Classification of Bacteria

Classification of Bacteria

Traditional Classification Methods

  • Phenotypic Classification: Initially, bacteria were classified based on the following characteristics:

    • Cell wall structure
    • Biochemical properties

  • Example Genera of Bacteria:

    • Aquifex pyrophilus
    • Fervidobacterium islandica
    • Thermotoga maritima
    • Chloroflexus aurantiacus
    • Thermomicrobium roseum
    • Thermus thermophilus
    • Deinococcus radiodurans
    • Synechococcus CIP 6301
    • Neisseria gonorrhoeae
    • Rhodocyclus gelatinosus
    • Pseudomonas aeruginosa
    • Chromatium vinosum
    • Escherichia coli
    • Rickettsia typhi
    • Rhodospirillum rubrum
    • Rhodobacter capsulatus
    • Agrobacterium tumefaciens
    • Wolinella succinogenes
    • Bdellovibrio stolpii
    • Desulfuromonas acetonitans
    • Chlamydia psittaci
    • Planctomyces staleyi
    • Treponema pallidum
    • Spirochaeta halophila
    • Serpula hyodysenteriae
    • Leptonema illini
    • Chlorobium limicola
    • Flexibacter flexilis
    • Mycobacterium tuberculosis
    • Propionibacterium acnes
    • Micrococcus luteus
    • Streptomyces lividans
    • Lactobacillus minutus
    • Megasphaera elsdenii
    • Heliobacterium chlorum
    • Clostridium pasteurianum
    • Clostridium aminovalericum
    • Clostridium thermosaccharolyticum

Modern Classification Methods

  • Phylogenetic Classification: In more recent times, bacteria classification has evolved to incorporate DNA sequencing, specifically from the 16s rRNA gene.
  • The 16s rRNA gene is used as a reliable genetic marker for determining genetic relationships among bacteria.

Definition of a Species

  • The definition of a species varies between sexually and asexually reproducing organisms:
    • Sexually Reproducing Species:
    • Defined as reproductively isolated populations.
    • Asexually Reproducing Species:
    • More complex definition, often involving phenotype or genetic similarity.
    • Phenotypic Definition: A population of cells demonstrating similar characteristics.
    • Phylogenetic Definition: Two bacteria are classified as the same species if:
      • Their DNA displays at least 70% homology.
      • Their 16s rRNA DNA shows at least 97% homology.

Bergey’s Manual of Systematic Bacteriology

  • 1st Edition (1980-1989):
    • Comprised of 4 volumes.
    • Relying on phenotypic classification methods.
  • 2nd Edition (2000-2010):
    • Expanded to 5 volumes.
    • Shifted to phylogenetic classification methods.

Classification Changes: Chlamydia and Rickettsia

  • Observations from the manual editions noted significant changes in classifications of Chlamydia and Rickettsia due to the following:
    • Both genera are obligate intracellular organisms, showcasing phenotypic similarities.
    • In the 1st edition, they were categorized within the same phylum.
    • Phylogenetic analysis revealed they are not closely related, leading to their separation into different phyla in the 2nd edition.

Bacterial Habitats

  • Bacteria occupy various habitats, including:
    • Soil
    • Water
    • Animal Hosts

Soil Bacteria

  • Rhizobium species:
    • Classification: Gram-negative bacteria.
    • Habitat: Form mutualistic relationships with leguminous plant roots.
    • Function: Nitrogen fixation, converting nitrogen gas to ammonia.
  • Streptomyces species:
    • Classification: Gram-positive bacteria exhibiting fungal behaviors.
    • Structure: Form hyphae and reproductive spores.
    • Function: Production of antibiotics.

Water Bacteria

  • Cyanobacteria:
    • Taxonomy: Phylum of Gram-negative, primarily aquatic bacteria.
    • Characteristic: Capable of photosynthesis and potential pathogenicity.
  • Bdellovibrio:
    • Classification: Genus of Gram-negative bacteria.
    • Behavior: Preys upon other Gram-negative bacteria.
  • Notable Processing Times:
    • Prey capture takes approximately 10 seconds.
    • Intracellular degradation of prey varies from 150-210 minutes.

Animal Bacteria

  • Epulopiscium fishelsoni:
    • Classification: Extremely large Gram-positive bacterium.
    • Habitat: Found within the intestines of fish.

Medically Relevant Bacteria

  • Different bacterial genera associated with human health, as either normal microbiota or pathogens:

    • Neisseria:

    • Classification: Gram-negative cocci.

    • Habitat: Animal.

    • Pathogens:

      • N. meningitidis
      • N. gonorrhoeae

    • Pseudomonas:

    • Classification: Gram-negative bacilli.

    • Habitat: Soil and animals.

    • Characteristic: Known for high resistance, common cause of hospital-acquired infections.

    • Vibrio:

    • Classification: Gram-negative curved rods.

    • Habitat: Aquatic.

    • Pathogen: V. cholerae (most pathogenic species)

    • Enterobacteriaceae:

    • Classification: Family of Gram-negative, facultative anaerobic bacilli.

    • Characteristic: Catalase positive and oxidase negative, primarily found in animal habitats.

    • Pathogens Include:

      • Yersinia pestis
      • Salmonella typhi
      • Shigella
      • Common opportunistic pathogens include E. coli, Klebsiella, and Proteus species.

    • Borrelia:

    • Classification: Gram-negative spirochete.

    • Habitat: Mostly animals.

    • Pathogen: Borrelia burgdorferi (causes Lyme disease).

    • Chlamydia:

    • Classification: Gram-negative coccoid.

    • Habitat: Animal.

    • Pathogen: Chlamydia trachomatis

Characteristics of Selected Gram-Positive Bacteria

  • Summary of various Gram-positive bacterial genera characterized by specific properties:

Firmicutes

  • Phylum/Class: Firmicutes
  • G+ C %: Low (less than 50%)
  • Representative Genera:
    • Clostridia: Obligate anaerobic rods; endospore formers (e.g., Clostridium).
      • Diseases: Contaminates dairy products, causes tetanus, botulism, severe diarrhea.
    • Bacillus: Aerobic or facultative anaerobic; endospore formers.
      • Diseases: Includes B. anthracis (causes anthrax).
    • Mycoplasma: Lack cell walls; smallest free-living cells; stain like Gram-negative bacteria.
      • Characteristics: Pleomorphic and part of oral biofilm.

Actinobacteria

  • Phylum/Class: Actinobacteria
  • G+ C %: High (greater than 50%)
  • Representative Genera:
    • Corynebacterium: Snapping division and contains metachromatic granules.
      • Diseases: Causes diphtheria.
    • Mycobacterium: Contains waxy cell walls with mycolic acid, highly resistant.
      • Diseases: Includes tuberculosis and leprosy.
    • Streptomyces: Filamentous; known for antibiotic production.
      • Characteristics: Degrades pollutants.
    • Nocardia: Rare sinus infections, forms filamentous structures.