Download Dynamics of Prokaryotic Growth

Dynamics of
Prokaryotic Growth
Chapter 4
4.1 Principles of Prokaryotic Growth
Robert Koch (18431910)
Developed the strategies for
cultivating bacteria
Defined growth requirements
Media formulations
First to use agar for
semisolid media
Bacterial replication
Binary fission
Doubling time varies by
species and conditions
Growth can be calculated
Nt = N0 x 2n
Nt = total cells in a given time
N0 = starting population of cells
n = number of cell divisions
Assume t = 20 min (3 per hour)
3 x 4 hours = 12 doublings
If N0 = 10 then
10 x 212 = 40,960 bacteria
4.2 Bacterial Growth in Nature
Biofilms
Bacteria in nature tend to attach to solid
surfaces
These bacteria encase themselves in
polysaccharide coatings to form
communities
Collectively, these communities are
referred to as biofilms
Biofilm communities are highly organized
Intracellular communication
Channels
Cellular movement within the biofilm
(usually mediated by pili)
Biofilm Activities
Ear infections
Dental decay
Bioremediation
Sewage treatment
Toxic waste sites
Heavy metals
Intracellular “warfare” (i.e., competition)
4.3 Obtaining a Pure Culture
Isolation of pure cultures is mandatory for
studying bacteria
This is usually accomplished using semisolid media composed with agar
Agar is a polysaccharide obtained from
marine algae
It melts at about 95° C and remains liquid
to 45° C
Media are made with agar and usually
sterilized in an autoclave
Media are cooled to 60° C, then dispensed
into Petri dishes or tubes
After cooling, the agar solidifies, providing a
semi-solid surface
The streak plate method for obtaining a pure culture
Agar media in Petri dish
Collect a broth culture with a sterile loop
Streak the plate to deposit individual bacteria at sites on the plate
Incubate for 24-48 hr
Single bacterium grows to millions, forming a colony on the plate
Maintaining a Stock Culture
Once a colony is obtained it is considered
pure
This colony can be picked and inoculated into
another tube or plate (subculturing) to provide
a stock of the purified culture for short-term
use (weeks)
Long-term storage (years)
This purified culture can also be grown in
broth and lyophilized (freeze-dried)
It can also be diluted 1:2 in glycerol and
frozen at -70° C
4.4 Bacterial Growth in Laboratory
Conditions
The Growth Curve
Bacteria exhibit distinct kinetic profiles of
growth in the laboratory in closed cultures
(systems)
These profiles generally are the same,
although time-course between species
can be different
Continuous (Open) Cultures
Removal of toxic
metabolites
Replenishment of medium
Conducted in culture tanks
called fermenters
Internal sensors
Computer controlled
4.5 Environmental Factors that
Influence Microbial Growth
4.6 Nutritional Factors that Influence
Microbial Growth
Heterotrophs
Carbon source is organic carbon
Medically important bacteria
Autotrophs
Carbon source is inorganic carbon
Perform carbon fixation, the conversion of
inorganic carbon into organic carbon
Photoautotrophs use photosynthesis
CO2 + H2O ➔ Glucose + O2
4.7 Cultivating Prokaryotes in the
Laboratory
Complex Media
Contain a variety of biomolecule precursors
Concentrations of precursors can vary between
media
Often, the source of the precursors are extracts,
which are water-soluble substances
Examples
Nutrient agar
Tryptic soy agar
Blood agar
Defined Media
Known amounts of chemicals and
biomolecules are formulated into the medium
More expensive than complex media
Selective Media
Generally used for the propagation of
particular bacteria
MacConkey agar selects for Gram- enterics
Bismuth sulfite agar cultures Salmonella and Proteus
species to the exclusion of other bacteria
Differential Media
Media that can
distinguish two or
more groups of
bacteria in a
mixed culture
Providing Appropriate
Atmospheric Conditions
Increased CO2
Capnophiles (15% CO2)
Hemophilus
Neisseria
Microaerophilic - low levels
of oxygen
Anaerobe - killed by
prolonged exposure to
oxygen