Lecture 4- Bacterial counts

Question 1

what is a continuous culture?

Answer 1

microbes in an open system with a fixed volume

Question 2

what is the most common type of continuous culture device?

Answer 2

chemostat

Question 3

what does a chemostat allow for? what is controlled?

Answer 3

growth rate of culture is controlled independently
population density of a culture is controlled independently
both occur simultaneously

Question 4

what is a dilution rate?

Answer 4

rate at which fresh medium is pumped in and spent medium is pumped out

Question 5

what controls population size and growth?

Answer 5

limiting nutrient

Question 6

microbial counts can be counted using a petroff- hausser counting chamber, how does it work?

Answer 6

it has a grid and in each grid you count how many microorganisms there are

Question 7

what are 8 limitations to microscopic cell counts?

Answer 7

1. cant distinguish between live and dead cells without special stains
2. small cells can be overlooked
3. precision is difficult to achieve
4. if stain is not used then need to use phase- contrast microscope
5. low density cells are hard to count
6. motile cells need to be immobilized
7. debris in sample can be mistaken for cells
8. cells may move, some form clumps

Question 8

what is flow cytometry? what does it use?

Answer 8

its an alternative method that can be used to count the total number of cells that uses laser beams, fluorescent dyes, and electronics

Question 9

what are viable cell counts?

Answer 9

counts that only measure living cells

Question 10

what are two main ways to perform viable cell counts?

Answer 10

spread plate method
pour- plate method

Question 11

what are the 3 issues with viable cell counts?

Answer 11

1. requires lots of preparation and incubation time
2. counts can be unreliable
3. a single medium will never grow every microbe (some will thrive and some will die, they all have different growth requirements)

Question 12

what is the great plate anomaly?

Answer 12

direct microscopic counts of viable (alive) cells reveal way more organisms then those on plates

Question 13

why do microscopic counts give more counts of microbes?

Answer 13

1. because they count dead cells
2. different organisms may have different requirements for growth (we dont know all the requirements for all organisms)

Question 14

what percentage of microbes are culturable?

Answer 14

1-10%

Question 15

what is a spectrophotometric count example?

Answer 15

turbidity

Question 16

how does turbidity spectrophotometry work?

Answer 16

light
light is absorbed
goes through curvette or tube
photocell detects how much comes through
spectrophotometer puts it in units of OD

Question 17

what is turbidity spectrophotometry based on?

Answer 17

fact that bacteria behave like small particles (absorb and scatter light)

Question 18

if theres a large number of particles what does that mean for spectrophotometry?

Answer 18

larger absorbance and lower light transmission to photocell

Question 19

turbidity is measured with measurements of optical density (OD), what are the problems with this? 4

Answer 19

1. has an infinite range of measurement
2. only works if the cells are evenly distributed (no clumps)
3. cuvette must not have scratches
4. culture may need to be diluted when the cells are at very high density (so that actual doesn’t separate from theoretical)

Question 20

why must we establish a standard curve to count turbidity value?

Answer 20

to compare the standard curve to another counting method

Question 21

what is the other counting method that has to do with mass?

Answer 21

a specific volume of cells are concentrated, washed to remove media and concentrated and dried
then we weigh. takes a long time but is normally more reliable