Medically-Important Gram-Negative Bacteria

Medically-Important Gram-Negative Bacteria

• This material is reproduced by the University of Sydney under the Australian Copyright Act.
• Specific learning outcomes involve discussing bacteria's role in medicine and human disease, naming relevant Gram-negative bacteria, describing their phylogeny, microscopic morphology, normal habitat, human diseases caused, and unique characteristics.

Classification of Bacteria

• Bacteria are categorized using both a phenetic (based on observable characteristics) and phylogenetic (based on genetic relatedness) approach.
• Phenetic characteristics include Gram reaction, morphology, carbon/energy sources, and electron acceptors (aerobic/anaerobic).
• Phylogenetic characteristics include ribosomal RNA sequence and other DNA, RNA, and protein sequences.
• In some cases, the classification based on genetics matches observed characteristics (e.g., endospore formers are Gram-positive). Other cases reveal differences between classification methods.
• Both methods are useful for different classification purposes.

Diversity of Gram-Negative Bacteria

• Several medically relevant Gram-negative bacteria are discussed, including:
  • Escherichia coli
  • Salmonella
  • Vibrio cholerae
  • Pseudomonas aeruginosa
  • Neisseria
  • Rickettsia
  • Bacteroides
  • Treponema pallidum

Specific Bacterial Species (Examples)

• Escherichia coli (E. coli):
  • Facultative anaerobe, heterotrophic, Gram-negative rods
  • Found in the gut; some strains cause infections (e.g., 0157, 0111), most are normal flora and beneficial.
  • Examples of virulence factors are endotoxins, enterotoxins
  • Used as a model organism in microbiology.
• Salmonella:
  • Facultative anaerobe, heterotrophic, Gram-negative rods
  • Found in animal guts; some strains are pathogenic to humans (e.g., foodborne infections, typhoid).
  • Includes S. enterica and S. typhi.
  • Virulence factors include endotoxin, enterotoxin, and cytotoxin.
• Vibrio cholerae:
  • Facultative anaerobic, heterotrophic, Gram-negative curved rods
  • Mostly found in marine environments; causes cholera (severe diarrhea)
  • A key virulence factor is cholera toxin.
• Pseudomonas aeruginosa:
  • Aerobic, heterotrophic, Gram-negative rods
  • Primarily opportunistic infections, common in nosocomial settings (e.g. burn injuries).
  • Innate antibiotic resistance, haemolysin, and proteases are key virulence factors.
• Neisseria species:
  • Aerobic, heterotrophic, Gram-negative diplococci
  • Exists as a carrier in the human throat and nose
  • Pathogenic species include N. gonorrhoeae (causes gonorrhea) and N. meningitidis (causes meningitis).
  • Virulence factors include capsule for immune evasion and fimbriae for tissue adhesion.
• Rickettsia species:
  • Aerobic, heterotrophic, Gram-negative coccobacilli
  • Obligate intracellular parasites of arthropods.
  • Cannot be cultured in vitro.
  • Vectors are arthropods (e.g., fleas, lice, ticks).
  • An example is R. prowazekii, responsible for epidemic typhus.
• Bacteroides species:
  • Obligate anaerobe, heterotrophic, Gram-negative rods
  • Significant component of human normal flora.
  • Plays a crucial role in digestion.
  • Opportunistic pathogens; example includes B. fragilis which cause infection in the gut (e.g., abscesses).
• Treponema pallidum:
  • Anaerobic, heterotrophic, Gram-negative spirochetes.
  • Obligate parasite; causes syphilis.
  • Classified by its morphology and motility.

Additional Bacterial Aspects

• Phylogenetic trees based on 16S rRNA alignments are important for classifying bacterias.
• Identifying specific bacteria using selective and differential microbiological media (e.g. XLD) is introduced.
• Pathogenesis, clinical outcomes and relevant public health are also discussed.