Chapter 11

CH 11: The Prokaryotes: Domains Bacteria and Archaea

Gram-Negative Bacteria: Pseudomonadota

The Pseudomonadota (formerly Proteobacteria) represent the largest taxonomic group of bacteria, characterized by their Gram-negative cell walls and chemoheterotrophic metabolism. This group is divided into five main classes, each with unique ecological and pathogenic roles.

• Alphaproteobacteria

• Betaproteobacteria

• Gammaproteobacteria

• Deltaproteobacteria

• Campylobacterota

Alphaproteobacteria

Alphaproteobacteria are adapted to low-nutrient environments and often possess stalks or buds called prosthecae. Many are important in environmental and pathogenic contexts.

• Pelagibacter: Extremely abundant in oceans; plays a key role in the carbon cycle.

• Acetobacteraceae (Acetobacter, Gluconobacter): Convert ethanol to acetic acid; important in food industry.

• Rickettsia: Obligate intracellular parasites; cause spotted fevers such as Rocky Mountain spotted fever; transmitted by ticks and insects.

• Ehrlichia: Tick-borne pathogens causing ehrlichiosis.

• Agrobacterium: Plant pathogen; induces crown gall tumors by inserting plasmids into plant cells.

• Bartonella: Human pathogen; B. henselae causes cat-scratch disease.

• Brucella: Causes brucellosis; survives phagocytosis; transmitted via raw dairy and undercooked meat.

• Wolbachia: Endosymbiont of insects; affects insect reproduction; potential for controlling mosquito-borne diseases.

Betaproteobacteria

Betaproteobacteria include several important human pathogens.

• Bordetella: Nonmotile rods; B. pertussis causes whooping cough.

• Neisseria: Includes N. gonorrhoeae (gonorrhea) and N. meningitidis (meningococcal meningitis).

Gammaproteobacteria

This class contains diverse genera, many of which are medically significant.

• Pseudomonas: Soil-dwelling, metabolically diverse, opportunistic pathogens; P. aeruginosa causes wound and urinary tract infections.

• Legionella: Found in aquatic environments; causes legionellosis.

• Coxiella: C. burnetii causes Q fever; transmitted via aerosols or contaminated milk.

• Vibrio: Aquatic habitats; V. cholerae causes cholera.

• Enterobacteriales (Enterics): Inhabit intestinal tract; ferment carbohydrates; facultative anaerobes; peritrichous flagella.

  • Escherichia: E. coli is an indicator of fecal contamination; some strains cause foodborne disease and UTIs.

  • Salmonella: Common foodborne pathogen; S. typhi causes typhoid fever.

  • Shigella: Causes bacillary dysentery (shigellosis).

  • Klebsiella: K. pneumoniae causes pneumonia.

  • Serratia: Produces red pigment; nosocomial infections.

  • Yersinia: Y. pestis causes plague; transmitted by fleas.

  • Enterobacter: E. aerogenes causes UTIs and nosocomial infections.

  • Haemophilus: H. influenzae causes meningitis, ear infections, and epiglottitis (not influenza).

Campylobacterota

These are slender, helical or curved Gram-negative rods, often microaerophilic.

• Campylobacter: Microaerophilic vibrios; C. jejuni causes foodborne intestinal disease (commonly from chicken).

• Helicobacter: Microaerophilic curved rods; causes peptic ulcers and stomach cancer.

Cyanobacteria (Oxygenic Photosynthetic Bacteria)

Cyanobacteria are important for oxygenic photosynthesis and nitrogen fixation, playing a crucial role in Earth's atmosphere and ecology.

• Contain heterocysts for nitrogen fixation.

• Gas vesicles provide buoyancy.

• Exist in unicellular, colonial, and filamentous forms.

• Significantly increased atmospheric O2 during Earth's history.

Chlamydiae

Chlamydiae are obligate intracellular bacteria lacking peptidoglycan in their cell walls.

• Chlamydia: Forms elementary bodies (infective stage); C. trachomatis causes trachoma (blindness) and urethritis.

Spirochaetes

Spirochaetes are coiled bacteria that move via axial filaments, enabling corkscrew motility.

• Treponema: T. pallidum causes syphilis.

• Borrelia: Causes Lyme disease.

Bacillota (Low G + C Gram-Positive Bacteria)

Bacillota are Gram-positive bacteria with low guanine-cytosine content, including several important genera.

• Clostridiales: Endospore-producing, obligate anaerobes; includes C. tetani (tetanus), C. botulinum (botulism), C. difficile (hospital-acquired infections).

