Counting microbial growth Flashcards
(19 cards)
why do we use log
it’s easy to interpret
Useful to bring large complicated numbers down to single digits.
Also used for calculation of reduction in numbers for disinfectants.
why do we use cultures
Count numbers of cells/colonies present
Identify the variety of microbes present
Detect presence of specific microbes
Isolate microbes with specific abilities:
To Produce things and To Degrade things
where are microbes grown (bacteria/fungi)
in a liquid or solid media
What is it when it’s grown in a solid
agar
What is it when it is grown in a liquid
Broth
plate cultures - Explain spread plate (draw the schematic)
Sample is spread over the agar surface.
Bacteria grow on the surface
Explain pour plate (draw the schematic)
Sample is placed in the plate and agar is poured over it.
Bacteria grow within the agar.
Why are plates inverted before incubation?
Prevents condensation falling onto the plates.
Culture Media - what does it contain
Has a mixture of nutrients in solution
Contain nutrients used by microbes when grown in laboratory
Contain buffer to maintain pH value
Liquid (referred to as broth) or semi-solid (agar)
what is agar derived from and what is it used to make
red algae polysaccharide
makes solid media
what is viable count
A common way assessing growth of single celled organisms such as bacteria is the viable count.
Bacteria are grown on an agar plate
Method relies on the fact that one bacteria can produce a colony large enough to be visible
Plating methods
what do they measure
measure number of viable cells
population size is expressed as colony forming units (CFU)
How do you conduct plating methods?
plate dilutions of population on suitable solid medium
THEN
count number of colonies
THEN
calculate number of cells in population
advantages and disadvantages of plating methods
simple and sensitive
widely used for viable counts of microorganisms in food, water, and soil
inaccurate results obtained if cells clump together
what can we do if a plate is inoculated with too many microbes
dilutions
how many colonies should be on the plate for us to count
Need plates with between 30 and 300 colonies
serial dilution is used to dilute samples in a
consistent manner
For plating viable counts can be determined by
Pour plates (0.1ml – 1ml per plate)
plating methods
Viable counts can be determined by:
Pour plates (0.1ml – 1ml per plate)
Following incubation the number of colonies are counted
Plates with between 30 and 300 used
30 is too low, 300 too high
