Lecture 3 - Classification of Microorganisms

Classification of Microbes

Bacteria and Archaea

• Microbes are classified into two primary groups: Bacteria and Archaea, each with distinct characteristics.

Early Earth and the Formation of Life

Origins of Life

• Life on Earth originated from microorganisms; however, it raises the question of how life emerged from non-living matter.

• The early Earth was extremely hot with a hypoxic atmosphere, lacking oxygen necessary for modern life.

• Biomolecules and macromolecules can spontaneously form under suitable conditions, driven by energy from ultraviolet radiation, lightning, and radioactivity.

• When gases such as methane, carbon dioxide, nitrogen, and ammonia are subjected to various energy sources, they can form macromolecules, including fatty acids, sugars, and nucleic acids.

Self-Replicating Entities

• Self-replicating entities require:

  • A means of obtaining and using energy.

  • A mechanism for heredity to pass genetic information on to future generations.

• Research suggests that early life forms were likely "naked" RNAs capable of self-replication, leading to the formation of the RNA World theory.

Formation of Cells

• Clays and pyrite on early Earth may have facilitated the creation of organic compounds and films containing various macromolecules.

• Experiments show that Montmorillonite clay can induce the formation of lipid vesicles, resembling early cells.

• RNA oligonucleotides can also spontaneously form within these vesicles, hinting at the emergence of the first cells.

Evolution of Cells

• RNA-containing vesicles could have eventually become self-replicating.

• As the Earth cooled, more complex macromolecules may have developed, leading to the formation of the first viable cell through a mixture of macromolecules and self-replicating RNA.

• Natural selection favored cells with optimal catalytic rates, promoting the evolution of proteins and enzymes that surpassed RNA in efficiency.

Early Microbial Life

Evidence of Microbial Existence

• Evidence suggests that microbial life existed on Earth around 3.5 billion years ago, found in sedimentary rocks that required liquid water, vital for life.

• Stromatolites, consisting of microbial mats developed from bacteria, provide fossil evidence of early microbial life.

Role of Early Bacteria

• Early bacteria likely fixed atmospheric chemicals like methane and nitrogen for metabolic processes.

• Cyanobacteria significantly influenced Earth's atmospheric composition through organic photosynthesis, contributing to the creation of the ozone layer and the availability of oxygen for aerobic organisms.

First Eukaryotic Cells

• Early eukaryotic cells were simpler than modern eukaryotes and lacked complex organelles.

• As early eukaryotic cells grew in size, they required more proteins, leading to the division of genes into chromosomes for efficient replication.

Endosymbiosis and Eukaryotic Evolution

Origin of Organelles

• Mitochondria and chloroplasts may have derived from early aerobic and photosynthetic microbes incorporated into larger cells.

• These organelles contain their own circular DNA and ribosomes similar to prokaryotic structures, and their incorporation represents a symbiotic relationship beneficial to both.

Phylogeny of Life Forms

Evolutionary Relationships

• Phylogeny studies evolutionary relationships among species that reveal classification based on shared characteristics.

• A phylogenetic tree constructed from rRNA comparisons demonstrates the relationship between Bacteria, Archaea, and Eukarya.

Domains of Life

• The tree presents three main domains representing all life:

  • Bacteria

  • Archaea

  • Eukarya

• Notably, all prokaryotes (Bacteria and Archaea) are closely related, with Archaea being more similar to Eukarya.

Microbial Growth and Environmental Factors

Conditions Affecting Microbial Growth

• Environmental conditions such as solutes and water, pH levels, temperature, oxygen concentration, and pressure significantly affect microbial growth.

Tonicity and Water Availability

• In hypertonic environments, cells lose water, leading to plasmolysis.

• In hypotonic environments, cells can absorb too much water and lysate; organisms adapt to changing osmolarity to survive.

• Examples include halophiles, which thrive in high salt concentrations.

pH and Temperature Preferences

• Microbes thrive in specific pH ranges:

  • Acidophiles: pH 0 to 5.5

  • Neutrophiles: pH 5.5 to 8.0

  • Alkalophiles: pH 8.0 to 11.5

• Temperature impacts microbial growth; optimal temperatures vary by species.

Microbial Temperature Classification

• Microbes are categorized based on their temperature preferences:

  • Psychrophiles (0-20°C)

  • Mesophiles (20-45°C, ideal growth is often at 37°C)

  • Thermophiles (45-80°C)

  • Hyperthermophiles (above 55°C)

Pressure Tolerance

• Most bacteria thrive at normal atmospheric pressure; exceptions include:

  • Barotolerant prokaryotes that can tolerate higher pressures.

  • Barophilic prokaryotes that favor high-pressure environments.

