Microbiology
Bacteria Structure
Prokaryotic cells — simple but ubiquitous; key components plasma membrane, ribosomes, DNA
Bacteria are single-celled prokaryotes — no nucleus, no membrane-bound organelles. Smaller than eukaryotic cells (~1-10 µm). Key components: plasma membrane (selective barrier), cell wall (peptidoglycan; rigid; protects against osmotic pressure), nucleoid (where DNA is concentrated; not membrane-enclosed), ribosomes (70S; smaller than 80S eukaryotic), cytoplasm. Often: capsule (outer protective layer), flagella (for movement), pili (attachment, conjugation), plasmids (extra DNA). Despite simplicity: ~10^30 bacteria on Earth; foundation of ecosystems; cause many diseases.
- Size~1-10 µm (smaller than eukaryotic cells)
- Cell wallPeptidoglycan (polymer of sugars + amino acids)
- Gram positiveThick peptidoglycan; stains purple
- Gram negativeThin peptidoglycan + outer membrane; stains pink
- Ribosome70S (different from 80S eukaryotic)
- Population~10³⁰ bacterial cells on Earth
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why bacteria matter
- Disease. Many human pathogens.
- Microbiome. Gut bacteria essential for health.
- Antibiotics. Treatment exploits bacterial structure.
- Food. Yogurt, cheese, fermentation.
- Environment. Decomposition, nitrogen cycle.
- Biotechnology. Recombinant proteins, drugs.
- Evolution. Antibiotic resistance fast evolution.
Common misconceptions
- Bacteria are simple. Surprisingly sophisticated.
- All bacteria pathogenic. Most are beneficial.
- Bacteria same as viruses. Very different — bacteria are cells.
- Bacteria don't have DNA. They do — just no nucleus.
- Bacteria don't have membranes. Have plasma membrane; some have outer membrane.
- Cell wall same in all. Differs by Gram type.
Frequently asked questions
How are bacteria different from eukaryotes?
(1) No nucleus — DNA in nucleoid (concentrated region but not enclosed). (2) No membrane-bound organelles — no mitochondria, ER, Golgi. (3) Smaller — ~1-10 µm vs 10-100 µm. (4) Different ribosomes — 70S vs 80S. (5) Cell wall: peptidoglycan vs cellulose (plants) or chitin (fungi). (6) Asexual reproduction — binary fission (vs mitosis + meiosis). (7) Rapid evolution.
What's the cell wall?
Peptidoglycan polymer. Repeating units of N-acetylglucosamine + N-acetylmuramic acid; cross-linked by amino acid bridges. Provides: rigidity (against osmotic pressure), shape, protection. Different thicknesses define gram staining: gram + thick wall (40 layers), purple stain; gram - thin wall + outer membrane, pink. Penicillin: targets peptidoglycan synthesis — kills bacteria but not human cells (no peptidoglycan).
What's gram staining?
Differential staining technique developed 1884 (Hans Christian Gram). Crystal violet (purple) → iodine → alcohol wash → safranin (pink). Gram + retain purple (thick peptidoglycan blocks alcohol wash). Gram - lose purple but pick up pink (thin peptidoglycan; outer membrane removed). Different bacteria: different antibiotics work. Used for: rapid bacterial classification.
What's the bacterial flagellum?
Long whip-like appendage for motility. Composed of flagellin protein. Rotates (powered by H⁺ flow through motor at base) — propels bacterium. Number/arrangement varies: monotrichous (one), peritrichous (many around cell), polar tufts. Rotation reverses for tumbling/turning. Allows: chemotaxis (movement toward attractants, away from repellents).
What are pili?
Hair-like appendages, shorter than flagella. Functions: (1) Attachment — to host cells, surfaces, biofilms. (2) Conjugation — sex pilus connects to other bacterium for DNA exchange (transfer plasmid). (3) Type IV pili: twitching motility on surfaces. Pathogenic: E. coli adheres to gut wall via pili; gonococcus to urethra. Important target for antibiotics, vaccines.
What are plasmids?
Small circular DNA molecules, separate from main chromosome. Carry: antibiotic resistance genes, virulence factors, metabolic genes. Replicate independently of chromosome. Can transfer between bacteria (conjugation, transformation, transduction). Important in: bacterial evolution (rapid spread of resistance), genetic engineering (vectors), biotechnology.
How do bacteria reproduce?
Binary fission. Single bacterium grows; DNA replicated; cell divides into two. Fast: E. coli can divide every 20 min in optimal conditions. Theoretical: 1 cell → 10⁹ in 9 hours. Plus: horizontal gene transfer (transformation, transduction, conjugation) — share DNA between bacteria; rapid evolution. Antibiotic resistance spreads quickly.