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PETRI DISH IN DIAGNOSTIC MICROBIOLOGY Basics in Diagnostic Microbiology Dr.T.V.Rao MD A Petri dish sometimes spelled "Petrie dish" and alternatively known as a Petri plate named after the German bacteriologist Julius Richard Petri is a shallow cylindrical glass or plastic lidded dish Microbiologists have many tools to perfect the knowledge in Microbiology and to make optimal use of their skills to diagnose many infectious diseases PETRI DISH is the basic tool we learn Microbiology however bacteriological diagnosis continues to be most need of the Hospitals and important need TO DIAGNOSE MANY INFECTIOUS DISEASES The Petri dish continues to be most important tool to our Microbiologists who report in the laboratory. An agar plate is a Petri dish that contains a growth medium used to culture microorganisms Individual microorganisms placed on the plate will grow into individual colonies each a clone genetically identical to the individual ancestor organism except for the low unavoidable rate of mutation. OR NON ADOPTING MICROBES AS ANAEROBES WHEN INCUBATED WITH ENVIRONMENT OF HIGH OXYGEN As a post graduated in 1980 petri dish was like a Stethoscope to a practicing Doctor I learnt real skills through putting dedication through observation of wonders in Bacteriology and Mycology. A Senior Microbiologist can assess how a student handles the Petri dish by primary observation One should know how to handle the petri dish in laboratory it is simple can be learnt with simple practices Sometimes we are very impressed how a young Microbiologist handles the petri dish Do not keep open your petri dish to the atmosphere at your work or during poring the plates Quickly replace the top half of the petri dish to prevent any airborne bacteria from contaminating the experiment. Set the petri dishes aside for 30 to 2 hours until the agar solution cools and hardens Label and seal the petri dishes. Once you have introduced or inoculated the bacteria you should replace the lid on the petri dish and seal it with some tape. • Make sure to label each petri dish with the source of the bacteria it contains otherwise you wont be able to tell which is which. You can do this using some tape and a marker. • as an extra precaution you can place each petri dish in a zipper-lock bag. This will provide an extra layer of protection against any hazardous bacteria colonies that may develop but will still allow you to view the contents of the petri dish. Thus the plate can be used either to estimate the concentration of organisms in a liquid culture or a suitable dilution of that culture using a colony counter or to generate genetically pure cultures from a mixed culture of genetically different organisms using a technique known as "streaking". In this technique a drop of the culture on the end of a thin sterile loop of wire known as an inoculator or a loop is streaked across the surface of the agar leaving organisms behind a higher number at the beginning of the streak and a lower number at the end. At some point during a successful "streak" the number of organisms deposited will be such that distinct individual colonies will grow in that area which may be removed for further culturing using another sterile loop Like other growth media the formulations of agar used in plates may be classified as either "defined" or "undefined" selective enriched media To know a little a defined medium is synthesized from individual chemicals required by the organism so the exact molecular composition is known whereas an undefined medium is made from natural products such as yeast extract where the precise composition is unknown. Samples can then be taken from the resulting colonies and a microbiological culture can be grown on a new plate so that the organism can be identified studied or tested. The streaking is done using a sterile tool such as a cotton swab or commonly an inoculation loop. Place the plates in a warm dark place to grow - not too warm but anything up to about 98 degrees F 37 degrees C should be fine. In a short time youll be greeted by an amazing variety of bacteria molds and fungi. - You’re likely to have a huge variety of colors sometimes fascinating shapes and smells in your tiny worlds. Count the number of colonies on the plate note the differences in color shape and other properties. Take the proper safety precautions. Before you attempt to dispose of your petri dishes you first need to take the proper safety precautions. •

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Although most of the bacteria you grow will not be hazardous large bacteria colonies may pose more of a risk - so you will need to kill them before disposal using household bleach. Protect your hands from the bleach by wearing rubber gloves protect your eyes with plastic goggles and protect your clothes by wearing an apron. HOWEVER THE MAJORITY OF THE WORK WE DO IN DIAGNOSTIC LABORATORY IN DEVELOPING COUNTRIES IS JUST ROUTING HANDLING OF THE CLINICAL SPECIMENS FROM URINE BLOOD SPUTUM CSF AND BODY FLUIDS IT IS JUST ESSENTIAL THAT WE SHOULD BE GOOD AT HANDLING A PETRI DISH TO THE OPTIMAL USE • Ref 1 Wikipedia on culturing bacteria and fungi • 2 Basic trends in diagnostic microbiology • Dr.T.V.Rao MD Professor of Microbiology Freelance writer IPhoto by Dr.T.V.Rao

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