3.4 Microbiology Flashcards
(25 cards)
What are bacteria distinguished from one another?
Bacteria can be distinguished from one another by their size, shape, metabolism, antigenic features, genetics and by their staining characteristics.
What are the 3 basic shapes of bacteria
- The cocci (singular coccus) are spherical,
- Bacilli (singular bacillus) are rod-shaped
- Spirilli (singular spirillum) are spiral in shape.
Why is the shape of bacteria the way it is?
The shape of bacteria is due to the rigid cell wall which has a unique structure of a 3D mesh of peptidoglycan and murein.
What are the two basic types of cell wall ?
Gram positive and gram negative
Describe the gram positive cell wall.
Gram positive bacteria have cell walls with a thicker layer of peptidoglycan and murein than gram negative bacteria.
Which can retain the crystal violet/iodine complex within their cells when washed with alcohol, staining PURPLE
What do gram positive bacteria stain as and why?
When washed with alcohol, they stain PURPLE.
This is becase they have a thicker peptidoglycan/murein layer which can retain the crystal violet/iodine complexes.
Describe the gram negative bacteria
- When treated with alcohol, the gram negative bacteria lose their outer LIPOPOLYSACCHARIDE MEMBRANE
- and the thin inner peptidoglycan layer is exposed.
- This means the crystal violet/iodine complexes are washed from the gram negative cell along with the outer membrane
- They stain RED with safranin
What colour do Gram Negative stain and why?
Red because the crystal violet/iodine complexes are washed along with the outer membrane because they lose their outer LIPOPOLYSACCHARIDE membranes.
What are the stages to staining a GRAM.
Heat fix a smear of bacteria
Stain using crystal violet stain
Fix the stain with iodine
Decolourise with alcohol
Counterstain with safranin.
What are some advantages gram negative cells have
Gram negative have extra layers in the complex cell wall (1)
Which gives resistance to penicillin/antibiotics (2)
What is the grain stain process?
Heat fix a smear of bacteria
Stain using crystal violet stain
Fix the stain with iodine
Decolourise with alcohol
Counterstain with safranin.
What are the conditions needed for bacteria to undergo binary fission?
Need
1. Nutrients
2. Growth factors
3. Temperature - as bacteria regulated by enzymes (25 - 24*)
4. PH- most bacteria favour slightly alkaline conditions
5. Oxygen
Why do some bacteria need oxygen and some dont?
Obligate bacteria require oxygen for metabolism, obligate anaerobes don’t need oxygen for metabolism.
Many species are Faculative aerobes meaning they can respire anaerobically if need too.
How are nutrients supplied
In a nutrient broth.
Including carbon and an enegy source (glucose)
Why are growth factors needed for bacteria and what are they
Growth factors are vitamin and mineral salts.
Eg Na2+, Mg2+
What is selective media
Selective media are media in which only certain bacteria will grow.
Why are antibiotics added to agar
Antibiotics can be added to agar to prevent the growth of non-resistant bacteria.
What is an aseptic technique?
Also called sterile technique to avoid containimation of environments by the microbes and contamination of microbial cultures by unwanted microbes in the environment.
What are some methods of sterilising
Heat eg autoclave at 121* for 15 minutes or heating an incolauting tube in Bunsen burner
Irradiation - heat labile (stable) plastics
What is a total count
A total count uses methods that count all the cells present but cannot distinguish between live and dead cells.
What is a viable count
A viable count only counts the cells that are capable of reproducing (forming colonies) and are therefore alive.
Whats the difference between a total and viable count.
Total counts for alive and dead cells whereas viable can only count alive and those who are capable of reproducing
What are some things that affect the accuracy of a bacteria sample
If the bacteria colonies are overlapping they cannot be counted accurately .
If there’s too little colonies, it’s NOT an accurate representation of the whole sample.
