Microbial growth

9.1 Binary Fission

Binary Fission

• Most common way bacteria reproduce.

• 1 cell → 2 daughter cells

• Steps: Grow → Copy DNA → Elongate → Divide

FtsZ & Z Ring

• FtsZ = protein that forms the Z ring.

• Z ring forms at the center and helps the cell divide.

Divisome

• Structure made of the Z ring + other proteins.

• Builds the septum.

Septum

• Wall that forms between the two new daughter cells.

Key Terms

• Binary fission = bacterial cell division

• FtsZ = forms Z ring

• Z ring = directs division

• Septum = divides the cells

Remember:
FtsZ → Z ring → Septum → 2 daughter cells

9.1 Generation Time

• Generation time = doubling time

• Time it takes a bacterial population to double through binary fission.

• Example: E. coli can double in about 20 minutes under optimal conditions.

Cell Growth

• Each division doubles the number of cells.

• Growth is exponential.

Formula

Nₙ = N₀ × 2ⁿ

• Nₙ = final number of cells

• N₀ = starting number of cells

• n = number of generations

Example

Starting with 1 cell:

1 → 2 → 4 → 8 → 16

9.1 The Growth Curve

Closed Culture

• Closed/batch culture = no nutrients added and most waste is not removed.

• Microbial growth follows 4 phases:

1. Lag Phase

• Cells grow and prepare to divide.

• Cell number does not increase.

• Cells are metabolically active.

2. Log Phase

• Cells divide rapidly by binary fission.

• Cell number increases exponentially.

• Cells have a constant growth rate.

• Bacteria are most susceptible to antibiotics and disinfectants during this phase.

3. Stationary Phase

• Nutrients become limited.

• Waste products accumulate.

• New cells = dying cells, so population stays about the same.

• Cells enter a survival mode.

4. Death/Decline Phase

• Nutrients are exhausted and toxic waste increases.

• More cells die than divide.

• Number of living cells decreases.

Easy Way to Remember

L → L → S → D

Lag → Log → Stationary → Death

Think:

Prepare → Grow → Stop → Die

Chemostat

• Maintains microbes in the log phase.

• Adds nutrients continuously.

• Removes waste/cells continuously.

9.1 Measurement of Bacterial Growth

Bacterial Count

• Bacterial count = estimating the number of bacteria in a sample.

• Used to measure infections and detect contamination.

Direct Cell Count

Direct count = actually counting cells.

🔬 Petroff-Hausser Chamber

• Special slide with a grid.

• Count bacteria in several squares and find the average.

• Can estimate cells/mL.

• Limitation: cannot normally tell live cells from dead cells or debris.

• Works poorly with very dilute cultures.

Viability Staining

• Live cells = green

• Dead cells = red

• Dead cells appear red because their damaged membrane allows the secondary red stain to enter.

Coulter Counter

• Counts cells by detecting changes in electrical resistance as cells pass through a small opening.

• Fast and accurate within certain concentrations.

• Cannot distinguish live vs. dead cells.

Remember

Petroff-Hausser = microscope + grid

Viability stain = green live, red dead

Coulter counter = electrical resistance

9.1 Plate Count

Viable Plate Count

• Counts living/viable cells.

• Results are reported as CFU/mL = colony-forming units per mL.

• Usually count plates with 30–300 colonies.

  • <30 = not reliable

  • >300 = too crowded

Serial Dilution

• Dilute the sample in steps, usually by 10×.

• Goal: get a plate with 30–300 colonies.

• Then use the dilution to calculate the original bacterial concentration.

Two Plate Methods

Pour plate

• Mix sample with warm agar (45–50°C).

• Pour into Petri dish.

• Colonies grow throughout the agar.

Spread plate

• Place sample on solid agar.

• Spread it across the surface.

• Colonies grow on the surface.

Membrane Filtration

• Used for very dilute samples, such as drinking water.

• Filter traps bacteria on a membrane.

• Put membrane on agar → incubate → count colonies.

Most Probable Number (MPN)

• Used when there are too few bacteria for a plate count.

• Estimates viable bacteria using statistical probability.

• Often used for water and food samples.

• Growth is detected by color/turbidity changes.

Remember

CFU = colony-forming unit

30–300 = countable plate

Dilute → Plate → Incubate → Count

Water samples:
Membrane filtration or MPN

9.1 Indirect Cell Counts

1. Turbidity

• Turbidity = cloudiness of a bacterial liquid.

• More bacteria → more cloudy → less light passes through.

• Measured with a spectrophotometer.

