Unit 2: Microbial Diversity - Subjective Questions

Microbial diversity refers to the wide variety of microorganisms found in nature, differing in their genetic composition, cellular structure, metabolism, morphology, and ecological functions.

Major criteria used for classification include:

• Cellular organization: Prokaryotic or eukaryotic.
• Cell structure: Presence or absence of a nucleus, cell wall composition, membrane structure, and organelles.
• Morphology: Shape, size, arrangement, and specialized structures.
• Nutritional pattern: Autotrophic, heterotrophic, phototrophic, or chemotrophic modes of nutrition.
• Reproduction: Binary fission, budding, fragmentation, spore formation, or sexual reproduction.
• Genetic relationships: Comparisons of DNA, RNA, ribosomal RNA sequences, and conserved genes.
• Ecological roles: Decomposition, photosynthesis, disease production, symbiosis, and nutrient cycling.

Modern classification combines structural, biochemical, physiological, ecological, and molecular characteristics.

Prokaryotic and eukaryotic cells differ in several fundamental ways:

• Nucleus: Prokaryotic cells lack a membrane-bound nucleus, whereas eukaryotic cells possess a well-defined nucleus.
• Genetic material: Prokaryotes usually contain a single circular chromosome in the nucleoid region. Eukaryotes generally contain multiple linear chromosomes inside the nucleus.
• Membrane-bound organelles: Prokaryotes do not possess membrane-bound organelles. Eukaryotes contain mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and other organelles.
• Ribosomes: Prokaryotes possess smaller ribosomes, while eukaryotic cytoplasm contains larger ribosomes.
• Cell size: Prokaryotic cells are usually smaller, commonly about –, whereas eukaryotic cells are often about –.
• Cell division: Prokaryotes generally reproduce by binary fission. Eukaryotic cells divide by mitosis or meiosis.
• Cell wall: Bacteria commonly possess peptidoglycan cell walls. Eukaryotic cell walls, when present, are made of cellulose, chitin, or other materials.

Thus, eukaryotic cells have a more compartmentalized and structurally complex organization than prokaryotic cells.

Prokaryotic microorganisms, including bacteria and archaea, are distributed almost everywhere because of their metabolic diversity and ability to tolerate extreme conditions.

• Soil: Numerous bacteria and actinomycetes decompose organic matter and participate in nitrogen, carbon, and sulfur cycles.
• Freshwater and marine environments: Cyanobacteria, heterotrophic bacteria, and archaea occur in lakes, rivers, oceans, and sediments.
• Extreme habitats: Archaea may live in hot springs, salt lakes, acidic environments, anaerobic sediments, and highly alkaline habitats.
• Human and animal bodies: Normal microbiota colonize the skin, mouth, intestine, and respiratory tract.
• Plant surfaces and roots: Rhizosphere bacteria promote nutrient availability, nitrogen fixation, and plant growth.
• Polar and deep-sea environments: Psychrophilic microorganisms survive at low temperatures, while specialized organisms inhabit deep ocean sediments.
• Industrial environments: Prokaryotes are found in wastewater, food-processing systems, oil reservoirs, and fermentation units.

Their distribution is determined by temperature, pH, salinity, oxygen availability, moisture, nutrients, pressure, and biological interactions.

A bacterial cell is a relatively simple prokaryotic cell containing the following structures:

• Capsule or slime layer: Protects the cell from desiccation and phagocytosis and helps in attachment to surfaces.
• Cell wall: Maintains cell shape and prevents osmotic lysis. In most bacteria, it contains peptidoglycan.
• Plasma membrane: Regulates transport and performs functions related to respiration, energy generation, and biosynthesis.
• Cytoplasm: Contains enzymes, metabolites, ribosomes, and storage inclusions.
• Nucleoid: Contains the main bacterial chromosome, usually a circular DNA molecule.
• Plasmids: Small, self-replicating DNA molecules that may carry genes for antibiotic resistance or special metabolic properties.
• Ribosomes: particles responsible for protein synthesis.
• Flagella: Provide motility.
• Fimbriae and pili: Help in attachment and, in the case of sex pili, genetic exchange.
• Endospores: Highly resistant dormant structures formed by some bacteria, such as Bacillus and Clostridium.

The absence of a membrane-bound nucleus and membrane-bound organelles is a defining feature of bacterial cells.

Gram-positive and Gram-negative bacteria differ in cell envelope structure and consequently show different Gram-staining reactions.

