The different methods used for Laboratory Diagnosis of Infection.
Discussion on the advantages and disadvantages of traditional testing compared to the more recent automated, and rapid molecular methods:
1. Automation
2. Molecular tests
3. Mass spectrometry: MALDI-TOF
With free cited revie...
Name: Karen Chisanga
Content: Lecture Notes
Module: Medical microbiology
Advances in Laboratory Diagnosis of Infectious Diseases
Learning Objectives:
• Describe the different methods used for Laboratory Diagnosis of Infection
• Discuss the advantages and disadvantages of traditional testing compared to the more recent
automated, and rapid molecular methods:
1. Automation
2. Molecular tests
3. Mass spectrometry: MALDI-TOF
Part 1 – Automation
Slide – Laboratory Diagnosis of Infection
If infection is suspected, the following samples of tissue of fluid are collected:
• Urine
• Blood
• Faeces
• Pus
• Sputum cerebrospinal fluid
Type of sample will depend on where doctor thinks the infection is i.e., if it is a urine infection, a urine
sample is collected. Specific tests are also requested according to the case clinical history of the patient.
Slide - Traditional methods of diagnosis in microbiology:
• Microscopy – e.g. gram stain method, tagging with fluorescent pigment
• Culture – specialised agar for certain types of bacteria, broths used. This method takes quite a while
to get the results.
• Identification: morphology, biochemical tests (API strips, positive or negative profile of the
microorganisms), serogrouping, phage typing etc – takes 24/48 hours to get results
• Antimicrobial susceptibility – zone of inhibition using antibiotics, takes time to get this result too
• Serology: host ab (IgG) response serum
Slide – Advantages of traditional methods:
• Catch-all – if two or more infections are happening at the same time in a patient, traditional
methods identify this
• Established methods and sensitivity determined (PHE standard microbiology investigations SIMs)
• Living organisms can be recovered and pure cultures can be obtained for further testing or archived
• Can quantify viable organisms in sample (cfu/ml)
Slide – Disadvantages of traditional methods
• Many methods are labour intensive
• Perceived as expensive and slow
• A very skilled operator is needed for procedure – colony morphology, takes time to know what the
different microorganisms look like.
, Name: Karen Chisanga
Content: Lecture Notes
Module: Medical microbiology
• Results may not have a definite identification
• Some organisms are difficult to culture
Slide – Automation
Automation methods are becoming more prevalent in microbiology testing. Following explains how
different traditional methods are being automated:
• Microscopy – urine analysers which either use photometric, or flow cytometry
• Broth culture – Often detect colorimetric change to spot growth earlier than visual detection, e.g.,
BacT/ALERT (usually for blood and TB cultures), microorganism is cultured and incubated in blood
samples in the machine, as organism grows, gas within the BacT/ALERT change in composition,
this is detected and notified via alarm.
• Biochemical ID and MIC – VITEK has automated these methods
Figure 1a Shows urine analyser used in
microscopy
Figure 1b Shows the BacT/ ALERT
automated method
Slide – VITEK 2
• VITEK 2 is a fully automated system for bacterial/ fungal identification and antibiotic susceptibility
testing.
• It reduces set-up time and minimizes manual steps
• Uses ID and antimicrobial susceptibility testing (AST) cards and process the samples (like the API
strip).
• Provides rapid microorganism identification
• Provides same-day antimicrobial susceptibility testing, no pipetting involved
• Advanced expert system validates IDs
• Data management software allows for generation of epidemiology reports and antibiograms
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