Chapter 4 Microbiology

Question 1

How do bacteria reproduce asexually?

Answer 1

Process called binary fission in which Plasmid DNA replicates and attaches to the mesosome. A Septum (cross wall) develops and two daughter cells are formed.

Question 2

What are the 3 types of bacteria and there shapes?

Answer 2

Coccus - spherical
Bacillus - rod shaped
Spirillum - spiral/corkscrew shaped

Question 3

What are some examples of useful bacteria?

Answer 3

Lactobacillus balgaricus - used to make yoghurt (Bacilli)
Streptococcus lactus - used to make cheese (cocci)
E. Coli - used to make insulin (bacilli)

Question 4

What are some examples of harmful bacteria?

Answer 4

Treponema pallidum - causes Syphilis (spirillum)
Streptococcus - causes strep throat (cocci)
Salmonella typhi - causes typhoid fever (bacilli)

Question 5

What is a gram stain?

Answer 5

A method of staining the cell walls of bacteria as an aid to their identification.

Question 6

Outline the process of gram staining?

Answer 6

Heat fix bacteria to a microscope slide and flood with crystal violet. Wash with iodine to fix the stain. Decolourise with alcohol and counterstain with safranin.

Question 7

What are the characteristics of gram positive bacteria?

Answer 7

After staining, appear violet/purple.
Thick walls of peptidoglycan
No outer layer of lipopolysachharide

Question 8

What are the characteristics of gram negative bacteria?

Answer 8

After counterstaining appears red
Thin walls of peptidoglycan
Outer layer of lipopolysachharide

Question 9

What are the conditions necessary for culturing bacteria?

Answer 9

Nutrients (carbon and nitrogen sources)
Growth factors (vitamins)
Suitable temperature
Suitable pH
Oxygen
Water

Question 10

What is a defined medium?

Answer 10

Contains only known ingredients

Question 10

What is an undefined medium?

Answer 10

Contains components that are not all known.

Question 12

What is a selective medium?

Answer 12

Medium that only allows certain bacteria to grow

Question 13

What are antiseptics, antibiotics and disinfectants?

Answer 13

Antiseptics inhibits growth of bacteria on the skin
Antibiotics inhibits growth of bacteria within the body
Disinfectants inhibits growth of bacteria on surfaces

Question 14

Define aseptic techniques

Answer 14

Laboratory practice that maintains sterility in apparatus and prevents contamination of the equipment, personnel and the environment.

Question 15

What are the 2 types of antibiotics? What do they do?

Answer 15

Bacteriostatic antibiotics inhibit the reproduction and growth of bacteria
bactericidal antibiotics kill bacteria

Question 16

How do you prevent the contamination of pure cultures and apparatus by bacteria?

Answer 16

Sterilising all apparatus and media before

Flaming the necks of culture vessels before opening and closing

Question 17

How do you prevent contamination to the environment by bacteria being used in the lab?

Answer 17

Sterilise work surfaces before and after experiments (using disinfectant)
Use correct handling technique (flaming mouth/neck of culture vessels, flaming inoculating loops, lifting petri dish to a 45 degree angle

Question 18

Define pathogen

Answer 18

An organism that causes disease in its host

Question 19

How do you discourage the growth of pathogenic microorganisms in the lab?

Answer 19

Only partially seal two sides of a Petri dish, otherwise sealing all the way creates anaerobic conditions and potentially encourages growth of pathogens.
Incubate at 25 degrees Celsius, if higher at around 37 degrees Celsius it is the ideal temperature for pathogens.

Question 20

What are the different ways of sterilising equipment?

Answer 20

Autoclave
Gamma radiation
Flaming

Question 21

What is a viable cell count?

Answer 21

The number of living bacteria cells

Question 22

What is a total cell count?

Answer 22

The number of living and dead cells

Question 23

What are the two ways of measuring bacteria in a liquid culture?

Answer 23

Directly by counting cells or indirectly by measuring turbidity (cloudiness)

Question 24

Define colony

Answer 24

A cluster of cells which arises from a single bacteria or fungal spore by asexual reproduction

Question 25

Outline the process of serial dilutions

Answer 25

The sample is diluted. 1cm3 suspension is added to 9cm3 medium, it has been diluted 10 times (10^-1). This process is repeated to make as many dilutions as needed. Each dilution is spread over a sterile agar plate and incubated at 25 degrees celsius. A dish containing 20-100 colonies is chosen and the colonies counted. The total cell count is found by multiplying the number of colonies by the dilution factor.

Question 26

What is are the problems of using serial dilutions to measure bacteria growth?

Answer 26

The separate colonies of bacteria counted ASSUMING that each has arisen from a single cell from the original culture which has divided asexually.
Colonies may merge or ‘clump’ and counting may be inaccurate resulting in an underestimate of numbers.

Question 27

How is a haemocytometer used to measure bacterial growth?

Answer 27

It uses a specialised microscope slide. However, it is not possible to distinguish between living and dead cells so the result is only a total cell count

Question 28

How is growth measured by turbidity?

Answer 28

A colorimeter can be used to measure the cloudiness of the culture as cell numbers increase. Measurements of the bacterial population are found by the light absorbance of the suspension against the number of bacterial cells. The result is a total cell count as the colorimeter cannot distinguish living and dead cells