Bacteria are microscopic, single-celled organisms that exist in their millions, in every environment, both inside and outside other organisms.

Some bacteria are harmful, but most serve a useful purpose. They support many forms of life, both plant and animal, and they are used in industrial and medicinal processes.

Bacteria are thought to have been the first organisms to appear on earth, about 4 billion years ago. The oldest known fossils are of bacteria-like organisms.

Bacteria can use most organic and some inorganic compounds as food, and some can survive extreme conditions.

A growing interest in the function of the gut microbiome is shedding new light on the roles bacteria play in human health.

In this post, Pritish Kumar Halder takes a brief look at Bacteria. Read below:

What are bacteria?

Bacteria are single-cell organisms that are neither plants nor animals.

They usually measure a few micrometers in length and exist together in communities of millions.

A gram of soil typically contains about 40 million bacterial cells. A milliliter of fresh water usually holds about one million bacterial cells.

The earth is estimated to hold at least 5 nonillion bacteria, and much of the earth’s biomass is thought to be made up of bacteria.


There are many different types of bacteria. One way of classifying them is by shape. There are three basic shapes.

  • Spherical: Bacteria shaped like a ball are called cocci, and a single bacterium is a coccus. Examples include the streptococcus group, responsible for “strep throat.”
  • Rod-shaped: These are known as bacilli (singular bacillus). Some rod-shaped bacteria are curved. These are known as vibrio. Examples of rod-shaped bacteria include Bacillus anthracis (B. anthracis), or anthrax.
  • Spiral: These are known as spirilla (singular spirillus). If their coil is very tight they are known as spirochetes. Leptospirosis, Lyme disease, and syphilis are caused by bacteria of this shape.

There are many variations within each shape group.


Bacterial cells are different from plant and animal cells. Bacteria are prokaryotes, which means they have no nucleus. A bacterial cell includes:

Bacteria structure

Capsule: A layer found on the outside of the cell wall in some bacteria.

Cell wall: A layer that is made of a polymer called peptidoglycan. The cell wall gives the bacteria its shape. It is located outside the plasma membrane. The cell wall is thicker in some bacteria, called Gram positive bacteria.

Plasma membrane: Found within the cell wall, this generates energy and transports chemicals. The membrane is permeable, which means that substances can pass through it.

Cytoplasm: A gelatinous substance inside the plasma membrane that contains genetic material and ribosomes.

DNA: This contains all the genetic instructions used in the development and function of the bacterium. It is located inside the cytoplasm.

Ribosomes: This is where proteins are made, or synthesized. Ribosomes are complex particles made up of RNA-rich granules.

Flagellum: This is used for movement, to propel some types of bacteria. There are some bacteria that can have more than one.

Pili: These hair-like appendages on the outside of the cell allow it to stick to surfaces and transfer genetic material to other cells. This can contribute to the spread of illness in humans.


Bacteria feed in different ways.

Heterotrophic bacteria, or heterotrophs, get their energy through consuming organic carbon. Most absorb dead organic material, such as decomposing flesh. Some of these parasitic bacteria kill their host, while others help them.

Autotrophic bacteria (or just autotrophs) make their own food, either through either:

photosynthesis, using sunlight, water and carbon dioxide, or

chemosynthesis, using carbon dioxide, water, and chemicals such as ammonia, nitrogen, sulfur, and others

Bacteria that use photosynthesis are called photoautotrophs. Some types, for example cyanobacteria, produce oxygen. These probably played a vital role in creating the oxygen in the earth’s atmosphere. Others, such as heliobacteria, do not produce oxygen.

Those that use chemosynthesis are known as chemoautotrophs. These bacteria are commonly found in ocean vents and in the roots of legumes, such as alfalfa, clover, peas, beans, lentils, and peanuts.

Reproduction and transformation

Bacteria may reproduce and change using the following methods:

Binary fission: An asexual form of reproduction, in which a cell continues to grow until a new cell wall grows through the center, forming two cells. These separate, making two cells with the same genetic material.

Transfer of genetic material: Cells acquire new genetic material through processes known as conjugation, transformation, or transduction. These processes can make bacteria stronger and more able to resist threats, such as antibiotic medication.

Spores: When some types of bacteria are low on resources, they can form spores. Spores hold the organism’s DNA material and contain the enzymes needed for germination. They are very resistant to environmental stresses. The spores can remain inactive for centuries, until the right conditions occur. Then they can reactivate and become bacteria.

Spores can survive through periods of environmental stress, including ultraviolet (UV) and gamma radiation, desiccation, starvation, chemical exposure, and extremes of temperature.

Some bacteria produce endospores, or internal spores, while others produce exospores, which are released outside. These are known as cysts.


Bacteria are often thought of as bad, but many are helpful. We would not exist without them. The oxygen we breathe was probably created by the activity of bacteria.

Human survival

Many of the bacteria in the body play an important role in human survival. Bacteria in the digestive system break down nutrients, such as complex sugars, into forms the body can use.

Non-hazardous bacteria also help prevent diseases by occupying places that the pathogenic, or disease-causing, bacteria want to attach to. Some bacteria protect us from disease by attacking the pathogens.

Nitrogen fixation

Bacteria take in nitrogen and release it for plant use when they die. Plants need nitrogen in the soil to live, but they cannot do this themselves. To ensure this, many plant seeds have a small container of bacteria that is used when the plant sprouts.

Food technology

Lactic acid bacteria, such as Lactobacillus and Lactococcus together with yeast and molds, or fungi, are used to prepare foods such as as cheese, soy sauce, natto (fermented soy beans), vinegar, yogurt, and pickles.

Not only is fermentation useful for preserving foods, but some of these foods may offer health benefits.

For example, some fermented foods contain types of bacteria that are similar to those linked with gastrointestinal health. Some fermentation processes lead to new compounds, such as lactic acid, which that appear to have an anti-inflammatory effect.

Bacteria in industry and research

Bacteria can break down organic compounds. This is useful for activities such as waste processing and cleaning up oil spills and toxic waste.

The pharmaceutical and chemical industries use bacteria in the production of certain chemicals.

Bacteria are used in molecular biology, biochemistry and genetic research, because they can grow quickly and are relatively easy to manipulate. Scientists use bacteria to study how genes and enzymes work.

Bacteria are needed to make antibiotics.

Bacillus thuringiensis (Bt) is a bacterium that can be used in agriculture instead of pesticides. It does not have the undesirable environmental consequences associated with pesticide use.