Obligate intracellular pathogenesis

Obligate intracellular bacterial pathogens are also called as Obligate Parasitism , are living and replicate extremely inside the host cells with out extracellular state , Their genome usually small is around ( 0.5 -2 MB ) . They are widely spreaded in the nature and they have diverse effect on their hosts. Species of obligate intracellular pathogens like , Chlamydia , Rickettsia and Coxiella are less characterized than those of Facultative intracellular pathogens , this is due to growing difficulty of obligate intracellular pathogens and there is no genetic tools to manipulate them. They both responsible for producing specific disease but Facultative intracellular pathogens are common .

Obligate intracellular species are can be deveided according to their way of transmission into : 1- Horizontal transmission (which means that bacteria can transmit from one species to another or between individuals of the same species . the bacteria replicate intracellular but have the ability to switch the host, horizontal transmission can be promoted by TROPHALLAXIS , Blood-feeding and parasite – host intraction ) . 2- Vertical Transmission ( The bacteria transmitted from the mother by inherity to offspring) .

A brief history of molecular biology methods: – The beginning of the molecular revolution was ignited when DNA was reported by Watson and Crick at 1953 , which was the initial understanding of gene function. – Then after purification of restriction enzyme further manipulation of DNA has been developed. Restriction enzyme has been isolated from bacteria , it cuts DNA at a specific sequence and produce restriction fragments . it has two major roles which is construction of recombinant DNA molecule and maps the restriction sites place in DNA .

– Southern blotting developed by Edward M. Southern at Edinburgh university in the year 1970. This locates an interested DNA sequence within a complex mixture by using radioactive probes to detect fragments on agarose gel which are complementary to a sequence of a given DNA. This method solved the problem of an ability of analyzing gene fragments – Northern Blotting that uses same technique of Southern blotting for analysis of RNA that’s can be detected by autoradiography and it’s also called as Northern hybridization or RNA hybridization.

– Then during the year 1984 polymerase chain reaction ( PCR ) developed by biochemist Kary Mullis , that amplify small amount DNA into large amount in an easy , quick and automated steps. which then followed by reverse transcriptase – PCR ( RT-PCR) for amplification of RNA . the product of RT-PCR will be dsDNA which is analogous to target segment of RNA molecule. – SDS POLYACRYLAMIDE GEL ELECTROPHORESIS: Its use as separation technique and to analytical study of the single charged species properties. It’s simple, rapid and highly sensitive. – The advance of molecular biology the techniques like PCR, Southern blotting, gene cloning and DNA sequencing provided a good understanding of many infectious disease and genetics of other organisms.


Intracellular pathogens are can be divided into two grouped : – obligate intracellular or facultative intracellular . Obligate intracellular pathogens are those organisms that they lost their ability to live outside the host cell example for these group is all viruses , protozoa and bacteria such as Chlamydia and Rickettsia spp. – Facultative intracellular pathogens are those organisms that can retain their ability to replicate outside their hosts , such as so many pathogenic bacteria and fungi.

– There are a suggestion that a loss in one gene differentiated between obligate and facultative pathogens which accompanies association of hosts and dependence. – The distinguish between obligate and facultative pathogens can be done through their acquisition means in which obligate pathogens can be acquired only from other hosts , while facultative pathogens can be acquired from either other hosts or directly from environment.

– The relation ship of obligate intracellular pathogens to their hosts are dependent , in such way that they adapted to escape from host response mechanism , but when response of host mechanism occurs the microbial disease will cause by the disruption of that relation ship. – There are varied relation ship between facultative intracellular pathogens to their hosts , some cause disease in a minority of hosts that infected which is related to transmission of infection , while others no needs for virulence of the hosts in their replication and survival.

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