During Gram staining, Gram-positive cells retain the crystal violet–iodine complex because of their thick peptidoglycan layer. Gram-negative cells lose this complex during decolorization and take up the counterstain, usually safranin.

Bacterial morphology is based mainly on cell shape and the arrangement of cells after division.

Common bacterial shapes:

• Cocci: Spherical or oval cells, such as Staphylococcus and Streptococcus.
• Bacilli: Rod-shaped cells, such as Escherichia coli and Bacillus.
• Vibrios: Comma-shaped cells, such as Vibrio cholerae.
• Spirilla: Rigid spiral-shaped cells.
• Spirochetes: Flexible, thin, spiral-shaped cells, such as Treponema.
• Coccobacilli: Short rods that resemble cocci.
• Filamentous forms: Long, thread-like cells, common among actinomycetes.

Common arrangements:

• Diplococci: Pairs of cocci.
• Streptococci: Chains of cocci.
• Staphylococci: Irregular grape-like clusters.
• Tetrads: Groups of four cocci.
• Sarcinae: Cubical packets of cells.
• Palisades: Rods arranged side by side.

Cell division planes, cell rigidity, and incomplete separation after division determine the final arrangement.

An endospore is a dormant, highly resistant structure formed inside certain bacterial cells, especially species of Bacillus and Clostridium.

Structure:

• Core: Contains DNA, ribosomes, enzymes, calcium dipicolinate, and very little water.
• Inner membrane: Provides a strong permeability barrier.
• Cortex: A specialized peptidoglycan layer surrounding the core.
• Spore coat: Protein layers that protect against chemicals and enzymes.
• Exosporium: An external covering present in some species.

Significance:

• Endospores help bacteria survive heat, drying, radiation, chemicals, and nutrient deprivation.
• They are not reproductive structures because one bacterial cell produces only one endospore, which later develops into one vegetative cell.
• Their resistance is associated with dehydration of the core, protective proteins, calcium dipicolinate, and the multilayered structure.
• Endospores are important in medical and food microbiology because they may survive inadequate sterilization.

Favorable conditions trigger germination, followed by outgrowth into a metabolically active bacterial cell.

Algae are predominantly photosynthetic organisms that may be unicellular, colonial, filamentous, or multicellular.

• They are mainly found in freshwater, marine environments, moist soil, and on damp surfaces.
• Their cells are generally eukaryotic, except for cyanobacteria, which are photosynthetic prokaryotes and are not true algae.
• Algal cells contain a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes, and chloroplasts.
• Chloroplasts contain chlorophyll and accessory pigments that enable photosynthesis.
• Cell walls may contain cellulose, silica, alginates, or other substances depending on the group.
• Storage products vary and may include starch, oils, laminarin, or other carbohydrates.
• Algae may reproduce by fragmentation, asexual spores, mitosis, or sexual reproduction.
• Unicellular algae include Chlamydomonas and Diatoms, while multicellular forms include Ulva and kelps.

Algae are ecologically important as primary producers, oxygen contributors, food sources, and participants in aquatic food webs.

Unicellular and multicellular algae differ in their structural organization and ecological roles.

• Organization: Unicellular algae consist of a single photosynthetic cell, whereas multicellular algae contain many cells organized into filaments, sheets, or thallus-like bodies.
• Size: Unicellular forms are usually microscopic, while multicellular forms may be visible and can reach considerable lengths.
• Examples: Chlamydomonas and diatoms are unicellular; Ulva, kelps, and many red algae are multicellular.
• Reproduction: Unicellular algae commonly reproduce by cell division, asexual spores, or sexual fusion. Multicellular algae may reproduce by fragmentation, specialized spores, and complex sexual life cycles.
• Differentiation: Unicellular algae generally lack specialized tissues. Multicellular algae may show differentiated reproductive or anchoring regions, although they do not possess true roots, stems, and leaves.
• Ecological importance: Both groups contribute to primary production and oxygen release. Large multicellular algae provide habitats and food for aquatic organisms, while unicellular algae are major components of phytoplankton.

Both forms possess chloroplasts and depend mainly on photosynthesis for energy production.

Fungi are eukaryotic, nonphotosynthetic organisms that obtain nutrients by absorption.