Oxygen Requirements

• Microbial oxygen tolerance varies:

  • Obligate aerobes require atmospheric oxygen.

  • Facultative anaerobes prefer oxygen but can grow without it.

  • Aerotolerant anaerobes grow better in an oxygen-free environment but can tolerate some oxygen.

  • Strict anaerobes cannot survive in oxygen.

  • Microaerophiles require reduced oxygen levels.

Fungi: Characteristics and Roles

Classification and Structure

• Fungi are heterotrophic eukaryotic organisms that were previously classified as primitive plants.

• All fungi share certain traits, including microscopic stages in their life cycles and being eukaryotic.

Decomposers of Ecosystems

• Fungi, along with heterotrophic bacteria, are principal decomposers, breaking down organic matter to release nutrients back into the environment and supporting soil fertility.

Pathogenic and Beneficial Roles

• Fungi can cause diseases in plants and animals, negatively impacting agriculture.

• However, they also provide benefits, including food sources like mushrooms, yeast for baking and fermentation, and production of antibiotics such as penicillin.

Fungal Cell Structure and Reproduction

• Fungal cell walls are primarily made of chitin, which offers resistance to degradation compared to plant cellulose.

• Fungi reproduce through both vegetative methods and spore production for dispersal, with spores formed as a survival strategy in adverse conditions.

Fungal Nutrition

• Fungi secrete enzymes to break down complex organic materials, absorbing the resulting nutrients. They are primarily absorptive feeders, growing in or on their food sources.

• Fungal growth is mainly restricted to moist environments as nutrient absorption occurs near the growing tips of hyphae, which are filamentous structures.

Diversity of Fungal Nutrition and Lifestyle

• Fungi can be saprophytes, parasites, or form symbiotic relationships. They possess varying methods for nutrient acquisition, including predacious behaviors.

• Certain fungi, like yeast, can reproduce both asexually and sexually, presenting diversity in reproductive strategies.

Mycoses and Mycotoxicoses

Fungal Infections

• Mycoses are diseases triggered by fungi growing on or inside a host, while mycotoxicoses are due to exposure to fungal toxins.

• Mycotoxins produced by fungi can lead to severe health issues in humans and animals, whose effects can be exacerbated by their persistence and stability even after cooking.

Categories of Mycotoxins

• Examples of mycotoxin classes include:

  • Fumonisins linked to esophageal cancer through contaminated food.

  • Trichothecenes potentially used in chemical warfare.

  • Patulin, noted for antibacterial properties but toxic to animals and humans.

Medical Mycology

Fungal Disease Groups

• Medical mycology identifies three fungal groups that affect humans, namely obligately parasitic fungi, thermal dimorphic saprobes, and opportunistic saprobes.

Types of Fungal Infections

• Infections can be categorized into three main types:

  • Cutaneous infections affecting the skin’s outer layers.

  • Subcutaneous infections introduced through wounds.

  • Systemic infections stemming from true pathogenic fungi or opportunistic fungi in immunocompromised individuals.

Specific Cutaneous Infections

• Diseases like tinea (ringworm) are prevalent, caused by keratinolytic fungi including species from the Epidermophyton, Microsporum, and Trichophyton genera.

Challenges with Fungal Infections

• Conditions such as excessive moisture can lead to rapid fungal growth, causing candidiasis and other skin infections.

Subcutaneous Infections and Complications

Types of Subcutaneous Infections

• Conditions like chromoblastomycosis, entomophthoromycosis, mycetoma, and sporotrichosis can develop from various fungi entering the skin through wounds or bites.

Acute Symptoms and Progression

• Mycetoma and sporotrichosis involve tumor-like growths and can spread through lymphatic systems, leading to systemic infections and complications.

Systemic Infections from Fungal Pathogens

Overview of Systemic Pathogenic Infections

• Systemic fungal infections can arise from specialized pathogens such as Histoplasma capsulatum and Coccidioides immitis or opportunistic saprobes related to compromised immune systems.

Notable Diseases

• Histoplasmosis primarily infects the lungs and can spread via macrophages; symptoms may remain undetected initially.

• Coccidioidomycosis shows flu-like symptoms and can lead to serious complications without treatment.

Other Systemic Infections

• Blastomycosis presents as an asymptomatic lung infection in most individuals, while paracoccidiomycosis predominantly affects males and can have severe outcomes.

Opportunistic Mycosis

Increased Incidence

• Opportunistic fungal infections have surged due to rising immunocompromised populations and include candidiasis and aspergillosis.

Infection Mechanisms and Risks

• Candidiases can spread to various organs, complicating underlying health issues; Aspergillosis primarily affects individuals with pre-existing respiratory abnormalities.