microbiology

Question 1

list the features of a bacteria and their functions

Answer 1

• plasma mem: controls exit/entry of substances
• cell wall: prevents cell lysis
• slime capsule: protects cell from attack, can stick bacteria to surface/each other
• pili: allows transfer of plasmids and attachment to surfaces
• flagellum: allows motility
• plasmid: circular DNA that contains codes for extra genes (eg. anti-biotic resistance, can be exchanged between bacteria)

Question 2

what is the difference between gram-positive and gram-negative bacteria?

Answer 2

• gp: cell wall has thicker layer of peptidoglycan which can retain crystal violet/iodine complex- stains purple
  -gn: cell wall has thinner layer of peptidoglycan and protective outer-layer of lipoprotein and lipopolysaccharide (which is removed by treatment with alcohol), thinner layer of pep means crystal violet/iodine complex is washed from cell- stains red with counter-stain safranin

Question 3

what conditions are necessary for growth of bacterium?

Answer 3

-water
- suitable temp and pH
- oxygen/anaerobic conditions (depending on mode of respiration)
- C compounds, nitrogen, mineral ions

Question 4

what is an obligate aerobe?

Answer 4

an organism that requires oxygen for metabolism

Question 5

what is an obligate anaerobe?

Answer 5

an organism that can only survive in the absence of oxygen

Question 6

what is a facultative anaerobe?

Answer 6

organisms that can respire anaerobically if needed

Question 7

list the precautions taken during the aseptic technique :)

Answer 7

• sterilise agar using autoclave (kills any bacteria/microbes present)
• flame loops/spreaders (kills any bacteria on surface)
• keep lid of petri dish open at an angle (prevent bacteria in air entering dish)
• flame bottlenecks when opening/before closing (creates convection current which sucks air out of bottle)
• work close to roaring flame (prevents contamination of desk and creactes convection current that uplifts air away)

Question 8

why is counting bacteria difficult?

Answer 8

• more than one type of bacteria present
• too many in a sample (solution: dilute)
• too small to see (solution: dilute and grow on plate)

Question 9

how is a viable count carried out?

Answer 9

-dilute with sterile distilled water
-spread known volume onto agar plate
- allow to grow fro 24-48 hours

Question 10

how is a total count carried out?

Answer 10

• dilute bacteria with sterile distilled water
• use a haemocytometer to count bacteria under microscope

Question 11

what are the advantages and disadvantages of doing a viable count?

Answer 11

AD: counts live bacteria
DIS: takes a while to get results, could underestimate as cannot be sure each colony has grown from a single bacterium

Question 12

what are the advantages and disadvantages of doing a total count?

Answer 12

AD: quicker, easier to count different types at one time
DIS: counts living and dead bacteria, may need oil immersion lens