GROWTH
In nature
Bacteria exist in two forms:
•Planktonic – individual, free-
floating bacterial cells
•Biofilm – polymer-encased
communities of one or more
bacterial species, which are
attached to a surface
•Comprise 70% of human
infections
Biofilms
•Benefits to bacteria:
•Sharing of nutrients
•Protection from harm
(desiccation, antibiotics,
immune system)
•Easier exchange of
genetic material
•Harder to clean and treat
for us, e.g. Pseudomonas
aeruginosa in lungs of CF
patients
In nature
Bacteria exist in two forms:
•Planktonic – individual, free-
floating bacterial cells
•Biofilm – polymer-encased
communities of one or more
bacterial species, which are
attached to a surface
•Comprise 70% of human
infections
•Quorum sensing – chemical
language through which bacteria
talk to each other
Quorum sensing
•Discovered in Vibrio fischeri, which produces light only at high cell density
Quorum sensing
•Discovered in Vibrio fischeri,
which produces light only at
high cell density
•At high cell density,
concentration of a particular
chemical signal called
autoinducer reaches a
threshold, thus turning on genes
responsible for group-specific
behaviors like light or toxin
production
Quorum sensing
•Allows bacteria to act in unison to
perform big tasks that are useful for
the community
•In the light organs of Hawaiian
bobtail squid, V. fischeri produces
light, minimizing the squid shadow
and allowing it to hunt stealthily
•Can be used to design anti-
microbials
In lab
•Often use “pure cultures” - cultures
with only one species of bacteria
•Culture (growth) medium
•Nutrient broth (NB) – liquid
•Turbidity - cloudiness
•Nutrient agar (NA) – solid
•Isolated colonies – colony is a distinct mass of
cells arising from a single cell by multiple
rounds of cell division
•Have all known bacteria been cultured in
lab?
The Great Plate Count Anomaly
•# cells observed under
microscope > # colonies
on a plate
•99% bacteria
unculturable
•Know identity through DNA
studies – can find their
DNA, but don’t know how
to grow them
Characteristics
•Growth = numbers
•Binary fission – splitting into two
•NO mitosis/meiosis
•Exponential growth
•Generation (doubling) time – time it
takes for 1 cell to divide into 2 daughter
cells
•E. coli on a salad at RT – 10 cells form
40,000 cells in 4 hours
Growth – exponential
Growth curve
•Closed (Batch) system – do not
replenish nutrients or remove wastes
•Broth vs. agar
•4 phases:
•Lag – prepping for division
•Log – increase in cell number
exponentially; max growth rate
•Stationary – no increase in cell number;
growth rate = death rate
•Death – decrease in cell number;
eventually all bacteria will die
How might you create
a continuous system?
Comprehension Check
Penicillin attacks growing bacteria, so in which phase of the
growth curve does penicillin work best?
1. Lag phase
2. Log/exponential phase
3. Stationary phase
4. Death phase
E. coli cells exposed to penicillin
ENVIRONMENTAL
FACTORS
1. Temperature
2. pH
3. Osmotic pressure
4. Oxygen
Temperature
•Psychrophiles and
Psychrotrophs
•Cold-loving
•Mesophiles (optimum = 370C)
•Pathogens
•Thermophiles
•Heat-loving
•Hyperthermophiles
•Superheat-loving
Temperature
•How do bacteria survive in very hot
environments?
•Rigid thermostable proteins – tightly folded to prevent
denaturation
•Rigid membranes – have tightly packing lipids
Temperature and food
*High temperatures are usually bactericidal = bacterial killing
*Low temperatures are usually bacteriostatic = bacterial growth
stopped
pH
Concentration of H+ – cytoplasm of
most bacteria has pH 7
•Neutrophiles
•pH 6-8
•Acidophiles
•pH < 5.5
•Alkaliphiles
•pH > 8.5
pH
•How do bacteria survive in low or high pH
environments?
•Pumping protons in (alkaliphiles) or out (acidophiles)
•Having membranes which are less permeable to
protons
•Having more buffers in the cytoplasm to maintain pH
Osmotic pressure
•Water availability
•In hypertonic environments, bacteria
undergo plasmolysis – used in food
preservation
•Facultative halophiles (= halotolerant)
•NaCl up to 10% or higher
•E.g. S. epidermidis can grow on
clean or sweaty skin
Osmotic pressure
•Obligate halophiles
•NaCl at least 9%; can live in
saturated salt (32%)
•E.g. Halobacterium salinarium
•How do bacteria prevent
plasmolysis in high salt
environments?
•Accumulate solutes in
cytoplasm so that they are
isotonic with their salty
environments
Oxygen
•Obligate aerobes – need O2, e.g.
Pseudomonas
•Obligate anaerobes – die in presence of
O2, e.g. Clostridium
•Facultative anaerobes – prefer O2, but
can survive without it, e.g. E. coli
Anaerobic jar
Anaerobic chamber
