MODULE 4 Flashcards
What is the difference between a viable and total count of bacteria?
Total count: all cells, viable count: living cells only
What are the advantages and disadvantages of using turbidity as a measure of bacterial growth?
Advantages: rapid, fairly accurate measure of growth
Disadvantages: any particulate matter interferes with optical density readings, does not directly measure cell numbers
Why should a standard curve be prepared when using turbidity as a measure of bacterial growth rates?
Turbidity does not measure actual cell numbers. Absorbance is proportional to cell numbers so a standard curve is needed to relate absorbance to numbers.
Why does growing in a salty environment pose a challenge?
A salty environment has higher solute concentration than the cytoplasm of a cell. Osmotic pressure is placed on the cell and water flows out. A halophile has strategies to overcome this, usually by increasing internal solute levels.
How do complex and defined media differ?
Complex media: precise composition is unknown
Defined media: precise composition is known
Give an example of a selective and differential medium. What are the components that make it selective and differential?
There are numerous, one is MacConkeys agar: contains bile salts and crystal violet that inhibit growth of gram-positive bacteria. Gram negative bacteria that ferment lactose produce red/pink colonies while those that do not ferment lactose produce white colonies.
How does sterilisation differ to disinfection?
Sterilisation: complete destruction or removal of all viable microbes
Disinfection: use of chemicals to kill or inhibit pathogenic microbes
What are the main methods of (i) dry heat and (ii) moist heat sterilisation?
Dry heat: incineration & burning to ashes, hot air oven
Moist heat: autoclaving, boiling/hot water, UHT, pasteurisation
Briefly explain the roles of time, temperature, and pressure in the functioning of the autoclave.
High pressure is used in an autoclave so that liquids do not boil at temperatures beyond 100°C. The heat is more important that the pressure in killing the microbes and is usually used for 15 minutes at 121°C as a balance to kill all organisms and not damage items autoclaved. The time must be increased for bulky samples to ensure the objects or liquids are heated with steam to 121°C for at least 15 minutes.
The following disinfectants were tested against S.aureus using the tube dilution test. Which is the most effective disinfectant? What is the MIC for each disinfectant?
Dilution Disinfectant A Disinfectant B Disinfectant C
1/20 - - -
1/40 + - -
1/80 + - +
1/160 + + +
1/320 + + +
Most effective: Disinfectant B
MIC: A – 1/20, B – 1/80, C – 1/40
What are the advantages and disadvantages of using radiation to sterilize items?
Advantages: If sufficient dosage is used, radiation is highly effective and quick in killing microbes. Very useful for anything effected/damaged by high temperature (e.g. plastics, foods, pharmaceuticals).
Disadvantages: The high costs associated with the specialized equipment make it feasible only for large industrial applications. UV radiation sterilizes only the surfaces of objects. Insufficient dosage can lead to mutations and survival of microbes. Due to the high energy of some radiation procedures, there are many concerns of potential radioactive contamination, production of carcinogenic or toxic products, and alteration of nutritional value.
Briefly describe the mode of action of the β-lactam antibiotics.
Target bacterial cell wall. More specifically, β-lactam ring binds to transpeptidase enzyme (penicillin binding protein) in peptidoglycan synthesis. Therefore cross-linking of the peptide chains can’t occur, resulting in degradation of the cell wall and lysis.
Growth factor analogs may inhibit microbial metabolism, though only a few are practically useful. Why would this be the case?
Many can be toxic, also inhibit host cell metabolism and not specifically microbes.
Where would you best place a sterilization indicator in an autoclave? Explain your reasoning.
Centre of the autoclave. Steam enters from the back and contacts external parts of the items first, so takes longer to reach the centre
Explain what is meant by selective toxicity. How does selective toxicity relate to the use of antimicrobial chemicals and the development of chemotherapeutic agents?
Selective toxicity is the idea that only certain organisms are killed while others survive. With antimicrobials, the target is the particular pathogen causing a disease. Ideally other microbes like normal flora are not killed. In developing chemotherapy drugs certain cells are killed (e.g. cancer cells) and healthy cells are not killed