• More bacteria = higher absorbance (optical density)

• More bacteria = lower % transmission

Memory:
More bacteria → more cloudy → less light → higher absorbance

2. Calibration Curve

• Compares turbidity/absorbance to the actual number of cells.

• Made using samples with known cell counts.

• Once created, it can estimate cell numbers from turbidity.

Purpose: → Use turbidity to estimate cell count.

3. Dry Weight

• Cells are filtered/centrifuged → washed → dried → weighed.

• Useful for filamentous microorganisms that are difficult to count directly.

4. Newer Methods for Live Cells

Instead of growing and counting cells, these measure cell activity, such as:

• ATP production

• Protein and nucleic acid production

• Oxygen consumption

9.1 Alternative Patterns of Cell Division

1. Fragmentation

• Common in filamentous cyanobacteria and Actinomycetes.

• A long filament breaks into smaller pieces.

• Each piece can grow into a new cell/colony.

2. Budding

• A small bud grows from the parent cell.

• The bud gets bigger and eventually detaches.

• Common in yeast and also occurs in some bacteria/cyanobacteria.

3. Spore Formation

• Actinomyces can form aerial filaments.

• Cells inside divide and form spores.

• Spores can develop into new colonies.

Main Difference

Fragmentation: the parent filament breaks apart into pieces.

Budding: a small new cell grows off the side of the parent cell and detaches.

Memory:
Fragmentation = breaks apart
Budding = grows a bud

9.1 Biofilms

What is a Biofilm?

• A biofilm = a structured community of microorganisms attached to a surface.

• Microbes in nature commonly grow in biofilms.

• Example: dental plaque is a biofilm.

Biofilm Structure

• EPS (extracellular polymeric substances) = sticky, hydrated material made mainly of polysaccharides.

• EPS helps:

  • Hold the biofilm together

  • Keep cells hydrated

  • Protect microbes

  • Move nutrients, waste, and gases through channels

• Streamers = biofilms shaped by fast-moving water.

• Slow/still water → often mushroom-shaped.

Biofilm Formation

1. Planktonic cells = free-floating cells.

2. Cells attach to a surface.

3. Attachment becomes irreversible.

4. Cells grow and produce EPS.

5. Biofilm matures with water channels.

6. Dispersal = cells leave the biofilm and colonize new locations.

Memory:
Attach → Grow → EPS → Mature → Disperse

Quorum Sensing

• Microbes communicate using quorum sensing.

• They release chemical signals called autoinducers.

• When enough microbes are present (quorum), the signals trigger changes in gene activity.

• Can activate things like virulence factors.

Biofilms & Human Health

• Some biofilms are beneficial.

• Harmful examples:

  • Dental plaque → dental/periodontal disease

  • Wound infections

  • Pseudomonas aeruginosa in cystic fibrosis airways

  • Biofilms on catheters and artificial joints

• Biofilms can be more resistant to antibiotics because EPS can slow antibiotic movement and some cells are metabolically inactive.

Most Important Terms

• Biofilm = attached microbial community

• EPS = protective matrix

• Planktonic = free-floating

• Sessile = attached

• Dispersal = leaving the biofilm

• Quorum sensing = microbial communication

• Autoinducers = signaling molecules

9.2 Oxygen Requirements of Microorganisms

Reactive Oxygen Species (ROS)

• O₂ is NOT required by all microorganisms.

• ROS = unstable molecules/ions that can damage cells.

• Examples: superoxide, hydrogen peroxide, hydroxyl radicals.

• Microbes that can detoxify ROS can survive in oxygen.

Thioglycolate Tube

Oxygen is highest at the top and lowest at the bottom.

Microbe	Where it grows	Easy way to remember
Obligate aerobe	🔝 Top	Needs O₂
Obligate anaerobe	🔽 Bottom	O₂ kills it
Facultative anaerobe	Everywhere, mostly top	Can live with or without O₂
Aerotolerant anaerobe	Evenly throughout	Doesn't use O₂, but isn't harmed
Microaerophile	Just below top	Needs a little O₂

The 5 You Need to Know

Obligate aerobe → MUST have oxygen
Obligate anaerobe → CANNOT tolerate oxygen
Facultative anaerobe → Can use oxygen, but can live without it
Aerotolerant anaerobe → Doesn't use oxygen but tolerates it
Microaerophile → Needs low levels of oxygen (about 1–10%)

Oxygen Levels

• Minimum permissive = lowest O₂ level that allows growth

• Optimum = best O₂ level for growth

• Maximum permissive = highest O₂ level tolerated

• Outside the minimum–maximum range → no growth.

