Microbiology Flashcards
estimating the number of bacteria present calculation
number of colonies counted x dilution factor x 2 (to estimate in 1cm3 if counted in 0.5cm3)
how correct is the estimate and why
underestimate - does not include non-viable/dead bacteria and cannot be sure each colony has grown from a single bacteria (clumps)
what is the assumption made during dilution plating
each colony has arisen from a single bacterium - may not be the case due to clumps
colony
cluster of cells from a single bacterium
pathogen
microorganism that causes disease in its host
aseptic technique
lab practice that maintains sterile equipment and prevents contamination
how do you count bacteria directly
in liquid culture (nutrient broth)
by counting each cell
how do you count bacteria indirectly
measuring turbidity (cloudiness of culture medium) with a colorimeter
total counts
living and dead cells
viable counts
only living or actively growing cells which underestimates the number
when is serial dilution/dilution plating used
when population density of the sample is too much to count
serial dilution method
- fill 5 test tubes with 9cm3 water
- add 1cm3 sample to first tube (1 in 10 dilution)
- mix dilution and pipette 1ml into second tube (1 in 100 dilution)
- repeat the process until 1 in 10000 dilution
- plate each on agar
why do you count the plate with a middle amount of bacteria
plates with lots of bacteria (1/10 and 1/100) is impossible to distinguish individual colonies and cannot count
plate 1/1000 cannot count reliably
plate 1/10000 has enough and easy to count * most reliable
plate 1/100000 does not have enough for reliable
3 types of bacteria
spherical coccus
rod shaped bacillus
spiral spirillum
what colour does gram positive bacteria stain and why
purple - thicker peptidoglycan cell wall which retains the crystal violet stain and becomes purple
