Bacterial growth

Bacterial growth

From a microbiological point of view, the word growth is defined as an increase in the no of microbial cells in a population which can also be measured as an increase in microbial biomass.The most common method of bacterial multiplication is binary fission.However, there are some bacteria which reproduce by budding.For eg Rhodopseudomonas acidophila,Hypomicrobium species Fragmentation 9eg;Nocardia) and formation of conidiospores or sporangiospores (eg; Streptomyces)

Bacteria divided by binary fission.The time taken by a single bacterial cell to divide from one to two is known as generation time.The bacterial growth of E. coli is 20 minutes.

Binary fission

In most prokaryotes, the growth of an individual cell continues until the cell divides into two cells.The process of forming two daughter cells from single parent cell is known as binary fission.The synthetic process of bacterial cell growth involves as many as 2000 chemical reactions which require a large amount of energy.The main reaction of cell synthesis is the polymerization reaction ie the process by which polymers (macronucleus) are made from monomers.So before the formation of two individual cell, there is the synthesis of macromolecules (nucleic acid, protein, carbohydrates, and lipids) later on these macromolecules are assembled and leads to the formation of the cellular structure such as cell wall, cytoplasmic membrane, flagella, ribosome, inclusion bodies and so on.The primary step of binary fission is the replication of DNA which is followed by septum formation.

Bacterial growth shows exponential growth pattern ie if there is a signified cell.The increase In cell number that occurs in an exponentially growing bacterial culture is a geometric progression of no of 2.

1 2 2² 2³ 2ⁿ after n no of generation.

Because of this geometric propagation there is direct relationship between a no of cells present after a period of express this is as below

So if there is number of cells initially ,final number N will be N=N_o Zⁿ

N_o 2ⁿ=N

The generation time can be calculated by g=t/n

Where t=hours or minutes of exponential growth.

Taking log on both sides

Log N=Log N_o +n log 2

Where,n=No of generation

n_o=Initial no of bacteria

N=Final no of bacteria.

Log N-Log N_o=n log 2

n=log N-log N_o/log 2

n= log N-log N_o/0.301

n=3.3 (log N-log N_o)

Substituting the value of n in generation time,we get

G=t/n=t/3.3 (log N-log N_o)

From this equation, we can also calculate the number of generations that the bacteria can divide.

Growth rate: it is defined as the damage in cell number or cell mass per unit time.During this cell division cycle,all the structural components of the cell double.The interval for the formation of two cells from a single cell is called generation time.

Exponential growth: Bacteria reproduces by binary fission thereby doubling the no. each time as it divides.This pattern of population increases where the no of cells doubles during each unit time period is known as exponential growth.

Bacterial nutrition

Bacteria need nutrition for their growth and functioning bacterial nutrition is classified into two types

• Macronutrients
• Micronutrients
• Macronutrients: Those nutritions that are very vital to the bacterial cell are known as macronutrients.The macro nutrients are carbon,nitrogen,oxygen,sulphur and phosphorous.The nutrients are required in very large amount.Carbon forms the basic skeletal structure of the cell.Nitrogen is the unit of DNA.oxygen is required for cellular respiration,phosphorous is required for ATP synthesis and sulphur acts as an electron donor.
• Micronutrients: Those nutrients are required by bacteria in very small amounts.They are also known as trace elements.Zinc,copper,magnesium,Molybdenum,nickel,boron,cobalt,iron,magnesium falls in this category.Fe,Mg,Mo,mn and Cu are known to be cofactors for various enzymes.Co-factor is responsible for assisting in enzymatic activities.In the laboratory, these nutrients are provided in culture media.The culture media could be either solid,liquid and semi-solid medium.Those bacteria which required special growth factors are known as fastidious organisms whereas those bacteria that can grow in basic growth conditions are known as nonfastidious organisms.

Nutritional classification of bacteria

On the basis of energy source:

Phototrophs: These bacteria can derive energy from sunlight and use inorganic and the organic compound as electron sources.These organisms can derive cellular energy by the process of photophosphorylation.They are further divided into two types ie photo-lithotrophic bacteria or photo-organotrophic bacteria on the basis of the electron source.

Chemotrophs: These organisms use chemical compound either organic or inorganic compounds as the source of energy.eg Nitrosomonas Europaea

On the basis of electron source

Lithotrophs: These organisms use inorganic compounds as the source of an electron.They are classified as photolithotrophs and chemolithotrophs.

Photolithotrophs are defined as the organism which uses light as a source of energy and the inorganic compound as a source of the electron.

Chemolithotrophs are defined as the organism which utilizes chemicals as a source of energy and the inorganic compound as a source of an electron.

Organotrophs: These organisms use organic compounds as a source of the electron.They are classified as photo-organotroph and chemoorganotroph.

Photoorganotroph is defined as the organisms which use light as a source of energy and the organic compound as a source of the electron.

Chemoorganotroph is defined as the organisms which use chemicals as a source of energy and the organic compounds as a source of the electron.

On the basis of carbon source

Autotrophs: They use CO₂ as a source of carbon.They utilize light energy to assimilate CO₂ such organisms are called photoautotrophs.They are further divided into two types i.e photolithotrophs bacteria or photo-organotrophic bacteria on the basis of the electron source.

Heterotrophs: Organisms that use an organic compound as a sole source of carbon are called heterotrophs. They lack the ability to fix CO_2. Most pathogenic bacteria of human beings, other animal and plant are heterotrophs. Some heterotrophs have simple nutritional requirements while some heterotrophs require a large number of vitamin and growth promoting substances. Such organisms are called fastidious heterotrophs.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.