Presentation on theme: "Diagnostic Testing in the Microbiology Laboratory Jane Wong Public Health Microbiologist September 30, 2003"— Presentation transcript:
Diagnostic Testing in the Microbiology Laboratory Jane Wong Public Health Microbiologist September 30, 2003
Topics Some basic principles of microbiology testing A crash course in microbiology Follow a specimen through the lab Laboratory staffing issues
Media and Culture Media: Nutrients (agar, pH indicators, proteins and carbohydrates) used to grow organisms outside of their natural habitats Culture: The propagation of microorganisms using various media
Direct and Indirect Testing Direct: Demonstration of the presence of an infectious agent –Culture –Microscopy –Molecular methods such as PCR Indirect: Demonstration of presence of antibodies to a particular infectious agent –Serology
Sterile versus Non-sterile Body Sites Sterile body sites: –These sites normally do not contain any bacteria, so any bacteria found there are significant Blood Spinal fluid Non-sterile body sites: –These sites are open to the external environment and normally contain bacteria Throat Feces
Specimens from Sterile Sites Any organism growing in a normally sterile site is significant Identify it
Specimens from Non-Sterile Sites Only look for specific pathogens Physician will order test for a specific organism, or group of organisms Other “normal flora” bacteria will be present, but are not be identified
Sensitivity The fraction of those with the disease correctly identified as positive by the test. Isolation and identification of a known pathogenic organism may not be a very sensitive test –If the organism is present, it may not be found 100% of the time There can be false negatives
Specificity The fraction of those without the disease correctly identified as negative by the test. Isolation and identification of a known pathogenic organism is a very specific test –If the organism is not present in the specimen it will not be found
Documentation Specimen is logged in upon arrival in laboratory All tests and results are recorded and initialed by microbiologist All media and reagents are batch tested with positive and negative controls All equipment is checked at least once a day to be sure it is operating within predetermined parameters
Specimen Appropriateness Collection Transport to lab Inoculation of media Culture and isolation Confirmation Report
Appropriate Specimen From relevant body site Adequate amount Quality
Collection No contamination Appropriate equipment Good instructions to patient
Transport to Laboratory Safe packaging Good labeling Temperature
Inoculation of Media Use appropriate culture media –What kind of specimen is it? –What test did the physician request?
Culture media Used to grow bacteria Can be used to: –Enrich the numbers of bacteria –Select for certain bacteria and suppress others –Differentiate among different kinds of bacteria
Microbiological Culture Media
Isolation of Individual Bacteria Specimen is “streaked”, using a sterile loop, onto solid media. The agar plates (media) are incubated at appropriate temperature and atmosphere –Often at 35º C. –Often at 5% CO 2 –Usually first examined after 24 hours
“Streaking a Plate”
Growth of Colonies Bacterial Colony –Result of one bacterium being isolated from others during “streaking procedure” –That bacterium grows in numbers exponentially –Many bacteria have a generation time of 20 minutes –2 72 organisms in one colony after 24 hours!
Classical bacterial identification can only be performed on pure cultures of bacteria (ideally, all descendants from one bacterial cell)
Mixed Culture of Soil Organisms Containing Bacillus anthracis
Colony “Picking” Sterile needle or loop is touched to surface of colony and transferred to fresh, sterile media Incubation for another 24 hours
Colonies of Bacteria in Pure Culture
Pure Culture of Francisella tularensis Colonies After 72 hours Growth
Pure Culture of Yersinia pestis Colonies on Blood Agar After 48 hours of Growth
Yersinia pestis Colonial Morphology Viewed With Transmitted Light
Confirmation Now we have a pure culture of bacteria Testing is now done to confirm the identification of the bacteria culture –Stains –Biochemical tests –Serological tests (using known antibodies) –Molecular tests (nucleic acid probes)
Gram Stain of Streptococcus sp.
Yersinia pestis Gram stain
Gram stain of Brucella sp.
B. anthracis Gram stain showing spores
Gram stain of B. anthracis from broth culture
Examples of Biochemical Tests Left: API 50 Test Above: Antimicrobial Sensitivity Test
Case Study Patient arrives in emergency room with f ever (temperature greater than 100 degrees F). The fever is accompanied by chills or night sweats. Flu-like symptoms. Non-productive cough, chest discomfort, shortness of breath, fatigue, muscle aches
Patient Admitted to Hospital Blood cultures ordered Blood drawn and immediately placed in blood culture bottles
Blood Bottles Incubated Bottles are automatically tested every 10 minutes. Positive results are tagged for quick processing. Negative bottles can be batch-scanned out of the system and unloaded at the end of protocol.
18 Hours of Incubation Blood culture incubator signals that there is growth in one of the bottles. It is removed and a Gram stain is performed
Microbiologist Suspects Bacillus anthracis Reports results so far to supervisor Streaks a fresh blood agar plate and incubates it May perform wet mount test with India Ink to see “capsule” around individual bacteria Inoculates media to observe motility
Bacillus anthracis India Ink Preparation
Growth on a Blood Agar Plate (Petri Dish) After Hours
Gram stain of B. anthracis from broth culture
Motility B. anthracis is non-motile. Other Bacillus species are motile.
Laboratory Cannot Rule Out Bacillus anthracis Refers the culture to a reference laboratory that is part of the Laboratory Response Network (LRN)
Report Final report goes to physician The validity of this report is dependent upon: –Appropriateness of specimen –Proper collection and adequacy of specimen –Appropriate transport to lab –Use of media of known quality –Culture and isolation by knowledgeable personnel using equipment known to be operating correctly –Confirmation by tests of known quality –Results interpreted and reported by professional staff –No transcription or computer errors
Molecular Tests Biotechnology has given diagnostic laboratories very powerful tools – for rapid detection and identification of human pathogens –for strain typing for epidemiological investigations
The Flip Side! Biotechnology companies attract recent college graduates –Majors in biology and allied fields –Salaries usually higher than clinical or government public health labs offer –Appeal to public service only goes so far! Result: public health and clinical laboratories have trouble recruiting and retaining laboratory personnel.
Other Factors in Personnel Shortage Training opportunities have been drastically reduced Pay is not competitive Much of the work force is approaching retirement age
Licensing Applications/Year For Clinical Laboratory Scientist Certification