• Cell type: Fungal cells contain a true nucleus and membrane-bound organelles.
• Cell wall: Usually composed mainly of chitin, glucans, and other polysaccharides.
• Cell membrane: Contains ergosterol as an important sterol.
• Hyphae: Most fungi grow as thread-like structures called hyphae. Hyphae may be septate or aseptate.
• Mycelium: A mass or network of hyphae is called a mycelium.
• Yeasts: These are unicellular fungi that commonly reproduce by budding or fission.
• Molds: Filamentous fungi that produce extensive hyphae and reproductive structures.
• Dimorphic fungi: Certain fungi exist as yeasts under one condition and molds under another, often depending on temperature.
• Reproduction: Fungi reproduce by fragmentation, budding, asexual spores, and sexual spores.
• Nutrition: They secrete digestive enzymes into their surroundings and absorb the resulting soluble nutrients.

Fungi function as decomposers, plant symbionts, pathogens, and producers of food, antibiotics, and industrial products.

Yeasts and molds are two major morphological forms of fungi.

Some fungi are dimorphic, meaning that they grow as molds under environmental conditions and as yeasts under host-associated conditions. This transition may contribute to pathogenicity in certain species.

Protozoa are unicellular, microscopic, eukaryotic organisms that are commonly classified as animal-like protists.

• They possess a true nucleus and membrane-bound organelles.
• Their cytoplasm is differentiated into ectoplasm and endoplasm in many species.
• Most protozoa lack a rigid cell wall and have a flexible plasma membrane or pellicle.
• Locomotion occurs through pseudopodia, flagella, or cilia.
• Some species are nonmotile or move by gliding.
• They obtain food by phagocytosis, pinocytosis, absorption, or photosynthesis in certain groups.
• Contractile vacuoles regulate water balance, especially in freshwater species.
• Food vacuoles digest ingested material, while specialized organelles eliminate wastes.
• Reproduction may be asexual, commonly by binary fission, multiple fission, or budding, and may also involve sexual processes.
• Many protozoa form resistant cysts during unfavorable conditions.

Protozoa occur in soil, freshwater, marine habitats, and animal hosts. Some are free-living, whereas others are important parasites.

Protozoa use specialized structures for movement:

• Pseudopodia: Temporary extensions of cytoplasm formed by cytoplasmic streaming. They enable amoeboid movement and also help in engulfing food by phagocytosis. Example: Amoeba.
• Flagella: Long, whip-like structures that produce movement through undulating waves. A flagellum may also help in sensing the environment. Example: Giardia.
• Cilia: Numerous short, hair-like structures that beat in coordinated waves. They propel the cell and direct food particles toward the cytostome. Example: Paramecium.
• Pellicle-associated movement: Some protozoa use a flexible pellicle and internal fibrils to produce gliding movement.

The type and arrangement of locomotory organelles are useful in identifying protozoan groups. These structures also assist in feeding, attachment, avoiding harmful conditions, and locating favorable environments.

A virus is an acellular infectious agent composed mainly of genetic material surrounded by protective structures.

• Viral genome: May consist of DNA or RNA, which can be single-stranded or double-stranded. It contains the information needed for producing viral components.
• Capsid: A protein coat surrounding the genome. It protects the nucleic acid and helps deliver it into a host cell.
• Capsomeres: Repeating protein subunits that assemble to form the capsid.
• Nucleocapsid: The combined structure of the viral genome and capsid.
• Envelope: A lipid membrane present in some viruses. It is usually acquired from the host cell membrane during release.
• Viral spikes: Glycoprotein projections on enveloped viruses that recognize and bind specific host-cell receptors.
• Viral enzymes: Some viruses carry enzymes required for genome replication or entry into host cells.

Viral shapes may be helical, icosahedral, complex, or enveloped. A complete infectious virus particle outside a host cell is called a virion.

Viruses have several unique characteristics that distinguish them from bacteria, fungi, algae, and protozoa:

• They are acellular, lacking cytoplasm, ribosomes, and independent cellular organelles.
• They contain only one type of nucleic acid, either DNA or RNA, but not both as their primary genome.
• They are obligate intracellular parasites and can multiply only inside suitable host cells.
• They lack independent metabolism and cannot generate energy or synthesize proteins outside a host.
• Their genomes may be extremely small or highly complex and may be linear, circular, segmented, single-stranded, or double-stranded.
• They are assembled from separately synthesized components rather than reproducing by binary fission or mitosis.
• They show host specificity and tissue tropism based on interactions between viral attachment proteins and host receptors.
• They can be crystallized under suitable conditions, demonstrating their noncellular nature outside the host.
• Some viruses mutate rapidly, especially those with error-prone RNA replication.

