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
Which two enterobacteriaceae are occasionally pathogenic
proteus and klebsiella
Which two enterobacteriaceae are normal flora
e. coli and enterobacter
What does a bracket indicate under the microtube
Fill both microtube and cupule
What does a line indicate under the microtubule
Fill cupule with mineral oil (anaerobic)
How are values allocated in the microtube tests?
For the first test, a positive result receives a value of 1. The second positive test receives 2 and the third positive test receives 4. Negative tests receive 0.
What are the advantages and disadvantages of multi-test systems?
Advantages: minimal storage space, less use of culture media, rapid results, application to a computerized system for microbe identification
Disadvantages: difficulty in obtaining proper inoculum size, media and reagent carryover, using inoculum that is too old, requires knowledge on how to inoculate and interpret results, reader bias, expensive
What Enterobacteriaceae are of medical significance? List and describe the infections caused by 3 of these organisms.
Salmonellae: enteric fevers, typhoid and gastroenteritis. Penetrate the intestinal mucosa to enter blood stream, infecting other organs. LPS endotoxin causes ulceration of the intestinal wall, abdominal pain, nausea, vomiting and diarrhoea.
Shigella: shigellosis (bacilliary dysentery) with ulceration of the large intestine, explosive diarrhoea, fever and dehydration.
E. coli can cause bloody diarrhoea (enterohaemorrhagic E. coli) and are the most common cause of UTIs.
Would similar results be obtained by use of the computer-assisted method and the traditional colour change method? Why?
Yes, because the same reagents/indicator reactions are involved. A species will either be positive or negative, no matter which method is used as long as the reagents and incubation conditions used are the same.
What is the clinical justification for the use of a rapid test procedure such as the Enterotube II system for the identification of enteric microorganisms?
When there are multiple tests needed to identify the bacteria or a need to differentiate between multiple species so that an appropriate treatment can be chosen and commenced in a timely manner.
Which growth media would you choose to differentiate a mixed culture of faecal enterococci, staphylococci and E. coli?
Staphylococci: Vogel Johnson or mannitol salt agar
E. coli and enterococci: MacConkey agar no. 3- E. coli would appear dark red to mahogany, enterococci would grow on this agar but be translucent to light brown in colour.
- Describe how a zone of inhibition is formed.
The disks used contain antibiotics which diffuse out of the disk and into the surrounding media which is inoculated with bacteria. Closest to the disk, the antibiotic concentration is greatest, and bacterial growth will be suppressed. Concentration of the antibiotic decreases with distance from the disk, eventually reaching a concentration that is insufficient to prevent bacterial growth. This results in a ring around the disk where no bacterial growth occurs, surrounded by bacterial growth where the antibiotic concentration is insufficient to prevent growth.
- Based on the size of inhibition, can you determine the antibiotic(s) that is (are) most effective against each organism?
Antibiotics that are the most effective against each organism will have a zone of inhibition of the greatest diameter.
- Some antibiotics can kill or inhibit many different kinds of pathogenic bacteria (i.e. they have a broad spectrum). In this experiment, did you observe an antibiotic that inhibits both organisms?
An antibiotic that inhibits both organisms will have a large, similarly sized zone of inhibition for both species.
- How can the Gram reaction be useful in prescribing antibiotic treatment?
Antibiotics are classified based on their spectrum of action. Some antibiotics will be effective against either gram negative or gram-positive bacteria due to antibiotic resistance properties of these two classes, while some will be effective against both. For this reason, the Gram-status of an organism needs to be known in order to prescribe an antibiotic that will be effective in inhibiting the growth of that organism.
- Based on the density of bacterial suspension, did you find any difference in the size of inhibition zone?
Areas with a high concentration of bacteria will have smaller zones of inhibition compared to areas with a low concentration of bacteria, as a higher concentration of antibiotic would be required to suppress bacterial growth
- Is the disk-diffusion technique measuring bacteriostatic or bactericidal activity? Briefly explain.
Bacteriostatic activity as once the antibiotic concentration is below its minimum inhibitory concentration, the bacteria is still able to grow. If it was bactericidal, no growth would occur even at concentrations of antibiotic below the minimum inhibitory concentration.
- In which growth phase is an organism most sensitive to an antibiotic?
The exponential growth or log phase where bacteria are rapidly multiplying.
- Why is the disk diffusion technique not a perfect indication of how the drug will perform in vivo? What other factors are considered before using the chemotherapeutic agent in vivo?
The agar plate does not involve factors such as body pH, how a drug is metabolized and excreted (interindividual variation in drug metabolism), concentration of bacteria in the body or the possibility of drug resistant strains.
What 3 tests is SIM media used for?
Tryptophan metabolism to indole, hydrogen sulfide production, motility
Describe a positive result in the indole test
Addition of Kovac’s produces cherry red colour