Easy Memory Trick

Aerobe = Air/O₂
Anaerobe = Away from O₂
Facultative = Flexible
Aerotolerant = Tolerates O₂
Micro = Minimum/low O₂

9.2 Detoxification of Reactive Oxygen Species

ROS

• ROS = reactive oxygen species

• They can damage cells, so microorganisms need enzymes to break them down.

3 Important Enzymes

Enzyme	What it does
Peroxidase	Breaks down hydrogen peroxide (H₂O₂)
Superoxide dismutase (SOD)	Breaks down superoxide (O₂⁻)
Catalase	Breaks down H₂O₂ → water + oxygen

Catalase Test

• Add 3% hydrogen peroxide to the bacterial sample.

• Bubbles = catalase positive 🫧

• No bubbles = catalase negative

Staphylococci → catalase positive
Streptococci → catalase negative

Capnophiles

• Capnophiles grow best with more CO₂ and less O₂.

• A candle jar helps create these conditions.

• The candle uses up oxygen and releases CO₂.

Memory Trick

SOD → Superoxide
Catalase → H₂O₂ + bubbles
Candle → ↓O₂ + ↑CO₂

9,3 The Effects of pH on Microbial Growth

pH Basics

• pH < 7 = acidic

• pH = 7 = neutral

• pH > 7 = basic/alkaline

• Extreme pH can damage proteins, DNA, and lipids.

• Proteins are especially sensitive because pH changes can cause them to denature and lose their function.

3 Important pH Levels

• Minimum pH = lowest pH the organism can tolerate and still grow.

• Optimum pH = best pH for growth.

• Maximum pH = highest pH the organism can tolerate.

Think: Minimum → Optimum → Maximum = lowest → best → highest.

3 Types of Microbes

Type	Best pH	Easy memory
Acidophile	Below 5.5	❤ Acid
Neutrophile	Around 7	⚖ Neutral
Alkaliphile	8–10.5	🔵 Basic

Acidophiles

• Grow best in acidic environments.

• Example: Sulfolobus.

• Some can survive around pH 2–3.5.

Neutrophiles

• Grow best around pH 7.

• Most bacteria are neutrophiles.

• Examples: E. coli and Salmonella.

Alkaliphiles

• Grow best in basic environments, around pH 8–10.5.

• Example: Vibrio cholerae

H. pylori Example

• H. pylori is actually a neutrophile, not an acidophile.

• It survives in the stomach by producing urease.

• Urease breaks down urea and creates NH₄⁺, which raises the pH around the bacteria.

• This creates a nearly neutral microenvironment.

9.4 Temperature and Microbial Growth

Temperature Growth Terms

• Minimum growth temperature = lowest temperature a microbe can grow.

• Optimum growth temperature = temperature where it grows best/fastest.

• Maximum growth temperature = highest temperature it can grow.

2. Types of Microbes Based on Temperature

Type	Temperature	Easy way to remember
Psychrophile	Very cold: ~0°C or below; optimum ~15°C	Cold
Psychrotroph	~4–25°C	Cool
Mesophile	~20–45°C	Moderate
Thermophile	~50–80°C	Hot
Hyperthermophile	~80–110°C+	Extremely hot

Most human pathogens = mesophiles because the human body is about 37°C.

3. Psychrophiles vs. Psychrotrophs

• Psychrophiles → truly cold-loving; can grow around 0°C and usually cannot survive above 20°C.

• Psychrotrophs → prefer cool temperatures and can grow in refrigerators.

• Psychrotrophs can cause refrigerated food to spoil.

9.4 Temperature and Microbial Growth prt2

4. Listeria

Listeria monocytogenes

• Gram-positive short rod

• Psychrophile

• Halotolerant = tolerates high salt

• Can grow at 4–10°C

• Can contaminate meat, fish, dairy, and unpasteurized foods.

• Especially dangerous during pregnancy because it can cross the placenta.

Memory:
Listeria = likes the refrigerator

5. Extreme Heat

Thermophiles

• Grow best around 50–80°C

• Examples: Thermus aquaticus, Geobacillus

Hyperthermophiles

• Grow around 80–110°C

• Some can survive 121°C, the temperature of an autoclave.

• Found near hydrothermal vents.

6. How Temperature Affects Cells

Cold

• Membranes become less fluid.

• Chemical reactions slow down.

• Proteins become too rigid.

• Ice crystals can damage cells.

Heat

• Proteins denature.

