1º parcial Inglés II 1 effect of temperature on microbial growth

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1º parcial Inglés II

Factors determining temperature limits for growth

The numerical values of cardinal temperatures (those at which microorganism growth is minimal, optimal and maximal) and the range of temperatures over which growth is possible, vary widely among bacteria. Some which have been isolated from hot springs are able to grow at temperatures as high as 95ºC, while others, isolated from cold environments, can grow at temperatures as low as -10ºC. On the basis of the temperature range of growth, bacteria are frequently divided into three broad groups: termophiles, which grow at elevated temperature (above 55ºC); mesophiles, which grow well in the midrange of temperature (20 to 45ºC); and psychrophiles, which grow well at 0ºC.This tripartite classification of temperature response does not take fully into account the variation among bacteria with respect to the extent of the temperature range over which growth is possible.
Represente gráficamente la información brindada por este texto, de manera que se vea claramente la relación entre los grupos de bacterias y sus rangos correspondientes de temperatura.
 The factors determining the temperature limits for growth have been revealed by both comparisons of the properties of organisms with widely different temperature ranges and analyses of the properties of temperature-sensitive mutants, the temperature range of which has been decreased by a single mutational change.

Studies on the kinetics of thermal denaturation of not only enzymes but also cell structures containing proteins (e.g., flagella, ribosomes) have shown that many specific proteins of thermophilic bacteria are considerably more heat stable than their homologues from mesophilic bacteria. It is also possible to make an approximate determination of the overall thermal stability of soluble cell proteins by measuring the rates at which the protein in a cell-free bacterial extract becomes insoluble as a result of heat denaturation at several different temperatures. It can be clearly demonstrated that virtually all the proteins of a thermophilic bacterium remain in the native state after a heat treatment, this denaturing virtually all the proteins of a related mesophile. It therefore follows that the adaptation of a thermophilic microorganism to its thermal environment can be achieved only through mutational changes, which are able to affect the primary structures of most cell proteins.

Although the evolutionary adaptations which have produced thermophiles must have involved mutations that increased the thermal stability of their proteins, most of the mutations affecting the primary structure of a specific protein decrease the thermal stability of that protein, even though many of these mutations could have little or no effect on its catalytic proteins. Consequently, in the absence of counterselection by a thermal challenge, the maximal temperature for growth of any microorganism should decline progressively as a result of random mutations that affect the primary structure of its proteins. This inference is supported by the observation that psychrophilic bacteria isolated from antarctic waters contain a large number of exceptionally heat-labile proteins.
Diga si las siguientes afirmaciones son verdaderas o falsas (marque los segmentos de texto donde ha encontrado su respuesta)
a) Las proteínas de las bacterias mesófilas son más resistentes al calor que las de las


b) Las proteínas que han sido objeto de mutaciones generalmente muestran mayor estabilidad al calor.

c) Lo más común, es que después de varias mutaciones la temperatura máxima de un

microorganismo disminuya.

d) Las proteínas de las bacterias halladas en lugares fríos son muy sensibles al calor.

e) Podemos decir que la mayor o menor estabilidad al calor de las proteínas es lo que determina la supervivencia de un microorganismo a diferentes temperaturas.

When present above a certain level and ingested in water, some chemical pollutants (e.g., nitrates, arsenic and lead) may constitute a direct toxic hazard, not posed by other water constituents, such as fluorides, which are beneficial, and may be essential to health in small concentrations ( although toxic when taken in larger amounts. Certain other substances or chemical characteristics may affect the acceptability of water for drinking purposes, including substances causing odours or tastes, acidity or alkalinity, anionic detergents and naturally occurring salts of magnesium and iron. Both international and national criteria and standards have been established, so that a basis for the control of human exposure to many of these substances through ingestion of polluted water can be provided.

Ingestion is, however, only one possible pathway to exposure, since man can be exposed to water pollutants through other types of direct contact that would include recreation or the use of water for personal hygiene. The possible health implications these non-drinking uses of water (including agricultural and industrial uses) would pose are less well understood, no international criteria or guidelines (based on the actual intake of drinking water and the body burden resulting from other sources) existing for the control of such exposure.

