Bacteria Calculators

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Bacteria are single-celled prokaryotic microorganisms — among the most abundant and diverse life forms on Earth. They lack a membrane-bound nucleus and membrane-bound organelles, have circular chromosomal DNA, and cell walls (peptidoglycan in most). Bacteria are classified by morphology (cocci, bacilli, spirilla), Gram stain reaction, metabolic type, and 16S rRNA phylogeny. They inhabit every environment — from ocean hydrothermal vents to the human gut — and play crucial roles in nutrient cycling, biotechnology, and disease. There are approximately 10^30 bacteria on Earth.

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Bacterial Cell Structure

  • Cell wall: Gram-positive: thick peptidoglycan layer (20–80 nm); Gram-negative: thin peptidoglycan + outer membrane containing LPS (endotoxin)
  • Cell membrane: Phospholipid bilayer; site of electron transport chain (no mitochondria)
  • Chromosome: Single circular dsDNA; typically 1–10 Mb; in nucleoid region (no nuclear envelope)
  • Ribosomes: 70S (50S + 30S subunits); target of many antibiotics (aminoglycosides, tetracyclines, macrolides, chloramphenicol)
  • Plasmids: Extrachromosomal circular DNA; carry antibiotic resistance genes
  • Flagella: Motility organelle; composed of flagellin protein; proton-motive force driven rotation
  • Pili/fimbriae: Adhesion to surfaces; sex pili for conjugation (horizontal gene transfer)

Bacterial Growth Phases

Lag phase → Exponential (log) phase → Stationary phase → Death phase. During log phase: N = N₀ × 2^(t/t_d), where t_d = doubling time (~20 min for E. coli in LB at 37°C).

Bacterial Metabolism

Classified by carbon source (autotroph/heterotroph), energy source (chemo/photo), and electron donor (litho/organo). Obligate aerobe; microaerophile; facultative anaerobe; obligate anaerobe. E. coli: facultative anaerobe — uses O₂ when available; ferments when not.

Koch's Postulates

Four criteria establishing causation between a bacterium and disease (Robert Koch, 1884). Foundation of medical bacteriology. Molecular Koch's postulates extend this to virulence gene level.

Glossary

Bacteria
Single-celled prokaryotes with circular DNA, 70S ribosomes, peptidoglycan cell walls, and no membrane-bound nucleus or organelles; classified by Gram stain, morphology, and 16S rRNA.
Peptidoglycan
The rigid polymer forming the bacterial cell wall; thick in Gram-positive bacteria; thin in Gram-negative bacteria (with an outer membrane); target of beta-lactam antibiotics (penicillins, cephalosporins).
Endotoxin (LPS)
Lipopolysaccharide from the outer membrane of Gram-negative bacteria; released when bacteria lyse; triggers fever, inflammation, and septic shock; heat-stable (not destroyed by autoclaving).

Frequently Asked Questions

Gram-positive: thick peptidoglycan cell wall (20–80 nm); no outer membrane; retain crystal violet after decolorization → purple on Gram stain. Examples: Staphylococcus, Streptococcus, Bacillus, Clostridium. Gram-negative: thin peptidoglycan + outer membrane containing lipopolysaccharide (LPS/endotoxin) → decolorized and counterstained pink. Examples: E. coli, Salmonella, Pseudomonas, Neisseria. Clinical significance: Gram-negative outer membrane contains LPS endotoxin (triggers septic shock); many antibiotics (penicillins) target peptidoglycan synthesis (more effective against G+ where it's the primary barrier); the outer membrane limits antibiotic entry into G− cells.

Lag phase: metabolic adaptation; cells increase in size; synthesize enzymes and biosynthetic machinery; no net cell division; length depends on culture history and environment. Exponential (log) phase: constant doubling time; fastest growth; cells most susceptible to antibiotics targeting cell wall synthesis (growing cells need new wall). Stationary phase: growth rate = death rate; nutrient depletion or waste accumulation; cells produce stress-resistance factors, secondary metabolites, spores. Death (decline) phase: death rate > growth rate; loss of viability. For experimental work: use late-log/early-stationary phase cells for consistency.

Multiple classification systems: (1) Morphology: cocci (spherical), bacilli (rod-shaped), spirilla (helical), vibrio (comma-shaped). (2) Gram stain: G+ or G−. (3) Oxygen requirement: obligate aerobe, microaerophile, facultative anaerobe, obligate anaerobe, aerotolerant anaerobe. (4) Spore formation: spore-forming (Bacillus, Clostridium) vs. non-spore-forming. (5) 16S rRNA phylogeny: definitive taxonomy; sequences classify bacteria into phyla, classes, orders, families, genera, and species. (6) MALDI-TOF mass spectrometry: clinical identification of cultured isolates to species level within minutes.

Bacterial pathogenesis requires: adherence (pili, adhesins attach to host surfaces); invasion (intracellular pathogens: Salmonella, Listeria, Mycobacterium); immune evasion (capsule inhibits phagocytosis — Streptococcus pneumoniae; protein A binds antibody Fc — Staphylococcus); toxin production: exotoxins (secreted proteins — cholera toxin, botulinum toxin, Shiga toxin) and endotoxin (LPS from G− outer membrane → fever, septic shock). Virulence genes often encoded on pathogenicity islands, plasmids, or phage — mobile genetic elements transferable between strains. Koch's postulates require demonstrating that the pathogen causes disease when introduced to a healthy host.