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Biology component 1 > microbiology > Flashcards

microbiology Flashcards

1
Q

How are bacteria classified

A
  • classified according to their shape, cell wall structure and metabolic,antigenic and genetic features
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2
Q

What are the three shapes of bacteria

A
  • coccus - spherical
  • bacillus - rod shaped
  • spirillium - spiral
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3
Q

What does the name of the bacteria mean

A

reflects the name of the disease

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4
Q

How can bacteria be further classified

A

using gram staining technique

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5
Q

Describe how the antibiotic pencillin work

A
  • prevents the cross links from forming within the peptidoglycan layer and so weakens the cell wall in newly divided bacteria
  • gram positive bacteria are most affected as they are then subject to osmotic lysis when water enters the bacterial cell causing the cell to burst
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6
Q

describe the differences between gram positive bacteria and gram negative bacteria

A

gram positive bacteria
* thicker cell wall
* thick layer of peptidoglycan
* no lipopolysaccharide layer (LPS) so vulnerable to penicillin and lysozyme action
* peptidoglycan layer retains cyrstal violet stain so stains purple
* staphylococcus and streptococcus
gram negative bacteria
* thinner cell wall
* thin layer of peptidoglycn
* lipopolyaccharide layer (LPS) protects against penicillin and lysozyme action
* lipopolysaccharide layer prevents uptake of cyrstal violet stain so only stains red once LPS removed and a counter stain used
* salmonella and e coli

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7
Q

Describe the gram staining technique

A
  • transfers a small sample of bacteria to a glass microscope slide using an inoculating loop. pass the slide through a bunsen flame a few times to fix bacteria to the slide (also kills them)
  • add a few drops of crystal violet stain and leave for 30 seconds
  • rinse excess using water
  • add grams iodine for 1 minute to fix stain
  • bacteria which stain purple are gram positive
    to stain remaining bacteria:
  • wash with acohol for 30 seconds to dissolve lipids in lipopolysaccharide layer and expose inner peptidoglycan layer
  • re stain using another stain eg safranin which stains unstained bacteria red
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8
Q

obligate aerobes

A

microbes that require oxygen for growth

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9
Q

facultative anaerobes

A

microbes that grow better with oxygen but can grow without it

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10
Q

obligate anaerobes

A
  • microbes that cannot survive in the prescence of oxygen
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11
Q

Describe conditions necessary for bacterial growth

A
  • nutrients- a source of carbon for respiration eg glucose, nitrogen for synthesis of nucleotides and proteins and vitamines and mineral salts
  • water
  • suitable temperature - 25-45 degrees celcius for most bacteria 37 degrees is optimum for mammalian pathogens. thermophiles can survive at 90 degrees evolved in hot springs
  • suitable pH- optimum is slightly alklaine for most bacteria some can survive acidic conditions
  • oxygen may or not be required depending upon the mode of repiration
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12
Q

aseptic technique / sterile technique

A

good laboratory practice that maintain sterile conditions and prevents contamination

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13
Q

pathogen

A

a diease causing microorganism

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14
Q

Why is aseptic technique used

A

to ensure that the desired bacterium is grown and that you don’t contaminate yourself or the environment

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15
Q

how is equipment and media sterilised for aseptic technique

A
  • heat at 121 degrees celcius for 15 minutes in an autoclave or pressure cooker or by passing the equipment through a bunsen flame for 2-3 seconds until it glows red eg inoculating loop. this works for inanimate objects (non living)
  • irradiation works well for heat labile plastics
  • benches cannot be sterilised but can be disinfected eg 3% lysol which reduces the numbers of microbes but not fungal spores
  • living tissues cannot be safely sterilised without kiling them so antiseptics are used which kill or inhibit microbes on the outside of living tissues only
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16
Q
A
17
Q

Why do we grow bacteria at 25 instead of 37 degree celcius

A
  • so that pathogenic microorganims aren’t grown
18
Q

what happens with all material after doing aseptic technique

A
  • safely disposed or afterward by sterlising in an autoclave
19
Q

What are the four phases of measuring bacterial growth

A
  • lag phase
  • logarithmic phase
  • stationary phase
  • death phase
20
Q

why do we use log numbers with bactieral growth

A

this is as the range of number is too large

21
Q

What happens during lag phase

A
  • population number increases very slowly because time is needed for enzyme synthesis
22
Q

Describe what happens during the log/exponential/growth phase

A
  • there are plenty of nutrients and few toxic by products so there are no limiting factors
  • allow rapid reproduction
23
Q

Describe what occurs during stationary phase

A
  • cells are reproducing but population is relativley constant fluctuating around the carrying capacity due to cell production equalling cell death
  • the population has reached its carrying capactiy because reduced resources (eg nutrients/space/toxic waste products) are now limiting factors
24
Q

Describe the death phase

A
  • more cells are dying than are being produced so the population decreases
  • death of cells is due to lack of nutrients, lack of oxygen or increases toxicity of the medium
25
Q

How is growth measured

A
  • directly where the total number of cells is calculated
  • indirectly by measuring the turbidity (cloudiness) of a culture
26
Q

What are the types of direct counts

A
  • direct counts can either be viable counts where only living cells are counted or total counts where both living and dead cells are counted
  • by using a haemocytometer originally developed to count bloos cells
27
Q

Describe the method of viable cell count and what they are

A
  • this counts the number of living cells and is particularly useful in medical and food hygeine applications
  • even in small cultures the total viable cell count can exceed several million per cm3 so a serial dilution must be performed first
  • this is often done in tenfold steps 1 in 10 but higher dilutions can be performed 1 in 100
  • for 1 in 10 dilution 1 cm3 is added to 9cm3 of sterile medium and mixed and repeated until a range of dilutions is obtained
  • plate out each dilution and incubate the plates at 25 degrees celciius for 24-48 hours to allow bacteria to grow
  • plates are examines and a plate is chosen to count
28
Q

How do you know the best plate to count

A
  • 20 - 100 colonies
  • more than 100 is difficult to count
  • fewer than 10 has increased error due to small numbers involved
29
Q

How is viable count calculated and what does it assume

A

multiplying the dilution factor by the number of colonies
assume each colony originated from a single bacterium that divided asexually

Biology component 1 (6 decks)

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