Control de compactación mediante el penetrómetro ligero PANDA

Viral Genus Dimension

Special Features Features

(with Represen- of Virion tative species) [Dia. in nm] and Unclassified

Members*

3 Double-stranded DNA, enveloped Poxviridae Orthopoxvirus (vaccinia and smallpox viruses) Molluscipoxvirus

200-350 Very large, complex, brick shaped viruses that cause diseases e.g. smallpox (variola), molluscum contagiosum (wartlike skin lesion), cowpox and vaccinia. Vaccinia virus provides specific immunity to smallpox.

4 Single-stranded RNA, nonenveloped + strand

Picornviridae 28-30 Upto 70 human entero-viruses

are known, including the po- lio-, coxsackie-, and echovi- ruses ; more than 100 rhinovi- ruses exist and prove to be the most common cause of colds.

5 Single-stranded RNA, enveloped + strand Togaviridae A l p h a v i r u s , Rubivirus, (ru- bella virus)

60-70 Essentially include several viruses transmitted by ar-

thropods (Alphavirus) ; dis-

eases include Eastern

Equine Encephalitis (EEE),

Rubella virus is transmitted by the respiratory route. 6 – strand, one strand

of RNA

Rhabdoviridae Vesiculovirus (ve-

sicular stomatitis virus), Lyssavirus (rabies virus)

70-180 Bullet-shaped viruses having a spiked envelope ; invari- ably cause rabies and several animal diseases. 7 – strand, multiple strands of RNA Orthomyxo- viridae Influenzavirus (Influenza viruses A and B), Influ- enza C virus.

80-200 Envelope spikes can agglu- tinate red blood cells (RBCs).

8 Produce DNA Retroviridae O n c o v i r u s e s

Lentivirus (HIV)

100-120 Includes all RNA neoplasm viruses and double-stranded RNA viruses. The

oncoviruses invariably cause

leukemia and neoplasms in animals, and the lentivirus

HIV causes AIDS.

9 Double-stranded

RNA nonenveloped

Reoviridae Reovirus Colo-

rado tick fever virus

60-80 Involved in mild respiratory infections ; an unclassified species causes Colorado tick fever.

E n t e r o v i r u s Rhinovirus (Com-

mon cold virus), Hepatitis A virus

5.5.2. Growth of Bacteriophages in the Laboratory

It is practically possible to grow the bacteriophages in two different manners, namely : (a) In suspensions of organisms in liquid media, and

(b) In bacterial cultures on solid media.

Advantages of using Solid Media : In actual practice, the use of solid media makes it feasible

and possible the plaque method for the easy detection and rapid counting of the viruses.

Methodology (Plaque Method) : The various steps that are involved in the ‘plaque method’

are as enumerated under :

(1) Sample of bacteriophage is duly mixed with the host bacteria and molten agar.

(2) The resulting agar countaining the various bacteriophages as well as the host bacteria is then poured carefully into a Petri-plate adequately containing a hardened layer of the agar growth

medium.

(3) In this manner, the ensuing mixture of virus-bacteria gets solidified into a thin top-layer that invariably comprises of a layer of organisms nearly one-cell thick. This specific step allows each virus to infect a bacterium, multiplies subsequently, and helps to release several hundred altogether new

viruses.

(4) Nevertheless, these newly generated viruses in turn duly infect other organisms that are present in the immediate close vicinity ; and hence, more new crop of viruses are produced ultimately.

(5) Thus, several accomplished virus multiplication cycles, all the organisms duly present in the area surrounding the original virus are destroyed finally. In this way, a good number of ‘clearings’ or

plaques are produced, which may be seen against a “lawn” of bacterial growth upon the surface of the

agar ; whereas, the plaques are observed to form uninfected microorganisms elsewhere in the Petri

dish (or Petri plate) undergoing rapid multiplication and giving rise to a turbid background finally. Note : Each plaque correspond theoretically to a single virus in the initial suspension. Hence, the concentra-

tions of viral suspensions measured by the actual number of plaques are invariably expressed in terms of plaque-forming units (pfu).

5.5.3. Bacteriophage Lambda : The Lysogenic Cycle

In a broader and precise perspective the bacteriophage may conveniently exist in three phages, namely :

(a) As a free particle virion,

(b) In a lysogenic state as a prophage, and (c) In the vegetative state i.e., lytic cycle.

One may, however, observe that virion is inert in nature ; and hence, cannot reproduce.

