What Term Best Describes Heterotrophic Bacteria That Feed on Living Tissue
Bacteria are microscopic, unicellular organisms that reproduce independently and are mostly free-living. Bacteria are found everywhere in nature. They are organisms that are structurally simple but functionally complicated, and they are the foundation of all life on Earth.
Except for a few, most bacteria are beneficial to their surroundings. They play a variety of important roles in the ecosystem, including toxic component breakdown, nutrient recycling, and nitrogen fixation in the soil from the air, among many others.
Characteristics
Bacteria are unicellular organisms. They are devoid of organelles such as chloroplasts and mitochondria, as well as the true nucleus found in eukaryotic cells. Instead, their DNA, which is consistent and circular double-stranded, is found in a nucleoid.
The nucleoid is a region with an irregular shape that lacks a nuclear membrane. Bacteria have a cell membrane as well as a cell wall, which is often made of peptidoglycan. The cell envelope is made up of the cell membrane and cell wall. Many bacteria require a cell wall to survive.
Reproduction occurs via binary fission, which refers to the splitting of bacteria after it reaches a specific size. Bacteria reproduce asexually, so the two daughter cells produced by binary fission contain the same DNA as the parent cell.
However, some bacteria also have the ability to exchange genetic material horizontally, or among one another. This method uses two pre-existing bacteria. It is not a method of transmission from parent to child.
Autotrophic bacteria
Autotrophic bacteria are a diverse group of self-sustaining organisms that can produce their own food. These microorganisms use inorganic nutrients to grow. They play an important role in the cycling of inorganic compounds.
Autotrophs not only meet their own requirements for reduced carbon monomer units from inorganic matter, but they can also feed pre-existing heterotrophs. As a result, autotrophic organisms are also known as primary producers.
They use light energy (photons) and chemicals in a variety of reactions to generate energy for their biological survival. To do so, they use inorganic compounds such as carbon dioxide, water, or hydrogen sulfide to convert them into organic compounds such as carbohydrates, proteins, and others, which are known to be energy-producing molecules in biological systems.
Types
Since autotrophic bacteria can produce their own food by using light (i.e. photosynthesis) or chemical compounds. As a result, there are two types of autotrophic bacteria based on the substrate used to produce food and energy. They are as follows:
Photoautotrophic Bacteria (photosynthetic): These bacteria use photosynthesis to produce food and energy. They use sunlight to create compounds that supply them with energy.
Chemoautotrophic Bacteria (chemosynthetic): These are bacteria that use chemical compounds to produce both food and energy.
The need for a carbon source, which is necessary for the synthesis of their own food, is a trait shared by both types of bacteria. The examples of various types of autotrophic bacteria are provided below, and their special characteristics are explained.
1. Photoautotrophs
Phototrophic organisms include many microorganisms, as well as green plants, algae, and protists. In the process of photosynthesis, they use light as an energy source. As a result of this mechanism, a proton motive force is generated, which can be utilized in the ATP synthesis while also reducing power consumption (e.g., NADPH).
The majority of photoautotrophs use the ATP-stored energy and the NADPH-stored electrons to absorb carbon dioxide as a carbon source for biosynthesis. These phototrophs are referred to as photoautotrophs. Phototrophs that can use organic compounds as carbon sources and light as an energy source are known as photoheterotrophs.
The following are examples of photoautotrophic bacteria:
1. Aerobic Photosynthetic Bacteria (Oxygenic Photosynthetic Bacteria):
This type of photosynthetic bacteria uses oxygen to perform photosynthesis. Examples of photosynthetic bacteria that use oxygenic photosynthesis include cyanobacteria, also known as blue-green algae. Water molecules are used as electron donors in this type of photosynthesis, and oxygen is produced in the process. They, unlike plants, have photosynthetic pigments such as Chlorophyll-a but no chloroplasts.
These bacteria are colonial and filamentous bacteria. They have the ability to fix nitrogen. Nostoc and Anabaena are examples of this category.
