Microbiology Study Notes: Prokaryotes, Bacterial Classification, and Major Bacterial Groups

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Prokaryotes

Definition and Classification

Prokaryotes are unicellular organisms lacking a membrane-bound nucleus. Bacteria and other prokaryotes are classified based on genetic information, especially ribosomal RNA (rRNA) sequences, as well as morphology and staining characteristics.

Classification: Based on genetic similarities (e.g., rRNA sequence) and physical traits.
Genetic Markers: rRNA genes are highly conserved and useful for phylogenetic studies.

Prokaryotic Ribosomes - 16S RNA

Role in Taxonomy and Phylogeny

The 16S rRNA gene sequence is the most commonly used genetic marker for bacterial taxonomy and phylogeny. It allows for differentiation and identification of bacterial species.

16S rRNA: A component of the small subunit of prokaryotic ribosomes, essential for protein synthesis.
Sequence Analysis: Differences in DNA (rRNA) sequences among species enable identification.
Phylogenetic Tree: Constructed based on rRNA sequence similarities.

Gram-Negative Bacteria Phyla: Proteobacteria

Overview and Classification

Proteobacteria is the largest taxonomic group of bacteria, named after the Greek god Proteus, who could change shape. This phylum includes five major classes, each with distinct characteristics.

Classes:
- Alphaproteobacteria
- Betaproteobacteria
- Gammaproteobacteria
- Deltaproteobacteria
- Epsilonproteobacteria

Alphaproteobacteria

Key Genera and Features

Alphaproteobacteria are capable of surviving in low-nutrient environments and include agriculturally important bacteria and some pathogens.

Agrobacterium: Significant for farming; capable of nitrogen fixation.
Pelagibacter: Most abundant marine bacteria; important in carbon and nutrient cycles.
Azospirillum: Microaerophilic; forms associations with plant roots, fixes nitrogen.
Wolbachia: Infects insects; influences reproduction and virus transmission.
Rickettsia: Human pathogen; transmitted by insect bites, causes spotted fever.
Bartonella: Human pathogen; causes cat scratch disease.

Betaproteobacteria (Gram Negative Proteobacteria)

Key Genera and Pathogenic Species

Betaproteobacteria include important soil bacteria and human pathogens.

Burkholderia: Found in soil; can cause serious illness, resistant to antibiotics.
Bordetella: B. pertussis causes whooping cough; highly contagious respiratory infection, preventable by Tdap and DTaP vaccines.
Neisseria: Infects mucous membranes; includes N. gonorrhoeae (gonorrhea) and N. meningitidis (meningococcal meningitis).

Gammaproteobacteria (Gram Negative Proteobacteria)

Major Genera and Diseases

Gammaproteobacteria include many medically significant bacteria, commonly found in soil and aquatic habitats.

Pseudomonas: Common in soil; includes genus Pseudomonas.
Legionella: Found in water systems; causes Legionnaires' disease.
Vibrio: Found in aquatic habitats; V. cholerae causes cholera, V. parahaemolyticus causes gastroenteritis.
Enterobacteriales: Inhabit the intestinal tract; include many pathogens.
- Escherichia: E. coli causes foodborne disease and urinary tract infections.
- Salmonella: Pathogenic; common in poultry and cattle; causes salmonellosis.
- Shigella: Causes dysentery (shigellosis).
- Klebsiella: Causes pneumonia.
- Serratia: Nosocomial infections; found in contaminated catheters.
- Yersinia: Y. pestis causes plague (Black Death).
- Haemophilus: Causes meningitis, can be prevented by Hib vaccine.

Epsilonproteobacteria

Characteristics and Pathogenic Species

Epsilonproteobacteria are helical or curved, microaerophilic bacteria, including important human pathogens.

Campylobacter: One polar flagellum; causes foodborne diarrheal disease.
Helicobacter: Multiple flagella; H. pylori causes peptic ulcers and stomach cancer.

Gram-Bacteria (The Oxygenic Photosynthetic Bacteria)

Major Groups and Features

These bacteria perform oxygenic photosynthesis and include important environmental and pathogenic species.