Viruses therefore occupy a distinctive position between chemical entities and living cellular systems.

Viral replication generally occurs through the following stages:

1. Attachment: Viral surface proteins bind to specific receptors on the host-cell membrane.
2. Penetration: The virus or its genome enters the host cell by membrane fusion, endocytosis, or direct injection.
3. Uncoating: The capsid is removed or dismantled, releasing the viral genome.
4. Biosynthesis: The viral genome directs the production of viral nucleic acids and proteins using host and viral enzymes.
5. Assembly: Newly synthesized genomes and structural proteins are assembled into mature virions.
6. Maturation: Viral components may undergo processing or structural changes that produce infectious particles.
7. Release: Virions leave the cell by lysis, exocytosis, or budding from the host membrane.

The exact pathway depends on the type of virus, its genome, the host cell, and whether the infection is lytic, persistent, latent, or lysogenic.

The lytic and lysogenic cycles are two different patterns of bacteriophage infection.

In the lysogenic cycle, environmental stress can induce the prophage to excise from the chromosome and enter the lytic cycle. Thus, lysogeny allows viral genetic material to persist through bacterial cell divisions.

The structure of each microbial group is closely related to its lifestyle:

• Bacteria: Their small size, simple organization, flexible metabolism, cell wall, and rapid binary fission support survival and multiplication in diverse habitats.
• Algae: Chloroplasts and photosynthetic pigments enable them to produce organic matter in aquatic and moist environments. Their unicellular, colonial, or multicellular forms allow adaptation to different ecological niches.
• Fungi: Hyphae provide extensive surface area for absorption, while secreted enzymes enable decomposition of complex organic materials. Yeast forms are suited to growth in nutrient-rich liquids.
• Protozoa: Flexible membranes, cilia, flagella, and pseudopodia support movement, feeding, and responses to environmental conditions. Cysts permit survival during unfavorable periods.
• Viruses: Minimal structures consisting of a genome and protective protein coat allow efficient transfer of genetic material into host cells. Surface proteins determine host recognition and specificity.

Thus, microbial morphology is not merely descriptive; it is associated with nutrition, reproduction, movement, environmental survival, and interaction with hosts.

Eukaryotic microorganisms, including algae, fungi, protozoa, and some other protists, occur in nearly all environments.

• Aquatic habitats: Algae and protozoa are abundant in freshwater, oceans, ponds, rivers, and wetlands.
• Soil: Fungi, protozoa, and algae contribute to decomposition, nutrient cycling, and soil formation.
• Decaying organic matter: Fungi grow on dead plants and animals, while protozoa feed on bacteria and organic particles.
• Host-associated habitats: Some fungi and protozoa live as commensals, mutualists, or parasites in plants and animals.
• Extreme environments: Certain algae, fungi, and protozoa tolerate high salinity, low temperature, acidic conditions, or limited nutrients.
• Air and surfaces: Fungal spores and some algal cells may be dispersed through air, water, or contact.

Their distribution depends on:

• Temperature and moisture.
• Light availability for photosynthetic algae.
• pH and salinity.
• Oxygen concentration.
• Organic nutrients and inorganic minerals.
• Competition, predation, and symbiotic relationships.
• Availability of suitable hosts for parasitic forms.

Resting structures such as fungal spores and protozoan cysts improve survival during unfavorable conditions.

The external coverings of microorganisms differ in composition and function:

• Bacteria: Most possess a cell wall containing peptidoglycan. Gram-positive bacteria have a thick peptidoglycan layer, whereas Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide.
• Algae: Cell walls vary among groups. Many contain cellulose, while diatoms possess silica-rich walls and some algae contain alginates or other polysaccharides.
• Fungi: Their cell walls are mainly composed of chitin, glucans, and mannoproteins. The wall provides shape and protection against osmotic stress.
• Protozoa: Most lack a rigid cell wall and possess a flexible plasma membrane or pellicle. This flexibility supports movement and engulfment of food.
• Viruses: They have no cell wall or plasma membrane of their own. Their external covering is usually a protein capsid, and some also possess a lipid envelope derived from the host cell.

These structural differences influence staining behavior, environmental resistance, shape, motility, host interaction, and susceptibility to antimicrobial agents.