Salient Features : The various salient features of the bacteriophage lambda are as stated

under :

(1) In the critical ‘lysogenic state’, the DNA of the phage is duly integrated very much within the

bacterial DNA. It usually exists in a non-infectious form known as the prophage, and adequately replicates in synchrony with the bacterial DNA.

(2) In the corresponding ‘lytic cycle’, the phage particle infects the susceptible host, undergoes multiplication, and ultimately causes the lysis of the bacterial cell with the concomitant release of the

(3) In a situation when the integrated phage is carefully induced to become the corresponding

vegetative phage, the lytic cycle comes into being.

(4) Such phages which specifically give rise to the phenomenon of ‘lysis’ are normally termed as the virulent phages, as opposed to such phages that may exist in a lysogenic state and are usually called as the ‘temperate phages’.

(5) The microorganisms that particularly carry the ‘temperate phages’ are invariably termed as the ‘lysogenic bacteria’, which are observed to be absolutely immune to the ensuing superinfection caused by the same phage.

Figure 5.8 diagramatically illustrates the lysogenic cycle of bacteriophage λ in E. coli.

However, it is pertinent to state here that whether decisively the ‘lytic’ or the ‘lysogenic’ response takes place immediately following infection by a temperate phage will solely depend upon both the

bacterium and the phage.

Phage DNA (double stranded) 1 4 5 2 6 Phage attaches to host cell and injects DNA

7 Occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle

Cell lyses, releasing phage virions

Phage DNA circularizes and enters lytic cycle or lysogenic cycle

Lysogenic bacterium reproduces normally

Phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage

3 New phage DNA and proteins are synthesized and assembled into virions

Lytic cycle Lysogenic cycle

Many cell divisions Bacterial

chromosome

Prophage

Fig. 5.8. Diagramatic Representation of Lysogenic Cycle of Bacteriophage λ in E. coli. FURTHER READING REFERENCES

1. Dimmock NJ et al. : Introduction to Modern Virology, Blackwell Science Inc., Cam- bridge Mass, 5th edn., 2001.

2. Fisher F and Cook N : Fundamentals of Diagnostic Mycology, WB Saunders, Philadel- phia, 1998.

3. Flint S et al. : Principles of Virology : Molecular Biology, Pathogenesis, and Control, ASM Press, Washington DC, 1999.

4. Gulbins E and Lang F : American Scientist, 89 :406-13, 2001.

5. Jotlik WK et al., Zinnser Microbiology, Appleton and Lange, E. Norwalk, Conn., 20th edn, 1992.

6. Mandell GL et al. : Principles and Practice of Infectious Diseases, Churchill Livingstone, New York, 2000.

7. Murray PR : Manual of Clinical Microbiology, ASM Press, Washington DC., 8th edn., 2003.

8. Rhen M et al., Trends Microbiol., 11 (2) : 80-86, 2003.

9. Richman D et al. : Clinical Virology, ASM Press, Washington DC, 2002.

10. Roberts LR and Jonovy J : Fundamentals of Parasitology, McGraw Hill, Dubuque, Iowa, 6th edn., 2000.

11. Salyers AA and Whitt DD, Bacterial Pathogenesis - A Molecular Approach, ASM Press, Washington DC, 2nd edn, 2001.

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6.1.

INTRODUCTION

The microbial genetics as well as the molecular biology specifically and predominantly focus upon the very nature of ‘genetic information’. Besides, it invariably modulates the precise development and function of various cells and organisms. In fact, the application of microorganisms has been enormously useful in mustering a definitely better and exceptionally vivid in-depth understanding of the actual mechanism of ‘gene function’.

Importantly, it has been adequately observed that practically most of the ‘microbial traits’ are either strategically controlled or logically influenced due to heredity. In true sense, the inherited traits

of microorganisms essentially comprise of the following cardinal aspects :

• shape and structural features i.e., morphology, • biochemical reactions i.e., metabolism,

• ability to move or behave in different manners, and

• ability to interact with other microorganisms — thereby causing human ailment.

In a rather broader perspective one may consider that the individual organisms prominently do transmit these characteristic features directly to their offspring via genes, that are nothing but the hereditary materials (DNA) which essentially possess relevant information(s) that precisely determines these typical characteristic features.

It has been amply proved and duly established that almost all ‘living organisms’ prominently find it rather advantageous to share the hereditary materials derived from a ‘genetic pool’. However, under the influence of an effective environmental change, the microorganisms that critically possess such ‘genes’ which are proved to be advantageous under these new conditions* shall definitely exhibit a better chance (scope) of reproduction thereby enhancing their actual numbers in the overall population. Both eukaryotic and prokaryotic organisms usually exhibit different types of reproductive means, such as :