Since chemical water pollutants –at the level now existing in water bodies- may influence man´s health indirectly by disturbing the aquatic ecosystems or by accumulating in aquatic organisms used in human food, it would be advisable to analyse public health aspects of water pollution related to substances such as compounds of toxic metals and organochlorine pesticides.

The various chemical and biochemical transformations that pollutants may undergo in the aquatic environment also deserve attention, as chemical change may affect their biological availability or toxicity, which may be either enhanced or reduced. Although essential to the understanding of the health implications of chemical water pollution, these physical, chemical and biological processes and their mechanisms have not been completely understood.

  1. Haga la traducción de este texto

  2. Elabore un resumen del mismo

  3. Responda las siguientes preguntas:

  1. ¿De qué formas puede el ser humano entrar en contacto con sustancias contaminantes provenientes del agua?

  2. Mencione algunos tipos de sustancia que suelen contaminar el agua.

  3. ¿Qué efectos pueden producir algunos de ellos en el agua?

  4. ¿Qué aspectos deberían tenerse en cuenta si se elaboraran normas internacionales para regular la exposición a contaminantes a través del agua?

  5. ¿Qué tipo de transformaciones pueden sufrir los contaminantes en el agua y cuáles aspectos de estos compuestos pueden ser afectados?


For years, it has been known that the enteric diseases caused by the coliform bacteria

are transmitted almost exclusively by the faecal contamination of water and food materials. Contaminated water supplies are by far the most serious source of infection, being responsible for the massive epidemics of the more serious enteric diseases (particularly typhoid fever and cholera) that periodically scourged all countries until the beginning of the XX century. Today these diseases are almost unknown in the Western world, although cholera has recently reappeared in the countries bordering the Mediterranean. Their eradication was achieved primarily by appropriate sanitary controls, an essential part of this operation being the development of bacteriological methods ascertaining the occurrence of faecal contamination in water and foodstuffs.

As they are usually present in small numbers, it is seldom possible to isolate enteric pathogens directly from contaminated water. To demonstrate the fact of faecal contamination, it is sufficient to show the sample under examination contains bacteria, which are specific inhabitants of the intestinal tract, even though they may not be agents of disease. Although the bacteria normally used as indices of such contamination are the faecal streptococci and E. coli, the methods of sanitary analysis developed by bacteriologists differ somewhat from country to country.

When trying to detect E. coli, lab workers first inoculate dilutions of the sample into tubes containing lactose broth, which are then incubated at 37ºC and examined after 1 or 2 days. Cultures showing acid and gas formation are then streaked on a special medium. One of the media most commonly used is a lactose-peptone agar containing two dyes, eosin and methylene blue (EMB agar). On this medium, E. coli produces blue-black colonies with a metallic sheen, whereas the other principal member of the group capable of fermenting lactose with acid and gas production, Enterobacter aerogenes (not necessarily indicative of faecal contamination) produces pale pink mucoid colonies without a sheen.

  1. Traduzca el texto

  1. Complete estas oraciones con información proveniente del texto

  1. Las enfermedades de las que se habla son transmitidas principalmente transmitidas por …………………. Y ……………………

  2. De las dos enfermedades que se mencionan, la que aún prevalece es …………….. especialmente en la región ……………………..

  3. El hallazgo de abcterias intestinales (normalmente, ……………… y …………… ) es un índice de ……………………………

  4. El EMB se usa para diferenciar …………………………. De ………………….

  5. En el medio con lactosa y agar, uno de los microorganismos produce colonias ……………….. mientras que el otro …………………………..

  1. Diga si estas oraciones son verdaderas o falsas.

  1. Se determina que la muestra está contaminada solamente si la bacteria aislada es patógena.

  2. Existe un consenso mundial con respecto a la metodología utilizada.

  3. La muestra no se inocula directamente en el caldo con lactosa.

  4. El EMB es un medio líquido.

  5. No todos los organismos productores de ácido y gas son índice de contaminación.

Antibiotics were the first industrially produced microbial metabolites that were not major metabolic end products, for the yields –which had to be calculated in terms of conversion of the major carbon source into antibiotic- are low and are greatly influenced by both the composition of the medium and the other cultural conditions.

