microbiology Flashcards
What fungal medium is most suitable and what other features should fungal culture have?
Sabaurauds Dextrose Agar (SDA) is commonly and agents are added to inhibit growth of contaminating organisms. Chloramphenicol and or gentamicin is added to inhibit bacteria and cyclohexamide is added to inhibit the growth of contaminating fungi.
Fungi is incubated with and without anti-microbes for the reason that some chloramphenicol inhibits certain pathogens and cyclohexamide inhibits the yeast form of some dimorphic fungi.
what identification techniques are used to identify yeast?
Greentube production test cyclohexamide resistant (wont grow with it) india ink Hydrolyses of urea Tween 80 agar- morphology
what identification techniques are used to identify moulds?
gross appearance of the thallus which is wolly and white from the top and yello pigmented from the bottom.
Sexual structures (macroconidia) and asexual structures (basidiospores)
Test for thermal diamorphism (test if grows at 37degrees)
What is the process of gram staining?
Cell is fixed to slide with heat and then stained with a basic violet dye which is taken up by similar amounts by all bacteria. It is then washed
Slide then treated with iodine-potassium iodide to fix the stain. This forms a dye iodine complex. It is then washed
Then it it is decolourised with accetone or alchol and gram negative cells have the dye removed form their thin walls
Finally cells are counter stained with a paler diff colour.
Name a gram positive bacteria
Staphylococcus pseudentermidius
Name a gram negative bacteria
Pseudomas Aeroginosa
why do gram positive cells retain the violet stain?
The gram positive will not be stained itself but present a barrier that prevents the dye-iodine complex from being eluted out by the alcohol.
electron microscopy techniques to visualise bacteria?
shadow casting, negative staining, freeze etching, thin sections
gram negative cocci exist. true or false
FALSE BITCH
what is a mesosome?
Large irregular convoluted invaginations of cytoplasmic membrane
o Septal mesosomes (middle) help in cell division
Act as site for DNA attachment
o Lateral mesosomes (anywhere else): help bacteria to secrete substances (eg exotoxins) and help in electron transport
what is binary fission and how does it happen?
MEans by which bacteria divide.
1. Cell elongates
2. Transverse cell membrane is formed by mesosome
3. Nucleoids which have doubled in number preceding the division, are distributed equally to two daughter cells
• › If cell remains temporarily attached following division certain characteristic groupings result eg a chain of cocci
What are endospores? and what is an example?
Most are in gram positive cells
THey form undder certain conditions such as inadequate nutrition.
Surrounding the mother cell is the sporangium.
With a spore, the bacteria is much more resistant to the effects of heat, drying, freezing and toxic chemicals
- Case: Clostridium tetani
- Causes tetanus
- Gram positive rod
- Strictly anaerobic→Hates oxygen
- Survives in environment via endospores
- Spores get out of their endospores when conditions favourable such as puncture wound at tip of nail
What are the phases of bacterial growth?
1) Lag phase: This is the preperation stage and the cells do not significantly increase in number (some may increase in size and metabolic activity)
20 Low and exponential phase: In this phase, active cell division is occurring at an exponential rate. The bacteria has high metabolic rate and this makes them sensitive to antimicrobial agents.
3) Stationary phase: There is a decrease in the rate of growth until number of cells are constant. This is usually due to exhaustion of essential nutrients and build up of toxins.
4) Death or decline phase: Gram positive bacteria start to appear gram negative as their cell wall disintegrates (usually are dark purple)
What are MaConkeys agar?
MaConkey agars are used as selective and differential media.
Only bile tolerant bacteria grow (gram negatives) and then they are differentiated into lactose and non lactose fermenters.
-they will be pink if they ferment and yellow if they do not
What are the means by which bacterial variation is achieved?
Through gene transfer also known as recombination:
- Transformation is where DNA is taken up directly from environment. For this the cell must have large enough pores
- Congugation is mediated by plasmids that replicate independently from chromosomes. Pilli are used to provide a one way transfer of DNA between 2 organisms
- Transduction has bacteriophages transport the DNA for them.
