Systems for Detection of Pathogens I Flashcards
Define pathogen
A microbe CAPABLE of causing a specific degree of host damage
What is a commensal non-pathogen?
PRESENT but NOT CAPABLE of causing disease in the host
What conditions must be met to obtain a sample?
• Sterile sites must be free from contamination
o e.g. Skin flora in blood cultures
• Non sterile sites require decontamination of normal flora
o e.g. Faeces, Mouth, Skin
• Samples with high volume or relatively low infected pathogen load require concentration (centrifugation, filtering)
o e.g. CSF, Ascites, 24 hr Urine
What needs to be done further after taking a culture or a direct sample?
- For enrichment, purification and amplification.
- Direct for concentration or sample treatment.
- Both use molecular DNA, gross morphology (microscopy) and chemical composition for identification. E.g. immunofluorescent staining with pathogen specific conjugated antibody.
- Bacteriology requires ability of test system to be able to grow pathogen using selective media with selective environment (temp/o2/co2) for that species.
- A microaerophile is a microorganism that requires oxygen to survive, but requires environments containing lower levels of oxygen than that are present in the atmosphere (i.e. <21% O2; typically, 2–10% O2).
What is chocolate blood agar?
Chocolated blood agar is a differential medium for gram-positive cocci- by adding antibiotics. Otherwise, it is non-selective and grows both aerobic and anaerobic bacteria. Used for respiratory pathogens- Neisseria meningitidis/gonorrhoea, haemophilus influenzae and brucella melitensis. 5% Co2. H influenzae does not grow on blood agar but will on chocolated agar.
What is an anaerobic agar?
Some pathogens are unable to survive in aerobic conditions due to the effects of oxygen on their metabolism
e.g. Clostridium tetani, Clostridium botulinum, Clostridium difficile, Bacteroides fragilis
Gram positive, spore forming, anaerobic,
rod/bacillus shaped.
Define obligate bacteria
Obligate (will die if not in fully aerobic or anaerobic environment) or facultative
(capable of switching ATP production in aerobic or anaerobic environments to
survive). Haemophilus influenzae is a facultative anaerobic.
Some bacteria need NO2 environment- clostridium perfringens.
What is alpha and beta haemolysis?
Alpha-haemolysis (α-haemolysis) is a partial or “green” haemolysis associated with reduction of red cell haemoglobin. Alpha haemolysis is caused by hydrogen peroxide produced by the bacterium, oxidizing haemoglobin to green methaemoglobin.
Beta haemolysis represents a complete breakdown of the haemoglobin of the red blood cells in the vicinity of a bacterial colony. There is a clearing of the agar around a colony. Area appears yellow and transparent.
What is Optochin?
Optochin is a chemical that is toxic to some bacteria but harmless to others. It is useful in the identification of Streptococcus pneumoniae, the alpha-haemolytic
Streptococcus most commonly susceptible to this chemical.
How is Streptococcus pneumoniae identified?
Streptococcus pneumoniae- identified by optochin sensitivity, gram stain microscopy
(gram positive) and colony morphology and colour.
Colony identification, systematic identification and colony resistance to antibiotics.
What is the classic metabolic test?
Can identify Catalase enzymes
What is phage typing?
The viruses that infect bacteria are called bacteriophages and some of these can only
infect a single strain of bacteria.
What is an antibiotic sensitivity test?
What does it determine?
What does it identify?
A test that determines the “sensitivity” of bacteria to an antibiotic. It also determines the ability of the drug to kill the bacteria.
Scale of concentration along paper.
Identify minimum inhibitory concentration where a ring begins to form- area cleared
of growth.
How do we test for food poisoning?
- Food poisoning- stool tests.
- Direct microscopy to look for eggs/cysts or amoebae or parasites.
- Incubate aerobically at 37oC and microaerophilically at 42oC.
- Aerobic incubation on MacConkey agar and desoxycholate agar plates.
- Identify bacteria through biochemical profiling and then antibiotic sensitivity testing, showing shigella and salmonella.
- Spread stool on campy CVA selective media plate.
- Positive growth (silver grey colonies).
- Gram stain shows gram negative curved rods.
- Oxidase test is positive- campylobacter jejuni.
How can we test for viruses?
Culture- requires permissive cell lines (Vero cell for herpes simples- kidney epithelial). Cytopathic effect. Immunofluorescent staining of culture.
Direct antigen detection- ELISA.