The wild type strain of Penicillum chrysogenum, which had been first used for penicillin production, yielded approximately 0.1 gram of penicillin per liter, so a mutant was selected which produced 8 grams per liter under the same growth conditions. Subsequent strain selection by means of chemical mutagenesis has led to the development of new strains with which even greater production capacity has been achieved, and so by such sequential genetic selection for antibiotic production, improvements of antibiotic yield as great as a thousandfold have often been obtained.

Yet most genetic improvement has been empirical and large numbers of mutagenized clones are evaluated for their abilities to produce large quantities of the antibiotic, with increase knowledge of the pathways of biosynthesis of antibiotics more rational approaches are being exploited. It is now possible to select strains in which control of the synthesis of known precursors of an antibiotic has been altered by mutations, so such strains will be able to produce large amounts of not only the precursor but also the antibiotic end product.

The synthesis of antibiotics begins only when growth of the microorganisms that produce them has virtually ceased, for they belong to a class of microbial products whose synthesis is not associated with growth and which have been called secondary metabolites. The control mechanisms that trigger the synthesis of secondary metabolites when growth ceases are a fascinating but almost completely unexplored aspect of biochemical regulation.

Yet the microorganisms which are used to produce antibiotics are all aerobes and are grown under conditions of vigorous aeration, the production process is generally referred to in the technical literature as “fermentation”. Antibiotics are produced by so-called submerged cultivation methods by means of deep stainless steel tanks that will have to be subjected to both continuous forced aeration and rapid mechanical agitation. Not only is the provision of adequate aeration of great importance to yield, but the energy which has to be expended for aeration contributes appreciably to the cost of production.

  1. Decida cuál de las curvas representa el desarrollo del P. chrysogenum y cuál la producción de penicilina, y subraye la o las pàrtes del texto que le proporcionan esa información

2º Parcial - Inglés II
Rabies, a human disease which is known to be transmitted by animal bite and for which man serves as accidental host, has been responsible in the past for serious epidemics that have radically altered the course of human history. By either eliminating or controlling both vectors and reservoirs of infection (the species from which the vector derives the infection) man has been able to eradicate this and other diseases from large areas.

Although two researchers, G. Zinke and G. Breschet, had already demonstrated that rabies was an infectious disease, it was not until modern techniques utilizing the electron microscope and tissue culture began to be employed about 30 years ago that the viral agent could be characterized. Taxonomically it belongs to the rhabdoviruses, a large group of viral agents affecting plants, insects and both warm-blooded and cold-blooded animals.

Not so many years ago, some researchers became aware of the impossibility of studying how these virus had interrrelated and evolved, unless reagents able to identify the specific antigenic determinants of the various components of the rhabdoviruses were developed. Now, however, the availability of monoclonal antibodies, particularly those detecting antigenic determinants of the nucleocapsids of viruses belonging to the rabids group may allow the interrelations among various rhabdoviruses to be studied.

Until recently, the antigens represented by glycoproteins and nucleocapsid proteins were thought to be similar, if not identical, regardless of the strain of rabies virus from which they could be isolated in relatively pure form. When studying virus preparations with rabies-specific monoclonal antibodies, however, researchers found that strains of rabies virus formerly considered as being identical or very closely related could be quite different antigenically. For example, two fixed laboratory-adapted virus strains derived from the rabies virus originally isolated by Louis Pasteur differed antigenically both from the Pasteur virus and from each other.

Monoclonal antibodies can potentially be produced relatively cheaply and in large quantities. Since they react more specifically than antibodies used for diagnosing rabies, they should replace animal sera for such purposes. In addition, they should replace antirabies gamma globulin of human origin -this being expensive and difficult to obtain- in the postexposure treatment of people bitten by rabid animals.
1) Traduzca el texto
2) Diga si estas afirmaciones son verdaderas o falsas y corrija las falsas.

  • En esta enfermedad, el hombre actúa como vector y reservorio de la enfermedad.

  • A principios del siglo XX todavía no se habían determinado todas las características del virus de la rabia.

  • Los anticuerpos monoclonales son altamente específicos.

