Study sheet: Bacterial Culture and Media

📋 Course Outline

1. Purpose and Limits of Culture
2. Bacterial Growth Requirements
3. Anaerobic Culture Systems
4. Culture Media Consistency
5. Media Components and Functions
6. Inoculation and Isolation
7. Colony Morphology and Media Examples

📖 1. Purpose and Limits of Culture

🔑 Key Concepts & Definitions

• Bacterial culture : Used to obtain a pure bacterial isolate for examination and identification of a species or strain.

★ Must-know

• Bacterial cultures can be examined for: colony morphology, microscopic appearance including Gram and diagnostic stains, biochemical properties, antibiotic resistance, serotype, MALDI-TOF mass spectrometry results, virulence- or resistance-associated genes detected by PCR

Further detail

• Examples of bacteria that are unculturable on nutrient media include:
  • Mycobacterium leprae
  • Treponema pallidum
  • Chlamydia spp.

💡 Memory Hook

Isolate → examine → identify

📖 2. Bacterial Growth Requirements

★ Must-know

• Bacterial growth requires:

  • available water and nutrients
  • an optimal pH of 7.2–7.4
  • an optimal salt concentration of 0.5%
  • a suitable atmosphere
  • an appropriate temperature

• Atmospheric categories include: aerobic, microaerophilic or capnophilic under increased carbon dioxide, obligately anaerobic, facultatively anaerobic

• Mesophilic bacteria have an optimal growth temperature of 30–40°C, whereas psychrotrophic bacteria are mesophilic bacteria able to grow at lower temperatures, sometimes close to 0°C.

Further detail

• Reported sodium chloride tolerances are:

  • Vibrio spp.: up to 3%
  • Enterococcus spp.: up to 6.5%
  • Staphylococcus spp.: up to 10%

• Listeria can grow at 4°C, Lactobacillus prefers a lower pH than most bacteria, Listeria tolerates pH 4.4, and Enterococcus spp. tolerate pH 9.6.

💡 Memory Hook

W-P-S-A-T: water, pH, salts, atmosphere, temperature

📖 3. Anaerobic Culture Systems

📝 Essential Points

• 🔄 An anaerobic culture system: uses an envelope that generates hydrogen and carbon dioxide, uses an indicator that changes colour when anaerobic conditions are reached or not reached, uses a palladium catalyst

💡 Memory Hook

Generate gases → indicate conditions → catalyze oxygen removal

📖 4. Culture Media Consistency

★ Must-know

• Culture media by consistency are: liquid media for enrichment, selective growth, and biochemical tests, semisolid media containing 0.1–0.5% agar for motility examination and transport, solid media containing 1.5–2% agar for single-colony isolation and colony morphology

Further detail

• Examples of media formats include:
  • peptone water
  • brain-heart infusion
  • meat-peptone broth
  • Petri-dish agar
  • slant agar

💡 Memory Hook

Broth enriches, semisolid tests motility, agar isolates colonies

📖 5. Media Components and Functions

🔑 Key Concepts & Definitions

• Selective medium : Favours a particular organism or group by suppressing the growth of other bacteria, while an enrichment medium usually uses liquid broth to multiply a pathogen present in low numbers before agar inoculation.
• Differential medium : Contains a diagnostic compound or coloured indicator that allows colonies of one organism to be differentiated from others growing in the same culture.

📝 Essential Points

• Basic culture-media components include:

  • peptone
  • water
  • additional nutrients
  • a solidifying agent
  • selective agents
  • diagnostic agents

• Chocolate agar is heated blood agar whose lysed blood cells release NAD and hemin required by Haemophilus and Actinobacillus spp.

💡 Memory Hook

Selective agents suppress competitors → target organisms recover

📖 6. Inoculation and Isolation

📝 Essential Points

• Streak inoculation progressively dilutes cells across a solid agar surface until individual cells form isolated colonies, which can be subcultured to obtain a pure culture of a single bacterial strain.

• Inoculation techniques include: stab inoculation for liquid or semisolid media, streak inoculation for solid media and single-colony isolation, pour plates for bacterial counting, spread plates for bacterial counting

💡 Memory Hook

Streak → dilute → isolate → subculture

📖 7. Colony Morphology and Media Examples

★ Must-know

• Colony morphology is assessed by:

  • size
  • shape
  • margin
  • elevation
  • consistence
  • colour or pigmentation
  • smell
  • colony texture
  • haemolysis

• Haemolysis categories are:

  • beta: complete haemolysis
  • alpha: incomplete haemolysis
  • gamma: no haemolysis

📌 Smooth colonies are typical of freshly isolated wild strains, mucous colonies indicate capsule or slime production and are typical of highly virulent fresh cultures, and rough colonies are dry and granulated and may occur in laboratory mutants lacking lipopolysaccharide and capsule, old cultures, or some Bacillus spp.

• XLD agar is selective-differential for Salmonella spp.; Salmonella colonies appear red with black centres because of hydrogen sulfide production, whereas lactose fermenters appear yellow to orange.

Further detail

• MacConkey agar is selective-diagnostic for Enterobacteriaceae: bile salts inhibit Gram-positive bacteria and neutral red indicates lactose fermentation.

• Bile-esculin agar differentiates Enterococcus spp. from Streptococcus spp. because Enterococcus tolerates bile salts and degrades esculin to black esculetin.

• Mannitol-salt agar is selective-differential for staphylococci because it contains 7.5% salt, and yellow colour indicates mannitol fermentation by Staphylococcus aureus.

• Baird-Parker agar is selective-differential for coagulase-positive staphylococci, which produce black colonies with a clear zone around them in the presence of tellurite and egg yolk.

• Other named media and their uses are:

  • Rappaport–Vassiliadis medium: Salmonella spp.
  • Deoxycholate-citrate agar: Salmonella and Shigella spp.
  • PALCAM agar: Listeria spp.
  • ALOA agar: Listeria spp.
  • Carmali agar: Campylobacter spp.

💡 Memory Hook

Smooth and mucous colonies contrast with dry, rough colonies

📊 Synthesis Tables

Culture media by consistency

Medium	Composition	Main use
Liquid broth	No agar	Enrichment, selective growth, biochemical tests
Semisolid	0.1–0.5% agar	Motility examination and transport
Solid agar	1.5–2% agar	Single-colony isolation and colony morphology

Selected differential media

Medium	Target or selection	Diagnostic result
MacConkey agar	Enterobacteriaceae; bile salts inhibit Gram-positive bacteria	Neutral red indicates lactose fermentation
XLD agar	Salmonella spp.	Red colonies with black centres indicate hydrogen sulfide production
Bile-esculin agar	Enterococcus spp. versus Streptococcus spp.	Black esculetin indicates esculin degradation
Mannitol-salt agar	Staphylococci; 7.5% salt	Yellow colour indicates mannitol fermentation