• Bacillales: Endospore-producing rods; B. anthracis (anthrax), B. thuringiensis (insect pathogen), B. cereus (food poisoning).

• Staphylococcus: Grapelike clusters; S. aureus causes wound infections, is often antibiotic resistant, and produces enterotoxins.

• Lactobacillales: Aerotolerant anaerobes; produce lactic acid; includes Lactobacillus (food production, vaginal pH balance), Streptococcus (pharyngitis, scarlet fever, pneumonia, dental caries), Enterococcus (surgical wound and UTI infections), Listeria (L. monocytogenes contaminates food).

Mycoplasmatota (Low G + C Gram-Positive Bacteria)

Mycoplasmatota lack cell walls and are highly pleomorphic, with sterols in their membranes.

• Mycoplasma: Smallest self-replicating organisms; M. pneumoniae causes mild (walking) pneumonia.

Actinomycetota (High G + C Gram-Positive Bacteria)

Actinomycetota are Gram-positive bacteria with high G + C content, often found in soil and notable for their branching filaments.

• Mycobacterium: Thick, waxy cell wall; slow-growing; M. tuberculosis (tuberculosis), M. leprae (leprosy).

• Corynebacterium: C. diphtheriae causes diphtheria.

• Streptomyces: Soil-dwelling; produce many antibiotics.

• Actinomyces: Form hyphae; found in mouth and throat; destroy tissue.

Diversity within the Archaea

Archaea are a distinct domain of prokaryotes, lacking peptidoglycan and often thriving in extreme environments.

• Halophiles: Require salt concentrations >25%.

• Thermophiles: Grow at temperatures >80°C.

• Acidophiles: Can grow at pH values approaching 0.

• Methanogens: Anaerobic; produce methane.

Summary Table: Major Bacterial Groups and Their Characteristics

Group	Key Features	Representative Genera	Diseases/Applications
Pseudomonadota (Gram-negative)	Chemoheterotrophic, diverse metabolism	Escherichia, Salmonella, Pseudomonas, Vibrio	UTIs, foodborne illness, cholera, nosocomial infections
Bacillota (Low G+C Gram-positive)	Endospore-forming, anaerobic/aerobic	Bacillus, Clostridium, Staphylococcus, Streptococcus	Tetanus, anthrax, pneumonia, food poisoning
Actinomycetota (High G+C Gram-positive)	Branching filaments, antibiotic production	Mycobacterium, Streptomyces, Corynebacterium	Tuberculosis, leprosy, diphtheria, antibiotics
Mycoplasmatota	No cell wall, pleomorphic	Mycoplasma	Walking pneumonia
Cyanobacteria	Oxygenic photosynthesis, nitrogen fixation	Cyanobacteria	Atmospheric O2 production
Archaea	Extremophiles, no peptidoglycan	Halophiles, Thermophiles, Methanogens	Extreme environment adaptation, methane production

Key Terms and Definitions

• Gram-negative: Bacteria with a thin peptidoglycan layer and an outer membrane; stain pink in Gram stain.

• Gram-positive: Bacteria with a thick peptidoglycan layer; stain purple in Gram stain.

• Chemoheterotrophic: Organisms that obtain energy and carbon from organic compounds.

• Facultative anaerobe: Can grow with or without oxygen.

• Obligate anaerobe: Cannot grow in the presence of oxygen.

• Nosocomial infection: Hospital-acquired infection.

• Endospore: Resistant, dormant structure formed by some bacteria for survival.

• Serovar: Distinct variation within a species of bacteria or virus, identified by serological methods.

• Extremophile: Organism that thrives in extreme environmental conditions.

Example: Antibiotic Production by Streptomyces

Streptomyces species are soil-dwelling actinomycetes that produce a wide range of antibiotics, including streptomycin, tetracycline, and erythromycin. Their ability to synthesize these compounds has made them invaluable in medicine.

Example: Oxygenic Photosynthesis in Cyanobacteria

Cyanobacteria use sunlight to convert water and carbon dioxide into oxygen and glucose, a process that dramatically increased Earth's atmospheric oxygen and enabled the evolution of aerobic life.

Additional info:

• Some genera, such as Mycobacterium, are slow-growing due to their thick, waxy cell walls.

• Many pathogens listed are important for clinical diagnosis and treatment, including multidrug-resistant strains.

• Archaea are not known to cause human disease but are important in environmental microbiology.