Microbiology Chapter 6: Microbial Nutrition and Growth

Measuring Bacterial Growth

• Microbiologists study microbial growth by replicating natural conditions in the lab, focusing on chemical, physical, and energy needs.
• Microbial growth refers to both metabolic activity and resulting population increase.

Microbial Needs

• Chemical Needs:
  • Nutrients: carbon, oxygen, nitrogen, phosphorus, sulfur, calcium, manganese, magnesium, copper, and iron.
  • Macronutrients (over 95%): carbon, oxygen, nitrogen, phosphorus, and sulfur.
  • Micronutrients: calcium, manganese, magnesium, copper, and iron.
• Physical Needs:
  • Temperature: affects protein shape and membrane fluidity.
  • pH: microbes have optimal pH ranges.
• Energy Needs:
  • Autotrophs: use carbon dioxide as the starting block, combining it with water and sunlight (or chemicals) through photosynthesis.
  • Heterotrophs: rely on pre-made organic molecules.
  • Chemotrophs: use chemicals for energy.
  • Phototrophs: use light for energy.

Nitrogen Requirements

• Nitrogen (N) is crucial for building biomolecules like amino acids and nucleotides, making up 14% of a microbe's dry weight.
• Nitrogen limitation can limit microbial growth.
• Microbes acquire nitrogen from organic and inorganic sources, and some can fix atmospheric nitrogen gas (N₂) into ammonia (NH₃) through nitrogen fixation.

Temperature and pH Preferences

• Psychrophiles: thrive below 15°C, can grow at 0°C, and die at higher temperatures.
• Psychrotolerants: tolerate cold but don't grow best in it.
• Mesophiles: grow best between 20°C and 40°C.
• Thermodurics: mesophiles that can survive brief periods at high temperatures.
• Thermophiles: grow best above 45°C.
• Hyperthermophiles: thrive at extremely high temperatures (above 80°C).
• pH: microbes have optimal pH ranges, with acidic environments below pH 7 and basic environments above pH 7.### Impact of Hydrogen Ions on Biomolecules
• Hydrogen ions can interfere with hydrogen bonding in proteins and nucleic acids, affecting their function
• pH influences the structure and function of biomolecules, including enzymes and nutrients

Microbial pH Preferences

• Neutrophiles: grow best at a neutral pH (around 6.5-7.5), includes most bacteria and protozoa
• Acidophiles: grow best in acidic environments (as low as pH 0.0), includes chemoautotrophic prokaryotes
• Obligate acidophiles: require an acidic environment and die near neutrality (pH 7.0)
• Acid-tolerant: survive in acidic environments but do not necessarily prefer them
• Alkalinophiles: live in alkaline environments (up to pH 11.5), includes Vibrio cholerae

Osmotic Pressure and Microbes

• Osmosis: the diffusion of water across a semi-permeable membrane driven by unequal solute concentrations
• Osmotic pressure: the pressure exerted on a membrane by a solution containing solutes that cannot freely cross the membrane
• Hypotonic environment: a solution with lower solute concentration than the cell
• Hypertonic environment: a solution with higher solute concentration than the cell
• Microbes need water to dissolve enzymes, nutrients, and participate in metabolic reactions

Microbial Response to Osmotic Pressure

• Hypotonic environment: cells gain water and swell, potentially leading to lysis
• Hypertonic environment: cells lose water and shrink, potentially leading to crenation
• Obligate halophiles: require high salt concentrations and will burst if placed in freshwater
• Facultative halophiles: can tolerate high salt concentrations but do not necessarily require them

Biofilms

• Complex, synergistic association of microorganisms attached to surfaces
• Biofilm formation: 6 steps, including settlement, attachment, EPS formation, quorum sensing, maturation, and dispersal
• Impacts of biofilms: disease, protection from antimicrobial drugs, and challenges to treatment

Microbial Communities and Relationships

• Associations range from antagonistic (harmful) to beneficial (synergistic or symbiotic)
• Antagonistic relationships: one organism harms or kills another
• Beneficial relationships: synergistic (both organisms benefit but can live independently) or symbiotic (close association where members depend on each other)
• Biofilms: complex, synergistic associations of microorganisms attached to surfaces

Clinical Sampling and Diagnosis

• Goal: diagnose and treat disease by isolating and identifying pathogens
• Importance of proper collection: avoiding contamination and protecting healthcare workers
• Standard precautions: CDC guidelines to protect healthcare workers from contamination
• Culturing microorganisms: 4 steps, including inoculum, medium, culture, and colony formation
• Obtaining pure cultures: isolating the culprit microbe and separating it from normal microbiota

Isolation Techniques

• Streak plate method: most common technique, involves streaking the inoculum across a solid medium
• Pour plate technique: involves diluting the sample and mixing it with agar in Petri dishes
• Key differences: streak plates have colonies on the surface only, while pour plates have colonies both on and within the medium