1. Microbial Growth

2. Requirements for Growth
Temperature pH
Osmotic pressure
Oxygen
Nutrients

3. Temperature Psychrophiles – cold loving Mesophiles – moderate
Thermophiles – heat loving
Most important human pathogens are mesophiles
Optimum growth temperature is 37°C

4. Growth Rates and Temperature

5. Food Spoilage

6. pH and Growth
Most bacteria and especially the important human pathogens grow best at a pH near neutral, 6.5 – 7.5
Neutrophiles
Acidophiles – bacteria that can survive and grow at low pH
Alkalinophiles – bacteria that can survive and grow at high pH

7. Osmotic Pressure Bacteria are 80-90% water
If the cells are placed in a high salt or sugar solution the cells lose water and cannot grow
PLASMOLYSIS
Few bacteria survive high salt
HALOPHILES

8. PLASMOLYSIS

9. How about some nice salt pork?
Because high salt or sugar prevents bacterial growth such foods as salted meats, honey, jellies, etc will usually not spoil if kept at room temperature
Salt preserved meats were used for long ocean trips
Molds can grow however in high sugar, so be careful!
Always refrigerate when in doubt

10. Chemical Requirements
Carbon source – needed to make all organic compounds
Nitrogen, sulfur, phosphorus – synthesis of proteins, nucleic acids, and ATP
Trace elements – iron, zinc, copper

11. Oxygen Requirements Strict aerobe – requires oxygen
Facultative anaerobe – grows in the presence of oxygen but can without oxygen
Obligate anaerobes – cannot use oxygen for growth
Aerotolerant anaerobes – do not use oxygen for growth but can tolerate it

12. Anaerobe jar

13. What’s the problem with oxygen?
During normal respiration bacteria produce oxygen free radicals O2- which combine with 2H+ to produce hydrogen peroxide H2O2 which is toxic
The enzyme that makes this reaction happen is SUPER OXIDE DISMUTASE (SOD)
Another enzyme CATALASE breaks down the H2O2 into H2O and O2
Obligate anaerobes usually have neither SOD or catalase and are killed by oxygen and toxic by-products

15. High CO2 - Capnophiles

16. Growth of Bacterial Cultures
Bacteria reproduce by BINARY FISSION
Time required for a cell to divide is the generation time
Generation time is highly variable
Generally 1-3 hours but can be as short as 30 min

17. Growth curve

18. Stages of Growth Curve
Lag – little increase in number, cells are “getting ready” to grow
Log – rapid, exponential growth, cells most metabolically active, most rapid generation time
Stationary – growth rate slows, equilibrium
Death – decline, cell death exceeds cells formed

19. Measuring Bacterial Populations
Plate counts
Most common way to measure bacterial number
Make a series of dilutions, plate selected dilutions, count colonies
Multiply number of colonies by dilution factor to get COLONY FORMING UNITS

21. On what do you grow bacteria?
Culture media
Culture media can either be solid or liquid and contains all the nutrients the microbe needs to grow
Adding bacteria to sterile media is called INOCULATION
The growth is called a CULTURE
Visible growth on solid media is called a colony

22. What is agar? Agar is the solid agent on which bacteria grow
Agar is made from seaweed
Bacteria cannot break down the agar so it is the perfect solid media on which to observe bacterial growth

24. Types of media Chemically defined media
Exact chemical composition is known
Complex media
Exact chemical composition if media is not known
Most common type of lab media

25. Chemically defined media

26. Complex media

27. Selective and Differential Media
Selective media
Special media that inhibits growth of unwanted bacteria and encourages the growth of desired bacteria
Differential media
Media that allows you to distinguish different types of bacteria growing on a plate usually by color of colonies or growth patterns
Many media are BOTH selective and differential