  • El grupo de los rabdovirus afecta específicamente al hombre y otros mamíferos.

  • Se pudo detectar que distintas cepas del virus tienen características antigénicas diferentes.

3) Responda estas preguntas:

  • ¿Cómo ha logrado el hombre el control de esta enfermedad?

  • ¿En qué se basaban las técnicas que se desarrollaron para caracterizar al virus?

  • ¿Por qué anteriormente no se podían conocer las características evolutivas de estos virus, o cómo se

relacionaban entre sí?

  • ¿Qué componentes del virus son especialmente útiles en la generación de anticuerpos monoclonales?

  • ¿Qué ventajas presentan estos anticuerpos?


2.2. Recrystallization
Crystallization is the deposition of crystals from a solution or melt of a given material. During the process of crystal formation, a molecule will tend to become attached to a growing crystal composed of the same type of molecules because of a better fit in a crystal lattice for molecules of the same structure than for other molecules. If the crystallisation process is allowed to occur under near-equilibrium conditions, the preference of molecules to deposit on surfaces composed of like molecules will lead to an increase in the purity of the crystalline material. Thus the process of recrystallisation is one of the most important methods available to the chemist for the purification of solids. Additional procedures can be incorporated into the recrystallisation process to remove impurities. These include filtration to remove undissolved solids and adsorption to remove highly polar impurities.

Recrystallisation depends on the differential solubility of a substance in a hot and cold solvent. It is desirable that the solubility of the substance be high in the hot solvent and low in the cold solvent to facilitate the recovery of the starting material. The solution remaining after crystals have deposited is known as the mother liquor. The proper choice of solvent is critical and may require trial tests with small quantities of the material in a variety of solvents or solvent pairs (combinations of two solvents)

Recrystallisation procedures

The solvent, or solvent pair, to be used in the recrystallisation of a substance is chosen in the following manner. A small amount of the substance is placed in a small test tube and a few drops of solvent are added. The test tube is gently heated to see if the sample dissolves in the heated solvent. In general, one should first use a nonpolar solvent, for example, hexane or petroleum ether. If the sample does not dissolve, try using a more polar solvent such as ethanol or acetone. Should the sample completely dissolve in any solvent, chill the solution to see whether crystals will form (sometimes it is necessary to chill the solution using a Dry Ice-acetone bath in order to cause crystallisation). If no crystals appear, the material is too soluble in that solvent, and that solvent should not be used for the recrystallisation. If no single solvent provides suitable results, a mixture of two solvents can be employed, one of the solvents being a good solvent for the sample, and the other being a poor solvent for the sample. The sample is first dissolved in the solvent in which the sample is most soluble, and then small portions of the other solvent are added until a cloudiness is formed upon addition of the second solvent. A small amount of the better solvent is added to remove the cloudiness, and the solution is allowed to cool. The correct proportion of the two solvents must be determined by trial and error. Once the proper solvent has been chosen, the remainder of the sample is recrystallised.

For gram- or multigram-scale recrystallisations, the material to be recrystallised is placed in a suitable container such as an Erlenmeyer flask.

  1. Traducir la sección del texto que se indica.

  1. Decidir si estas afirmaciones, RELACIONADAS CON EL TEXTO TRADUCIDO, son verdaderas o falsas.

  1. Para seleccionar el solvente más adecuado, se puede trabajar con el total de la muestra

  2. Al seleccionar el solvente más adecuado se empieza a probar con los más polares.

  3. Cuando con uno solo de los solventes no se logran los resultados deseados se recurre al uso de dos solventes

  4. Si se usan dos solventes, la muestra se disuelve primero en aquel en el que es menos soluble.

  5. Existe una fórmula precisa que permite calcular las proporciones en que deben agregarse los dos solventes.

  1. Las siguientes preguntas se refieren a las secciones QUE NO ESTÁN INCLUÍDAS EN LA TRADUCCIÓN del texto. Respóndalas.

  1. ¿Por qué se produce el proceso de cristalización?

  2. ¿Para qué sirve este proceso?

  3. ¿Con qué otros métodos puede combinarse y para qué se recurre a ellos?

