Bacteria and archaea
Microbes are too small to see with the naked eye, yet they are the most abundant and varied life on Earth. Bacteria and archaea are the two prokaryotic domains.
The three domains
All life is classified into three domains: Bacteria, Archaea, and Eukarya. Bacteria and archaea are both prokaryotes, but differ in their chemistry: archaea have distinct membrane lipids and some genes more like eukaryotes. Many archaea live in extreme environments such as hot springs and salt lakes.
Bacterial structure
A typical bacterium has a cell membrane, a cell wall containing peptidoglycan, ribosomes, and a circular chromosome; many also carry small DNA loops called plasmids. Some have a protective capsule, flagella for swimming, or pili for attaching to surfaces.
Shapes and Gram staining
Bacteria come in three common shapes: cocci (spheres), bacilli (rods), and spirilla (spirals). The Gram stain separates them by wall type: Gram-positive bacteria have thick peptidoglycan walls and stain purple; Gram-negative have thin walls plus an outer membrane and stain pink. This guides choice of antibiotics.
Reproduction and growth
Bacteria reproduce by binary fission, splitting in two. Under ideal conditions, E. coli divides about every 20 minutes. In a culture, growth follows phases: a lag phase, exponential (log) growth, a stationary phase as resources run low, and a death phase.
Gene transfer
Bacteria swap genes horizontally, not just from parent to offspring. In transformation they take up DNA from the environment; in transduction, viruses carry DNA between them; in conjugation, they pass plasmids through a connecting tube. This spreads antibiotic resistance quickly between species.
Metabolic diversity
Bacteria use almost every energy source imaginable. Cyanobacteria photosynthesise and produced Earth's early oxygen. Some bacteria get energy from chemicals such as sulfur or iron. Some need oxygen, some are poisoned by it, and some can switch.
Beneficial bacteria
Most bacteria are harmless or helpful. Gut bacteria help digest food and make vitamins. Bacteria fix nitrogen, decompose waste, and make yogurt and cheese. Industry uses them to produce insulin, enzymes, and biofuels.
Endospores
Some bacteria, such as Clostridium and Bacillus species, form endospores when conditions are harsh: tough, dormant structures that survive boiling, drying, and disinfectants for years. This is why sterilisation in autoclaves uses pressurised steam at 121 degrees C.
Key takeaways
Life has three domains; bacteria and archaea are prokaryotes. Bacteria have peptidoglycan walls, often plasmids, and come in cocci, bacilli, and spirilla. The Gram stain separates thick-walled from thin-walled types. Binary fission can be fast; horizontal gene transfer spreads resistance. Most bacteria are beneficial, and endospores resist harsh conditions.
Flashcard
Bacteria, Archaea, Eukarya
Check your understanding
How can antibiotic resistance spread between different species of bacteria?