Prokaryotes Study Notes

Chapter 11: Prokaryotes

Overview of Bacterial Classification

  • Morphological Classification: Classification has traditionally relied on morphology, staining reactions, endospore formation, and oxygen requirements.
  • Bergey's Manual: References to the 1st edition of Bergey's Manual provide practical classification guidelines. The 2nd edition updates focus on molecular levels due to advancements in DNA sequencing.
  • Molecular Classification: It is now possible to classify species based on reliable comparisons of rRNA subunits, which leads to the realization that previously classified organisms based on physical and nutritional needs may not be closely related on a genetic level.
  • Focus of the Chapter: The chapter will discuss different genera of bacteria with specific species pertinent to medical concerns.

Phylum Proteobacteria

  • General Information:

    • Largest and most diverse group of bacteria, with over 2000 species.
    • Presumed to have arisen from a common photosynthetic ancestor, though few are currently photosynthetic.
    • Name derives from the Greek god Proteus, embodying many shapes (pleomorphic).

  • Metabolic Types:

    • Phototrophs: Obtain energy from sunlight to synthesize organic compounds.
    • Autotrophs: Make their own food from inorganic materials (light, water, carbon dioxide, or other chemicals).
    • Chemotrophs: Obtain energy by the oxidation of electron donors.
    • Heterotrophs: Depend on others for carbon sources.
Characteristics of Proteobacteria
  • Gram Status: Mostly Gram-negative bacteria. Includes anaerobes, facultative anaerobes, and obligate aerobes.
  • Classes: Divided into five classes: Alpha, Beta, Gamma, Delta, and Epsilon.
  • Note: Class Zeta will not be covered.

Alpha Proteobacteria

  • General Characteristics:
    • Includes a variety of aerobes and often has unusual cellular extensions called prosthecae.
    • Important soil organisms, including
    • Azospirillum: Forms symbiotic relationships with the roots of tropical grasses such as sugar cane and corn.
    • Rhizobium: Forms symbiotic relationships with the roots of legumes, fixing atmospheric nitrogen into ammonium (NH₄⁺), which is then utilized by the plant for amino acid and protein synthesis.

Nitrifying Bacteria

  • Importance:
    • Noted for their role within the nitrogen cycle, specifically through nitrification, a two-step process involving the following microbes:
    • Nitrobacter: First step, oxidizes ammonia (NH₃) into nitrite (NO₂⁻).
    • Nitrosomonas: Second step, oxidizes nitrite (NO₂⁻) into nitrate (NO₃⁻).
  • Chemotrophs: Utilize organic chemicals as their energy source and CO₂ as their carbon source.

Pathogenic Alpha Proteobacteria

  • Agrobacterium: Inserts a plasmid into plants, causing them to develop tumors that provide nutrients to the bacterium.
  • Bartonella: Causes Cat Scratch Fever through bites or scratches from cats, leading to symptoms such as fever and loss of appetite.
  • Rickettsia: Small intracellular obligate parasites, usually transmitted by insect and tick bites, associated with the spotted fever group (e.g., Rocky Mountain Spotted Fever).
  • Wolbachia: Found in insects; influences egg development, used in releasing infected male mosquitoes to combat diseases like Zika and Dengue Fever.
  • Brucella: Obligate parasites causing undulant fever in humans; transmitted via livestock or dairy products.

Beta Proteobacteria

  • General Characteristics:
    • Diverse and thrive in environments with low nutrient levels.
  • Notable Genera:
    • Thiobacillus: Recycles sulfur in soils by oxidizing hydrogen sulfide (H₂S).
    • Zoogloea: Important in wastewater treatment plants.
    • Sphaerotilus: Forms floating flocs that are problematic in wastewater management.
Pathogenic Beta Proteobacteria
  • Burkholderia: Previously classified under Pseudomonas, includes species responsible for hospital-acquired infections (B. cepacia, B. pertussis - causes whooping cough).
  • Neisseria: Comprised of human pathogens, notably Neisseria gonorrhoeae (gonorrhea) and Neisseria meningitidis (meningitis).

Gamma Proteobacteria

  • Characteristics: Largest and most diverse class of proteobacteria, encompassing various shapes, arrangements, metabolic types with the following sub-groups:
    1. Purple sulfur bacteria
    2. Intracellular pathogens
    3. Methane oxidizers
    4. Facultative anaerobes
    5. Pseudomonads
  • Legionella: Found in water systems; L. pneumophila causes Legionnaires' Disease.
  • Coxiella: Causes Q fever, obligate intracellular parasite affecting humans (at-risk groups: farmers, veterans).

Enterobacteriaceae Family

  • Characteristics: Commonly known as Enterics, inhabit the intestinal tract of humans and other mammals. Predominantly glucose fermenters.
  • Pathogenic Members:
    • Escherichia coli: Indicator of fecal contamination, with some strains (like E. coli O157:H7) causing severe illnesses (e.g., hemolytic uremic syndrome).
    • Salmonella: Includes multiple serovars, with Salmonella enterica causing typhoid fever.
    • Shigella: Causes shigellosis, spread via oral-fecal routes.
    • Yersinia: Includes Yersinia pestis, the causative agent of the Bubonic plague.

Firmicutes

  • Characteristics: Lack a cell wall, are classified based on DNA composition, and are a major source of antibiotics.
  • Pathogenic Members:
    • Clostridium: Includes species like C. tetani (tetanus), C. botulinum (botulism), and C. difficile (nosocomial infections).
    • Lactobacillus: Important probiotics known for their role in food production.
    • Staphylococcus: Includes S. aureus, known for antibiotic resistance and hospital-acquired infections.
    • Streptococcus: Includes various pathogenic groups, such as S. pyogenes (causes strep throat) and Group B Streptococcus (causes neonatal infections).

Actinobacteria

  • Characteristics: Obligate aerobes or facultative anaerobes, often filamentous with a distinct smell. Known for antibiotic production (e.g., Streptomyces spp.).
  • Pathogenic Members:
    • Mycobacterium: Includes M. tuberculosis (causes tuberculosis) and M. leprae (causes leprosy).

Miscellaneous Bacteria

  • Chlamydia: Obligate intracellular parasites, causing diseases such as trachoma.
  • Spirochetes: Includes Borrelia burgdorferi (causes Lyme Disease) and Treponema pallidum (causes Syphilis).
    • B. burgdorferi: Transmitted by ticks, characterized by a bullseye rash.
    • T. pallidum: Causes progressive symptoms, including nervous system and cardiovascular damage.

Conclusion

  • This chapter highlights the diversity and medical significance of prokaryotes, with a focus on pathogenic potentials of specific genera. It sets the foundation for understanding interactions between bacteria and host organisms, the implications of various diseases, and the importance of proper classification and recognition of these microorganisms.