2.6- BACTERIA; ECOLOGY AND DIVERSITY

 

2.6 Bacterial Ecology (Interactions with environment) and Diversity (Variety in forms, habitats, and functions)

Introduction to Prokaryotes (Archaea and Bacteria)

• The fossil record shows that prokaryotes (single-celled organisms without nucleus; includes archaea [extremophiles] and bacteria [diverse microbes]) were abundant 3.5 billion years ago.
• They evolved and remained all alone on Earth for the next 2 billion years (before eukaryotes appeared).
• Today, prokaryotes (archaea and bacteria) are found wherever there is life (ubiquitous in biosphere).
• Bacteria are found in water, air, soil, food, and in the bodies of animals and plants (e.g., gut microbiome).
• They outnumber all eukaryotes (prokaryotes vastly more abundant than plants/animals/fungi).
• They can survive in extreme habitats (e.g., hot springs, deep sea vents, acidic environments).

Diversity in Bacteria and Their Ecology

• Margulis and Schwartz proposed a useful classification system for all prokaryotes, dividing them into 16 phyla (major taxonomic groups based on morphology, physiology, and genetics).
• The following discussion focuses on important groups of the domain Bacteria (one of three domains of life; excludes Archaea here; refer to Figure 2.9 for visual representation of major groups).
• Key note: Recent discovery shows that the bulk of modern petroleum deposits were formed by masses of decayed cyanobacteria (ancient photosynthetic bacteria contributing to fossil fuels).

1. Omnibacteria

• These are rigid, rod-shaped (bacilli), heterotrophic (obtain nutrition from organic compounds), Gram-negative (thin peptidoglycan layer, stain pink) bacteria.
• Many important pathogens (disease-causing agents) are included in this group.
• Most have flagella (for motility).
• They do not produce spores (non-spore formers, less resistant to harsh conditions).
• Usually aerobic (require oxygen).
• Example: Escherichia coli (E. coli; common gut bacterium, can cause food poisoning).
• This group also includes vibrios (comma-shaped rods).

2. Cyanobacteria (Blue-green algae; prokaryotic photosynthesizers)

• These are photosynthetic bacteria (use light for energy via chlorophyll).
• Played the most important role in Earth's history by increasing free oxygen in the atmosphere (Great Oxidation Event ~2.4 billion years ago).
• Contain chlorophyll-a (primary pigment for photosynthesis) and accessory pigments like carotenoids (protect against UV), and blue and red phycobilins (absorb light in low-light conditions).
• Many fix atmospheric nitrogen in special cells called heterocysts (anaerobic sites for nitrogenase enzyme to convert N₂ to usable forms).
• Common in soil as mats (filamentous colonies).
• Cyanobacteria-containing lichens (symbiotic with fungi) are found on rock surfaces.
• Mats on sediments in the sea are dominated by cyanobacteria.
• Note: Colourful blooms may occur in polluted water due to rampant growth of cyanobacteria; colors result from photosynthetic pigments (can cause eutrophication and toxins).

3. Mycoplasmas and Spiroplasmas

• These groups differ from all other bacteria by lacking cell walls (pleomorphic shape, flexible membranes).
• Due to lack of cell walls, they are resistant to penicillin and other antibiotics that inhibit cell wall growth (e.g., beta-lactams target peptidoglycan synthesis).
• Some mycoplasmas cause diseases in mammals, e.g., certain types of pneumonia in humans (walking pneumonia).
• Spiroplasmas cause significant plant diseases, e.g., lethal yellowing disease of coconuts (affects palms, transmitted by insects).

4. Spirochaetes

• These are long spirilla (helical, corkscrew-shaped) with Gram-negative cell walls.
• They may have 2 to more than 100 flagella (internal, endoflagella for twisting motility called "corkscrew" movement).
• Important example: Treponema; causes syphilis (fatal sexually transmitted disease; stages include chancre, rash, and neurosyphilis).

