Prokaryotic Diversity and Chemotrophs

Prokaryotic Diversity

• Scientists are just beginning to understand the vast diversity of microbial life
• Only a fraction of over a million prokaryotic species have been described
• The majority of prokaryotes have not been isolated
• New molecular techniques are aiding in the discovery and characterization of prokaryotes
• The goal of this class is to provide an overview of the diversity of characteristics and habitats of prokaryotes, focusing on medically important species.

Anaerobic Chemotrophs

• The atmosphere was anoxic for approximately 1.5 billion years when prokaryotes first inhabited Earth
• Early chemotrophs likely used anaerobic respiration
• Terminal electron acceptors, such as sulfur, were used
• Anaerobic photosynthesis increased life on the surface
• After glucose became the standard energy molecule, other species likely used fermentation and passed electrons to organic molecules like pyruvate
• This produced, for example, ethanol and CO2

Cyanobacter and Two-Stage Photosynthesis

• Prokaryotes introduced large amounts of oxygen into Earth's atmosphere
• This was a poison to obligate anaerobes
• Anaerobic habitats remain common, even today, and are found in places like mud and tightly packed soil, where gases diffuse with limitations
• Aquatic environments can become limiting, as can any area of limited oxygen replenishment
• Anaerobic conditions are present in human and animal bodies, especially the intestinal tract and in microenvironments such as the skin and oral cavity
• Facultative aerobes contribute by depleting oxygen and maintaining anaerobic conditions.

Anaerobic Chemolithotrophs

• Oxidize inorganic chemicals for energy
• Use alternative electron acceptors, such as sulfur
• Many are in the Archaea domain
• Methanogens are one group
• No major medical agents are associated with this group

Anaerobic Chemoorganotrophs - Anaerobic Respiration

• Chemoorganotrophs oxidize organic compounds (such as glucose) for energy
• Anaerobes often use sulfur or sulfate as electron acceptors
• Produce hydrogen sulfide (rotten-egg smell)
• Important in sulfur cycles
• Desulfovibrio is discussed

Anaerobic Chemoorganotrophs - Fermentation

• Many current-day anaerobic bacteria ferment
• ATP is produced via substrate-level phosphorylation
• Many different organic energy sources are used, and various end products result
• Clostridium and Clostridioides are in this group
• Endospore formers can survive long exposure to oxygen, heat, drying, and chemicals

Medically Important Clostridia

• Clostridium tetani is ubiquitous in rich soils, common in animal intestines
• It causes a classic infection by deep puncture wounds
• It produces a neurotoxin (tetanospasmin), composed of two protein chains
• First protein chain binds to neuron membranes, while the second protein is internalized
• This blocks the release of inhibitory neurotransmitters, thereby permanently preventing muscle relaxation
• Treatment involves antitoxin vaccination against the toxin

Medically Important Clostridia

• Clostridium botulinum is ubiquitous in rich soils, and is present in low levels in the human gut
• It is important that infants' gut flora is developed
• Improper canning can allow botulinum spores to grow in airtight environments
• It releases neurotoxins that block excitatory neurons
• Classic diseases include headaches, blurred vision, dry mouth, and progressive weakness, which can be treated with antitoxin and ventilation support

Medically Important Clostridia

• Clostridium difficile is commonly found in normal intestinal flora
• Antibiotic treatment can disrupt normal flora in the gut, allowing Clostridium difficile to overgrow
• This often leads to mild disease that resolves when antibiotics are discontinued
• However, severe disease may require high-dose antibiotics

Aerotolerant Fermenters - Lactic Acid Bacteria

• Gram-positive bacteria produce lactic acid as a fermentation product
• Most can grow in aerobic environments, but lack catalase, and only ferment
• Medical important genera include Lactobacillus (part of healthy flora, e.g., vaginal) and Streptococcus (in oral cavity)
• Others, such as Streptococcus pyogenes, are pathogens (e.g., strep throat, scarlet fever)

Intervening Prokaryotic Types (Environmental Impact)

• Various prokaryotic types are involved in environmental impacts, including Anoxygenic Phototrophs, Oxygenic Phototrophs, Aerobic Chemolithotrophs

Aerobic Chemoorganotrophs

• Oxidize organic compounds for energy
• Use oxygen as a terminal electron receptor
• Obligate aerobes cannot ferment

Mycobacterium

• Additional components to the cell wall, such as mycolic acids (long-chain fatty acids), help them survive in dry environments and withstand chemical attack, as in various harsh environments
• Identification is accomplished using acid-fast staining
• Examples of key medical agents include M. tuberculosis and M. leprae

Pseudomonas

• Wide range of degradative activities, including the metabolism of complex molecules (e.g. plastics)
• Widespread in soil and water
• medically important microbes, such as P. aeruginosa, grow in nutrient-poor environments, and can contaminate "just water"
• Resistant to various disinfectants. Can be opportunistic infections in a hospital environment

Side Bar: Scientific Interest

• Thermus, a prokaryotic type, grows at high temperatures and produces heat-stable enzymes
• Deinococcus is extremely radiation resistant
• Has mechanisms for multiple genomes and active DNA repair

Facultative Anaerobes

• Preferentially use aerobic respiration but can ferment if oxygen is limited
• Play a major role in maintaining anaerobic environments
• Useful for understanding the environment because it can readily grow in a test tube

Medically Important Family: Enterobacteriaceae

• Common intestinal residents in humans and other animals
• Include Enterobacter, Klebsiella, Proteus, and Escherichia
• Can cause intestinal or systemic disease in certain individuals
• Salmonella and Shigella cause food poisoning or systemic infections
• Yersinia pestis causes bubonic and pneumonic plague

Other Medically Important Genera

• Corynebacterium and Vibrio are important genera
• Require some sodium ions for growth, especially in high-saline environments
• Medical examples include V. cholerae

Note on Environmental Factors

• A few prokaryotes have evolved to live in the human body and often take advantage of what humans offer them, including food and environment
• Generally, the human body can outcompete them to prevent them from growing in unwanted ways.

Out of Balance

• Prime examples of terrestrial environments include Clostridium spp.
• Another prime example of aquatic environments is Legionella
• Gram-negative bacteria thrive in various aquatic environments
• Bacteria that are ingested in the lung are immediately attacked by macrophages
• Occasionally, bacteria have evolved to successfully fight back against the human macrophage immune system

Animals as Habitats

• Skin, Mucus Membranes, and Intracellular Environments are key locations in animals

Skin

• Staphylococcus epidermidis and aureus are key bacteria species present in skin, commonly associated with higher salt/low-water conditions.

Mucous Membranes: Respiratory, Genitourinary, and Intestinal Tracts

• Important bacterial species include Streptococcus, Lactobacillus, and Enterobacteriaceae
• Other medically important species are Helicobacter and Campylobacter
• Bacteroides is a commensal species in the intestines, also one of the most common bacterial species in feces

Other Mucosal: Respiratory

• Mycoplasma is a unique species of bacteria that lack cell walls.

Other Mucosal -- Genitourinary

• Neisseria gonorrhoeae and Treponema pallidum are important species

Obligate Intracellular Bacteria

• Bacteria that cannot replicate outside a host cell
• Possess advantages because of the abundance of resources within a host
• Example: Chlamydia
• Invades epithelial cells of mucous membranes
• Can survive or invade white blood cells
• Range of infections, including STIs

Explore the fascinating world of prokaryotic life, focusing on the vast diversity and significance of these microorganisms. This quiz covers key concepts about their habitats, characteristics, and the role of anaerobic chemotrophs in early Earth's anoxic environment. Deepen your understanding of bacteria that contribute to ecological and medical advancements.