Bacteria

Bacteria are prokaryotic microorganisms belonging to the domain Bacteria, characterised by the absence of a membrane-bound nucleus and other membrane-enclosed organelles found in eukaryotic cells. First observed through early microscopes by Antonie van Leeuwenhoek in the 1670s, bacteria have since been recognised as one of the three domains of life alongside Archaea and Eukarya. They are estimated to number in the range of 10^30 individual cells globally, colonising soil, water, air, deep-ocean hydrothermal vents, and the bodies of plants and animals.

Structure and cell biology

Bacterial cells are typically between 0.5 and 5 micrometres in length, making them invisible to the naked eye. Despite their simplicity relative to eukaryotic cells, bacteria possess a sophisticated internal organisation.

Cell envelope

Most bacteria are enclosed by a cell wall composed of peptidoglycan, a mesh-like polymer that provides structural rigidity and protection. The composition and thickness of the cell wall underpin the Gram stain classification:

• Gram-positive bacteria have a thick peptidoglycan layer that retains crystal violet dye, staining purple.
• Gram-negative bacteria have a thinner peptidoglycan layer surrounded by an outer membrane, staining pink after the counterstain.

Some bacteria, such as Mycoplasma species, lack a cell wall entirely.

Internal organisation

The bacterial chromosome typically consists of a single, circular DNA molecule located in an irregularly shaped region called the nucleoid. Many bacteria also carry smaller, circular DNA elements known as plasmids, which often encode traits such as antibiotic resistance. Ribosomes (70S type) translate messenger RNA into protein and are the target of several important antibiotics.

Motility structures

Many bacteria move using flagella, rotating protein filaments driven by a molecular motor embedded in the cell membrane. Others use pili to enable a form of surface movement called twitching motility. Some species are non-motile.

Metabolism and nutrition

Bacteria display extraordinary metabolic diversity, allowing them to exploit an enormous range of energy sources.

Autotrophs and heterotrophs

Phototrophic bacteria, including cyanobacteria, harvest light energy via photosynthesis. Chemolithotrophs oxidise inorganic compounds such as ammonia, hydrogen sulphide, or ferrous iron. Heterotrophic bacteria obtain energy by breaking down organic molecules and are central to nutrient cycling in every ecosystem.

Anaerobic and aerobic respiration

While many bacteria require molecular oxygen (obligate aerobes), others are obligate anaerobes that are killed by oxygen. Facultative anaerobes, including Escherichia coli, switch between aerobic and anaerobic metabolism depending on oxygen availability. This metabolic flexibility is a major reason bacteria thrive in such varied environments.

Reproduction and genetics

Bacteria reproduce primarily through binary fission, a form of asexual reproduction in which a single cell enlarges and divides into two genetically identical daughter cells. Under optimal conditions, some species such as E. coli can complete a division cycle in as little as 20 minutes, enabling rapid population growth.

Genetic exchange

Although bacteria do not undergo sexual reproduction in the eukaryotic sense, they exchange genetic material through three main processes:

1. Conjugation — direct transfer of DNA between cells through a physical connection called a pilus.
2. Transformation — uptake of free DNA fragments from the surrounding environment.
3. Transduction — transfer of bacterial DNA by bacteriophages (viruses that infect bacteria).

These mechanisms accelerate horizontal gene transfer, driving rapid evolution and the spread of traits such as antibiotic resistance across bacterial populations.

Role in ecosystems and human health

Ecological importance

Bacteria are indispensable to the nitrogen cycle, with nitrogen-fixing species such as Rhizobium converting atmospheric nitrogen into forms usable by plants. Decomposer bacteria break down dead organic matter, recycling carbon, nitrogen, phosphorus, and other elements. Photosynthetic cyanobacteria are believed to have been responsible for the Great Oxidation Event roughly 2.4 billion years ago, fundamentally altering Earth's atmosphere.

Human microbiome

The human body harbours a vast community of bacteria collectively termed the microbiome. The gut microbiome alone is estimated to contain trillions of bacterial cells and influences digestion, immune function, and even neurological processes. Disruption of this community (dysbiosis) has been associated with conditions ranging from inflammatory bowel disease to metabolic disorders, though the precise causal relationships remain an active area of research.

Pathogenic bacteria

A minority of bacterial species are pathogenic, causing diseases in humans, animals, and plants. Notable examples include Mycobacterium tuberculosis (tuberculosis), Vibrio cholerae (cholera), Clostridioides difficile (C. diff infection), and Staphylococcus aureus (a cause of skin and systemic infections). Bacterial infections are generally treated with antibiotics, though the rise of antibiotic-resistant strains represents a major global health challenge.

Beneficial applications

Beyond ecology, bacteria are exploited extensively in biotechnology. E. coli is a workhorse of genetic engineering, used to produce insulin, vaccines, and research proteins. Lactic acid bacteria underpin fermentation processes in the production of yogurt, cheese, and bread. Bacteria are also employed in bioremediation, degrading pollutants such as oil spills and heavy-metal contamination.

Classification and diversity

Bacterial taxonomy has been transformed by molecular phylogenetics, particularly 16S ribosomal RNA gene sequencing. The domain Bacteria contains dozens of recognised phyla, among the best-studied being:

• Proteobacteria — the largest phylum, including E. coli, Salmonella, and nitrogen-fixing rhizobia.
• Firmicutes — includes Bacillus, Lactobacillus, and pathogenic clostridia.
• Actinobacteria — includes Streptomyces (source of many antibiotics) and Mycobacterium.
• Cyanobacteria — oxygenic photosynthesisers, critical to global primary production.
• Bacteroidetes — abundant in the human gut and soil.

Phylogenetic boundaries continue to be revised as metagenomics uncovers vast numbers of uncultivated bacterial lineages.

Antibiotic resistance

Antibiotic resistance in bacteria is driven by natural selection: exposure to antibiotics kills susceptible strains while selecting for resistant mutants. Resistance genes spread rapidly through horizontal gene transfer. The World Health Organization has identified antimicrobial resistance as one of the greatest threats to global public health; pathogens classified as priority threats include carbapenem-resistant Klebsiella pneumoniae and methicillin-resistant Staphylococcus aureus (MRSA). Strategies to address this include antibiotic stewardship programmes, development of novel antimicrobials, and phage therapy.

Frequently asked questions

Are bacteria prokaryotes or eukaryotes?

Bacteria are prokaryotes, meaning their cells lack a membrane-bound nucleus. This distinguishes them from eukaryotes such as fungi, plants, and animals, whose cells contain a defined nucleus.

Are all bacteria harmful to humans?

The vast majority of bacterial species are harmless or actively beneficial to humans. Pathogenic bacteria represent a small fraction of known species; many bacteria in the human microbiome support digestion, immunity, and overall health.

Is antibiotic resistance in bacteria reversible?

Resistance can diminish in a population if antibiotic selective pressure is removed, but resistance genes often persist, particularly when encoded on stable plasmids. Complete reversal at a population level is rarely observed in clinical settings.

Is bacteria singular or plural?

Bacteria is the plural form; the singular is bacterium. The word derives from the Greek bakterion, meaning "small staff" or "rod," reflecting the rod-shaped appearance of early-observed species.

Are bacteria visible without a microscope?

Almost all bacteria require a microscope to be seen, as most species measure less than 5 micrometres. A notable exception is Thiomargarita namibiensis, one of the largest known bacteria, whose cells can reach up to 0.75 millimetres — visible to the naked eye under ideal conditions.