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