The trends of nitrate in rain in
Scandinavia have remained constant for
two decades while those for sulfate have
decreased (6). Nitrate levels in ice cores
in southern Grenland have continued to
increase whereas sulfate levelled off in
1970 or thereabouts (7). The Thousand
lakes Survey in Norway has recorded a
doubling in the nitrate content of 305
lakes over the period 1974-75 to 1986,
despite little changes in pH and sulfate
(8). More recently, from 1986-1994,
sulfate levels in these lakes have
declined yet nitrate levels remained
constant(9).
NOx plays an important role in an
increasing range of environmental
impacts over a widening range of
spatial scales from the local and urban
to the global scale (10); some of these
impacts are illustrated in Figure 1. NOx
emissions are implicated in wintertime
urban
pollution
episodes
(11),
photochemical ozone formation (12),
acidification of remote soil and
freshwater ecosystems (13), fertilization
of sensitive soil and plant ecosystems
leading to changes and reductions in
biodiversity (14), stimulation of
plankton blooms in marine waters (1516),
global
tropospheric
ozone
production (17), and the increase in the
levels of ozone since preindustrial times
over large parts of the troposphere and
the build up of greenhouse gases, such
as ozone, methane and the HFCs and of
some ozone depleting trace gases such
as methyl chloroform and HCFC-22
(19-20).
In all these impacts, NOx does not act
alone. Suspended fine particulate
matter, along with NO2 is an important
component of wintertime urban
pollution episodes (11). Both NOx and
particles are emitted by diesel vehicles
and some measures to control one can
lead to exacerbation of the other (21).
NOx and hydrocarbon emissions both
contribute to photochemical smog
formation (12). NOx, SO2, HCl and
ammonia
together
control
the
acidification of remote ecosystems and
the sutrophication of terrestrial and
marine ecosystems (10). NOx, carbon
monoxide, methane and hydrocarbons
together control the oxidizing capacity
of the troposphere (17-19). Nitrate
aerosol is only one component of
summertime haze, others include
sulfuric acid and ammonium sulfate
(22). In adressing these impacts
therefore care has been taken to include
the role of NOx and of other pollutants
in quantifying the likely impacts of
NOx emission reductions.
Here, computer modelling studies are
described which assess the likely
impacts in the United Kingdom of
measures to reduce emissions of NOx in
the UK and in the rest of Europe. The
aim is to use the UK as an example, to
discuss the different aspects of a mulletpollutant mullet-effect approach to a
Second NOx Protocol. Table 1 contains
a summary of models employed. These
studies are set in the context of the other
pollutants involved and address the
environmental problems listed below:
- acidification and eutrophication of
sensitive soil ecosystems;
- photochemical ozone formation and
its impacts on human health;
- regional-scale ozone levels and
vegetation damage;
-
wintertime NO2 levels and human
health effects; and
summertime fine particle formation
and human health effects.
ACIDIFICATION
EUTROPHICATION
ECOSYSTEMS
OF
AND
SOIL
Soil acidification is mostly concentrated
in European regions north of the Alps
and it is here that acidification is known
to have caused a severe reduction in
base cation saturation of soil in many
areas. Where this reduction has
occurred, lakes and streams may receive
acidified waters containing elevated
concentrations of potentially toxic
forms of aluminum. Sulfur deposition
may pass through the sensitive soils of
north and central Europe almost entirely
in the form of sulfate, contributing to
acidification of soils by enhanced
leaching of aluminum.
[Acidification effects in the UK have
been detected in the upland areas of
Wales (24), the penines, Galloway, and
the Highlands of Scotland (25).
The situation with nitrogen deposition is
much more complicated. Most forest
ecosystems are nitrogen deficient and
nitrogen will initially act as a nutrient.
Nitrogen may accumulate in the forest,
either in the living biomass or in the
soil. The increased availability of
nitrogen will lead to enhanced growth
and a balancing increase in the uptake
of base cations, contributing to soil
acidification (26). If high nitrogen
deposition loads persist, nitrate leaching
will occur with the associated leaching
of base cations and soil acidification in
the same way as sulfur. In Scandinavia
and in the UK, nitrate leaching is
generally minimal whilst in continental
Europe, high nitrate leaching levels are
common (10).
Nitrogen eutrophication effects are far
less well-documented than acidification
effects, and lead to changes in the
competitive interactions between plant
species and hence to changes in species
composition in sensitive ecosystems. In
The Netherlands, severe effects due to
nitrogen deposition have been observed.
In the UK and southern Scandinavia,
acosystem changes have been observed
in areas with high nitrogen deposition
(27), but in the remote north and
western fringes of Europe, no
significant eutrophication effects have
been observed.
The sensitivity of the soil ecosystems to
acidification
and
eutrophication
depends on a complex range of
physical,
chemical,
ecological,
mineralogical,
geological
and
hydrological factors, which are highly
spatially variable. For sensitivity to be
quentified, the concept of critical load
may be used. This represents a
quantitative measure of the exposure to
one or more pollutants below which
harmful effects do not occur according
to present knowledge. Methodologies
have been described to quantify and
map critical loads for acidification and
eutrophication for sulfur and nitrogen
across Europe and within the UK.
Ecosystems are likely to be protected in
the long term if deposition loads are
below critical loads and are likely to
suffer long-term deterioration if the
converse is the case. The exceedance of
these critical loads can be ascertained
by overlaying the critical loads maps
with maps of the deposition loads of the
particular damaging pollutants. In this
study, critical loads maps are overlaid
with model estimates of current and
future deposition loads to assess the
impact of NOx controls in the UK and
in the rest of Europe.]
1) Nombre al menos siete problemas
ambientales relacionados con la
emisión de NOx.
2) Decida si las siguientes afirmaciones
son verdaderas (V) o falsas (F) e
indique en el texto las líneas que
justifican su decisión.
a)
b)
c)
d)
e)
El smog es producto de la
combinación de NOx y de las
emisiones de los hidrocarburos.
El 2º Protocolo de NOx pretende
mantener los niveles de emisión en
los valores registrados en 1987, al
igual que el protocolo de Sofía.
Los NOx y partículas emitidas por los
automóviles
pueden
controlarse
independientemente, sin que se
produzcan efectos adversos.
Al analizar los episodios ambientales
basta con considerar el efecto de los
NOx.
Los estudios que menciona el
artículo,
basados
en modelos
computacionales, pretenden tomar al
Reino Unido como ejemplo para
debatir los probables impactos sobre
el mismo de la reducción de los NOx,
tanto en el reino Unido como en
Europa.
3) Responda las siguientes preguntas:
a) Considerando Europa noroccidental,
¿cuál es el gas acídico predominante?
Enumere sintéticamente los datos que
brinda el artículo y que le permitan
justificar su respuesta.
b) Cuál es la causa de la disminución de
la saturación de cationes básicos en
los suelos?
c) Cómo se puede determinar la
sensibilidad de un suelo a la
acidificación y a la eutroficación.
4) Traduzca las partes
señaladas entre corchetes.
del
texto