Nutrition in Bacteria

Nutrition in bacteria is primarily categorized into two major groups, namely autotrophic and heterotrophic. Classification of bacteria in terms of nutritional requirement typically depends on the energy sources, whether in the form of light or chemical energy. Some bacteria obtain energy directly from the sunlight. However, a few bacteria get chemical energy by utilizing inorganic and organic carbon compounds as their sole energy source.

Autotrophic bacteria are photosynthetic organisms that can prepare food for themselves in the presence of sunlight and bacteriochlorophyll pigments. Oppositely, heterotrophic bacteria represent a group of organisms that depend on another host for food and survival.

Therefore, bacteria’s heterotrophic mode of nutrition is classified further into saprophytic, symbiotic and parasitic modes of nutrition based on the feeding mechanisms and the host type. This post describes the definition of nutrition and nutritional types of bacteria with examples.

Content: Nutrition in Bacteria

1. Definition of Nutrition
2. Nutrient Sources
3. Energy Sources
4. Nutritional Types
5. Conclusion

Definition of Nutrition

Nutrition is a process in which living organisms acquire energy and nutrition through their surroundings to carry out various cellular processes. All living organisms have different nutritional requirements for growth and development. Bacteria are prokaryotic organisms that need energy and nutrients to undergo activities like growth, reproduction, and metabolism.

Nutrient Sources in Bacteria

Bacteria necessarily require macronutrients (such as carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur) to build important biomolecules (proteins and nucleic acids) that constitute most cellular components.

• Carbon is the essential macronutrient that constitutes a significant portion of the cell. It builds biomolecules like carbohydrates, proteins, nucleic acids, lipids, etc.
• Nitrogen is necessary to build proteins and nucleic acids. Bacteria may utilize atmospheric nitrogen or other inorganic forms.
• Hydrogen and oxygen constitute many organic compounds, and they are part of water.
• Phosphorus is necessary to synthesize nucleotides and phospholipids.
• Sulfur is a part of amino acids (like cysteine, methionine).
• Besides all of the above, potassium, magnesium, calcium and sodium are essential to maintain the bacterial cell structure and function.

Bacteria need micronutrients (like iron, boron, chromium, and manganese) in trace or minute amounts to perform electron transport chain reactions and other processes.

• Bacteria need iron for the functioning of cytochromes and to undergo electron transport reactions.
• Some bacteria utilize elements like boron, chromium and manganese as they function as primary cofactors.

Energy Sources in Bacteria

They obtain energy from direct light and the oxidation-reduction reactions of different chemical compounds present in our environment. Depending upon the energy sources, the nutrition in bacteria is mainly autotrophic and heterotrophic. Phototrophic bacteria possess bacteriochlorophyll pigments to synthesize food, while heterotrophic bacteria rely on another host for food and energy.

Nutritional Types of Bacteria

Depending upon the energy, electron and carbon sources, bacteria can be classified into different groups, as mentioned in the diagram below. Phototrophs and chemotrophs are the two groups of bacteria that obtain energy through direct sunlight and chemicals.

Then, lithotrophs and organotrophs represent a bacterial group that use reduced inorganic carbon and organic compounds as electron donors, respectively. At last, bacteria utilizing carbon sources are grouped into autotrophs and heterotrophs.

Photoautotrophic and chemoautotrophic bacteria exhibit autotrophic nutrition, whereas photoheterophic and chemoheterotrophic bacteria exhibit heterotrophic nutrition. Chemoheterotrophs either undergo saprophytic, symbiotic and parasitic modes of nutrition.

Based on Energy Source

1. Chemotrophs includes organisms that rely on chemical energy obtained through the oxidation of chemical compounds. Such organisms use a chemosynthesis mechanism where they use the energy released by the oxidation of inorganic molecules to make organic substances. Based on the cellular carbon source, chemoautotrophs and chemoheterotrophs are the two common types. Conversely, chemoorganotrophs and chemolithotrophs are the two groups of chemotrophs based on the origin of a hydrogen donor.
2. Phototrophs include organisms that utilize light energy or capture energy photons. Such organisms use a photosynthesis mechanism where they use light energy to make organic molecules. Based on the cellular carbon source, photoautotrophs and photoheterotrophs are the two common types.

Below, you can look into the table that mentions all the types of bacteria based on different energy sources.

