Module 3: Bacterial Growth

physical

• temperature

• pH

• Osmotic pressure

Chemical

• carbon

• Nitrogen

• Sulfur

• Phosphorus

• Oxygen

Requirements for growth

Temperature

Certain bacteria are capable of growing at extremes of temperature that would certainly hinder the survival of almost all eukaryotic organisms

• Psychrophiles

• Mesophiles

• Thermophile

Classification of temperature

Psychrophiles

• Cold loving microbes

• Organisms capable of growing at 0°C

• Most of these organisms are so sensitive to higher temperature that they cannot grow in warm room (25°C)

• Found mostly in the ocean’s depth or polar regions thus seldom cause problems in food preservation

Mesophiles

• Moderate temperature loving microbes

• The most common type of microbe

• Optimum temperature for many pathogenic bacteria is about 37°C

• Include most of the common spoilage and disease organisms

Thermophiles

• Heat loving microbes

• Microorganisms capable of growth at high temperatures

• Optimum growth: 50-60°C

• Not considered a public health concern

• Important in organic compost piles

Psychrotrophs

• Can grow at 0°C but optimum growth temp. is 20-30°C and cannot grow above 40°C

• Mostly encountered in low-temperature food spoilage because they can grow at refrigerator temperatures

• ‘spoilage microorganisms’

Hyperthermophiles or extreme thermophile

Microbes that have an optimum growth temperature of 80•C

pH

• Most bacteria grow best in a narrow pH range near neutrality, between pH 6.5 and 7.5

• Very few bacteria grow at an acidic pH below about pH 4

Acidophiles

Microorganisms that are tolerant of acidity

Osmotic pressure

• Most microorganisms, however, must be grown in a medium that is nearly all water (1.5% concentration of agar)

• If osmotic pressure is low (environment is hypotonic), microbes with weak cell wall may be lysed

Halophiles

Can adapt to high salt concentrations

• carbon

• trace elements (iron, copper, molybdenum, zinc)

• oxygen

  • obligate aerobes

  • facultative anaerobes

  • aerotolerant anaerobes

  • microaerophiles

chemical requirement

Obligate aerobes

Organisms that require oxygen to live

Facultative anaerobics

• can use oxygen when it is present but are able to continue growth by using fermentation or anaerobic respiration when oxygen is not available

• E.g. E. coli

Anaerobic

• bacteria that are unable to use molecular oxygen for energy- yielding reactions

• E.g. Clostridium

Aerotolerant anaerobic

• fermentative and cannot use oxygen for growth, but they tolerate it fairly well

Microaerophiles

• aerobic; they do require oxygen

• they grow only in oxygen concentrations lower than those in air

Bacterial growth

• Refers to an increase in bacterial numbers, not an increase in the size of the individual cells

• Bacteria reproduce by binary fission

• Few bacterial reproduce by budding (forming a small initial outgrowth that enlarges until its size approaches that of the parent cell, then it separates)

Generation time

• The time required for a cell to divide

• Varies considerably among organisms and with environmental conditions such as temperature

• Uses logarithmic scales

• lag

• Log

• Stationary

• Death

Phases of growth

Lag phase

• Period of little or no cell division

• Can last for 1 hour or several days

• Cells are not dormant

• Period of intense metabolic activity such as synthesis of enzyme

Log phase

• cells begin to divide and enter a period of growth or logarithmic increase

• Cellular reproduction is most active

• Generation time reaches a constant minimum

Stationary phase

• The growth rate slows

• Number of microbial deaths balances the number of new cells

• Population stabilizes

• Period of equilibrium

• The population exceeds the carrying capacity (the no. of organisms that an environment can support) and run out of nutrients and space

Death phase / logarithmic decline phase

• Number of deaths exceeds the number of new cells formed

• The population is diminished to a tiny fraction of the number of cells or until the population dies out entirely