Characterizing and Classifying Prokaryotes

Prokaryotic Cell Morphology

Prokaryotes exhibit a variety of cell shapes and arrangements, which are important for identification and classification.

• Cocci: Spherical-shaped bacteria (e.g., Staphylococcus).

• Bacilli: Rod-shaped bacteria (e.g., Bacillus).

• Spirilla: Spiral-shaped bacteria.

• Arrangements: Cocci may form chains (streptococci), clusters (staphylococci), or pairs (diplococci). Bacilli may be single, in chains (streptobacilli), or palisades.

Example: Streptococcus pyogenes forms chains of cocci.

Bacterial Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria to survive harsh conditions.

• Formation: Triggered by nutrient depletion; involves asymmetric cell division and spore coat formation.

• Function: Ensure survival in extreme environments (heat, desiccation, chemicals).

• Genera: Bacillus and Clostridium are notable endospore formers.

Reproduction in Prokaryotes

Prokaryotes reproduce asexually by several methods:

• Binary fission: Most common; cell divides into two identical daughter cells.

• Budding: New cell develops from a small outgrowth.

• Fragmentation: Filamentous bacteria break into fragments, each developing into a new cell.

Bergey’s Manual of Systematic Bacteriology

This manual is a key reference for bacterial classification and identification, organizing bacteria based on phenotypic and genetic characteristics.

• Purpose: Provides descriptions, identification keys, and taxonomic information.

Modern Prokaryotic Classification

Classification aims to reflect evolutionary relationships but faces limitations due to horizontal gene transfer and phenotypic plasticity.

• Goals: Group organisms by genetic relatedness and evolutionary history.

• Limitations: Convergent evolution and gene transfer can obscure relationships.

Domain Archaea

Archaea are prokaryotes distinct from bacteria, often inhabiting extreme environments.

• Cell wall: Lacks peptidoglycan; unique membrane lipids.

• Extremophiles: Includes thermophiles (heat-loving) and halophiles (salt-loving).

• Methanogens: Produce methane; important in carbon cycling and anaerobic environments.

Comparison: Archaea vs. Bacteria

• Cell wall composition: Archaea lack peptidoglycan; bacteria have it.

• Membrane lipids: Ether-linked in archaea, ester-linked in bacteria.

• Genetic machinery: Archaea share similarities with eukaryotes.

Deeply Branching Bacteria

These bacteria are considered to be among the earliest forms of life, branching near the root of the bacterial evolutionary tree.

Phototrophic Bacteria

Phototrophic bacteria use light as an energy source. Major groups include:

• Cyanobacteria: Oxygenic photosynthesis; phylum Cyanobacteria.

• Green sulfur bacteria: Phylum Chlorobi.

• Green non-sulfur bacteria: Phylum Chloroflexi.

• Purple sulfur and non-sulfur bacteria: Phylum Proteobacteria.

Example: Anabaena (cyanobacterium) forms heterocysts for nitrogen fixation.

Heterocysts in Cyanobacteria

Specialized cells for nitrogen fixation, providing an anaerobic environment for the enzyme nitrogenase.

Mycoplasmas

These are bacteria lacking a cell wall, making them pleomorphic and resistant to antibiotics targeting cell wall synthesis.

Gram-Positive Bacteria: Low G+C vs. High G+C

Classification based on the proportion of guanine and cytosine nucleotides in DNA.

• Low G+C: Includes Clostridium, Bacillus, Listeria, Lactobacillus, Streptococcus, Staphylococcus.

• High G+C: Includes Corynebacterium, Mycobacterium, Actinomycetes, Nocardia, Streptomyces.

Proteobacteria

Largest and most diverse group of Gram-negative bacteria, divided into six classes:

• Alphaproteobacteria: Nitrogen-fixers, some pathogens (e.g., Rickettsia).

• Betaproteobacteria: Includes pathogens like Neisseria.

• Gammaproteobacteria: Largest class; includes Escherichia, Pseudomonas.

• Deltaproteobacteria

• Epsilonproteobacteria: Includes Helicobacter, Campylobacter.

• Zetaproteobacteria: Recently discovered, iron-oxidizing bacteria.

Other Gram-Negative Bacteria

• Chlamydia: Obligate intracellular pathogens.

• Spirochetes: Spiral-shaped, motile bacteria (e.g., Treponema).

• Bacteroids: Abundant in the human gut; important in digestion.

Characterizing and Classifying Eukaryotes

Eukaryotic Reproduction

Eukaryotic reproduction is more complex than prokaryotic reproduction due to the presence of a nucleus and organelles.

• Mitosis: Produces two genetically identical diploid cells.

• Meiosis: Produces four genetically unique haploid cells.

• Diploid: Two sets of chromosomes.

• Haploid: One set of chromosomes.

• Cytokinesis: Division of the cytoplasm.

• Schizogony: Multiple fission; nucleus divides several times before the cell divides.

Classification of Eukaryotes

Major groups have been classified differently over time, reflecting advances in understanding evolutionary relationships.

• 18th century: Plants, animals, protists.

• 20th century: Five-kingdom system (Monera, Protista, Fungi, Plantae, Animalia).

Problems: Protists are highly diverse and not a natural group.

Protozoa

• Characteristics: Unicellular, lack cell walls, motile at some stage.

• Taxonomy: Many schemes due to diversity.

• Examples: Plasmodium (malaria), Trypanosoma (sleeping sickness).

• Euglenids: Have flagella, photosynthetic, flexible pellicle.

• Alveolates: Have membrane-bound sacs (alveoli) under the plasma membrane.

• Amoebas: Move by pseudopodia, lack fixed shape.

Fungi

• Distinguishing features: Chitin cell walls, absorptive nutrition, spore formation.

• Benefits: Decomposition, antibiotics (e.g., Penicillium), food production.

• Body types: Yeasts (unicellular), molds (multicellular, hyphae).

• Dimorphic: Can exist as yeast or mold depending on conditions.

• Hyphae: Septate (with cross-walls) or aseptate (without cross-walls).

• Mycelium: Mass of hyphae.

• Nutrition: Absorptive heterotrophs.

• Reproduction: Asexual (spores: conidia, sporangiospores, chlamydospores), sexual (dikaryon stage).

• Pathogens: Candida, Aspergillus.

• Deuteromycetes: Fungi with no known sexual stage; not a formal taxon.

Lichens

• Members: Fungus and photosynthetic partner (alga or cyanobacterium).

• Benefits: Soil formation, bioindicators.

• Shapes: Crustose, foliose, fruticose.

Algae

• Distinguishing feature: Photosynthetic eukaryotes.

• Alternation of generations: Life cycle alternates between haploid and diploid forms.

• Groups: Green algae, brown algae, red algae, diatoms.

Water Molds

• Differences from fungi: Cellulose cell walls, diploid dominant, flagellated spores, related to algae.

Parasitic Worms and Arthropod Vectors

• Importance: Many are human pathogens; vectors transmit microbial diseases.

Table: Comparison of Fungi and Water Molds

Table: Examples of Pathogenic Genera

Key Terms and Definitions

• Dikaryon: Fungal cell with two genetically distinct nuclei.

• Mycelium: Mass of fungal hyphae.

• Dimorphic: Fungi that can exist as yeast or mold.

• Heterocyst: Specialized cell for nitrogen fixation in cyanobacteria.

• Schizogony: Asexual reproduction by multiple fission.

Equations

• Binary Fission (Bacterial Growth):

Where is the final number of cells, is the initial number, and is the number of generations.

• Alternation of Generations (Algae):

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard microbiology curricula.