Medical Microbiology: Systems for Detection of Pathogens I Flashcards
What is taxonomy?
- The naming, defining and classiftying of organisms
What can the name of a pathogen imply about it?
- Names only imply capacity for pathogenicity
- They do not include an assessment of virulence
How can you define what a pathogen is?
- A pathogen is biological agent that causes disease or illness to its host
What is the problem with defining a pathogen as something that definitely causes disease?
- Some pathogens may be latent or not at a high enough concentration to cause disease
What would a more accurate way of defining a pathogen be?
- A microbe that is CAPABLE of causing a specific degree of host damage
What is a commensal non-pathogen?
- A pathogen that’s present but not capable of causing disease in the host
- E.g. E.coli
What is a zoonotic non-pathogen?
- A pathogen that’s present but only capable of causing disease in another host
- E.g. E.coli O157:H7 is subclinical in cattle
What is a commensal opportunist?
- A pathogen that’s present and capable of causing disease in the host but only in certain circumstances
- E.g. Bacteroides fragilis and Coagulase Negative Staphylococcus (CNS)
Are all positive samples diagnostic of active disease, why is this?
- No
- This is because the body’s immune system may fight off the pathogen before it’s able to cause disease
- Also, some pathogens can become latent within the body and cause disease later
- You may not have been exposed to a large enough amout of pathogen for it to cause disease
When taking a sample of a pathogen from a patient what things must be done to ensure the sample is good?
- Sterile sites must be free from contamination e.g. Skin flora in blood cultures
- Non sterile sites require decontamination of normal flora e.g. Faeces, Mouth, Skin
- Samples with high volume or relatively low infected pathogen load require concentration (centrifugation, filtering) e.g. CSF, Ascites, 24 hr Urine
Do you need to culture all samples of a pathogen?
- No, you don’t always have to culture an organism/pathogen to prove that it’s there
- This is especially true if you have an idea of what organisms may be present within a particualr sample
What is light microscopy used for when identifying organisms in samples?
- Light microscopy used to visualise large organisms
What is electron microscopy used for when identifying organsisms in samples?
- Electron microscopy is used to visualise really small organisms, mainly viruses
- Not usually used in diagnostic setting
Why would you use electron microscopy to visualise an organism when there more modern techniques that can be used?
- Organism/pathogen can’t be cultured
- PCR for oragnsim hasn’t be defined yet
- Unable to identify what disease a patient has
Bacterial staining can be used in conjunction with light microscopy when identifying organisms in samples. What can bacterial staining be used for?
- Used to:
- Identify whether a bcateria is gram negative or gram positive
- Identify shape of bacteria
- Idenify presence of spores or capsules
- Identify bacterial species
How can you view viruses using light microscopy?
- Add specific antibody to the viral antigen tagged to a fluorochrome to a sample and then view that sample under microscope
- Antibody will bind to cells with viral antigens so those cells will fluoresece which can be seen on light microscope
- This is called immunofluoresence staining
What is classical culture?
- Technique that involves use of agar plates with either blood or different types of electrolytes in it as well as some sort of carbon source which allows bactria to grow and form colonies
What is a selective medium?
- Growth medium that selects and only grows pathogens
What is a selective atmosphere medium?
- Growth medium that’s used to grow pathogens in specific conditions, usually representative of conditions they would usually grow in
- E.g. Aerobic atmosphere; CO2 rich culture; Microaerophilic Culture - low oxygen levels and Anaerobic culture
Metabolic testing can also be conducted in this bacteria culture system, what are some examples of metabolic tests that can be done?
-
Catalase test
- E.coli = +ve
- Clostridium perfringens –ve
-
Indole test - Can cleave indole from tryptophan
- E.coli = +ve
- Clostridium perfringens –ve
What is antibiotic sensitivity testing?
- Test used to identify which antibiotics particular bacteria are sensitive/insensitive to
- Allows for an antibiotic profile to be made for a particular bacteria
- Can also be sued to find out how much of an antibiotic is needed to kill bacteria (E.test)
How can viruses be identified when growing them using permissive cell lines?
- Can be identified via cytopathic effect they produce, changes in the way the cells they infect grow
What technique is used to identify viruses instead of ciewing cytopathic effect in permissive cell lines?
- Antigen detection via direct ELISA or antibdoy detection via serological ELISA
