Diagnostic Bacteriology Flashcards
What are the five types of infection?
Bacteria
Fungi
Viruses
Protozoa
Helminth Worms
What colour does gram positive bacteria stain?
Purple
What is the cell wall of a gram positive bacteria?
Thick peptidoglycan cell wall
What colour does gram negative bacteria stain?
Pink
What is the cell wall of gram negative bacteria like?
two outer membranes with a thin peptidoglycan cell wall between it
What part of the bacterial cell wall “holds” the stain?
The peptidoglycan
In what order are the gram stain substances applied?
Crystal Violet
Iodine
Alcohol
Safranin
Explain fully how gram positive bacteria stains purple?
After the crystal violet stain has been applied, it is held by the peptidoglycan layers. Then iodine is added, and alcohol. The alcohol dehydrates the peptidoglycan so it tightens it, meaning in gram positive since there are alot of layers, the purple crystal violet cannot be washed out
How do gram negative bacteria stain pink?
After the alcohol has washed all the crystal violet out, safranin is added which stains any remaining bacteria pink
What are the different shapes of bacteria?
coccobacilli, bacilli, diplococci
What does the shape coccus describe?
Sphere
What does the term bacillus describe?
The rounded oval shape/ rod-like
What shape can coccus bacteria sometimes form?
Groups / clusters like grapes
Describe the shape of diplococci
Spherical in pairs
What does staphylcocci look like
Purple, spherical in grape like clusters
What does streptococci look like?
Purple, spherical in long chains
What does the hemolytic test do?
tests capacity of bacteria to produce hemolysins which are enzymes that damage RBCs
How do you carry out the haemolytic test?
samples are grown overnight on blood agar plates
What is gamma hemolysis?
No hemolysis around the colonies therefore no yellow zone
What is beta hemolysis?
Full hemolysis therefore large clear zone of haemolysis around the bacterial colonies
What is alpha hemolysis?
Partial hemolysis, therefore colonies appear more green
What medium is the hemolytic test conducted on?
medium containing sheep red blood cells
What type of hemolysis producing greening?
Alpha
What is the major cause of haemolysis in new born babies?
Group B Streptococcus or Streptococcus agalactiae is the commonest cause of meningitis in babies under 3 months
What type of agar is used for the lactose fermentation test?
MacConkey Agar
What does the MacConkey Agar contain?
bile salts, crystal violet and lactose
What colour does lactase positive bacteria show as?
Pink/red - this is because they are lactose fermenting which produces lactic acid, lowers the pH and therefore forms pink colonies
What colour does lactase negative/ lactose non-fermenter bacteria appear as?
Yellow/colourless colonies
Which type of gram (negative or positive) is the lactose fermentation test used to differentiate between?
Negative
What test is used to differentiate initially between types of gram positive bacteria?
Catalase Test
What will be seen in catalase positive bacteria?
Bubbles of oxygen
What will be seen in catalase negative bacteria?
No bubbles
What does catalase do?
Breaks down hydrogen peroxide
What test is used to differentiate between different catalase positive bacteria?
Coagulase test
What will be seen in coagulase positive bacteria?
Clumps
What will be seen in coagulase negative bacteria?
No clumps
How is the coagulase test conducted?
Add bacteria and plasma together
How do clumps form in the coagulase test?
Coagulase catalyses prothrombin to thrombin which causes fibrinogen to be cross linked, resulting in fibrin which forms a clot
How does the antibiotic susceptibility test show resistance?
There will be no zone of inhibition
What does a larger zone of inhibition suggest?
A more successful antibiotic as it has killed more bacteria
Which bacteria is gram positive, catalase positive and coagulase positive?
Staphylococcus aureus
How do you carry out the haemolytic test?
samples are grown overnight on blood agar plates
What are 4 different growth mediums for bacteria?
Nutrient agar, blood agar, MacConkey agar, Diagnostic sensitivity test agar (DST)
What is nutrient agar?
A simple medium used to grow bacteria. usually contains peptone, meat extract, and sodium chloride plus agar.
What is blood agar?
Nutrient agar base with sterile blood (either horse or sheep) added. This medium is suitable for testing for haemolysis (the destruction of red blood cells). Haemolysis is important for diagnosing certain microorganisms.
what is macconkey agar?
A selective media used to isolate coliforms and intestinal pathogens. It contains bile salts to inhibit the growth of non-intestinal pathogens. The media also contains lactose, peptone and the dye neutral red. Neutral red acts as a pH indicator; when the pH is acidic (< pH 6.8) it turns red/pink and when the pH is basic (> pH 8) it turns yellow.
MacConkey agar is commonly used to determine lactose fermentation by bacteria. If the bacteria ferment lactose, then acid is produced as a by-product which decreases the pH of the media. If the bacteria are lactose non-fermenters then they use the peptone instead, then ammonia is formed as a by-product which increases the pH of the media.
What is diagnostic sensitivity test agar?
A medium used for antibiotic sensitivity testing. None of the ingredients in the medium interfere with the action of the antibiotics and the agar base allows the diffusion of large molecule antibiotics.
