Characterizing and Classifying Prokaryotes: Study Guide for Microbiology Students

General Characteristics of Prokaryotes

Introduction to Prokaryotes

Prokaryotes are the most diverse group of organisms on Earth, inhabiting a wide range of environments from extreme cold to extreme heat, and from animal colons to rocks. Despite their ubiquity, relatively few prokaryotes colonize humans and animals to cause disease.

• Prokaryotes: Organisms lacking a membrane-bound nucleus and organelles.

• Ubiquitous: Found everywhere in nature.

• Fomite: Inanimate objects that can transmit microorganisms.

Prokaryotic Morphologies and Arrangements

Cell Shapes and Arrangements

Prokaryotes exhibit a variety of cell shapes (morphologies) and arrangements, which are important for identification and classification.

• Cocci: Spherical cells.

• Bacilli: Rod-shaped cells.

• Coccobacilli: Short rods, intermediate between cocci and bacilli.

• Vibrio: Comma-shaped cells.

• Spiral/Spirochete: Helical or spiral-shaped cells.

• Arrangements arise from the pattern of cell division and how cells remain attached:

  • Single: Individual cells.

  • Diplo-: Pairs.

  • Strepto-: Chains.

  • Staphylo-: Clusters.

  • Tetrad: Groups of four.

  • Sarcina: Cubes of eight.

  • Palisade: Side-by-side arrangement, often seen in bacilli.

Endospores

Formation and Function

Some Gram-positive bacteria can form endospores, which are highly resistant structures that allow survival in harsh conditions.

• Endospores: Dormant, tough, and non-reproductive structures produced by certain bacteria.

• Formed by Bacillus and Clostridium genera.

• Each vegetative cell forms one endospore as a defensive strategy against unfavorable conditions.

Modern Prokaryotic Classification

Domains and Classification Methods

Prokaryotes are classified based on genetic relatedness and phenotypic characteristics. The three domains of life are:

• Archaea

• Bacteria

• Eukarya

Classification methods include:

• rRNA sequence relatedness: Organisms with more identical rRNA sequences are more closely related (method developed by Carl Woese).

• Cultural, morphological, and physiological characteristics: As described in Bergey’s Manual of Systematic Bacteriology.

Survey of Domain Bacteria

Gram-Positive Bacteria

Gram-positive bacteria are divided into low G+C and high G+C groups based on their genomic DNA content.

Low G+C Gram-Positives

• Clostridium: Obligate anaerobes, endospore-formers. Pathogens include:

  • C. botulinum: Produces botulinum toxin (exotoxin) causing botulism.

  • C. tetani: Produces tetanus toxin (exotoxin) causing tetanus.

  • C. perfringens: Causes gas gangrene.

  • C. difficile: Causes antibiotic-associated diarrhea.

• Mycoplasma: Lack cell wall, cause pneumonias and urinary tract infections.

• Listeria: Survives refrigeration and phagocytosis; L. monocytogenes is a foodborne pathogen.

• Lactobacillus: Gut microbe, probiotic, involved in microbial antagonism.

• Staphylococcus: Includes S. aureus, S. epidermidis, S. saprophyticus; causes bacteremia, food poisoning, toxic shock, wound infections.

• Streptococcus: Classified by hemolysis on blood agar:

  • Alpha hemolytic: S. pneumoniae

  • Beta hemolytic: S. pyogenes

  • Non-hemolytic: Other streptococci

Bacillus

• Common in soil; endospore-formers.

• B. thuringiensis: Produces a toxin used as an insecticide.

• B. anthracis: Causes anthrax; produces three exotoxins and a capsule. Endospores survive in soil.

• Anthrax forms in humans/animals:

  • Cutaneous (not often fatal)

  • Gastrointestinal (sometimes fatal)

  • Inhalational (usually fatal)

High G+C Gram-Positives

• Corynebacterium: Pleomorphic, normal flora of the upper respiratory tract. C. diphtheriae causes diphtheria (exotoxin kills cells in trachea and bronchi).

• DTaP vaccine (children) and TDaP (adults) prevent diphtheria.

• Mycobacterium: Acid-fast bacilli; includes M. leprae (leprosy) and M. tuberculosis (tuberculosis).

Survey of Gram-Negative Bacteria

Proteobacteria

Proteobacteria is the largest and most diverse group of Gram-negative bacteria, divided into several classes.

• Alphaproteobacteria:

  • Rhizobium: Nitrogen fixation in plants.

  • Rickettsia rickettsii: Causes Rocky Mountain Spotted Fever (tick vector).

  • Brucella abortus: Causes brucellosis in animals and humans.

• Betaproteobacteria:

  • Neisseria gonorrhoeae: Causes gonorrhea, pelvic inflammatory disease.

  • Neisseria meningitidis: Causes meningitis.

  • Bordetella pertussis: Causes whooping cough.

  • Bordetella bronchiseptica: Causes kennel cough.

  • Legionella pneumophila: Causes Legionnaire’s Disease.

• Gammaproteobacteria:

  • Family Enterobacteriaceae: Facultative anaerobes, opportunistic pathogens.

  • Salmonella typhi: Typhoid fever.

  • Shigella dysenteriae: Dysentery.

  • Proteus vulgaris: UTIs.

  • Escherichia coli: Diarrhea, UTIs, gastroenteritis.

  • Klebsiella pneumoniae: Pneumonia.

  • Yersinia pestis: Plague.

  • Pseudomonas aeruginosa: Opportunistic pathogen, highly resistant, causes blue pus.

• Family Vibrionaceae: Vibrio cholerae (cholera).

• Family Pasteurellaceae: Haemophilus influenzae (meningitis, pneumonia), Pasteurella multocida (respiratory infections).

• Deltaproteobacteria: Environmental organisms.

• Epsilonproteobacteria: Helicobacter pylori (gastric ulcers), Campylobacter jejuni (diarrheal illness).

• Zetaproteobacteria: Mostly autotrophic.

Other Gram-Negative Bacteria

• Chlamydia trachomatis: Intracellular pathogen, causes blindness, pneumonia, STIs.

• Spirochetes: Helical, motile bacteria.

  • Treponema pallidum: Syphilis.

  • Borrelia burgdorferi: Lyme disease (tickborne).

• Bacteroides: Obligate anaerobes, most numerous in human colon, digest polysaccharides and cellulose, can cause abdominal infections if intestines rupture.

Key Terms and Concepts

Term	Definition
Binary fission/Budding	Methods of asexual reproduction in prokaryotes
Endospores	Dormant, resistant structures formed by some bacteria
Extremophile	Organism that thrives in extreme environments
Hemolysis	Destruction of red blood cells, used to classify Streptococcus
Chemotroph/Phototroph	Organisms obtaining energy from chemicals/light
RNA sequencing	Method for determining genetic relatedness
Probiotic	Live microorganisms that confer health benefits
Endotoxin/Exotoxin	Toxins produced by bacteria (endotoxins: Gram-negative cell wall; exotoxins: secreted proteins)