Cyanobacteria: Blue-green algae; produce oxygen via photosynthesis.
Chlamydia: Elementary body is the infective form; causes trachoma and sexually transmitted infections.
Bacteroidales: Anaerobic; found in mouth and large intestine, important in human microbiome.
Fusobacteria: Anaerobic; found in mouth, cause dental abscesses.

Other Gram-Negative Bacteria

Spirochetes and Pathogenic Species

Spirochetes are spiral-shaped bacteria with unique motility mechanisms.

Treponema: T. pallidum causes syphilis (STI).
Borrelia: Causes Lyme disease, transmitted by ticks/lice.
Leptospira: Causes leptospirosis; symptoms include jaundice and fever.

Gram-Positive Bacteria

Classification by G+C Ratios

Gram-positive bacteria are divided into two groups based on the guanine-cytosine (G+C) content in their DNA.

High G+C Ratios:
- Streptomyces: G+C content of 69-73%.
- Mycobacterium: G+C content of 62-70%.
Low G+C Ratios:
- Streptococcus: G+C content of 33-44%.
- Clostridium: G+C content of 21-54%.

Low G+C Ratio Gram-Positive Bacteria

Major Genera and Pathogenic Species

These bacteria include important pathogens and endospore-forming species.

Clostridiales: Endospore-producing; includes C. tetani (tetanus), C. botulinum (botulism), C. perfringens (food poisoning, gas gangrene), C. difficile (severe diarrhea).
Bacillales: Endospore-producing rods; includes B. anthracis (anthrax), B. cereus (food poisoning).
Staphylococcus: Cocci; causes wound infection, antibiotic resistance, and food poisoning.
Lactobacillales: Used in food production; includes Lactobacillus (fermentation), Streptococcus (pneumonia, strep throat, scarlet fever, impetigo, cellulitis), S. mutans (dental caries).
Enterococcus: Found in intestinal tract; common hospital contaminant.
Listeria: Contaminates food; causes listeriosis, dangerous for pregnant women and immunocompromised individuals.

High G+C Ratio Gram-Positive Bacteria

Major Genera and Features

High G+C ratio bacteria include soil inhabitants and important antibiotic producers.

Actinomycetota: Highly pleomorphic; common in soil.
Mycobacterium: Waxy cell wall (mycolic acids); acid-fast, drug-resistant, slow-growing; includes M. tuberculosis (tuberculosis) and M. leprae (leprosy).
Corynebacterium: C. diphtheriae causes diphtheria.
Streptomyces: Produces most commercially available antibiotics; lives in soil.

Summary Table: Major Bacterial Groups and Key Features

Group	Key Genera	Notable Diseases/Features
Alphaproteobacteria	Agrobacterium, Pelagibacter, Azospirillum, Wolbachia, Rickettsia, Bartonella	Nitrogen fixation, insect pathogens, cat scratch disease, spotted fever
Betaproteobacteria	Burkholderia, Bordetella, Neisseria	Whooping cough, meningitis, antibiotic resistance
Gammaproteobacteria	Pseudomonas, Legionella, Vibrio, Enterobacteriales	Legionnaires' disease, cholera, foodborne illness, plague
Epsilonproteobacteria	Campylobacter, Helicobacter	Diarrheal disease, peptic ulcers, stomach cancer
Gram-Positive (Low G+C)	Clostridium, Bacillus, Staphylococcus, Streptococcus, Enterococcus, Listeria	Tetanus, anthrax, food poisoning, pneumonia, listeriosis
Gram-Positive (High G+C)	Actinomycetota, Mycobacterium, Corynebacterium, Streptomyces	Tuberculosis, leprosy, diphtheria, antibiotic production

Key Terms and Definitions

rRNA (ribosomal RNA): RNA component of ribosomes, used for genetic classification.
Phylogeny: Evolutionary history and relationships among organisms.
Endospore: Resistant, dormant structure formed by some bacteria for survival.
Nosocomial infection: Infection acquired in a hospital setting.
Antibiotic resistance: Ability of bacteria to survive and proliferate despite antibiotic treatment.

Important Equations

G+C Ratio Calculation:

Examples and Applications

Example: Escherichia coli is used as a model organism in molecular biology and genetics.
Application: Streptomyces species are the source of many antibiotics, such as streptomycin.

Additional info: Some context and definitions have been expanded for clarity and completeness.