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DIAGNOSTIC MEDICAL MICROBIOLOGY

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PRINCIPLES OF DIAGNOSTIC MEDICAL MICROBIOLOGY: INTRODUCTION
Diagnostic medical microbiology is concerned with the etiologic diagnosis of infection.  Laboratory procedures used in the diagnosis of infectious disease in humans include the following:
1.   Morphologic identification of the agent in stains of specimens or sections of tissues (light and electron
microscopy).
2.   Culture isolation and identification of the agent.
3.   Detection of antigen from the agent by immunologic assay (latex agglutination,  enzyme immunoassay
[EIA],  etc) or by fluorescein-labeled (or peroxidase-labeled) antibody stains.
4.   DNA -DNA or DNA -RNA hybridization to detect pathogen-specific genes in patients' specimens.
5.   Detection and amplification of organism nucleic acid in patients' specimens.
6.   Demonstration of meaningful antibody or cell-mediated immune responses to an infectious agent.
In the field of infectious diseases,  laboratory test results depend largely on the quality of the specimen, …

CONTROL OF MICROBIAL INFECTIONS

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ANTIVIRAL DRUG RESISTANCE:
When an antibacterial or antiviral agent acts at a single target site then resistance can develop through 
mutations at that site. The high mutation rates in RNA viruses will mean that such problems will occur 
more readily than with DNA-based bacteria and this is witnessed in HIV therapies. Again, as with 
antibiotic policies, certain manoeuvres can be employed to minimise resistance developing. These 
include the use of combinations of antiviral compounds, switching between two unrelated agents 
(although sequential use of antivirals has been criticised for simply promoting resistance rather than 
preventing it) and the use of high concentrations to prevent any viral replication (‘knock out’). This, 
theoretically, prevents any viable mutants being released. 
Resistance to antiviral agents will result from mutations in the viral genome. The modified proteins may 
act on cell events that occur upstream of the active compound (e.g. mutations in viral thymidine kinase 

CONTROL OF MICROBIAL INFECTIONS

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ANTIBIOTIC RESISTANCE(BACTERIA):
The most pressing problem facing medicine is the growing number of micro-organisms that are now 
resistant to a wide range of antibiotics. The term antibiotic resistance describes the condition where 
bacteria have developed or acquired means to overcome the inhibitory effect of one or more antibiotics. A chilling example is multidrug-resistant Mycobacterium tuberculosis. Without the development of new antibiotics, thoracotomy (removal of the affected lobes of the lung) appears to be
the only treatment option. The mechanisms by which bacteria develops resistance to antibiotics are either due to mutation or acquisition of resistance genes from other organisms.  
MUTATION
Clinically, mutation leading to resistance has not been a major problem compared with the acquisition of 
resistance genes. In general, the effect of mutation will be to modify the target protein such that the 
binding affinity of the antibiotic is reduced. The protein will tolerate a certain l…

CONTROL OF MICROBIAL INFECTIONS

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CONTROL OF ZOONOSES:
Zoonotic sources of infections will be impossible to eradicate without killing all the animal reservoirs. 
Alternative strategies are the only hope and, with over 200 organisms that can cause zoonoses in 
humans, control rather than eradication is of great importance. The transmission of many zoonoses from 
animals to humans occurs via insect vectors, for example louse-borne infections by Rickettsia
spp. (typhus), tick borne infections by Borreliaspp. (e.g. Lyme’s disease), yellow fever virus is 
transmitted by mosquitoes of the genus Aedes, and plague (Yersinia pestis) transmitted by fleas. Perhaps 
the most famous zoonosis, rabies is caught directly from the bite of infected animals, typically dogs. 
With deforestation and increased expansion of cities (the slum regions, that is!) humans are encountering 
arthropod vector-borne diseases more frequently. The displaced vectors are driven from forests into new 
geographical locations and the micro-organisms spread into new v…

CONTROL OF MICROBIAL INFECTIONS

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VACCINATION(CONTD...)

HETEROLOGOUS VACCINE
The use of cow pox virus as a vaccine for smallpox by Edward Jenner is an example of a live, 
heterologous vaccine. The term ‘heterologous’ is used to describe how a micro-organism (cow pox 
caused by Vaccinia virus) that causes infections in other hosts (cows) is able to induce protective immunity to the human form of the disease (smallpox caused by Variola virus) if used as a vaccine.


LIVE, ATTENUATED VACCINE
The BCG is a live strain of Mycobacterium tuberculosis that has been cultivated on culture medium containing bile, thereby reducing the virulence of the organism compared with the wild type. The Sabin polio vaccine is another example of a live, attenuated vaccine. 

KILLED ORGANISMS
Influenza and rabies vaccines are killed organisms, achieved by treating the virus with β-propiolactone, 
a chemical not dissimilar to formaldehyde, which does not destroy the protective antigens on the virus. 
The difference between influenza and rabies vaccine is th…