1. Un CULTIVO PURO es aquel que contiene una sola clase de microorganismos. Para obtenerlo es necesario recurrir a las llamadas técnicas de aislamiento.
En esta practica emplearemos dos métodos para obtener un cultivo puro:
• Aislamiento por agotamiento por estrías
• Aislamiento por siembra de diluciones seriadas (que sera el objeto de otra practica mas adelante).

• Agotamiento por estrías.

Se trata de un método rapido y simple de agotamiento progresivo y continuo del inóculo sobre un medio sólido contenido en una placa de Petri. El objetivo es obtener, a partir de un elevado número de bacterias, un número reducido de ellas distribuidas individualmente sobre la superficie de la placa. Cada una de estas bacterias originara una colonia.

La muestra debe diseminarse de manera tal que los diferentes microorganismos queden lo suficientemente separados sobre la superficie de un medio de cultivo sólido, de manera que luego de la incubación ellos formen colonias visibles aisladas. Este proceso se conoce como aislamiento. En esta placa tendremosdiferentes tipos de colonias correspondientes a los diferentes microorganismos presentes en la población original.
Luego de tener las colonias aisladas, éstas deben transferirse con el filamento a un tubo que contenga agar nutritivo estéril para cultivar esa colonia aislada; este procedimiento se conoce como transplante.

4. Nutritivo Agar

Medio de cultivo utilizado para propósitos generales, para el aislamiento de microorganismos poco exigentes en lo que se refiere a requerimientos nutritivos.
Su uso esta descripto en muchos procedimientos para el analisis de alimentos, aguas y otros materiales de importancia sanitaria.

Fundamento
Por las características de sus componentes es un medio usado para el cultivo de microorganismos poco exigentes en sus requerimientos nutricionales. No contiene inhibidores del desarrollo bacteriano. La pluripeptona es la fuente de carbono y nitrógeno para el desarrollo bacteriano. El agregado de cloruro de sodio permite el enriquecimiento con sangre de carnero u otras sustancias para facilitar el cultivo de microorganismos exigentes. Cross-talk

nature, however, the plant encounters stress combinations concurrently or separated temporally and must present an integrated response to them. In the case of phytochrome signalling, the two pathways leading to red-light-induced CHS and CAB gene expression negatively regulate flux through one another1,2. Seemingly separate abiotic stress signalling pathways are also likely to interact in a similar manner. In addition, several abiotic stress pathways share common elementsthat are potential ‘nodes’ for cross-talk. Cross-talk can also occur between pathways in different organs of the plant when a systemic signal such as hydrogen peroxide moves from a stimulated cell into another tissue to elicit a response3.
Specificity

When stress signalling pathways are examined in the laboratory, they are usually considered in isolation from other stresses to simplify interpretation. In

In spite of considerable overlap between many abiotic stress signalling pathways, there might, in some instances, be a benefit to producing specific, inducible and appropriate responses that result in a specific change suited to the particular stress conditions encountered. One advantage would be to avoid the high energy cost of producing stress-tolerance proteins, exemplified by the dwarf phenotype of plants constitutively overexpressing the frost tolerance protein DREB1A (Ref. 4). In some cases, the signal transduction pathways triggered by different stresses are common to more than one stress type. One possible reason for this is that, under certain conditions, the two stresses cannot be distinguished from one another. Alternatively, each stress might require the same protective action (or at least some common elements). The discovery of separate sensing mechanisms for each stress would invalidate the first suggestion but the second is true in several cases. For example, dehydration protection is required in plants undergoing either freezing or drought and the production of antioxidants and scavenging enzymes (e.g.catalase and peroxidases) that protect against oxidative damage affords protection against a variety of different abiotic (and biological) stresses5. Most abiotic stresses tested have been shown to elicit rises in cytosolic free calcium levels ([Ca2+]cyt) and to involve protein phosphatases and kinases [including mitogen-activated protein kinase (MAPK) cascades]. However, are any of these components truly specific to one stress and which of them are ‘nodes’ at which cross-talk occurs? In the following sections, we consider different classes of signalling component in turn, and examine their potential

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Review

TRENDS in Plant Science Vol.6 No.6 June 2001

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(a)

Stimulus 1 A B C D

Stimulus 2 W X Y Z Response

these molecules themselves have the potential to encode specificity of response. An early event in the response to many different environmental stresses is an elevation in [Ca2+]cyt (Refs 7,8), which is thought to be the primary stimulus-sensing event for several stresses (e.g. cold)9–11. If this is the case then mechanisms could exist for encoding the information that relates to the particular stress through the calcium signature (see below). Alternatively, the stress might be sensed through other components either in parallel to or upstream of Ca2+ in the pathway. It has been postulated that cold is sensed via changes in membrane fluidity12 and cytoskeletal reorganization13affecting calcium channels.
(iii) Stimulus Stimulus A B +

(b) Stimulus
A

(i) Stimulus B

(ii) Stimulus Stimulus A B –

Calcium

(c)

(i) Stimulus 1

(ii) Stimulus 1 Stimulus 2

Signalling component

Response X

Response X

Response Y
TRENDS in Plant Science

Fig. 1. Cross-talk in signalling pathways. (a) Two different stimuli (1 and 2) evoke the same end response via different signalling pathways, using different signalling intermediates (A–D and W–Z, respectively). (b) Positive and negative reciprocal control. Two different stimuli (A and B) activate two signalling pathways (broken arrows), leading to different end responses. (i) Pathways operating totally independently of each other. (ii) Flux through the stimulus-A-mediated pathway negatively regulates the stimulus-B-mediated pathway and inhibits its flux. An example of this is in phytochrome-mediated expression of a chlorophyll a/b binding protein gene (CAB) and a chalcone synthase gene (CHS) by independent pathways, each negatively regulati