Microbial Cell Structure and Function

Prokaryotic and Eukaryotic Cells

Understanding the differences and similarities between prokaryotic and eukaryotic cells is fundamental in microbiology. Both cell types possess essential cellular components, but their organization and complexity differ.

• Prokaryotic cells: Lack a membrane-bound nucleus; genetic material is in the nucleoid region.

• Eukaryotic cells: Have a true nucleus and membrane-bound organelles.

• Cell wall: Present in most prokaryotes (e.g., Bacteria), absent in animal cells.

• Photosynthetic pigments: Found in bacteria such as cyanobacteria and in chloroplasts of eukaryotic algae.

Example: Cyanobacteria are prokaryotes that perform oxygenic photosynthesis using pigments like chlorophyll a.

Microbial Metabolism and Growth

Types of Microorganisms Based on Energy and Carbon Source

Microorganisms are classified by how they obtain energy and carbon for growth.

• Phototrophs: Use light as an energy source.

• Chemotrophs: Obtain energy from chemical compounds.

• Autotrophs: Use CO2 as a carbon source.

• Heterotrophs: Use organic compounds as a carbon source.

• Thermophiles: Thrive at high temperatures.

• Psychrophiles: Grow at low temperatures.

• Halophiles: Require high salt concentrations.

• Facultative anaerobes: Can grow with or without oxygen.

• Obligate aerobes: Require oxygen for growth.

• Facultative anaerobes: Can use oxygen but also grow without it.

• Thermal death point: The lowest temperature at which all microbes in a liquid suspension are killed in 10 minutes.

Example: Thermus aquaticus is a thermophile used in PCR due to its heat-stable DNA polymerase.

Microbial Control Methods

Sterilization and Disinfection

Controlling microbial growth is essential in medical and food industries. Sterilization eliminates all forms of microbial life, while disinfection reduces the number of pathogens.

• Autoclave: Uses steam under pressure to sterilize materials. Limitations include inability to sterilize heat-sensitive materials.

• Disinfectants: Chemicals that disrupt plasma membranes, leading to cell death. Limitations include reduced efficacy in the presence of organic matter.

• Hand scrubs: Chlorhexidine is commonly used for surgical hand scrubs due to its broad-spectrum activity.

• Food preservation: Chemical agents such as nitrates and sulfites are used to inhibit microbial growth in foods.

Example: Autoclaving is the preferred method for sterilizing surgical instruments.

Microbial Genetics and Biotechnology

Cyanobacteria and Genetic Diversity

Cyanobacteria are photosynthetic prokaryotes important for oxygen production and nitrogen fixation. Genetic diversity among bacteria is studied using Gram staining and molecular techniques.

• Gram-positive bacteria: Have thick peptidoglycan cell walls; stain purple.

• Gram-negative bacteria: Have thin peptidoglycan and outer membrane; stain pink.

• Commercial benefits: Cyanobacteria are used in biofertilizers and as sources of bioactive compounds.

Example: Spirulina (a cyanobacterium) is used as a dietary supplement.

Microbial Transport Mechanisms

Cell Transport and Diffusion

Microbial cells use various mechanisms to transport substances across membranes.

• Facilitated diffusion: Movement of molecules across membranes via transport proteins, without energy input.

• Active transport: Requires energy (usually ATP) to move substances against their concentration gradient.

• Extracellular enzymes: Secreted to break down large molecules for uptake.

• Aquaporins: Channel proteins that facilitate water transport.

• Simple diffusion: Passive movement of molecules from high to low concentration.

Example: Glucose uptake in bacteria often occurs via facilitated diffusion.

Additional info:

• In green and purple bacteria, electrons to reduce CO2 can come from inorganic molecules such as H2S.

• Strictly fermentative bacteria produce energy by substrate-level phosphorylation during glycolysis.

Key Table: Microbial Classification by Energy and Carbon Source