Microbial Growth

What is and isn’t microbial growth 

it is NUMBER of cells, NOT size (observed in colonies/pop) 

4 requirements for microbial growth (ideal)

• temp

• pH

• osmotic pressure

• organic growth factors

Psychrophiles:

• growth range

• optimal growth 

• & what they cause 

• -10-20 °C

• ~12 °C

• food spoilage 👹🤢

example of Psychrophiles & 2 things they do 

• Chlamydomonas nivalis 🧊☃🧤❄

  • grow on Alaskan glaciers & 

  • photosynthetic pigment makes snow red 

Psychrotrophs:

• growth temp range 

• optimal growth 

• 0-30 °C

• ~20s °C

Mesophiles:

• growth temp range 

• optimal growth 

• 10s-50s °C

• ~30s °C

Thermophiles:

• growth temp range 

• optimal growth 

• 40-70s °C

• 60s °C

Hyperthermophiles

• growth temp range 

• optimal growth 

• 60s-110  °C

• 90s °C

Optimal pH for most bacteria?

6.5–7.5

Optimal pH for molds and yeasts?

5.0–6.0

Microbes that grow in acidic environments.

Acidophiles

What happens to microbes in hypertonic environments?

↑ salt/sugar🧂 cause plasmolysis.

Difference between:

• extreme/obligate halophiles &

• facultative halophiles?

• require high osmotic pressure

• tolerate high osmotic pressure

Barophiles survive under ___, but rupture at ___

• extreme pressure,

• normal atmospheric pressure 

4 examples of organic growth factors 

• purines 😇

• pyrimidines 🛕

• amino acids 🧬 

• vitamins 💊

Why do some microbes live in oxygen free habitats

bc they cant detoxify toxic products

4 examples of toxic products

• 👱🏻‍♀️Singlet oxygen (O2)

• 🦸‍♂ Superoxide ion (O2-) 

• 👋 Hydroxyl (OH-) 

• ❤‍🩹 Peroxide (H2O2) 

Label these 5 left → right 

a) Obligate Aerobes 

b) Facultative Anaerobes

c) Obligate Anaerobes 

d) Aerotolerant Anaerobes

e) Microaerophiles

Which 2 enzymes neutralize toxic oxygen?

• Catalase

• Superoxide Dismutase

Biofilms:

• mechanism used

• form _ 

• share _ 

• communicating microbial communities🗣🌳

• form hydrogels/slime💧🫟

• share nutrients/shelter 🍔🏕

Hydrogels:

• mechanism used

• formed by

• attracted to

• quorum sensing

• biofilms

• chemicals 🧪

Biosafety Level Labs (BSL) precautions

• no special, but 

  • cabinets for airborne pathogens 🌬🗄

  • sealed,neg pressure rooms w filtered exhausts🔒𖣘

4 ways bacteria reproduce thru

• conidiospores (actinomycetes🎭🎬)

• filament/fragmentation

• binary fission 

• budding 

Phase: Cells not dividing yet

Lag Phase

Phase: Cells dividing rapidly bc metabolically active

Log Phase

Phase: Net growth zero bc death and birth rate equal & limited nutrients

Stationary Phase

Phase: Cells dying exponentially

Death Phase

What are the 3 direct methods of measuring microbial growth?

• filtration 🚿

• plate count 🍽

• direct microscope count 🔬

Direct Plate Count: 

• counts _ 

• how many _ 

• involves _ 

• colonies after incubation 🍽

• 25-250 CFUS

• serial dilutions 1:10 1:100 1:1000 till all counted 

Pour Plate Method:

• describe

• where colonies grow

• dilution mixed with melted agar;

• colonies grow inside AND on surface.

Spread plate

• describe

• where colonies grow

• dilution spread on agar surface;

• colonies grow only on surface.

Direct membrane filtration

• what kind of bacteria

• what used

• how counted

1. bacteria low conc (i.e. water)

2. 100ml water filtered thru mem w holes 

3. those stuck on mem incubated & counted 

Direct microscopic count: 

• instrument

• counts all _

  • disadvantage

  • advantage

• hemocytometer grid 🩸🧫📏

• counts all cells in fixed volume, →

  • can't distinguish live/dead cells

  • fast

Direct Microscopic Count: 

• Viable Colony Count 

• Direct Cell COunt 

• Viable: counts only living cells able to form colonies

• Direct: counts all cells (living/dead)

most common indirect method of measuring microbial growth & describe 

spectrophotometry 📸

• measures turbidity (cloudiness) 

• more turbidity= ↑ bacterial pop