Diagnostics Flashcards
What are the types of methods used for identification of microorganisms associated with infectious disease
- Direct
2. Indirect
Examples of direct methods of microorganism identification
- Blood
- Faeces
- Tissue biopsy
- Mucosal swab
Examples of indirect methods of microorganism identification
- blood sample and search for an antibody against a suspected pathogen
what happens after collected sample using direct method
- speciment collection
- Microscopic examination
- Microbial detection using
a) culture based techniques
b) non culture techniques
When collecting specimen using direct technique how should you collect?
- Aseptically
- representative site
- sufficient quantity
- Specimens collected prior to antimicrobial therapy to maximise pathogen recovery
- If cultures not immediately initiated after collection, specimens must be refrigerated
How do you obtain a representative sample?
- specimen must be from site containing pathogen
2. adequate volume
Definition of aseptic technique
refers to a procedure that is performed under sterile conditions, or to prevent contamination.
Samples are submitted to clinicians. What else must be submitted?
- complete submission form presumptive diagnosis
- must specify: what is the sample type, where swab collected from
- Case history
- Required test
How is a sample stored?
- Only do so if necessary
- 4oC better than anything cooler or warmer
- long-term storage at -80oC or below
How is sample transported?
- stop bugs from dying
get the sample to the lab fast! - use “transport medium
Pros of direct method: microscopic observation
- cost effective lab test
- Immediate info on presence/ absence and number of organisms
- Provides info on host cellular response
3 ways to prepare a slide for microscope
- smear using swab
- smear using aspirate (spread sample)
- Tissue impression smear (biopsy)
What is the msot common sued stain of bacteria?
What categories are the bacteria then divided into?
- Gram stain
- Gram positive = purple stain
- Gram negative = pink stain
- Before staining what must happen to the smears made from the specimens?
- What is the slide then covered by? and how long?
- Smears must be fixed, usually by passing them trough a Bunsen burner flame 2-3 times.
- The slide is then covered with cristal violet for 30 sec, iodine, 60 sec, then decolorized by covering the slide with alcohol for few sec.
- Iodine is also used in Gram’s staining, it enhances dye to enter through the pore present in the cell wall/membrane.
What are phenotypic trains of bacteria that help with identification?
- Staining properties: gram, acid fast, polychrome methylene blue
- Oxygen requirements: anaerobic or aerobic?
- Culture characteristics: colonial morphology, size, pigmentation
- Microscopic morphology: shape, size, cellular arrangement, presence of spores
- Biochemical reactions: fermentation reactions, catalase, oxidase, urease
What methods to microbiologists use to identify an unknown pathogen?
Methods that fall into 3 categories
- Phenotypic based: describing morphology (micro and macroscopic)
- Immunological: serological analysis = identification of bac, fungi, virus
- Molecular techniques: genetic techniques suitable for slow growing, non culturable, viruses
What are different types of media used for bacteria cultivation?
- Enrichment media are general purpose media supplemented by blood or other special nutrients to encourage the growth of fastidious organisms
- Selective media favour the growth of particular microorganisms and inhibits the growth of others.
- Differential media distinguish between different groups of bacteria on the basis of their biological characteristics
When interpreting the test what are 2 sources of error?
- test itself: false positive and false negative
2. interpreter of test
A perfect test is:
- never positive in a patient who hasn`t got the disease
- never negative in a patient who is in fact diseased
