Microbial Life: Prokaryotes

Introduction to Microbial Communities

Microorganisms, including prokaryotes and protists, are abundant in and on the human body, outnumbering human cells. Disruption of these microbial communities can increase susceptibility to infectious diseases and contribute to conditions such as asthma, allergies, irritable bowel syndrome, Crohn’s disease, and autism.

Prokaryotic Diversity and Widespread Presence

Prokaryotes are among the most diverse and widespread organisms on Earth. Their collective biomass is at least ten times greater than that of all eukaryotes. Prokaryotes are classified into two domains: Archaea and Bacteria. They play crucial roles in environmental processes and human health.

External Features and Classification of Prokaryotes

Cell Shapes and Identification

Microscopic examination of cell shape is essential for identifying prokaryotes. The three most common shapes are:

• Cocci: Spherical cells

• Bacilli: Rod-shaped cells

• Spirilla and Spirochetes: Spiral-shaped cells, with spirilla being short and rigid, and spirochetes longer and flexible

Cell Wall Structure and Gram Staining

Almost all prokaryotes possess a cell wall. Gram staining differentiates bacteria based on cell wall composition:

• Gram-positive bacteria: Stain purple, have thick peptidoglycan walls

• Gram-negative bacteria: Stain pink, have thinner peptidoglycan and an outer membrane of glycolipids

Surface Projections and Their Functions

• Capsule: Sticky layer that helps bacteria adhere to surfaces and evade immune responses

• Flagella: Used for movement

• Fimbriae: Hair-like structures for attachment to surfaces

Adaptation and Survival Strategies

Rapid Adaptation and Genetic Variation

Prokaryotes reproduce rapidly, generating significant genetic variation. This increases their ability to adapt to changing environments. Spontaneous mutations during DNA replication contribute to this diversity.

Endospores

Some prokaryotes form endospores, which are dormant, tough, and non-reproductive structures that allow survival through harsh conditions.

Nutritional Diversity of Prokaryotes

Energy and Carbon Sources

Prokaryotes exhibit unparalleled nutritional diversity, enabling them to inhabit nearly every environment. They utilize two main energy sources:

• Phototrophs: Capture energy from sunlight

• Chemotrophs: Harness energy from chemicals

Prokaryotes are further classified by their carbon source:

Biofilms and Environmental Impact

Biofilms

Biofilms are complex communities of microbes that adhere to surfaces and are difficult to eradicate. They cause medical and environmental problems due to their resistance to removal and treatment.

Bioremediation

Bioremediation is the use of organisms, often prokaryotes, to remove pollutants from soil, air, or water. Prokaryotes are essential in sewage treatment facilities, where biofilms of aerobic bacteria and fungi break down organic material in waste.

Prokaryotic Evolution: Bacteria and Archaea

Major Domains and Their Differences

Prokaryotes lack nuclei and membrane-enclosed organelles. Advances in molecular genetics have revealed that some prokaryotes are more closely related to eukaryotes than to other prokaryotes. The three domains of life are Bacteria, Archaea, and Eukarya. Key differences include:

Archaea: Life in Extreme Environments

Types of Archaea

Domain Archaea includes:

• Extreme halophiles (“salt lovers”)

• Extreme thermophiles (“heat lovers”)

• Methanogens: Thrive in anaerobic conditions and produce methane

Many archaea are referred to as extremophiles due to their ability to survive in extreme environments.

Bacterial Diversity: Major Groups

Proteobacteria

All gram-negative, sharing a particular rRNA sequence. Includes:

• Photoautotrophs: Use H2S to generate organic molecules from CO2

• Chemoautotrophs: Important for the nitrogen cycle

• Symbiotic proteobacteria: Live in close association with eukaryotic hosts

• Endosymbionts: Live within another organism

Gram-Positive Bacteria

Includes actinomycetes (branched chains, common in soil), decomposers, antibiotic producers, and pathogenic species such as Streptococcus, Staphylococcus, and Bacillus anthracis.

Cyanobacteria

The only prokaryotes with plant-like, oxygen-generating photosynthesis. They are a source of food in aquatic ecosystems and contain photosynthetic pigments. Anabaena has specialized cells for nitrogen fixation and forms symbiotic relationships with fungi, mosses, and marine invertebrates.

Chlamydias

Live inside eukaryotic hosts and cause diseases such as urinary tract infections and blindness (Chlamydia trachomatis).

Spirochetes

Helical bacteria with rotating internal filaments. Includes pathogens such as Treponema pallidum (syphilis) and Borrelia burgdorferi (Lyme disease).

Pathogenic Bacteria and Disease

Mechanisms of Pathogenicity

Pathogenic bacteria cause disease by producing:

• Exotoxins: Proteins secreted into the environment

• Endotoxins: Lipid components of the outer membrane of gram-negative bacteria, released when the cell dies or is digested

Certain bacteria and their toxins, such as those causing anthrax and botulinum, can be used as biological weapons.

Example:

Staphylococcus aureus produces exotoxins, while botulinum toxin is used in medical applications (e.g., Botox).

Additional info:

Botulinum toxin is one of the most potent biological toxins known and is used in minute amounts for cosmetic procedures.