Physical factors that influence microbial growth

1. Physical factors that
influence microbial growth
Khashayar Moarefian

2. Growth
 As all living organisms, microorganisms also require a combination of various physical and
chemical factors for their growth and multiplication.
 Although individual cells approximately double in size during their lifetime this change is not
very significant. Microbial growth actually refers to increase in numbers of the cells.
 The requirements for microbial growth can be divided into two main categories: physical
and chemical.
 Physical aspects include temperature, pH, and osmotic pressure.
 Chemical requirements macromolecules (carbon, nitrogen, hydrogen, sulfur, phosphorus,
oxygen) and micro molecules (trace elements and organic growth factors as magnesium,
potassium, sodium, calcium and iron in their ionised forms)

3. Physical factors influencing the growth of microorganisms
 Temperature
 pH
 Osmotic Pressure
 Hydrostatic Pressure
 Radiation

4. Temperature
 Temperature is the most 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.

7. Types of temperature
1. Minimum growth temperature
The lowest temperature at which organisms grow is the minimum growth
temperature.
2. Optimum growth temperature
The temperature at which the most rapid rate of multiplication occurs.
3. Maximum growth temperature
The highest temperature at which growth occurs. A temperature only slightly above
this point frequently kills the microorganisms by inactivating critical enzymes.

8. classification of bacteria according to growth
temperature

9. Psychrophiles/cryophiles(cold- loving
microbes),
1. The term psyrophile was first used by S.Schmidt-Nelson.
2. Extremophilic organisms that are capable of growth and reproduction in cold
temperatures.
3. Temerature range:-20℃ to +10 ℃.
4. Examples: Oscillatoria, Chlamydomonas nivalis, Methanogenium, etc.

10. Mesophiles-(moderate-temperature-
loving microbes),
 Grows best in moderate temperature.
 Temperature range: 20℃ to 45 ℃ .
 Examples: Escherichia coli, Streptococcus pneumonia, etc.

11. Psychrotrophs-
1. Cold-tolerant bacteria.
2. Have optimal and maximal growth temperatures aborespectivelyve 15 and
20℃, respectively.
3. Pshychrotrophic bacteria and fungi are the principal cause of spoilage of
refrigerated food.
4. Examples: Pseudomonas, Aeromonas, Bacillus, Clostridium, etc.

12. Thermophiles- (heat-loving microbes).
1. Derived from Greek word thermotita meaning heat and philia meaning love.
2. Heat- loving microorganisms.
3. Grow at 50 ℃ or higher. Their growth minimum is usually around 45 ℃ and often
optima between 50 and 80 ℃.
4. Examples: Thermus aquaticus, Geogemma barossii, etc.

13. Hyperthermophiles-
1. Thrives in extremely hot environments.
2. Temperature rang: 80 ℃ to 113 ℃.
3. The cell membrane contains high levels of saturated fatty acids to retain its shape
at high temperatures
4. Examples: Sulfolobus, Methanococcus jannaschii, Thermotoga, etc.

15. Thermal death point (TDP)
 Thermal death point (TDP) of bacteria is basically the study of effect of heat on
the growth of bacteria. In simple terms it is the time needed to kill bacteria in a
medium of liquid culture at a particular temperature.

18. pH
 pH refers to negative logarithm of hydrogen ion concentration.
 Microbial growth is strongly affected by the pH of the medium.
 Drastic variations in cytoplasmic pH disrupt the plasma membrane or inhibit the
activity of enzymes and membrane transport proteins.

19. classification of bacteria according to pH

20.  Acidophiles- Grow between pH 0 and 5.5.
Examples: Ferroplasma, Thiobacillus, Suljolobus acidocaldarius, etc.
 Alkalophils- Grow between pH range of 7.5 to 14.
Examples: Thermococcus alcaliphilus, etc.
 Neutrophiles- Grow between pH 5.5 to 8.0
Examples: Lactobacillus acidophilus, E. coli, Pseudomonas aerunginosa, etc.
Bacteria prefer media of pH near neutrality, and usually can not tolerate pH values
much below 4-5.

22. Oxygen requirements
 Bacteria show a great deal of variation in their requirements for gaseous oxygen. Most can be
placed in one of the following groups:
 1. Obligate aerobes are organisms that grow only in the presence of oxygen. They obtain their
energy through aerobic respiration .
 2. Microaerophils are organisms that require a low concentration of oxygen (2% to 10%) for
growth, but higher concentrations are inhibitory. They obtain their energy through aerobic
respiration .
 3. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in fact, are
often inhibited or killed by its presence. They obtain their energy through anaerobic respiration
or fermentation .
 4. Aerotolerant anaerobes , like obligate anaerobes, cannot use oxygen to transform energy
but can grow in its presence. They obtain energy only by fermentation and are known as
obligate fermenters.
 5. Facultative anaerobes are organisms that grow with or without oxygen, but generally better
with oxygen. They obtain their energy through aerobic respiration if oxygen is present, but use
fermentation or anaerobic respiration if it is absent. Most bacteria are facultative anaerobes.

24. Osmotic pressure(solutes and water
activity)
 Osmotic pressureis the minimum pressure which needs to be applied to a solution
to prevent the inward flow of water across a SPM.
 Types of solution:
1. Hypotoic
2. Isotonic
3. Hypertonic
 Water activity of a solution is 1/100 the relative humidity of the solution. It is
equivalent to the ratio of the vapour pressure of solution to thet of pure water.
Water activity = Vapour pressure of solution
Vapour pressure of pure water

25. Hydrostatic Pressure
 Force acting per unit area.
 Classification of bacteria on the basis of hydrostatic pressure
 Barotolerant-Does not get affected by increased pressure.
 Barophiles/Piezophiles-Bacteria which grow at moderately high hydrostatic
pressures.
Examples: Halomonas salaria, xenophyophores, etc.

26. Radiation
 Sunlight-The major source.
 Infrared is the major source of Earth’s heat.
 Lonising rays can produce mutations which may result in death.
 Visible light is beneficial because it is the source of energy for photosynthesis.

27. References
 Banwart GJ. 1979. Basic Food Microbiology. Westport, Conn.: AVI. Chapter 4, Factors that
affect microbial growth in food; p 115 (table 4.6).
 JA, Showalter RK. 1981. Infiltration of tomatoes by aqueous bacterial suspensions.
Phytopathology 71(5):515-8.
 KC, Goepfert JM. 1970. Growth of Salmonella at low pH. J Food Sci 35:326-8. CE, Board RG.
1991. Growth of Salmonella enteritidis in artificially contaminated hens' shell eggs. Epidemiol
Infect 106:271-81.
 Davidson PM, Branen AL, editors. 1993. Antimicrobials in foods. 2nd ed. New York: Marcel
Dekker. 647 p. (Food Science, 10).
 Rosario BA, Beuchat LR. 1995. Survival and growth of enterohemorrhagic Escherichia coli
0157:H7 in cantaloupe and watermelon. J Food Prot 58:105-7.
 MP, Beuchat LR, Montville TJ, editors. 2001. Food microbiology: fundamentals and frontiers.
2nd ed. Washington (DC): American Society for Microbiology.
 Microbiology,P.D.Sharma
 www.slideshare.net
 General Microbiology,C.P Power