Nutritional Class	Nutritional Types of Bacteria	Energy Source	Electron Source	Carbon Source	Examples
Phototrophs	Photolithotrophs	Light	Inorganic	CO2 or organic	Thiospirillum, Chromatiium etc.
	Photoorganotrophs	Light	Organic	Organic	Rhodospirillum rubrum
	Photoautotrophs	Light	Inorganic or organic	CO2	Cyanobacteria, Green sulfur bacteria and Purple bacteria etc.
	Photoheterotrophs	Light	Organic	Organic	Green nonsulfur bacteria, Purple nonsulfur bacteria etc.
Chemotrophs	Chemolithotrophs	Chemical	Inorganic	CO2 or organic	Hydrogen, Iron and Sulfur oxidizing bacteria etc.
	Chemoorganotrophs	Chemical	Organic	Organic	Psuedomonas pseudoflova
	Chemoautotrophs	Chemical	Inorganic or organic	CO2	Sulfobolus
	Chemoheterotrophs	Chemical	Organic	Organic	Clostridium

Based on Electron Source

1. Lithotrophs use reduced organic compounds as electron donors and thereby using a lithotrophic mode of nutrition. They can be further categorized into chemolithotrophs and photolithotrophs.
   • Chemo-lithotrophic bacteria gain energy from the reduced inorganic compounds. For example, Nitrosomonas use NH₃ as a source of an electron.
   • Photo-lithotrophic bacteria gain energy from light and use reduced inorganic compounds. For example, Chromatium okeinii uses H₂S as an electron source.
2. Organotrophs use organic compounds as electron donors and thereby undergoing the organotrophic mode of nutrition. They can be further classified into chemoorganotrophs and photoorganotrophs.
   • Photo-organotrophic bacteria capture light energy and utilize organic compounds. For example, Rhodospirillum uses succinate as a source of an electron.
   • Chemo-organotrophic bacteria solely obtain energy from organic compounds. For example, Pseudomonas pseudoflora use glucose and amino acids as a source of an electron.

Based on Carbon Source

Autotrophs

They typically make food from inorganic sources (H₂O, CO₂, H₂S salts). The autotrophic bacteria are of two types:

1. Photoautotrophs capture the photons of sunlight through bacteriochlorophylls and convert them into chemical energy. They generally reduce CO₂ to carbohydrates. Inorganic substances serve as hydrogen donors.
2. Unlike photoautotrophs, chemoautotrophs do not require light and chlorophyll pigments. These bacteria undergo oxidation-reduction reactions of certain inorganic substances. As a result of this, the energy is released or the reaction is exothermic. Chemoautotrophic bacteria derive this energy to carry out the synthetic processes of the cell.

Heterotrophs

They obtain their-ready made food from organic substances, living or dead. These are of two types:

1. Photoheterotrophs harness light energy but do not utilize CO₂ as the sole carbon source. They fulfil carbon and electron requirements through organic compounds. Such bacteria possess bacteriochlorophyll pigment.
2. Chemoheterotrophs derive energy from organic sources like carbohydrates, lipids and proteins. Depending upon the mechanism of how chemoheterotrophs get their organic nutrients, there are three main categories listed below.
   • Saprophytic bacteria obtain nutrition from dead organic matter such as decomposed leaves and vegetables. These bacteria secrete enzymes to degrade complex food, such as carbohydrates and protein, into simple absorbable compounds. Examples are Bacillus mycoides, B. ramosus, Acetobacter etc.
   • Parasitic bacteria obtain their nutrition from the living host tissues. They may be harmless or deleterious that may cause severe illnesses and diseases. Examples are Rickettsia, Chlamydia, Bacillus typhosus, Vibrio cholerae, Pseudomonas citri etc.
   • Symbiotic bacteria are closely associated with some organisms called symbionts. They are beneficial to organisms. The common examples are the nitrogen-fixing bacteria (like Rhizobium, Clostridium etc.), those living inside the roots of leguminous plants. Nitrifiers perform a primary role in fixing free atmospheric nitrogen into nitrogenous compounds, which plants utilize. In return, the plant harbours the bacteria by giving essential nutrients and a favourable environment for growth.

Conclusion

Therefore, we can conclude that the nutritional types of bacteria rely upon different energy sources like light energy and chemical energy to undergo various cellular processes.