1. Physical & chemical Factors
Microbial growth
Physical & chemical Factors

2. Physical factors:
- Temperature
- pH
- osmotic pressure
- radiation.
Chemical factors :
- Oxygen
- carbon

3. Physical factors :Temperature
Temperature is an important factor that determines the rate of growth, multiplication, survival, and death of all living organisms.
High temperatures damage microbes by denaturing enzymes, transport carriers, and other proteins.
Microbial membrane are disrupted by temperature extremes.
At very low temperatures membranes also solidify and enzymes also do not function properly.

4. Types of temperature Maximum growth temperature
- The highest temperature at which growth occurs
Optimum growth temperature
- The temperature at which the most rapid rate of multiplication occurs.
Minimum growth temperature
- The lowest temperature at which organisms grow

6. Psychrophilic organisms:
- are capable of growth and reproduction in cold temperatures
- Temperature range: −20°C to +10°C.
Mesophile :
- grow best at 37c
Thermophilic
- heat-loving microorganism
- ~ 50 to 100 c
Hyperthermophilic
> 100

7. pH

8. Acidophilic :
- grow at low pH
Neutrophilic :
- grow at neutral pH ~7
Alkalophilic:
- grow at high pH

9. pH Yeast and molds can tolerate low pH
that’s why SDA( pH ~ 5.6 ) medium is considered a selective factor to allow fungi to grow and inhibit bacterial growth.
sabouraud dextrose agar SDA

10. Osmotic pressure
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a PM.
Types of solution:
1. Hypotonic
2. Isotonic
3. Hypertonic

12. Classification of bacteria according to osmotic pressure
1. Osmotolerant ( salt-tolerant) are those microorganisms which can grow at relatively high salt concentration.
2. Halophiles- Grow in the presence of high salt concentration
Normal saline : 0.9% NACL ( 9 g/L)

13. Radiation UV
Ionizing radiation
- causes mutations

14. Chemical Mannitol salt agar High salt conc.
Mannitol as carbohydrate source
S.aureus gives yellow colonies

15. Chemical Neisseria spp. : Carbohydrate Utilization test
N. Gonorrhea
N. Meningitides
Glucose
Maltose
Lactose
N. Gonorrhea + -
N. Meningitides

16. Oxygen

18. oxygen Obligate aerobes: Obligate anaerobes: Facultative anaerobes:
They have absolute requirement for oxygen in order to grow. Ex. MTB
Obligate anaerobes:  
These bacteria grow in the absence of oxygen and for which oxygen is toxic. Ex. Clostridium.
Facultative anaerobes:  
They are versatile organisms,  capable of growth under both aerobic and anaerobic conditions. They preferentially use oxygen as terminal electron acceptor. e.g.S. aureus
Microaerophilic:
need only 5-10% O2 ex. campylobacter

19. Carbon requirement Capnophlic bacteria:
- require increased concentration of carbon dioxide (5% to 10%) and approximately 15% oxygen.
Air contains 21% oxygen and small amount of (0.03%) of carbon dioxide

20. Change in these factors affect the growth rate of microbes