2. Anaerobic Photosynthetic Bacteria (Anoxygenic Photosynthetic Bacteria):
Anaerobic photosynthetic bacteria are the most common type of photosynthetic bacteria. They substitute hydrogen sulphide or thiosulphate instead of water as a reducing agent. They use bacteriochlorophyll, which performs photosynthesis, rather than chlorophyll.
Green sulphur bacteria, purple sulphur bacteria, purple non-sulphur bacteria, phototrophic acidobacteria and heliobacteria, and FAPs (filamentous anoxygenic phototrophs) are some examples of anaerobic photosynthetic bacteria.
a. Purple Sulphur bacteria:
They can be found in sluggish water and hot sulphur springs. These bacteria flourish in anaerobic or low-oxygen environments. They are members of the proteobacteria order Chromatiales. They reduce with hydrogen sulphide or thiosulphates and release sulphur.
The main pigments in the plasma membrane are bacteriochlorophyll a and b. Their photosystem is comparable to that of higher plants' PSII. They are classified into two types:
i. Chromatiaceae that contains sulphur granules between cells.
ii. Ectothiorhodospiraceae that contains sulphur granules extracellularly.
b. Purple Non-sulfur bacteria:
They primarily employ hydrogen as a reducing agent. They are members of the Rhodospirillales order.
Purple photosynthetic bacteria are extremely important because they produce a variety of beneficial substances for plants, including polyphosphates, vitamins, pigments, hydrogen, extracellular nucleic acids, and growth-promoting substances. They can improve biomass quality, boost plant yield, and make plants more resilient to environmental stress.
They are also beneficial in the bioremediation of heavy metals and the reduction of greenhouse gas emissions.
c. Green Sulphur Bacteria:
They are anoxic-loving microorganisms that can be found in the deep oceans and in areas with little light. They also thrive close to thermal vents. The electron donor in this case is sulphide, hydrogen, or ferrous ion.
The plasma membrane and chlorosomes of these organisms contain bacteriochlorophylls 'c', 'd', and 'e' in addition to bacteriochlorophyll 'a. They use reverse tricarboxylic acid to fix carbon. They resemble PSI (Photosystem I) of higher plants in many ways.
2. Chemoautotrophs
The discovery of autotrophy in chemolithotrophic bacteria had a significant impact on the development of our understanding of cell physiology because it demonstrated that organic carbon could be produced from CO2 without photosynthesis.
Chemoautotrophs, such as methanogens, which produce methane, and nitrifiers, which convert ammonia to nitrate, play an important role in the cycling of inorganic compounds on Earth.
They are classified into various types based on the source of their energy, such as sulphur bacteria, hydrogen bacteria, ferrous bacteria, nitrogen bacteria, methanotrophs, and so on. They are crucial in the recycling of nutrients such as nitrogen, phosphorus, sulphur, iron, and so on.
1. Sulphur Bacteria:
These bacteria convert hydrogen sulphide or thiosulphates into molecular sulphur or sulphates. Beggiatoa, Thiothrix, Sulfolobus, and Thiobacillus, are examples of chemosynthetic bacteria.
2. Nitrogenous Bacteria:
These bacteria use ammonia to produce nitrite. Nitrite is next, followed by nitrate. The oxidation reaction causes energy to be released. The plants then utilize the converted nitrate. Nitrobacter and Nitrobacter are two examples.
3. Hydrogen bacteria:
The term "hydrogen bacteria" refers to organisms that oxidize molecular hydrogen. Aerobic hydrogen-oxidizing bacteria use oxygen as an electron acceptor, whereas anaerobic hydrogen-oxidizing organisms use nitrogen dioxide or sulphate. Helicobacter pylori, Hydrogenobacter thermophilus, and Hydrogenovibrio Marinus are examples.
4. Methanotrophs:
They generate energy from methane and use it as a source of carbon. They can be aerobic or anaerobic in nature. Aerobic methanotrophs oxidize methane to formaldehyde, which is then used in a number of different metabolic processes to create organic compounds. Anaerobic methanotrophs use other compounds as electron acceptors.
