Classification and Taxonomy of Prokaryotes

Prokaryotic Species and Strains

Prokaryotes are classified based on their genetic, morphological, and physiological characteristics. Understanding these classifications is essential for identifying and studying microbial diversity.

• Prokaryotic species: A population of cells with similar characteristics.

• Culture: Microbial population grown in laboratory media.

• Clone: Population of cells derived from a single cell.

• Strain: Genetically different cells within a clone.

Modern Prokaryotic Classification

Classification is now primarily based on genetic relatedness, especially ribosomal RNA (rRNA) sequences. The three domains of life are:

• Archaea

• Bacteria

• Eukarya

G+C Ratio

The G+C ratio refers to the percentage of guanine and cytosine bases in a DNA molecule. This ratio varies among organisms and is used in microbial classification.

• Formula:

Domain Archaea: The Other Prokaryotes

General Characteristics of Archaea

Archaea are prokaryotic organisms with unique features distinguishing them from bacteria and eukaryotes.

• Certain genetic sequences found only in their rRNA.

• AUG codon codes for methionine (like eukaryotes).

• Cell membrane lipids have branched hydrocarbon chains.

• Lack true peptidoglycan in cell walls.

• Unique cell wall construction.

• Most primitive of all life forms; closely related to the first cells on earth.

Extremophiles

Archaea often inhabit extreme environments and are classified as extremophiles.

Hyperthermophiles

• Require temperatures above 80°C.

• Enzymes used in applications such as PCR.

Thermoacidophiles

• Prefer hot and acidic environments.

• Found in acidic sulfur springs and undersea vents.

Extreme Halophiles

• Require NaCl concentrations of 9% or above.

• Many contain red or orange pigments.

• Optimal growth at 17-23% NaCl.

• Example: Halobacterium salinarum

Methanogens

• Obligate anaerobes.

• Convert organic wastes in ponds and lakes to methane.

Low G+C Gram-Positive Bacteria

Mycoplasmas

Mycoplasmas are unique Gram-positive bacteria that lack a cell wall, making them resistant to antibiotics like penicillin.

• Gram positive only because they take up dye in the cell membrane.

• Transitional – doesn’t hold stain well.

• Tough cytoplasmic membranes.

• Examples: Mycoplasma pneumoniae (causes LRTI), Ureaplasma urealyticum (causes UTI).

Clostridia

Clostridia are Gram-positive, endospore-forming rods known for their toxin production.

• Difficult to eradicate due to endospores.

• Toxin producers.

• Examples: Clostridium tetani, Clostridium botulinum.

Other Low G+C Gram-Positive Bacilli and Cocci

• Bacillus sp. – includes B. cereus (food poisoning, especially in high-carb foods).

• Listeria sp. – Gram-positive rods, does not form endospores, opportunistic pathogen.

• Lactobacillus sp. – Gram-positive, facultative anaerobic or microaerophilic rods, used for food and probiotics.

• Streptococcus sp. and Enterococcus sp. – Gram-positive cocci, classified by hemolytic activity.

• Staphylococcus sp. – Gram-positive bacteria in grape-like clusters, catalase positive, includes MRSA strains.

High G+C Gram-Positive Bacteria

Mycobacteria

Genus Mycobacterium includes aerobic, slow-growing rods with mycolic acids in their cell walls, making them acid-fast.

• Examples: Mycobacterium leprae (leprosy), Mycobacterium tuberculosis (TB).

Corynebacterium sp.

• Gram-positive, rod-shaped, pleomorphic, catalase positive, nonspore-forming, nonmotile.

• C. diphtheriae causes diphtheria.

Actinomycetes: Nocardia sp.

• Gram-positive, catalase-positive, rod-shaped bacteria.

• Found in soil, can cause opportunistic infections (pneumonia, pleurisy, encephalitis).

Gram-Negative Bacteria: Proteobacteria

General Characteristics

Proteobacteria are Gram-negative, chemoheterotrophic bacteria with diverse shapes and metabolic capabilities. They include many pathogens.

• Subgroups: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Epsilonproteobacteria, Zetaproteobacteria.

Pathogenic Alphaproteobacteria

• Rickettsia sp. – Gram-negative aerobic rods, cause typhus and Rocky Mountain Spotted Fever.

• Brucella sp. – Coccobacillus, causes spontaneous abortions in cattle (zoonosis).

Betaproteobacteria

• Neisseria sp. – Gram-negative diplococci, causes gonorrhea (N. gonorrhoeae) and meningitis (N. meningitidis).

Gammaproteobacteria

• Purple sulfur bacteria, methane oxidizers, intracellular pathogens (e.g., Legionella, Coxiella), glycolytic facultative anaerobes (e.g., Enterobacteriaceae, Vibrionaceae, Pasteurellaceae), pseudomonads.

Legionella sp.

• Gram-negative aerobic rods, transmitted via aerosols, cause Legionnaires’ Disease.

Enterobacteriaceae

• Gram-negative, facultative anaerobes, includes Salmonella, Shigella, E. coli.

Vibrionaceae

• Vibrio cholerae – causes severe dysentery and dehydration.

• Vibrio vulnificus – can cause flesh-eating disease.

• Vibrio parahaemolyticus – found in shellfish, self-limiting.

Pasteurellaceae

• Includes Haemophilus influenzae, opportunistic pathogens causing respiratory infections.

Pseudomonads

• Gram-negative, aerobic flagellated rods, includes Pseudomonas aeruginosa, common in hospitals, causes UTI, skin, and lung infections.

Epsilonproteobacteria

• Gram-negative rods, includes Campylobacter (causes diarrhea, periodontitis) and Helicobacter (survives in acidic stomach by producing urease).

Other Gram-Negative Bacteria: Non-Proteobacteria

Chlamydias

• Obligate intracellular parasites, cannot synthesize their own ATP.

• Most common bacterial sexually transmitted infections.

• C. trachomatis – causes STD and eye infection.

Spirochetes

• Helical bacteria, includes Treponema pallidum (syphilis) and Treponema pertenue (yaws).

Bacteroids

• Gram-negative, rod-shaped anaerobes.

• Major component of mammalian gastrointestinal flora.

• Process complex molecules to simpler ones in the host intestine.

Summary Table: Major Types of Prokaryotes

Key Points to Know

• Recognize major types of prokaryotes and their representative organisms.

• Understand general characteristics and clinical significance.

• Be able to classify prokaryotes based on genetic, morphological, and physiological traits.