Lecture 5 Flashcards
bacteria reproduce by:
binary fission
time required for one binary fission event
generation time (G)
G depend on:
species, environment
why measure bacterial numbers?
monitor growth, diagnose, determine loads in food
methods for counting bacteria vary depending on:
living vs all, crude vs accurate, all or certain species
this method uses calibrated chamber and microscope to visually count all bacteria in liquid of known vol
direct microscopic count
pros of direct micro count?
simple and rapid
cons of direct micro?
crude, need large # cells for accuracy, can’t distinguish living/dead
based on concept that bacteria are particles that block light; amount of light blocked is proportional to # cells
turbidity
turbidity measured as
absorbance units in spectrophotometer
pros of turbidity?
fast, non-destructive, can follow growth in sample over period of time
cons of turbidity?
not sensitive, can’t distinguish live or dead
based on principle that each living cell in pop forms colony on solid media, uses dilution, spread on solid media, allow cells grow into colonies, count # of colonies
viable count (plate count)
bacterial concentration expressed as ___
Colony Forming Unit/mL (or L)
colonies on plate x ____ = # bacteria (CFU)/mL
dilution factor
pros of viable count:
very sensitive, widely used in food industry and clinical lab where number of live bacteria super important
cons of viable count:
very slow, some bacteria not easily or cannot be grown (underestimates)
to carry out binary fission, all bacteria need:
nutrients, water, proper pH, proper temp, proper gas atm
nutrients provide sources of :
carbon and minerals
most bacteria obtain all required nutr and gen nrg from breakdown of :
organic materials
can use any one of >90 diff org nutr
pseudomonas
can only use urea for growth
bacillus fastidiosus
requires complex factors in host cell
chlamydia
needs only water, minerals, and gluc
e. coli
