Biology: Prokaryotes and Cell Morphology

Prokaryotes

• Most diverse group of organisms
• Can be found in various habitats, including:
• Antarctic glaciers to thermal hot springs
• Colons of animals to cytoplasm of other prokaryotes
• Distilled water to supersaturated brine
• Disinfectant solutions to basalt rocks
• Only a few capable of colonizing humans and causing disease

Morphology of Prokaryotic Cells

• Various shapes, including:
• Coccus
• Coccobacillus
• Bacillus
• Vibrio
• Spirillium
• Spirochete
• Pleomorphic

Reproduction of Prokaryotic Cells

• All reproduce asexually
• Three methods:
• Binary fission (most common)
• Snapping division
• Budding

Arrangements of Prokaryotic Cells

• Result from two aspects of division during binary fission
• Planes in which cells divide:
• Cocci divide in multiple places, bacilli in only one
• Separation of daughter cells

Modern Prokaryotic Classification

• Based on genetic relatedness of rRNA sequences
• Three domains:
• Archaea
• Bacteria
• Eukarya

Survey of Archaea

• Common features:
• Lack peptidoglycan
• Cell membrane lipids have branched hydrocarbon chains
• Lack tRNA specific for thymine
• AUG codon codes for methionine
• Reproduce by:
• Binary fission
• Budding
• Fragmentation
• Most are:
• Cocci
• Bacilli
• Spiral forms
• Unusual shapes exist
• Not known to cause disease in humans or animals

Extremophiles

• Require extreme conditions of:
• Temperature
• pH
• Salinity
• To survive
• Prominent members:
• Thermophiles
• Halophiles

Thermophiles

• DNA, RNA, cytoplasmic membranes, and proteins do not function properly below 45°C
• Hyperthermophiles require temperatures over 80°C

Halophiles

• Inhabit extremely saline habitats
• Depend on greater than 9% NaCl to maintain integrity of cell walls
• Contain red or orange pigments for protection from:
• Visible light
• UV light
• Most studied: Halobacterium salinarium

Methanogens

• Convert carbon dioxide, hydrogen gas, and organic acids to methane gas
• Largest group of archaea
• Convert organic wastes in:
• Pond sediments
• Lake sediments
• Ocean sediments
• To methane
• Some live in colons of animals, contributing to environmental methane
• Have produced ~10 trillion tons of methane buried in mud on ocean floor

Low G+C Gram-Positive Bacteria

Clostridia

• Rod-shaped, obligate anaerobes
• Important in medicine and industry
• Produce toxins that cause diseases in humans
• Endospores survive harsh conditions
• Related microbes:
• Epulopiscium
• Sulfate-reducing microbes
• Selenomonas

Mycoplasma

• Facultative or obligate anaerobes
• Lack cell walls
• Smallest free-living cells

Bacillus

• Endospore-forming aerobes and facultative anaerobes
• Many common in soil
• Bacillus thuringiensis toxin is used as an insecticide
• Bacillus anthracis causes anthrax
• Other species synthesize antibiotics

Listeria

• Contaminates milk and meat products
• Can reproduce under refrigeration
• Survives inside phagocytic white blood cells
• Rarely causes disease in adults
• Can kill the fetus if crosses the placenta in pregnant women

Lactobacillus

• Grows in the human mouth, stomach, intestinal tract, and vagina
• Rarely causes disease
• Inhibits the growth of pathogens within the body
• Used in the production of various foods

Streptococcus and Enterococcus

• Cause numerous diseases
• Various strains of multi-drug-resistant streptococci

Staphylococcus

• One of the most common inhabitants of humans
• Produces toxins and enzymes that contribute to disease

High G + C Gram-Positive Bacteria

Corynebacterium

• Pleomorphic aerobes and facultative anaerobes
• C.diptheriae causes diptheria

Mycobacterium

• Aerobic rods that sometimes form filaments
• Slow growth, partly due to mycolic acid in cell walls
• Some species are pathogens of animals and humans

Actinomycetes

• Form branching filaments resembling fungi
• Cause disease primarily in immunocompromised patients
• Important genera