Microbial Growth

What do they need to grow?

• Physical needs– Temperature, proper pH,

etc.• Chemical needs

– Molecules for food, ATP production, coenzymes, etc.

• Growth = increase in number, not size– Binary fission, some by

budding– E coli growing

• Generation time: doubling time– 1-2 hrs for most bacteria– E. coli can divide in 20

minutes in optimum conditions!

What is the bacterial growth curve?• Graph shows a closed system• Lag phase

– Cells acclimating– Preparing to divide

• Log phase– Exponential growth– Generation time reaches constant minimum– Must susceptible to adverse conditions

• Heat, radiation, drugs – Total growth = # of cells X 2n (n = number of

generations)

• Stationary phase– # dividing = # dying– Population stabilizes– Decrease in nutrients, increase in

wastes• Death phase

– # deaths > # new cells

How can I measure bacterial growth directly?

• Direct count– Plate

counts– Serial

dilutions• Then

either pour plates or spread plates

– Pro: only measures viable cells

– Con: time consuming!

Pro: only measures viable cells

– Con: time consuming!

How can I measure bacterial growth directly?

• Direct microscopic count– Use gridded

slide to count• Pro: no

incubation time• Con: counts

dead cells, too; hard to count moving bacteria

– Take average and calculate back from dilution

• Coulter counter

How can I measure bacterial growth indirectly?

• Turbidity– Cloudiness and more bacteria present– Use spectrophotometer

• Percentage of transmission• Only works if you have at least a 1M cells per milliliter

What are the physical requirements for growth?

• Temperature– Each species has

preferred temp range (over about 30 degrees C spread)

– Dies quickly outside range

• Minimum growth temp

• Maximum growth temp

• Optimum growth temp– Usually near top

of range (close to max)

• Temperature: Three groups

• Psychrophiles (cold loving): 5-20 degrees C

– Psychrotrophs: 20-30 degrees C

• Contribute to food spoilage in refrigerator

• Mesophiles (mod.-temp): 25-40 degrees C

– Most common– Often in animals

• Thermophiles (hot): 45-60 degrees C

– Obligate thermophiles: only above 50 degrees C

– Extreme thermophiles: above 80 degrees C

• Archaea

What are the physical requirements for growth?

What are the physical requirements for growth?

• pH– Most bacteria prefer 6.5-7.5

• When growing, pH changes• Additive to growth medium buffer pH

– Phosphate salts, amino acids

– Yeast, molds more tolerant of greater range• Optimum 5-6

• Acidophiles– Not many

What are the physical requirements for growth?• Osmotic pressure

– Bacteria are 80% to 90% water– Hypertonic solutions ________

water ____ cell• Results in plasmolysis

– PM pulls away from cell wall

– Retards bacterial growth– Why meat, butter, etc.

salted– Extreme halophiles

• Obligate halophiles– Bacteria in Dead Sea,

30% salt• Facultative halophiles

– Can grow in up to 2% salt– Some in up to 15%– Salt not required for

growth

Anaerobic Culture Methods

• Reducing media– Anaerobic jar– Contain chemicals (thioglycollate or oxyrase)

that combine O2

– Heated to drive off O2

What are all the different “troph” types?

Energy source: nonliving environment

• Photoautotroph• Chemoautotroph

Energy source: other organisms or sunlight

• Photoheterotroph• Chemoheterotroph• Saprobe• Parasite

What are the chemical requirements for growth?• Oxygen

– Aerobe– Obligate aerobe– Anaerobe– Obligate anaerobe

• Often harmed by oxygen• Clostriudium

– Facultative anaerobe• Uses O2 when present• Can use anaerobic path or fermentation• E. coli

– Aerotolerant anaerobes• Tolerate oxygen but don’t use it• Lactobacilli for cheese, pickles

– Microaerophiles• Require oxygen at low concentration

• Others include nitrogen, sulfur, phosphorus, etc.

What are symbiotic relationships?

• Mutualism– lichen

• Commensalism– Satellitism– Microbial flora– Lactobacillus– E. coli

• Parasitism

What are non-symbiotic relationships?

• Synergism– Roots &

bacteria

• Antagonism– Penicillium

Photo from: http://scientificteaching.wisc.edu/products/PeanutFiles/imagesforsite/penicillium.jpg