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Unit 4 of 4 · B.Sc MLS Sem 1

Unit 4: Bacterial classification, growth and culture

General Microbiology notes · PTU syllabus (BMLS102-18)

3 min read7 topics10 exam questions
On this page
  1. Unit summary
  2. Classification of bacteria and fungi
  3. Bacterial anatomy
  4. Nutrition and growth requirements
  5. The bacterial growth curve
  6. Synchronous, static and continuous culture
  7. Culture media
  8. Aerobic and anaerobic culture methods
  9. Key terms
  10. Quick revision
  11. Important questions

Unit summary

Identifying bacteria starts with their structure and growth. This unit covers the characteristics and classification of bacteria and fungi, bacterial anatomy, nutrition and growth requirements, the growth curve, synchronous, static and continuous culture, culture media and aerobic and anaerobic culture methods.

After this unit you can

  • Classify bacteria and fungi
  • Describe bacterial anatomy
  • Explain nutrition, growth requirements and the growth curve
  • Classify culture media and describe aerobic and anaerobic culture

PTU syllabus topics

  • General characteristics and classification of bacteria and fungi
  • bacterial anatomy
  • nutrition and growth requirements
  • the bacterial growth curve
  • synchronous/static/continuous cultures
  • classification of culture media (solid, liquid, semisolid, synthetic, basal, enriched, selective, differential, transport, preservation, anaerobic)
  • aerobic and anaerobic culture methods
ProcessBacterial growth curve
  1. 1Lag phase

    Adjusting, no division yet

  2. 2Log (exponential) phase

    Rapid division

  3. 3Stationary phase

    Growth = death

  4. 4Death (decline) phase

    Cells die faster than they divide

1

Topic 1

Classification of bacteria and fungi

ComparisonClassifying bacteria
Basis
Examples

Shape

Cocci (round), bacilli (rods), vibrios (comma), spirilla, spirochaetes

Staphylococcus, E. coli, Vibrio cholerae, Treponema

Arrangement of cocci

Pairs, chains, clusters, tetrads

Pneumococcus, Streptococcus, Staphylococcus

Gram stain

Gram-positive (purple), Gram-negative (pink)

Staphylococcus vs E. coli

Oxygen requirement

Obligate aerobes, facultative anaerobes, microaerophiles, obligate anaerobes

M. tuberculosis, E. coli, Campylobacter, Clostridium

Key termsFungi
Yeasts
Unicellular, budding — Cryptococcus
Yeast-like fungi
Pseudohyphae — Candida
Moulds (filamentous)
Hyphae and mycelium — Aspergillus
Dimorphic fungi
Yeast at 37 °C, mould at 25 °C — Histoplasma
2

Topic 2

Bacterial anatomy

ComparisonBacterial structures
Structure
Function

Cell wall

Peptidoglycan; thick in Gram-positive, thin with outer membrane (LPS) in Gram-negative

Shape, protection; target of penicillin

Cell membrane

Phospholipid bilayer

Transport, respiration

Capsule

Polysaccharide layer

Resists phagocytosis — virulence

Flagella

Protein filaments

Motility

Pili (fimbriae)

Hair-like projections

Adhesion; sex pili transfer DNA

Spores

Resistant dormant forms

Survival — Bacillus, Clostridium

Nucleoid and plasmids

Circular DNA; extra DNA

Genetics; antibiotic resistance

Ribosomes

70S

Protein synthesis; target of antibiotics

3

Topic 3

Nutrition and growth requirements

Key termsGrowth requirements
Nutrients
Carbon, nitrogen, sulphur, phosphorus, growth factors (vitamins)
Temperature
Mesophiles (most pathogens, 25–40 °C, optimum 37 °C), psychrophiles, thermophiles
pH
Most 7.2–7.6; Vibrio prefers alkaline (8.2)
Oxygen
Aerobes to anaerobes
Carbon dioxide
Capnophiles need 5–10% CO₂ — Neisseria
Moisture and osmotic pressure
Water essential; halophiles tolerate salt
  • Nutritional types: autotrophs (CO₂ as carbon) and heterotrophs (organic carbon — all pathogens); phototrophs and chemotrophs.
4

