Microbial Cell Structure

• Microbiology revolves around understanding basic life processes and applying that knowledge to benefit humans.
• Microbes serve as excellent models for understanding cellular processes in both unicellular and multicellular organisms.
• Microbes play important roles in medicine, agriculture, and industry.
• Microorganisms are the oldest form of life and comprise the largest mass of living material on Earth.
• Microorganisms carry out major processes for biogeochemical cycles.
• They can inhabit places unsuitable for other organisms, and other life forms depend on microbes to survive.

Microbial Cell Structure

• A cell is a dynamic entity forming the fundamental unit of life.
• All cells commonly have a cytoplasmic membrane, cytoplasm, ribosomes, and cell wall structure.
• The cytoplasmic membrane acts as a barrier separating the cell's interior from the external environment.
• Cytoplasm consists of an aqueous mixture containing macromolecules, ions, and ribosomes.
• Ribosomes are structures responsible for protein synthesis.
• A cell wall, present in most microbes, contributes to the structural strength of the cell.

Cell Types: Prokaryotes vs. Eukaryotes

• Prokaryotes lack membrane-enclosed organelles and a nucleus.
• Prokaryotic cells are generally smaller than eukaryotic cells (Figure 1.2a).
• Eukaryotes feature DNA enclosed within a membrane-bound nucleus (Figure 1.2b).
• Eukaryotic cells are generally larger and more structurally complex, containing organelles.

Genes, Genomes, Nucleus, and Nucleoid

• A genome is the complete set of genes within a cell.
• Eukaryotic DNA is linear, resides within the nucleus, and associates with proteins to aid DNA folding.
• Eukaryotic cells usually have multiple chromosomes, typically two copies of each.
• During cell division in eukaryotes, the nucleus divides through mitosis
• During sexual reproduction, the genome is halved by meiosis.
• Prokaryotic cells typically possess a single, circular DNA molecule known as a chromosome.
• Prokaryotic DNA aggregates to form the nucleoid region (Figure 1.2a).
• Prokaryotes may also contain small amounts of extrachromosomal DNA called plasmids, which can confer special properties like antibiotic resistance.
• The Escherichia coli genome contains 4.64 million base pairs and 4,300 genes.
• Human cells have 1,000 times more DNA and 7 times more genes than E. coli.

Characteristics of Living Cells

• Living cells are known to exhibit characteristics such as metabolism, reproduction, differentiation, communication, movement, and evolution (Figure 1.3).
• Metabolism involves the chemical transformation of nutrients.
• Reproduction is the generation of two cells from one.
• Differentiation is the synthesis of new substances or structures that modify the cell (only in some microbes).
• Communication is the generation of, and response to, chemical signals (only in some microbes).
• Movement is the self-propulsion via various forms in microbes.
• Evolution includes genetic changes in cells that are transferred to offspring.

Metabolism and Genetics

• Cells carry out chemical reactions facilitated by enzymes, which are protein catalysts that accelerate these reactions.
• Cells store and process information passed on to offspring during reproduction and evolution via DNA.
• During transcription,DNA produces RNA.
• During translation, RNA produces protein.

Growth

• Growth links the function of cells as machines and cells as coding devices.

Evolution of Microbial Cells

• The first self-replicating entities may not have been cells.
• Last universal common ancestor (LUCA) represents the common ancestral cell from which all cells descended.

Life on Earth

• Earth is approximately 4.6 billion years old.
• The first cells emerged between 3.8 and 3.9 billion years ago.
• The atmosphere was anoxic (lacking free oxygen) until about 2 billion years ago.
• Metabolisms were exclusively anaerobic until oxygen-producing phototrophs evolved.
• Life was exclusively microbial until around 1 billion years ago.

Evolution and Phylogeny

• Evolution is defined as the process of cumulative change over time, resulting in new varieties and species of organisms.
• Phylogeny refers to the evolutionary relationships between organisms.
• These relationships can be deduced by comparing genetic information in the different specimens.
• Ribosomal RNA (rRNA) is excellent for determining phylogeny.
• Relationships are visualized on a phylogenetic tree.

Domains

• Comparative rRNA sequencing has defined three distinct cell lineages, called domains: Bacteria, Archaea, and Eukarya.
• Bacteria and Archaea are prokaryotic.
• Eykarya are eukaryotic.
• Archaea and Bacteria are distant relatives.
• Archaea are more closely related to Eukarya than Bacteria.
• Eukaryotic microorganisms were the ancestors of multicellular organisms.
• From the last universal common ancestor (LUCA), evolution proceeded to form two domains which were Bacteria and Archaea.
• Archaea later diverged to form two domains which were Archaea and Eukarya.

Microorganisms and Their Environments

• Microorganisms exist in nature as populations of interacting assemblages called microbial communities.
• The environment in which a microbial population lives is its habitat.
• An ecosystem is defined by all living organisms along with the physical and chemical constituents of their environment.
• Microbial ecology studies microbes in their natural environment.
• The diversity in microbial cells is the product of nearly 4 billion years of evolution.
• Microorganisms demonstrate differences in size, shape, motility, physiology, pathogenicity, etc.
• Microorganisms exploit every conceivable means of obtaining energy.
• Diversity and abundances of microbes are controlled by resources and environmental conditions.
• Microbial communities can affect the chemical and physical properties of their habitats.
• Microbes interact with their physical and chemical environment.
• Ecosystems are greatly influenced by microbial activities.
• Microorganisms alter the chemical and physical properties of their habitats by removing nutrients and excreting waste products.

Extent of Microbial Life

• Microbes are found in almost every environment imaginable.
• Global estimate is 5 × 1030 cells.
• Extremophiles such as certain Bacteria and Archaea thrive in harsh conditions.
• Most microbial cells are found in oceanic and terrestrial subsurfaces.
• Microbial biomass is significant, and cells are key reservoirs of essential nutrients.

Impact on Humans

• Microorganisms can be both beneficial and harmful to humans.
• Many more microorganisms are beneficial than harmful.
• Harmful microorganisms are considered infectious disease agents or pathogens.
• Control of infectious diseases increased in the last century.

Microorganisms and Agriculture

• Many aspects of agriculture depend on microbial activities.
• Microbes have positive effects with nitrogen-fixing bacteria, cellulose-degrading microbes in the rumen, and the regeneration of nutrients in soil and water.
• Microbes have negative impacts such as diseases in plants and animals.

Microorganisms and the Human Gastrointestinal Tract

• High numbers of microorganisms occur in the colon and oral cavity.
• Positive impacts consist of synthesizing vitamins, nutrients, competing with pathogens for space, and obtaining resources.

Microorganisms and Food

• Negative impacts consist of microorganisms that cause food spoilage, requiring methods of preservation.
• Positive impacts involve microbial transformations (typically fermentations) that yield dairy and other food products.

Microorganisms and the Environment

• Microbes play a role in production of biofuels, for example, methane, ethanol, and hydrogen.
• Microbes help in cleaning up pollutants through bioremediation.

Microorganisms and their Genetic Resources

• Microbes aid in exploitation for the production of antibiotics, enzymes, and various chemicals.
• Microbes assist in the genetic engineering of microbes to generate products of value to humans.
• An example of a product of use would be insulin through biotechnology.