Types of Bacteria Based on Nutrition and Respiration

Autotrophy refers to organisms that are capable of self-nourishment. An autotrophic organism can obtain all the carbon it needs from atmospheric CO₂ that dissolves in water. There are two main groups of autotrophs:

A) Photosynthetic: These organisms obtain energy from sunlight.

B) Chemoautotrophic: These organisms obtain energy by oxidizing inorganic substances such as sulfur, nitrate, nitrite, or ferrous iron.

Photosynthetic bacteria: Photosynthetic bacteria contain chlorophyll, which can be one of six different types: chlorophyll a, b, c, or d, found in photosynthetic plants, or two other types found only in photosynthetic bacteria. Chlorophyll is located in the membrane of their mesosomes or dispersed in the cytoplasm. Like green plants, photosynthetic bacteria use the energy of sunlight to convert CO₂ into carbohydrates. Examples of photosynthetic bacteria include green sulfur bacteria, purple sulfur bacteria, and purple non-sulfur bacteria. Instead of using water, these bacteria use hydrogen sulfide (H2S) in the following reaction:

2H₂S + CO₂ à (CH₂O)N + H₂O + 2S

Chemoautotrophic bacteria: Certain colorless bacteria are capable of synthesizing carbohydrates from inorganic substances without using light energy. They oxidize inorganic substances, and the energy produced from this oxidation is used to synthesize carbohydrates. Examples of chemoautotrophic bacteria include nitrifying bacteria and sulfur bacteria. Sulfur bacteria oxidize sulfur to produce energy in the following reaction:

2H₂S + O₂ à 2S + H₂O + energy

The energy produced is then used by bacteria to synthesize carbohydrates (CH₂O)n.

2H2S + CO2 à (CH2O)N + H2O + 2S

 

Heterotrophy

Heterotrophic organisms, in contrast, must obtain carbon and metabolic energy from organic molecules that have already been synthesized by autotrophs. They are not capable of self-nourishment and instead rely on consuming autotrophs, other heterotrophs, or organic waste for their nutrition.

Heterotrophic bacteria: These bacteria are not capable of synthesizing their own organic compounds from simple inorganic substances and depend on organic compounds present in their environment for nutrition. There are two types of heterotrophic bacteria: saprotrophic bacteria and parasitic bacteria.

 

Saprotrophic bacteria: These bacteria possess an extensive enzyme system that breaks down the complex substances of humus, which is the decay matter of plants and animals, into simpler compounds. The bacteria then absorb these simpler compounds. Examples of saprotrophic bacteria include many soil bacteria.

Parasitic bacteria: These bacteria obtain their food from a host organism and depend on the enzymes of the host cells to synthesize food. Examples of parasitic bacteria include pathogenic (disease-causing) bacteria, among others.

Note: Pseudomonas is an anaerobic bacterium, Spirochete is an anaerobic bacterium, E.coli is a facultative anaerobic bacterium, and Campylobacter is a microaerophilic bacterium.

Respiration in Bacteria: Bacterial respiration can be either aerobic or anaerobic, and accordingly, bacteria are categorized as aerobic bacteria or anaerobic bacteria. Some bacteria are facultative, meaning they can grow in the presence or absence of oxygen. Bacteria that require a low concentration of oxygen for growth are known as microaerophilic bacteria.