Bacterial Growth By Dr. Marwa Salah

Learning objectives Definition of bacterial growth. Requirements of bacterial growth. Types of respiration in bacteria. Phases of bacterial growth curve.

Growth involves an increase in the size and number of organisms. In the laboratory, bacterial growth is seen in two main forms:  Development of colonies: macroscopic products of divisions of a single bacterium on solid media  Conversion of a clear fluid medium to a turbid suspension

Bacteria growth on solid and liquid culture media

Growth Requirements 1- Nutrients Heterotrophs:Autotrophs: Organic sourcesInorganic materials e.g. CO2 Source of carbon They cannot synthesize them from inorganic materials from inorganic materials Synthesis of complex organic substances Most bacteria of medical importance No or little medical importance Medical importance

2- Oxygen (O 2 ) Strict or obligate aerobes: require oxygen e.g. Pseudomonas aeroginosa. Strict or obligate anaerobes: Require complete absence of oxygen e.g. Bacteroids fragilis. Facultative anaerobes: grow better in absence of oxygen but still are able to grow in its absence e.g. staphylococci, E. coli Microaerophilic: requires reduced oxygen level e.g. Campylobacter, Helicobacter. Aerotolerant: have anaerobic pattern of metabolism but can tolerate the presence of oxygen because they possess superoxide dismutase e.g. Clostridium perfringens.

3.Respiration and energy production A.Aerobic cellular respiration: In the presence of oxygen. The final electron acceptor is molecular O 2. Superoxide radicals and hydrogen peroxide are formed which are highly toxic, aerobic organisms have developed two enzymes ; superoxide dismutase and catalase that detoxify these molecules. Produces 38 ATP molecules.

B.Anaerobic cellular respiration In the absence of oxygen The final electron acceptor is an inorganic molecule such as NO 3, SO 4 or CO 2. They lack enzymes that detoxify superoxide radicals and hydrogen peroxide, so they cannot grow in the presence of oxygen. Less yield of ATP B.Fermentation Used by facultative anaerobes when the environment lack suitable inorganic final electron acceptor. The least efficient method in generating energy

4.Carbon dioxide (CO2) Most bacteria require the minute amount of CO 2 present in air. Some species require higher concentration (5-10%) (cabnophilic)e.g. neisseria, Brucella abortus.

5.Temperature Mesophiles: able to grow in temperature o C. Pathogenic bacteria are able to grow within this range with optimum temperature of 37oC (the normal body temperature). Psychophiles (cold-loving): grow at refrigeration temperature (0-8 o C e.g. Flavobacterium. Thermophiles (heat-loving): grow at high temperature (>60 o C) e.g. Bacillus stearothermophilus.

6.Hydrogen ion concentration (pH) Most pathogenic bacteria grow at pH that is close to the human body (pH 7.2) Some bacteria grow better at alkaline pH (8-9) e.g. Vibrio cholera. Some bacteria grow better at acidic pH (4 or less) e.g. lactobacilli.

Bacterial growth curve If a small number of an organism is placed in a suitable fluid nutrient medium under appropriate physical and chemical conditions, then the number of viable cells per milliliter is determined periodically and blotted, a characteristic growth curve with four phases is obtained.

Phases of Bacterial Growth Curve Lag phase: The cells adapt to the new environment, enzymes and intermediates are formed to permit growth, the initial number of bacterial cells is constant. Exponential (logarithmic phase): Marked increase in cell number which is accelerated exponentially with time giving a characteristic linear blot, the organism shows typical morphology in this phase.

Stationary phase: Exhaustion of nutrients and accumulation of toxic products decreases growth, cell death is balanced with the formation of new cells, so the number of viable bacteria remains constant. Decline phase: The death rate increases and exceeds the multiplication rate due to nutrient exhaustion and accumulation of toxic metabolites, so the number of viable bacteria decreases.

Phases of Bacterial Growth Curve

Generation time (doubling time) The time between two successive divisions. It may be short as 13 minutes e.g. V. cholera and may reach 24 hours e.g. M. tuberculosis.