Examples are Methylomonas, Methylococcus capsulatus, and so on. It absorbs formaldehyde via the RuMP pathway. It's used to make animal feed. Some methanotrophs use the serine pathway to assimilate formaldehyde.
5. Iron Bacteria:
They convert ferrous ions to ferric ions by oxidation. They are found in iron-rich environments such as hot lava beds and hydrothermal vents. Thiobacillus ferrooxidans, Geobacter metallireducens, Zetaproteobacteria, Gallionella, Ferrobacillus, and others are examples.
Heterotrophic Bacteria
Bacteria classified as heterotrophic need organic substrates to obtain the chemical energy necessary for growth and development. The most prevalent bacteria in nature are heterotrophic ones. They are a type of microorganism that feeds on carbon.
Heterotrophic bacteria consume decomposing plants and animals. They can be found in any type of water. A heterotrophic bacterium has a significant impact on humans, both as a helper and as a cause of harm. A heterotrophic bacterium acts as a decomposer and aids in the decomposition process.
Every day, heterotrophic bacteria help us by producing curd from milk, producing antibiotics, fixing nitrogen in legume roots, and so on. However, they also play the role of parasites, harming humans, crops, farm animals, and pets by spreading diseases like cholera, typhoid, citrus, tetanus, canker, etc.
Heterotrophic Bacteria can be found in food, soil, and water. Because heterotrophic bacteria cannot produce their own food, they act as parasites, relying on the host for nutrition. Every type of bacterium that relies on organic nutrients for growth is referred to as heterotrophic.
Types
Heterotrophic bacteria are classified into three groups based on their habitat, food source, and association with other organisms:
Parasitic – They eat living organisms
Saprophytic – They eat dead and decaying organic matter
Symbiotic – They live in symbiotic relationships with other organisms
1. Parasitic Bacteria
Parasitic Bacteria can live as parasites on either plants or animals. Not all parasitic bacteria are harmful, such as the microbial flora found in the human body's skin, digestive tract, mucous membranes, and other organs.
Numerous diseases have been linked to pathogenic heterotrophic bacteria. They infect and spread in different ways. The below table shows parasitic bacteria along with their infection.
| Parasitic bacteria | Disease |
| Xanthomonas axonopodis | Citrus canker |
| Xanthomonas campestris | bean blight |
| Vibrio cholerae | Cholera |
| Bordetella pertussis | Whooping cough |
| Corynebacterium diphtheriae | Diphtheria |
| Agrobacterium tumefaciens | Crown gall |
| Xanthomonas campestris | Blight of beans |
| Pseudomonas syringae | Wildfire of Tobacco |
| Pseudomonas solanacearum | Granville wilt |
| Neisseria gonorrhoeae | Gonorrhea |
2. Symbiotic Bacteria:
These bacteria live in symbiosis with one another or with other organisms. Symbiotic bacteria live in close proximity to other living organisms, benefiting from one another. Neither of them is harmed.
Symbiotic bacteria convert free atmospheric nitrogen into nitrogenous compounds that plants can use, and in exchange, the plant provides nutrients and protection to the bacteria. Symbiotic bacteria include nitrogen-fixing organisms like Rhizobium, which are found in the root nodules of leguminous plants. Zoamastogopera is another example.
3. Saprophytic Bacteria
These are bacteria that feed only on dead and decaying organic matter. They convert complex organic matter into simple compounds. They also aid in the fermentation and purification processes.
These bacteria feed on the decomposition of dead people, animal and plant excreta, as well as their parts. Enzymes are secreted, which break down complex organic compounds into simpler products.
Bacillus mycides, Acetobacter, and other such bacteria are examples of this type.
4. Pathogen Bacteria
These are bacteria that cause diseases and illnesses in their hosts.
References
- https://biologydictionary.net/bacteria/
- https://www.embibe.com/exams/nutrition-in-bacteria/
- https://www.vedantu.com/neet/heterotrophic-bacteria
- https://collegedunia.com/exams/heterotrophic-bacteria-types-characteristics-and-examples-biology-articleid-1646
Source: https://www.biochemgems.com/autotrophic-and-heterotrophic-bacteria/
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