MICROBIOLOGY

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 qual

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 vir

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

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 Borrelia spp . (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 micr

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 tuberculosi s 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 i