Topic 4

The bacterial growth curve

ProcessPhases of growth
  1. 1Lag phase

    Adapting; no increase in number

  2. 2Log (exponential) phase

    Rapid binary fission; most sensitive to antibiotics

  3. 3Stationary phase

    Nutrients run out; growth equals death; spores and exotoxins form

  4. 4Decline (death) phase

    Cells die exponentially

  • Generation time: time for the population to double — about 20 minutes for E. coli, about 18 hours for M. tuberculosis.
5

Topic 5

Synchronous, static and continuous culture

ComparisonTypes of culture
Description
Use

Static (batch) culture

Fixed medium in a closed vessel; follows the growth curve

Routine diagnostics

Continuous culture

Fresh medium added and culture removed at the same rate — chemostat, turbidostat

Keeps cells in log phase; industry

Synchronous culture

All cells divide at the same time (by filtration or temperature shifts)

Studying the cell cycle

6

Topic 6

Culture media

ComparisonTypes of culture media
Description
Examples

Basal (simple)

Basic nutrients

Nutrient broth, peptone water

Enriched

Basal medium plus blood, serum or egg

Blood agar, chocolate agar

Selective

Inhibits unwanted organisms

MacConkey (bile salts), Löwenstein–Jensen, TCBS

Enrichment (liquid)

Favours one organism in broth

Selenite F broth (Salmonella), alkaline peptone water (Vibrio)

Differential (indicator)

Shows a reaction as colour

MacConkey — lactose fermenters pink

Transport

Keeps organisms alive without growth

Stuart's, Amies, Cary–Blair

Anaerobic

Supports anaerobes

Robertson's cooked meat, thioglycollate broth

Synthetic

Chemically defined

Davis–Mingioli medium

  • By consistency: solid (1.5–2% agar), semisolid (0.2–0.5% agar — motility) and liquid (broth). Preservation media: glycerol broth, cooked meat, lyophilisation for stock cultures.
7

Topic 7

Aerobic and anaerobic culture methods

Key termsCulture methods
Streak plate
Isolates pure colonies
Lawn culture
Antibiotic sensitivity testing
Pour plate
Counts viable bacteria
Stab culture
Motility and storage
Liquid culture
Blood culture, large yields
Key termsAnaerobic methods
McIntosh–Fildes anaerobic jar
Hydrogen with palladium catalyst removes oxygen
Gas-Pak system
Envelope generating H₂ and CO₂
Reducing agents
Thioglycollate, cooked meat
Anaerobic chamber (glove box)
Oxygen-free workspace
Indicator
Methylene blue or resazurin colourless when anaerobic

Key terms

Peptidoglycan
Polymer forming the bacterial cell wall
Capsule
Polysaccharide layer resisting phagocytosis
Generation time
Time for a bacterial population to double
Selective medium
Medium inhibiting unwanted organisms
Facultative anaerobe
Organism growing with or without oxygen

Quick revision

  • Shapes and arrangements; Gram reaction; oxygen needs; yeasts, moulds, dimorphic fungi.
  • Cell wall, capsule, flagella, pili, spores, plasmids.
  • Growth requirements; growth curve phases.
  • Batch, continuous, synchronous cultures.
  • Media types; streak, pour, lawn, stab; anaerobic jar and Gas-Pak.

Important exam questions

Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).

Short-answer questions

  1. Q1.Name the shapes of bacteria.
  2. Q2.Give one difference between Gram-positive and Gram-negative cell walls.
  3. Q3.In which phase are bacteria most sensitive to antibiotics?
  4. Q4.Distinguish selective and differential media.
  5. Q5.Give an example of a transport medium.
  6. Q6.How does a Gas-Pak create anaerobic conditions?

Long-answer questions

  1. Q1.Describe the anatomy of a bacterial cell.
  2. Q2.Explain the bacterial growth curve.
  3. Q3.Classify culture media with examples.
  4. Q4.Describe aerobic and anaerobic culture methods.

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