• DNA/nucleic acids can be damaged.

• Membranes become too fluid.

7. Adaptations

Psychrophiles:

• More unsaturated lipids → keeps membrane fluid.

• Flexible proteins.

• Antifreeze proteins/solutes help prevent freezing.

Thermophiles:

• More saturated lipids → keeps membrane from becoming too fluid.

• More G-C DNA → G-C has 3 hydrogen bonds vs. A-T's 2.

• Proteins have extra bonds/structures that make them more heat-stable.

8. Important Application: Taq Polymerase

Taq polymerase comes from Thermus aquaticus.

It is heat-stable, so it can survive the high temperatures used during PCR.

Memory:
Taq = Thermus = heat-stable enzyme = PCR

Know These for the Test

• Cold → psychrophile

• Cool → psychrotroph

• Human body → mesophile

• Hot → thermophile

• Extremely hot → hyperthermophile

• Listeria → psychrophile + halotolerant

• Taq polymerase → thermophile → PCR

• Thermophiles → more saturated lipids + higher G-C

• Psychrophiles → more unsaturated lipids + flexible proteins

9.5 Osmotic and Barometric Pressure

1. Osmotic Pressure & Salt

When there is too much salt outside the cell, water leaves the cell.

➡ This causes plasmolysis = the cell's cytoplasm shrinks away from the cell wall.
➡ Too much water loss can cause cell death.

Why salt preserves food:
Salt removes water from microbial cells → microbes can't grow easily.

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2. Halophiles 🧂

Halophile = “salt-loving”

• Require high salt to grow.

• Found in salty environments.

• Example: Halobacterium

• Found in places like the Great Salt Lake and Dead Sea.

Memory:
🧂 Halo = salt-loving

Halotolerant

Halotolerant = can tolerate salt but doesn't require it.

Examples:

• Staphylococcus

• Micrococcus

• Corynebacterium

Some halotolerant bacteria can cause food poisoning, including:

• S. aureus

• Bacillus cereus

• V. cholerae

Easy difference:

Type	Salt
Halophile	NEEDS high salt
Halotolerant	Can SURVIVE high salt

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3. Water Activity (aw) 💧

Water activity = amount of water available for microorganisms to use.

• Pure water = aw = 1.0

• Bacteria need high aw: 0.97–0.99

• Fungi can survive with less water.

Food preservation methods:

• Drying → jerky

• Freeze-drying

• Salt/brine

• Jams → high osmotic pressure

🧠 Memory:
Less available water = less microbial growth

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4. Barophiles 🌊

Barophile = microorganism that requires high pressure to grow.

• Found deep in the ocean.

• Must withstand very high pressure.

🧠 Memory:
Baro = pressure

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⭐ Test Review

Halophile: requires high salt
Halotolerant: tolerates salt but doesn't require it
Plasmolysis: water leaves cell → cytoplasm shrinks
Water activity (aw): available water for microbial growth
Bacteria: need high aw
Fungi: tolerate lower aw
Barophile: requires high pressure

🔑 Super Easy Memory Trick

HALO = SALT 🧂
BARO = PRESSURE 🌊
aw = WATER 💧
PLASMOLYSIS = WATER LEAVES CELL

9.5 Osmotic and Barometric Pressure

1. Osmotic Pressure & Salt

When there is too much salt outside the cell, water leaves the cell.

➡ This causes plasmolysis = the cell's cytoplasm shrinks away from the cell wall.
➡ Too much water loss can cause cell death.

Why salt preserves food:
Salt removes water from microbial cells → microbes can't grow easily.

2. Halophiles

Halophile = “salt-loving”

• Require high salt to grow.

• Found in salty environments.

• Example: Halobacterium

• Found in places like the Great Salt Lake and Dead Sea.

Halotolerant

Halotolerant = can tolerate salt but doesn't require it.

Easy difference:

Type	Salt
Halophile	NEEDS high salt
Halotolerant	Can SURVIVE high salt

3. Water Activity (aw)

Water activity = amount of water available for microorganisms to use.

• Pure water = aw = 1.0

• Bacteria need high aw: 0.97–0.99

• Fungi can survive with less water.

Food preservation methods:

• Drying → jerky

• Freeze-drying

• Salt/brine

• Jams → high osmotic pressure

Memory:
Less available water = less microbial growth

4. Barophiles

Barophile = microorganism that requires high pressure to grow.

• Found deep in the ocean.

• Must withstand very high pressure.