  4. ¿Cuál es el fundamento del proceso de recristalización?

  5. ¿Qué se define en el texto como “mother liquid”

  6. ¿Cómo se elige el solvente más adecuado?

  7. ¿Cuándo se utiliza el carbón activado en este tipo de procesos?

3) Air Pollution and Climate
While acid rain and acid cloud or fog water can significantly alter the structure and function of various compounds of terrestrial ecosystems, these may be simultaneously stressed by a variety of other pollutants, such as ozone, toxic metals, hydrocarbons and dry deposited particles, the common source for many of these air pollutants being the combustion of fossil fuels.

Because of the relatively recent recognition of this diversity of air pollutants and their interactions with terrestrial ecosystems, we have suggested that regional air pollution is in reality more severe than it appeared to be some 20 years ago when acid rain was discovered and was the primary regional air pollutant of concern. It was only during these past 20 years that we became aware of the diversity and severity of air- pollution climates in different regions in the Earth. For example, acid cloud and fog water are known to be widespread in North America, particularly in the north-eastern US.

Although ozone, like acid rain, was initially considered to be a problem with only local dimensions, it is now clear that ozone damage to crops and natural vegetation is widespread throughout Europe and North America. Increased dosages of ozone have been shown to cause visible damage to plants and to reduce net photosynthesis in tree seedlings in controlled chamber experiments. Other studies have shown statistically significant reductions in net photosynthesis and growth of tree seedlings from ambient ozone concentrations in the eastern US. Apparently, these ambient levels of ozone currently may have deleterious effects on native trees showing no visible symptoms such as stippling.

Yet long-term records of ozone concentration in the lower atmosphere are rare, average concentrations in some rural areas of Europe appear to have increased during the last two decades. The widely held view that ozone concentrations will peak during the daytime hours may not hold for summits of mountains where vegetation damage is often the most severe. For example, if we measure ozone concentrations at 1900 meters mean sea level, these may frequently remain high throughout the day and night during the summer, whereas at a lower elevation the diurnal pattern is different. Because ozone concentrations are higher at higher elevations, if total ozone stress to plants were measured, it should be more severe there than at sea level.

1) Traduzca el texto

  1. Responda las siguientes preguntas.

  1. ¿Cuáles son, específicamente, los efectos perjudiciales producidos por el incremento de ozono en la vegetación?

  2. ¿Bajo qué condiciones el daño debido al ozono sería mayor?

  3. ¿De dónde provienen mayoritariamente los contaminantes del aire?

  4. ¿En qué sentido ha cambiado la visión de esta problemática en los últimos 20 años?

  1. De acuerdo con el texto, seleccione la opción correcta:

  1. El texto pone énfasis principalmente en la problemática de la contaminación debida a un grupo de contaminantes/ la lluvia ácida/ un tóxico en particular.

  2. El texto plantea el tema de la contaminación de los ecosistemas terrestres como una situación actualmente más controlable/ más compleja/ más incontrolable que hace 20 años atrás.

  3. El texto pone en duda/ expone/ clasifica las causas de la contaminación del aire.

  4. El texto generaliza/ compara/ especifica la acción del ozono en los ecosistemas terrestres.

  1. I - Diga si los siguientes enunciados son V o F. Corrija los falsos.

  1. El ozono en particular, como contaminante, se ha propagado a lo largo de Europa y América del Norte y se ha demostrado que su incremento ha causado daños visibles y no visibles en la vegetación.

  2. Varios contaminantes del aire, muchos de ellos provenientes de la combustión de restos de combustibles, alteran a los ecosistemas terrestres porque actúan en forma simultánea.

  3. Durante las últimas décadas se ha incrementado el nivel de ozono a nivel mundial y se ha demostrado que el daño producido en la vegetación es mayor a nivel del mar, donde la concentración de este tóxico es mayor.

  4. Durante mucho tiempo se desdeñó el efecto de otros contaminantes frente al de la lluvia ácida. Ahora todos los contaminantes han dejado de considerarse como un problema de índole local.

III- Ordene los párrafos verdaderos y los corregidos según se corresponda con el texto de modo que quede conformado un resumen del mismo.

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