Prokaryotes and Protists

Microbiome and Types of Bacterial Cells

The microbiome refers to the collective genomes of all microbial species in a particular environment. Bacterial cells are classified by their shape into three main categories:

• Cocci: Spherical-shaped prokaryotic cells. Examples include Streptococcus (chains), Staphylococcus (clusters), and Meningococcus.

• Bacilli: Rod-shaped cells, which may also be filamentous. Examples: Lactobacilli, Escherichia coli (E. coli), Bacillus anthracis.

• Spirilla and Spirochetes: Spiral, corkscrew-shaped cells. Examples: Helicobacter, Oceanospirillum, Treponema pallidum.

Example: Streptococcus forms chains of cocci, while Bacillus anthracis is a rod-shaped bacterium responsible for anthrax.

Bacterial Gram Stain

The Gram stain is a differential staining technique that distinguishes bacteria based on cell wall composition:

• Gram-Positive Bacteria: Thick peptidoglycan cell wall; stains purple.

• Gram-Negative Bacteria: Thin peptidoglycan layer and an outer membrane of lipids bonded to carbohydrates; stains pink/red.

• Gram-negative bacteria are generally more resistant to antibiotics and more pathogenic.

Example: Staphylococcus aureus is Gram-positive, while Escherichia coli is Gram-negative.

Exotoxins and Endotoxins

Bacteria can produce toxins that contribute to disease:

• Exotoxins: Toxic proteins secreted by bacteria into their environment. Example: Staphylococcus aureus secretes potent exotoxins.

• Endotoxins: Lipid components of the outer membrane of Gram-negative bacteria, released when the cell is destroyed. Example: Salmonella releases endotoxins.

Example: Food poisoning can result from exotoxins produced by Staphylococcus species.

Bacterial Structures

Bacteria possess specialized structures that aid in survival and colonization:

• Capsules: Sticky polysaccharide layer that helps bacteria adhere to surfaces and evade the immune system.

• Flagella: Structures for movement; prokaryotic flagella lack microtubules but enable chemotaxis.

• Fimbriae: Hairlike projections that facilitate attachment to host cells.

• Endospores: Dormant, resistant cells containing a copy of the chromosome; can survive harsh conditions and germinate when favorable.

Example: Bacillus anthracis forms endospores that can persist in soil for decades.

Biofilms

Biofilms are organized communities of prokaryotes attached to surfaces and embedded in a self-produced matrix. They are medically and environmentally significant:

• Can form on rocks, soil, metals, plastics, stagnant water, and tissues.

• Common on medical devices (e.g., catheters, pacemakers) and teeth (dental plaque).

• Biofilms are resistant to antibiotics and disinfectants due to their protective matrix.

• Some biofilms are beneficial, such as those decomposing organic matter in wastewater treatment.

Example: Dental plaque is a biofilm that can lead to tooth decay.

Proteobacteria and Prokaryotic Modes of Nutrition

Prokaryotes exhibit diverse nutritional strategies, classified by their carbon and energy sources:

• Autotrophs: Use CO2 as a carbon source.

• Heterotrophs: Obtain carbon from organic compounds.

• Photoautotrophs: Use light as an energy source and CO2 as a carbon source.

• Photoheterotrophs: Use light for energy but require organic compounds for carbon.

• Chemoautotrophs: Obtain energy from inorganic chemicals and use CO2 as a carbon source.

• Chemoheterotrophs: Obtain both energy and carbon from organic compounds.

Proteobacteria are a major group of Gram-negative bacteria with varied nutritional modes and often live in symbiosis with eukaryotes.

Symbiosis is a close association between different species; endosymbiosis refers to one organism living inside another.

Gut Microbiota and Ghrelin Output

The gut microbiota plays a crucial role in human health, including regulation of hunger hormones:

• Helicobacter pylori is a bacterium associated with gastric ulcers.

• Ghrelin is a hormone that stimulates hunger; its levels rise before eating and fall after eating.

• Presence of H. pylori is linked to lower ghrelin levels and reduced hunger after meals.

• Absence of gut bacteria can lead to persistently high ghrelin, increased hunger, overeating, and higher BMI (body mass index).

Example: Disruption of gut microbiota may contribute to obesity due to altered hormone regulation.

Archaea – Extremophiles

Archaea are prokaryotes distinct from bacteria, often inhabiting extreme environments and termed extremophiles:

• Halophiles: Thrive in highly saline environments (e.g., Great Salt Lake, Utah).

• Thermophiles: Live in hot, acidic environments (e.g., hot springs in Yellowstone National Park).

Example: Some archaea can survive in boiling acidic pools where few other organisms can live.

Protists & Modes of Nutrition

Protists are eukaryotic microorganisms, often called "protozoans," with diverse nutritional strategies:

• Autotrophs: Produce food via photosynthesis (e.g., green algae).

• Heterotrophs: Ingest other organisms for nutrition (e.g., Giardia).

• Mixotrophs: Can photosynthesize or ingest food depending on environmental conditions (e.g., Euglena).

Example: Euglena can switch between photosynthesis and heterotrophy based on light availability.

Protist Diversity

Protists are classified into four major eukaryotic supergroups, reflecting their evolutionary relationships and diversity:

• SAR Group: Includes diatoms, brown algae (kelp), water molds, dinoflagellates, and ciliates.

• Excavata: Organisms with modified mitochondria, often found in anaerobic environments.

• Unikonta: Includes amoebas, slime molds, fungi, and animals.

• Archaeplastida: Encompasses green algae, red algae, and higher plants; all possess chloroplasts and mitochondria.

Archaeplastida are believed to have evolved from endosymbiosis with cyanobacteria. Most multicellular algae reproduce via spores, and their life cycle includes haploid gametophytes and diploid sporophytes.

Example: Green algae are autotrophic protists within Archaeplastida, capable of photosynthesis.

Additional info: The alternation of generations in algae involves haploid (n) gametophytes producing gametes, which fuse to form diploid (2n) sporophytes.