5. Pseudomonads

• These are straight or curved Gram-negative rods with one or many flagella at one end (polar flagella for swimming).
• Found in soil and water (ubiquitous environmental bacteria).
• Can easily break down organic compounds (versatile decomposers).
• Some are autotrophic (self-feeding via inorganic sources), but many are plant pathogens (e.g., cause blights).
• Some play a role in denitrification (convert nitrates to N₂ gas, part of nitrogen cycle).
• Example: Pseudomonas aeruginosa; occurs in soil, water, and raw vegetables; usually harmless but can form serious infections in weak people (e.g., burn victims, cystic fibrosis patients; opportunistic pathogen).

6. Actinomycetes

• These have filamentous growth forms (branching hyphae-like structures, mold-like appearance).
• Produce spores that are resistant to unfavourable conditions (dormant forms for survival).
• Some are nitrogen fixers and found in root nodules of many flowering plants (symbiotic, e.g., with legumes).
• Some responsible for dental plaque, where enamel of teeth is destroyed (oral biofilms leading to cavities).
• Members: Mycobacterium leprae causes leprosy (chronic skin/nerve disease); Mycobacterium tuberculosis causes tuberculosis (TB; lung infection).
• Many antibiotics (e.g., tetracycline, chloramphenicol, erythromycin, neomycin) were originally derived from actinomycetes (soil-derived producers).
• Note: About 150 new antibiotics from actinomycetes are being discovered each year (source of novel drugs).

7. Nitrogen-fixing Aerobic Bacteria

• This group includes economically important bacteria (enhance soil fertility).
• They are Gram-negative and most are flagellated (motile).
• Example: Azotobacter; found in soil and water; converts atmospheric nitrogen into nitrates (free-living N-fixer using nitrogenase).

8. Chemosynthetic Bacteria

• These bacteria derive energy from the oxidation of inorganic compounds of nitrogen, sulphur, and iron (chemolithotrophy).
• Use this energy for the synthesis of their food (autotrophic, no light needed).
• Examples: Nitrosomonas and Nitrobacter; oxidize nitrogen compounds (NH₃) to gain energy.
  • NH₃ is converted to nitrite (NO₂⁻) and nitrate (NO₃⁻).
• Thus, they play a vital role in the nitrogen cycle (nitrification process, making nitrogen available to plants).

Figure 2.9: Major groups of bacteria (includes examples like Escherichia coli, Mycoplasmas, Pseudomonas, Azotobacter, Anabaena [cyanobacterium], Treponema, Mycobacterium tuberculosis, Nitrosomonas).

Table: Characteristics of Some Groups of Bacteria

Name of Group	Form (Shape)	Motility	Nutrition	Ecological Role
Omnibacteria	R (rods/bacilli)	N, F (nonmotile or flagellated)	H (heterotrophic)	Pathogens and decomposers
Cyanobacteria	R, C, M (rods, cocci, mats/aggregations)	G, N (gliding or nonmotile)	P (photosynthetic)	Carbon and nitrogen fixers
Mycoplasmas and Spiroplasmas	No wall (pleomorphic)	N (nonmotile)	H (heterotrophic)	Pathogens
Spirochaetes	S (spirilla/helical)	F (flagellated)	H (heterotrophic)	Decomposers and pathogens
Pseudomonads	R (rods)	F (flagellated)	H, C (heterotrophic or chemosynthetic)	Decomposers and plant pathogens
Actinomycetes	M, R (mats/chains or rods)	N (nonmotile)	H (heterotrophic)	Pathogens and nitrogen fixers
N-fixing Aerobes	R (rods)	N, F (nonmotile or flagellated)	H (heterotrophic)	Free-living and mutualistic nitrogen fixers
Chemosynthetic	R, C (rods or cocci)	N, F (nonmotile or flagellated)	C (chemosynthetic)	Oxidize nitrogen and sulphur compounds; role in nitrogen cycle

Key to Table:

• Form: R = rods (bacilli); C = cocci (spherical); S = spirilla (helical); M = regular chains or aggregations.
• Motility: F = flagellated; N = nonmotile; G = gliding.
• Nutrition: H = heterotrophic; C = chemosynthetic; P = photosynthetic.