Bacterial Growth

How do bacteria grow in populations?

Individual cells

Cells grow in size,

DNA is replicated,

Then they divide by binary fission

Binary Fission

One cell makes 2 identical cells

Population Growth

Measured by measuring cell number of cell mass

Population Growth Equation

Nn​=N0​⋅2n

n stands for what

number of generations

N0​

number of starting cells

Nn

number of cells after n generations (final cells)

Doubling Time

Time for the population to double in cell numbers (generation time)

Depends on the environment (certain factors)

Varies from bacterium to bacterium (species)

Finding generations formula

n=3.3⋅log(Nn​/N0​)

Growth rate formula

k=n/t

Doubling time formula

g=1/k

what does k stand for

growth rate (generations/hr) aka how fast they grow

What does t stand for

total time (hr)

What does g stand for

doubling time (hr)

Exponential growth

Curved graph = growth is exploding

Fast increase

Semi-log scale

Straight line = constant/same growth rate the whole time

Continuous increase

All bacteria are dividing at the same rate

How do we measure growth?

Total Cell Count

Viable Count

Cell Mass

Total Cell Count

Uses the Petroff-Houser chamber

Counts everything (live and dead cells)

Can’t tell the difference between the living or the dead cells

Viable Count

Plate dilutions and count colonies

ONLY counts living cells

Cell Mass

Spectrophotometer

More cells/higher mass = higher absorbance

Counts live and dead cells

4 Phases of a Microbial Growth Curve

1. Lag phase

2. Exponential (Log) Phase

3. Stationary Phase

4. Death phase

Lag → Log → Stationary → Death

Lag Phase (adjusting phase)

Cells are adapting to their new environment to optimize their own growth

Making enzymes

NOT dividing yet, no increase in cell number

Exponential (Log) Phase

Cells are dividing at a constant, maximum rate

Population is doubling (2 → 4 → 8 → 16…)

Fastest growth phase

Straight line on log scale = constant growth rate

cells are healthiest here

Stationary Phase

Growth stops increasing

BUT cells are still active

Steady state where they divide at the same rate (population stays the same)

Why?

Nutrients run out

Waste builds up

Space is limited

Death Phase

Cells die faster than they can divide

Population decreases

Why?

No nutrients

Toxic waste

Harsh conditions

Decline in viable cells

Simplest way to remember Microbial Growth Curves

• Lag = “getting ready”

• Log = “growing like crazy”

• Stationary = “balanced”

• Death = “dying off”

Antibiotics work best in what phase?

Log phase bc cells are actively dividing

Stationary phase is like what?

A stress response, harder to kill

Factors that affect growth

Nutrient availability

Temperature

pH

Oxygen availability

Water availability

Major Elements for Nutrient Availability

CHNOPS

used for proteins, DNA, and membranes

Oligotrophic

Nutrients in Nature

Nutrients are limited (LOW)

Lots of competition

Slow growth

Caulobacter

Exists in low-nutrient environments

Makes a stalk composed of a membrane

-Stalk forms with more membrane so that nutrients can be transported

-Grows in tap water

Nutrients in the Laboratory

Chemically defined

Complex medium

Chemically defined

Exact amount of composition is known of each nutrient

Complex medium

The amount of the composition of each nutrient is unknown

More nutrients =

Faster growth AND shorter doubling time

• Rich media → fastest growth

Effect of Nutrient Shift on Growth

Where the bacteria come from (their phase) + what medium you put them into = affects how fast they grow

Red Line, From log phase into fresh BHI broth

Almost NO lag phase

Growth continues quickly

Why?

Cells are actively dividing, enzymes are already made, metabolism is ON

FASTEST GROWTH

Green Line, From stationary phase to fresh BHI broth

Lag phase appears

THEN growth resumes

Why?

Cells were stressed, low nutrients and slower metabolism

Moderate growth (after delay)

Blue Line, From log phase into glucose salts medium

Long lag phase

Slower growth (less premade nutrients = less steep slope)

Why?

