ICS - MIcrobiology 2 Flashcards
What does gram staining do?
Differentiate between gram positive and gram negative bacteria
Describe the gram staining process
- Fixation to microscope slide (heat/methanol)2. Primary stain - crystal violet3. Mordant - iodine4. Decolourisation - ethanol or acetone5. Counterstain - safranin
What colours does gram staining turn gram positive and negative bacteria?
Gram positive - purpleGram negative - pink
What bacteria does not stain with gram staining?
Acid fast bacilli, mainly mycobacteria (e.g. TB)
Why do mycobacteria not stain with gram staining?
Have a waxy, lipid cell wall that is impenetrable to the gram stain (also allows them to withstand phagolysosomal killing)
What stain is used for mycobacteria?
Ziehl-Neelsen stain
Describe the Ziehl-Neelsen stain process
Use heat to drive a dye (carbon fuchsin) inside the cell wall
What colours does Ziehl-Neelsen staining turn bacteria?
Acid fast bacteria (positive mycobacterium) - redNon-acid fast bacteria - blue
Define pathogen
Organism that causes or is capable of causing disease
Define commensal
Organism which colonises the host but causes no disease in normal circumstances
Define opportunist pathogen
Microbe that only causes disease if host defences are compromised
Define virulence/pathogenicity
How easily a pathogen is spread E.g. high virulence = easily spread e.g. measles
Define asymptomatic carriage
When a pathogen is carried harmlessly at a tissue site where it causes no disease
Describe cocci
Round-shaped bacteria. Can be individual, as pairs, (diplococci), in chains or clusters
Describe bacilli
Rod-shaped bacteria. Can be individual, chains, vibrio (curved), spirochaete (spiral)
Describe blood agar
- Contains sheep/horse blood- Provides a good medium for growing many different types of bacteria (non selective)
Describe chocolate agar
- Blood agar heated to 80 degrees celsius for 5 minutes- Releases nutrients into agar and makes it easier to grow certain organisms (fastidious bacteria)
What are 2 examples of bacterias that grow better on chocolate agar?
- Haemophilus influenza2. Neisseria meningitidis
Define fastidious bacteria
Bacteria that is hard to grow because they have complex/restricted nutritional and/or environmental requirements
Describe MacConkey agar
- Contains bile salts (inhibits gram positive), lactose and pH indicator- Designed primarily to grow gram negative bacilli- Enables differentiation of gram negative bacilli into lactose fermenters and non-lactose fermenters
What colour are gram negative bacilli on MacConkey agar?
Lactose fermenting - redNon-lactose fermenting - white/colourless
Describe CLED agar
- Cysteine lactose electrolyte deficient- Use to differentiate microorganisms in urine - Enables differentiation of lactose fermenting and non lactose fermenting gram negative bacilli
What colour are gram negative bacilli on CLED agar?
Lactose fermenting - yellowNon lactose fermenting - blue
Describe Sabouraud’s agar
- For fungi- Selective media
Describe XLD agar
- Xylose lysine deoxycholate agar- Selective media- Contains indicator
Describe Lowenstein-Jensen agar
- Specifically used for culture of mycobacterium species- Selective media
Which sites are sterile in the body?
- Blood- CSF- Joints- Lower resp tract- Pleural fluid- Peroneal cavity- Urinary tract
Which sites are colonised with flora in the body?
- GI tract- Skin- Oral cavity (upper resp tract)- Vagina- Urethra- Bladder
What is alpha beta gamma haemolysis?
Used to indicate different types of streptococci on blood agar plates
What test is used to differentiate beta haemolytic streptococci?
Streptoccoci Lancefield grouping test
What are the results of streptococcus lancefield grouping test?
Group A = strep. pyogenesGroup B = strep. agalactiaeGroup C = strep. dysgalactiaeGroup D = enterococcus
Describe the results of alpha beta gamma haemolysis
- Alpha - green/brown- Beta - clear- Gamma - neither (may be slightly discolouration in the medium)
What is the optochin test?
- Used in the identification of streptococcus pneumoniae- Filter paper discs are impregnated with ethylhydrocupreine hydrochloride and applied directly to inoculated plates before incubation
Describe the results of the optochin test
- If a culture is resistant to optochin, the disc won’t prevent cultures growing- If a culture is sensitive (susceptible) to optochin, the disc prevents cultures growing (streptococcus pneumoniae)
What is the coagulase test?
Used to differentiate bacteria that produce coagulase (e.g. staphylococcus aureus) from those that do not (e.g staphylococcus epidermidis/saprophyticus)
What is coagulase?
Enzyme-like protein that causes plasma to clot by converting fibrinogen to fibrin
Describe the results of the coagulase test
If coagulase is present, clotting will occur
What is the catalase test?
Used to differentiate bacteria that produce catalase (e.g. staphylococci) from those that do not (e.g streptococci)
What is catalase?
Enzyme that catalyses the release of oxygen from hydrogen peroxide
Describe the results of the catalase test
Hydrogen peroxide (H2O2) is added and if catalase is present, H2O2 will be broken down into 2H2O + O2 (gas bubbles)
Describe the characteristic features of gram positive bacteria
Single membrane and thick peptidoglycan layer
Describe the characteristic features of gram negative bacteria
Double membrane, thin peptidoglycan layer and lipopolysaccharides (endotoxin area)
What does the endotoxin area of gram negative bacteria comprise?
Terminal sugars. O antigen and Lipid A
Between what temperatures and pH range can bacteria grow?
-80 to +80 degrees celsius and pH of 4-9
What are the 3 phases of bacterial growth?
- Lag phase2. Exponential phase3. Stationary phase
Give an example of slow growing bacteria
TB
Give 2 examples of fast growing bacteria
E.coli and Staphylococcus. aureus
Give 2 functions of pili
- Help adhere to cell surfaces2. Plasmid exchange
What is the primary function of flagelli?
Movement
What is the primary function of the polysaccharide capsule?
Protection - prevents MAC or opsonisation molecules attacking
What types of bacteria release endotoxin?
Gram negative bacteria
What types of bacteria release exotoxin?
Gram positive and gram negative bacteria
What are endotoxins?
Component of bacteria’s outer membrane released when bacteria are damaged. Less specific and toxic to the host. Heat stable
What are exotoxins?
Proteins secreted that are specific and heat labile
What are plasmids?
Circular pieces of DNA that often carry genes for antibiotic resistance
Gram positive cocci can be divided into…?
Staphylococcus (clusters) and streptococcus (chains)
How do you distinguish between staphylococcus and streptococcus
Catalase test:Staph = catalase +ve (gas bubbles)Strep = catalase -ve (no change)
How do you further distinguish between staphylococci bacteria?
Coagulase test:+ve = Staphylococcus aureus-ve = all others (e.g. staph. epidermidis, staph. saprophyticus )