Chapter 6: Microbial Growth

Requirements for Microbial Growth

Physical:

-Temperature

-pH

Osmotic Pressure

Requirements for Microbial Growth

Chemical:

-CHONPS

-Growth factors and trace elements

Temperature

M/O can be classified by their optimum temp.

-Psychrophile-

2 different groups...

1. Can grow at 0 C, optimum growth is 15 C, won't grow above 25 C.

2. Can grow at 0 C, optimum growth is 20-30 C, Won't grow above 40 C.

Responsible for most food spoilage -> Psychotroph.

Temperature Continued...

Mesophiles- "Moderate-temp- loving"

-Optimum growth temp. 25-40 C (37 C).

Most common group, includes most *PATHOGENS.

Thermophiles- "Heat- loving"

Optimum growth temperature is 50-60 C.

Hyperthermophiles (Extreme thermophiles)

-Optimum growth temperature is 80 C, or higher.

-Usually members of the Archaea.

Physical Requirements- pH

Most M/O grow near neutrality-> 6.5-7.5

-pH of blood -7.38, ideal for most pathogens.

-Few bacteria can grow below a pH of -> 4

Acidophiles

"Acid-loving"

-very small group

-several foods are preserved by acids produced by fermentative bacteria. ex. cheeses, sauerkraut, pickles, ect.

Physical Requirements-> Osmotic Pressure

Osmotic Pressure: The pressure required to stop the flow of water across a selectively permeable membrane.

Plasmolysis

Osmotic loss of water-> shrinkage of plasma membrane away from cell wall.

Principle of food preservation using salts and sugars-> increase the osmotic pressure.

Plasmoptysis

Osmotic gain of water-> Cell Bursting!

Chemical Requirements: Sources of the Essential Elements

Carbon: Structural backbone of all living matter.

-50% of the cell is made up of carbon.

Nitrogen (N) 14%, Sulfur (S), and Phosphorous (P)

-N+S Required for protein synthesis

-N+P Required for DNA, RNA, and ATP.

-N Sources include proteins, ammonium ions (NH4+), nitrate ions (NO3-), nitrogen gas (N2)

-S sources include sulfate ions (SO42-), hydrogen sulfide gas (H2S), and amino acids (cysteine).

-P sources include phosphate ions (PO43-)

Trace Elements: Other minerals such as Fe, Cu, Zn, Mg, ect. act as cofactors for enzymes.

Chemical Requirements- Oxygen

-Aerobic respiration is more efficient than anaerobic

Any organism that can grow in the presence of oxygen must have...

-Superoxide dismutase (SOD) to neutralize harmful forms of oxygen radicals (O2-->H2O2 hydrogen peroxide) and

-Catalase H2O2-> H20 + O2 or Peroxidase H2O2 + 2H+-> 2H20

Culture Medium (food)

Material used to grow M/O

Media must meet 3 criteria...

1. Contain appropriate nutrients, proper pH, moisture.

2. Must be sterile!

3. Must be incubated at the proper temp for growth.

Microbes introduced into media are the...

inoculum

The inoculum that grows on the media is the...

culture

Microbial Growth

Refers to an increase in the number of cells, not the size of the cell.

-Most bacteria reproduce by binary fission: results in 2 individual cells ("daughter" cells) identical to the "parent" cell.

Binary fission:

four steps:

1. DNA divides

2. Cell wall and plasma membrane begin to divide

3. Cross wall forms completely around divided DNA

4. Cells separate.

Generation time

The time required for a cell to divide (or a population to double)

-Varies among organisms...

-Typically 1-3 hours, but varies.

-Myobacterium leprae-> 12 days

-under ideal conditions... Escherichia coli -> 20mins.

Bacterial growth curve:

Demonstrates the growth of cells over time.

-Bacterial growth is exponential.

-Difficult to graph arithmetically.

-Use logarithmic scales

-Distorted perceptions

Phases of Microbial Growth...

4 stages:

1. Lag phase-little or no change in bacterial numbers.

-> Metabolic activity is "gearing up"

2. Log phase (aka Exponential phase)- Great increase in number of cells. Generation time reaches a minimum.

-Cells are metabolically active and most sensivive to adverse conditions, antimicrobials, ect.

3. Stationary Phase: Cell deaths balance the number of new cells

-growth rate slows

-exhaustion of nutrients, waste build up. (decrease pH)

4. Death Phase- Logarithmic decline.

Cell death predominates.

Phases process

Lag, log, stationary phase