prac exam Flashcards
Why did more bacteria grow on the washed hands side
Washing removed layer of skin lipids, exposing more bacteria to the agar surface
Differentiate between the appearance of staph aureus and epidermidis on mannitol salt agar
Epidermidis does not ferment mannitol, stays pink
Aureus ferments mannitol, turns agar yellow
Three characteristics of staph aureus. How are these characteristics visualised?
Coagulase positive: clot formation in serum
Catalase positive: bubbles with addition of hydrogen peroxide
Gram positive: purple
Why is mannitol salt agar used to select for normal skin flora?
Replicates conditions of the skin: high salinity
Differentiates between pathogenic and nonpathogenic staphylococci: fermentation of mannitol yields yellow colour, indicating presence of pathogenic strains like S. aureus.
What additional information should be obtained before performing a coagulase test on Gram positive cocci?
The coagulase test differentiates S. aureus from other strains of staphylococci. To determine whether the gram positive cocci is staphylococci, a catalase test should be conducted as staphylococci are catalase positive.
List 3 factors that protect the skin from infection:
Many layers of keratinized cells, dryness, high salinity, lipids (sebum) from oil glands
What is coagulase and how does it relate to the pathogenicity of staphylococci?
Coagulase is an enzyme which clots fibrin in blood to form blood clots. This may protect the bacterium from phagocytosis and isolate it from other defenses of the host.
Why is the swab moistened with saline for a skin swab?
So that the bacteria will be transferred from the skin to the swab and onto the agar plate.
What test differentiates the three major species of Staphylococcus?
Coagulase test. Aureus and Intermedius are coagulase positive.
How could S. epidermidis and S. saphrophyticus be distinguished in the Micrococcacae identification key?
They will have different 5 digit keys.
Why are staphylococcal infections frequent among hospital patients?
Weakened immune systems and presence of resistant bacteria like MRSA.
Why is blood agar a differential medium for streptococci?
Allows for differentiation between alpha, beta and gamma haemolytic species.
Is Gram staining of significant importance in the identification of organisms studied in this exercise?
No, only streptococci were used, which are all gram positive.
What enzymatic test would be used to differentiate between staph and strep if the sample is known to be a gram-positive coccus?
Catalase test. Staph is catalase positive while strep is catalase negative.
Which streptococcus is susceptible to optichin?
S. pneumoniae
Which streptococcus is resistant to optichin?
S. pyogenes
Which streptococcus is resistant to bacitracin
S. pneumoniae
Which streptococcus is susceptible to bacitracin?
S. pyogenes
What type of haemolysis is produced by S. pneumoniae?
Alpha- incomplete green
What type of haemolysis is produced by S. pyogenes?
Beta- complete
Name at least 4 types of disease caused by S. pyogenes:
Streptococcal pharyngitis (strep throat), scarlet fever, impetigo, streptococcal toxic shock syndrome, rheumatic fever.
How would you differentiate between alpha and beta haemolysis?
Beta haemolysis is complete haemolysis with a clear zone with a clean edge able to be visualized around the colony while alpha haemolysis is incomplete, producing methemoglobin and a green, cloudy zone around the colony.
Enterobacteriaceae characteristics:
Short, gram negative, non-spore forming bacilli
Which two enterobacteriaceae are pathogenic
salmonella and shigella