Examining the Diversity, Structure, and Function of Prokaryotes

Habitats and Diversity

Prokaryotes are found in nearly every environment on Earth due to their remarkable resilience and adaptability. Their diversity is mapped through genetic and ecological studies, revealing a vast array of species and functions.

• Habitats: Soil, water, extreme environments (hot springs, salt lakes), and within living organisms.

• Classification: Prokaryotes are classified into Bacteria and Archaea, each with unique genetic and structural features.

• Relationships: Prokaryotes interact with each other and with eukaryotes in complex ecological networks.

The Importance of Prokaryotes

Ecological and Biological Roles

Prokaryotes play essential roles in global biogeochemical cycles and human health. Despite their abundance, less than 1% are pathogenic to humans.

• Carbon fixation: Conversion of atmospheric CO2 into organic compounds by photosynthetic bacteria.

• Nitrogen fixation: Transformation of atmospheric nitrogen (N2) into ammonia (NH3), making nitrogen accessible to plants.

• Biological remediation: Breakdown of pollutants and toxins in the environment.

• Resident microbiotas: Communities of prokaryotes living in and on the human body, contributing to health and disease prevention.

Ecological Associations Among Microorganisms

Types of Symbiosis

Microorganisms interact in various ways within communities, influencing ecosystem dynamics and health.

• Symbiosis: Any interaction between different species within a community.

• Mutualism: Both species benefit from the relationship.

• Amensalism: One population is harmed while the other is unaffected.

• Commensalism: One organism benefits, the other is unaffected.

• Neutralism: Neither organism is affected.

• Parasitism: One organism benefits while harming the other.

Human Microbiome

Microbial Colonization and Variation

The human microbiome consists of all prokaryotic and eukaryotic microorganisms associated with the human body. Colonization begins at birth and varies by body region and over time.

• Resident microbiota: Long-term inhabitants of the body.

• Transient microbiota: Temporary microbial residents.

• Microbiome composition is influenced by factors such as mode of delivery, diet, and antibiotic use.

Domain Archaea

Characteristics and Ecological Roles

Archaea are prokaryotes that often thrive in extreme environments and possess unique genetic and structural features.

• Extremophiles: Archaea that live in hostile conditions (high temperature, salinity, acidity).

• No known archaeal pathogens.

• Genomes are larger and more complex than those of bacteria.

• Cell walls lack peptidoglycan, but may contain similar substances.

Clinical Case: Tuberculosis

Case Study and Diagnosis

A clinical scenario illustrates the transmission, symptoms, and diagnosis of tuberculosis (TB), a disease caused by Mycobacterium tuberculosis.

• Symptoms: Fatigue, cough, night sweats, loss of appetite, low-grade fever, blood in sputum, chest pain.

• Diagnosis: Chest radiograph, sputum sample, Mantoux tuberculin skin test, and X-rays.

• TB is transmitted via inhalation of respiratory droplets; infectious dose is only 10 cells.

Mycobacterium Tuberculosis

Pathogenesis and Disease Progression

TB bacteria enter alveoli, survive in macrophages, and can cause chronic inflammatory responses. Latent TB can reactivate, leading to severe symptoms and high mortality if untreated.

• Formation of tubercles (walled-off lesions in the lungs).

• Necrotic caseous lesions may heal by calcification (Ghon complex).

• Symptoms: Persistent cough, hemoptysis, chest pain, weight loss.

• If untreated, mortality rate can reach 60%.

Diagnosis of TB

Testing Methods

• Mantoux tuberculin skin test: Intradermal injection of PPD; positive result indicated by redness, swelling, or hardness after 48–72 hours.

• Chest X-rays are used to rule out false positives and confirm diagnosis.

TB Prevention and Treatment

Therapies and Vaccination

• Multi-month antibiotic therapy (isoniazid, rifampin); combination therapy prevents resistance.

• Drug resistance issues: MDR-TB (multi-drug resistant) and XDR-TB (extensively drug resistant).

• BCG vaccine (based on M. bovis) is used outside the US and for high-risk groups; modestly effective.

Vibrio Cholera

Pathogenesis and Treatment

Vibrio cholerae causes cholera, a disease associated with poor sanitation and water quality. The bacterium releases cholera toxin, leading to severe dehydration.

• Symptoms: Secretory diarrhea ("rice water stool"), muscle and neurological symptoms due to dehydration.

• Treatment: Oral rehydration therapy (ORT); timely administration leads to >99% survival.

• Prevention: Safe water, handwashing, sanitation, Vaxchora vaccine.

Chlamydia trachomatis

Characteristics and Life Cycle

Chlamydia trachomatis is a small, Gram-negative, obligate intracellular parasite. Many infections are asymptomatic but can cause serious reproductive and urinary tract diseases.

• Females: Urethritis, salpingitis, pelvic inflammatory disease (PID), associated with cervical cancer.

• Males: Urethritis, epididymitis, orchitis.

• Lymphogranuloma venereum: Swollen lymph nodes (buboes) in the groin.

• Treatment: Antibiotics (azithromycin or doxycycline).

Life Cycle of Chlamydia

• Elementary body: Infectious, extracellular, metabolically inactive form.

• Reticulate (inclusion) body: Noninfectious, actively dividing form within host cell vacuoles.

Borrelia burgdorferi and Lyme Disease

Pathogenesis and Prevention

Borrelia burgdorferi, a spirochete, causes Lyme disease, which is transmitted by blacklegged (Ixodes) ticks.

• Characteristic bull's eye rash (erythema migrans) at tick bite site.

• Other symptoms: Fever, chills, headache, fatigue, muscle/joint aches, swollen lymph nodes.

• Prevention: Remove ticks within 24 hours to reduce risk; avoid tick habitats.

• Treatment: Early antibiotics lead to rapid recovery.

Anthrax

Forms and Treatment

Bacillus anthracis causes anthrax, which can present in cutaneous, pulmonary, gastrointestinal, or injection forms.

• Cutaneous anthrax: Endospores enter skin, form nodules and eschar; least dangerous, 20% mortality without treatment.

• Pulmonary/inhalation anthrax: High mortality, bioterrorism risk.

• Gastrointestinal anthrax: Up to 40% mortality even with treatment.

• Injection anthrax: Rare, associated with intravenous drug use, very high mortality.

• Treatment: Broad-spectrum antibiotics; vaccine exists but not routinely used.

Mycobacterium leprae and Hansen's Disease (Leprosy)

Characteristics and Disease Course

Mycobacterium leprae is a strict parasite causing Hansen's disease (leprosy), which progresses slowly and affects skin and nerves.

• Transmission: Not fully verified; person-to-person via inhalation or prolonged skin contact; armadillos may be reservoirs.

• Most people (95%) have natural immunity.

• Incubation: 2–5 years; untreated cases lead to nerve damage, ulcers, deformities, and loss of digits.

• Global prevalence reduced by effective antibiotic treatment.

Forms of Hansen's Disease

• Tuberculoid: Asymmetrical, shallow lesions, nerve damage, local loss of pain reception; immune system contains infection.

• Lepromatous: Deeply nodular infection, severe disfigurement, widespread dissemination; nodules filled with bacilli, chronic mucosal damage.