Test Review

Halophile: requires high salt
Halotolerant: tolerates salt but doesn't require it
Plasmolysis: water leaves cell → cytoplasm shrinks
Water activity (aw): available water for microbial growth
Bacteria: need high aw
Fungi: tolerate lower aw
Barophile: requires high pressure

Super Easy Memory Trick

HALO = SALT
BARO = PRESSURE
aw = WATER
PLASMOLYSIS = WATER LEAVES CELL

9.5 Light

1. Microbes That Use Light

Photoautotrophs

• Use light for energy

• Use CO₂ as a carbon source

• Examples: cyanobacteria and green sulfur bacteria

Photoheterotrophs

• Use light for energy

• Use organic compounds for carbon

• Example: purple nonsulfur bacteria'

2. Photosynthetically Active Radiation (PAR)

PAR = light that microorganisms can use for photosynthesis.

• Mainly 400–700 nm (visible light)

• Some photosynthetic bacteria can also use near-infrared light.

🧠 Memory:
PAR = Photosynthesis-Active Radiation

3. Accessory Pigments

Some pigments allow microbes to capture more wavelengths of light, especially when light is weak deeper in water.

Examples:

• Fucoxanthin → brown algae

• Phycobilins → cyanobacteria

4. Halobacteria

Some Halobacteria use light differently.

• They use light to power proton and sodium pumps.

• Light is absorbed by bacteriorhodopsin.

So, they use light energy but not necessarily for photosynthesis.

5. Watermelon Snow

Chlamydomonas nivalis

• A green microalga

• Contains the red pigment astaxanthin

• Astaxanthin gives snow a pink/red color.

Memory:
Watermelon snow = Chlamydomonas + astaxanthin

Test Review

• Photoautotroph → light energy + CO₂ carbon

• Photoheterotroph → light energy + organic carbon

• PAR → 400–700 nm

• Fucoxanthin → brown algae

• Phycobilins → cyanobacteria

• Bacteriorhodopsin → Halobacteria

• Watermelon snow → Chlamydomonas nivalis + astaxanthin

9.6 Nutritional Requirements

1. Culture Media

Culture media = materials used to grow microorganisms in the laboratory.

Different microbes need different nutrients and environmental conditions.

2. Types of Media General-Purpose Media

Supports many different microorganisms.

Example: Tryptic Soy Broth (TSB)

Enriched Media

Contains extra nutrients to help organisms that have special nutritional needs.

Fastidious organisms = microbes that cannot make certain nutrients themselves, so those nutrients must be added.

Enriched = extra nutrients

Chemically Defined Media

• Every chemical ingredient is known.

• Exact amounts are known.

Example: EZ medium

Defined = you know exactly what's inside

Complex Media

• Contains extracts/digests from things like meat, yeast, or plants.

• Exact chemical composition is not known.

• Amounts can vary.

Examples:

• Nutrient broth

• Tryptic soy broth

• Brain heart infusion

Complex = composition isn't completely known

Defined vs. Complex

Defined	Complex
Exact ingredients known	Exact ingredients unknown
Exact amounts known	Amounts can vary
Example: EZ medium	Example: TSB

3. Selective vs. Non-Selective Media

Allows many/all microorganisms to grow.

Example: Mueller-Hinton agar

Used for antibiotic testing, including the Kirby-Bauer test.

Selective Media

Stops/inhibits some microbes while allowing others to grow.

Example: MacConkey agar

• Inhibits many Gram-positive bacteria

• Favors Gram-negative bacteria

Selective = selects who gets to grow

4. Enrichment Culture

Enrichment culture = encourages the growth of a specific microorganism that is only a small part of the original sample.

Enrichment = increase the organism you want

Selective vs. Enrichment

• Selective → inhibits unwanted organisms.

• Enrichment → gives the organism you want an advantage.'

5. Differential Media

Differential media = makes different bacteria look different.

Usually happens through a color change.

MacConkey Agar

Can differentiate bacteria based on lactose fermentation.

• Lactose fermenter → produces acid → pink colonies

• E. coli → bright pink

• Serratia marcescens → cream-colored because it doesn't ferment lactose

Differential = different colors help you tell bacteria apart

MUST-KNOW

Enriched → extra nutrients
Defined → exact ingredients known
Complex → exact composition unknown
Selective → inhibits certain microbes
Enrichment → favors the microbe you want
Differential → makes microbes look different
MacConkey → selects Gram-negative + differentiates lactose fermenters
Mannitol salt agar → helps identify S. aureus
S. aureus → Gram+ cocci in clusters + catalase positive + halotolerant