Prokaryote Diversity

Overview of Prokaryotes

Prokaryotes are among the most diverse and widespread organisms on Earth. They are classified into two distinct domains based on genetic and biochemical differences: Archaea and Bacteria. This chapter surveys major groups within the domain Bacteria, focusing on their structure, metabolism, and ecological roles.

• Archaea: Prokaryotes often found in extreme environments; distinct from bacteria in cell wall composition and genetics.

• Bacteria: The most numerous and diverse prokaryotes, classified into groups based on genetic sequence comparisons.

Gram-Negative Bacteria

General Characteristics

Gram-negative bacteria possess a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides, making them more resistant to certain antibiotics. They are identified by the Gram stain technique, which stains them pink/red.

Escherichia coli (E. coli)

• Habitat: Commonly found in the human intestines.

• Significance: Used as a model organism in research; some strains can cause foodborne illness.

Vibrio cholerae

• Pathogenicity: Causes cholera, a severe diarrheal disease.

• Transmission: Spread through contaminated water and food.

• Symptoms: Severe diarrhea, vomiting, dehydration.

Thiomargarita namibiensis

• Metabolism: Uses hydrogen sulfide () to generate organic molecules from carbon dioxide ().

• Byproducts: Produces sulfur wastes, visible as greenish globules.

Rhizobium Species

• Symbiosis: Live in root nodules of legumes, forming a mutualistic relationship.

• Nitrogen Fixation: Convert atmospheric nitrogen () into ammonia (), usable by plants.

• Endosymbiosis: Rhizobium lives within plant cells.

Example: Formation of root nodules in soybean plants, where bacteria reside within vesicles in infected cells.

Neisseria gonorrhoeae

• Pathogenicity: Causes gonorrhea, a common sexually transmitted disease (STD).

• Transmission: Spread through vaginal, oral, or anal sexual contact.

• Symptoms in Men: Burning sensation when urinating, discharge, painful/swollen testicles (less common).

• Symptoms in Women: Often mild or absent; can include painful urination, increased discharge, vaginal bleeding between periods.

• Complications: Untreated infections can lead to pelvic inflammatory disease and infertility.

• Antibiotic Resistance: Penicillin-resistant strains exist; treated with other antibiotics.

Cyanobacteria

• Photosynthesis: Only prokaryotes with plantlike, oxygen-generating photosynthesis.

• Specialized Cells: Some, like Anabaena, have heterocysts for nitrogen fixation.

Example: Anabaena forms filaments with photosynthetic and nitrogen-fixing cells.

Chlamydias

• Obligate Intracellular Parasites: Live inside eukaryotic host cells.

• Chlamydia trachomatis: Causes trachoma (eye infection) and nongonococcal urethritis (STD).

• Symptoms: Often asymptomatic; can lead to pelvic inflammatory disease and infertility in women.

• Treatment: Easily cured by antibiotics; early detection is important.

Spirochetes

• Shape: Helical bacteria with axial filaments for movement.

• Pathogens: Cause syphilis (Treponema pallidum) and Lyme disease (Borrelia species).

• Syphilis: Characterized by genital sores; can progress to severe symptoms if untreated.

• Lyme Disease: Transmitted by ticks; initial rash followed by flu-like symptoms.

Helicobacter pylori

• Shape: Gram-negative, curved bacillus with multiple flagella.

• Pathogenicity: Causes peptic ulcers and stomach cancer.

Gram-Positive Bacteria

General Characteristics

Gram-positive bacteria have a thick peptidoglycan cell wall and lack an outer membrane. They are diverse and include many important soil organisms and pathogens.

Actinomycetes

• Role: Common in soil as decomposers of organic matter.

• Streptomyces: Source of many antibiotics, including streptomycin.

Pathogenic Gram-Positive Bacteria

• Staphylococcus: Includes Staphylococcus aureus, a common cause of skin infections and food poisoning.

• Streptococcus: Includes species causing strep throat, scarlet fever, and other diseases.

• Bacillus anthracis: Causes anthrax.

Clostridium Species

• Endospore Formation: Produce resistant spores; obligate anaerobes.

• C. tetani: Produces exotoxins causing tetanus (muscle spasms, lockjaw); spores persist in soil.

• C. botulinum: Causes botulism; toxin attacks nerves, leading to paralysis and death; can contaminate food, wounds, and infant intestines.

• C. perfringens: Causes food poisoning by producing toxins in contaminated food.

Table: Comparison of Major Bacterial Groups

Key Terms and Concepts

• Gram Stain: Differential staining technique to classify bacteria as Gram-positive or Gram-negative.

• Endospore: Highly resistant, dormant structure formed by some bacteria for survival in harsh conditions.

• Symbiosis: Close association between organisms of different species.

• Nitrogen Fixation: Conversion of atmospheric nitrogen () into ammonia () by certain bacteria.

• Exotoxin: Toxins secreted by bacteria into their environment, often causing disease symptoms.

Summary

This chapter highlights the diversity of prokaryotes, focusing on major bacterial groups, their ecological roles, and their significance in health and disease. Understanding these organisms is fundamental to microbiology, biotechnology, and medicine.

Additional info: Some scientific terms and disease mechanisms were expanded for clarity and completeness.