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Microbiology section of clinical laboratory is adopting
automation more than hematology and chemistry
section.
Two main tasks of microbiology laboratory are:
 Identify the causative agents of infections
 To determine Antibiotic susceptibilities.
History:
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Traditionally it has been accomplished by test tube
biochemical reaction and agar diffusion plates.
In 1960 and 1970 microorganism identification kits
becomes popular kirby-Bauer antimicrobial
susceptibility testing.
In 1980 and 1990 automatic susceptibility testing
founded on the principle of nephelometry,
spectrophotometry, fluorometry and radiometry.
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There are two systems for detecting microorganisms in
the blood and other fluids.
 Becton microbiology system, BACTEC.
 Organon Teknika’s BacT/Alert.
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There are four systems for detecting bacteria and their
antibody susceptibility:
One immunoassay system for identifying organisms
antigens and hosts antibodies:
 BioMerieux’s VIDAS.
BACTEC system to detecting Microorganisms in blood.
In early 1970s, introduction of BACTEC 460.
 It is based on the automated measurement of
carbon dioxide produced by growing
organisms.
 It has reading capacity of 60 vials.
 The culture medium: carbon 14 labeled
glucose and other substrates.
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Components:
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This systems uses 4 components:
 Disposable culture vials inoculated with blood.
 The BACTEC instrument that continually
tests vials to asses microbial growth.
 Culture gas and Adopters to regulate gas
flow
 Shaker to agitate the vials.
Working principle:
The microorganisms break down the
labeled chemicals and release C-14
labeled carbon dioxide.
 Detection of C-14 labeled carbon
dioxide indicates growth of
microorganisms, fungi, myobacteria.
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Block Diagram:
Procedure:
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Aerobic and anaerobic culture bottles are
incubated offline in standard microbiology
incubator.
Aerobic Bottles are placed on shaker for
first 24 hours, which simulates faster
growth.
Blood culture vials are placed on BACTEC
instrument in four vial rack.
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Testing of vial begins with a pump
producing a partial pressure in ion
chamber.
Ion chamber contains positive and negative
probes from and electrometer.
Radioactivity in gas changes the
impedance between probes.
Change in impedance is converted in
growth index, GI number.
This GI number is displayed on front panel
and printed on paper.
Output indication:
If GI value increases the threshold
value, red light illuminates. This
identifies vials with microorganisms.
 Then test head rises and leaves the vial.
 The head flush and ion chamber outlets
valves are opened.
 CO2 is retained in soda lime trap.
 Ion chamber and electrometer is
prepared for next reading cycle.
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Maintenance:
Checking of display lights
 Examining the needles
 Changing the tubes
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