Even though cells were active, the new medium has:

-fewer nutrients

-requires new enzymes

SO

Cells must adapt metabolically

Black/purple line aka original culture in BHI

Normal growth curve in rich media

Used as a reference

Previous condition matters

Log phase cells = ready

Stationary phase cells = need recovery

New environment matters

Rich media (BHI) = fast growth

Minimal media = slow growth + adaptation

Lag phase = adaptation time

Making enzymes

Adjusting metabolism

Log with rich media →

No lag, then growth

Stationary with rich media →

lag, then growth

Log with poor media →

lag and slower growth

Temperature can also affect

Bacterial growth

Bacteria/prokaryotes can grow within a range of

-7°C to 120°C

Each organism has a NARROW growth temp range at

~30-40°C

Every bacterial group has different temps

Minimum

Optimal/optimum

Maximum

Minimum temp

Too cold → no growth

Optimum temp

Best growth!

Maximum temp

Too hot → proteins denature (lose structure therefore function) → death

Psychrophiles

EXTREME TEMP

“Cold lovers”

Range: ~ -5°C to 15°C

more unsaturated fatty acids in membrane

Found in the Arctic, glaciers, and the deep ocean

Psychrotrophs

Refrigerator bacteria

Grow in cold BUT prefer ~20–30°C

Causes food spoilage in the fridge (Listeria) aka these are why food goes bad

Mesophiles

Range is roughly (~20–45°C)

• Optimal ≈ 37°C

Most human microbes/pathogens

Meso = middle = us

Thermophiles

EXTREME TEMP

“Heat lovers”

Range is ~50–80°C

more saturated fatty acids in the membrane

Found in hot springs, compost piles

Hyperthermophiles

Extreme heat

~80–110°C

Found in deep ocean vents, volcanoes

Bacterial growth can also be affected by what?

pH

Most bacteria grow around a neutral pH, these are called:

Neutralophiles, ph (5.5–8)

Buffers are needed to maintain a neutral pH of media

Fungi are more acid tolerant at what pH?

5

Some bacteria can grow outside of the neutral range

Acidophiles

Alkaliphiles

Neutralophiles

Neutral pH

Acidic outside and positively charged

The proton gradient is balanced

Acidophiles

“Acid-loving”

Acidic aka lots of H+

pH optimum 0-5.5; acid hot springs

Protons want to rush INTO the cell to make the inside acidic (BAD)

Solution:

Inside becomes positively charged and repels H+ from entering

ALSO brings in K+ to help create a positive interior

K+ affects charge, NOT pH

Alkaliphiles

“Alkaline-loving”

Basic

pH optimum 8.5-14; high carbonate soils and soda lakes

Not enough H+ available

Cell needs H+ to maintain pH and energy

Solution:

Use Na+/H+ antiporters to bring H+ INTO the cell and push Na+ OUT

neutralizes the effects of the protons

Bacteria keep internal pH neutral

No matter what the pH of the environment is

Even if outside is acidic/basic

Obligate aerobe

Grows at the top of the test tube with O2

Facultative anaerobe

Can grow in the test tube without O2 but prefer O2

Aerotolerant anaerobe

Bacterium is distributed throughout the test tube

Strict anaerobe

Grows at the bottom of the test tube without O2

Microaerophile

Only needs low amounts of O2 to grow

Why O2 can be toxic

Oxygen forms:

Superoxide (O2-)

Hydrogen peroxide (H2O2)

Hydroxyl Radical (OH-)

Which all of these damage cells

How do aerobic bacteria protect themselves from toxic O2 products?

They possess enzymes that can remove toxic O2 products like:

Catalase

Superoxide dismutase

NADH Peroxidase

Water activity (aw)

aw= relative humidity/100

aw of water = 1

Factors that can affect aw

Osmolarity

Evaporation

Osmolarity

As osmolarity increases, aw decreases

More solute (salt) → lower aw

Evaporation

As evaporation increases, aw decreases

Less water → less growth

Types of resistant bacertia in low aw environments

Halotolerant

Halophile

Halotolerant

S. aureus

Can tolerate high salt concentrations

Needs to increase the internal solute concentration

Produces compatible solutes inside the cell

Halophile

Halobacterium

Loves/ thrives in high salt concentration

Proteins and membranes adapted to high salt concentration

Cannot survive in low salt environments

Found in salt lakes!