Bacteria overview

Last updated: 11.06.2019

Summary

The nomenclature of bacteria is complex; human pathogenic bacteria are classified according to their form (cocci, bacilli, or coccobacilli) and Gram staining properties (gram‑positive, gram‑negative, and atypica). Further classification is based on metabolic activity (aerobic or anaerobic) and virulence factors (e.g., formation of coagulase or enterotoxins), among other traits. The most important human pathogenic bacteria are discussed in this learning card. An overview of the basics of bacteriology – including diagnosis and genetics – may be found in the learning card on general bacteriology.

Overview

The following tables provide an overview of the nomenclature of important human pathogenic bacteria, according to their form and Gram staining properties.

References:[1][2]

Gram-positive cocci

Staphylococcus

Important species Reservoir Characteristics and important virulence factors

Diseases

Antibiotic of choice
Staphylococcus aureus
  • Skin
  • Detectable in the nostrils in ∼ 30% of the healthy population
Staphylococcus epidermidis
  • Natural skin flora

Staphylococcus saprophyticus

  • Natural (mucosal) skin flora

Streptococcus and enterococcus

Genus Important species Reservoir Characteristics and important virulence factors Diseases Antibiotic of choice
Streptococcus

Streptococcus pneumoniae (pneumococci)

  • Nasopharynx
Viridans streptococci

Streptococcus gallolyticus

  • Isolated in stool in up to 10% of the population (higher rates in patients with colorectal cancer)
Streptococcus pyogenes (Group A Streptococcus)
  • Humans are the sole host; nasopharynx
Streptococcus agalactiae (Group B Streptococcus)
  • Colonizes the human gastrointestinal and genital tracts
Peptostreptococcus Numerous peptostreptococcus species (e.g., P. anaerobius, P. magnus)
  • Part of the natural flora of mucocutaneous surfaces
Enterococcus

E. faecium and E. faecalis

“B for baby”: Group B streptococci primarily affect neonates.

References:[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]

Gram-negative cocci

Genus Important species Reservoir Characteristics and important virulence factors Diseases Antibiotic of choice
Neisseria
  • Humans are the only hosts; nasopharynx
  • Humans are the only hosts
Moraxella
  • Humans are the only hosts; nasopharynx

References:[3][22][23][24][25][26][27][28][29]

Gram-positive bacilli

Genus

Important species

Reservoir

Characteristics and important virulence factors

Diseases

Antibiotic of choice

Clostridia (spore‑forming bacteria)

  • Natural gut flora of ∼ 5% of all adults
  • Part of the natural skin and gastrointestinal flora
  • Found in soil, worldwide
  • Found in soil, intestinal tracts of birds and fish, and agricultural products (e.g., vegetables)
  • Administration of a botulism antitoxin is the most important treatment measure. Antibiotics are of secondary importance.
Listeria
  • Most commonly found in unpasteurized milk products
Corynebacterium
  • Humans are the only hosts
Bacillus
  • Found in soil and mammals
  • Proliferate quickly in contaminated food; rice that is kept warm and/or reheated is a particularly good breeding ground.
  • There is no specific treatment against the enterotoxin available.
  • Antibiotic treatment is not indicated.

References:[30][31][3][32][33][34][35][36][37][38]

Gram-negative coccobacilli

Genus Important species Reservoir

Characteristics and important virulence factors

Diseases Antibiotic of choice
Haemophilus
  • Haemophilus influenzae
  • Colonization of the nasopharynx is relatively common in young children.
  • Humans are the only hosts
Brucella
  • Goats and sheep serve as hosts
  • Facultative intracellular
  • Cattle are the only hosts
Bordetella
  • Humans are the only hosts; particularly adults and adolescents

References:[39][40][41][42][43][44]

Gram-negative bacilli

Enterobacteriaceae

Genus Species/serotype Reservoir Characteristics and important virulence factors Diseases Antibiotic of choice
Yersinia
  • Primarily rodents
  • Contaminated pork and milk products
Shigella
  • Humans are the only host
Salmonella
  • Salmonella enterica
  • Humans and animals serve as hosts
  • Humans are the only hosts
Klebsiella
  • Klebsiella pneumoniae
  • Human genital tract (endemic in tropical and subtropical developing countries)

Escherichia coli (E. coli)

  • Part of the natural gut flora
  • EAEC (enteroaggregative E. coli)
  • UPEC (uropathogenic E. coli)
Proteus
  • Proteus vulgaris
  • Proteus mirabilis

Further gram‑negative bacilli

Genus Important species Reservoir Characteristics and important virulence factors Diseases Antibiotic of choice
Helicobacter
  • Humans are the predominant hosts

Legionella

  • Natural aquatic habitats
  • Facultative intracellular
  • Aerobic
  • Waterborne bacteria; often symbiotic with amebae

Campylobacter

  • Component of the gut flora in birds. Transmission often occurs via contaminated poultry.
Pseudomonas
  • Pseudomonas aeruginosa
  • Common in the environment; grows in water and humid conditions, e.g., hot tubs, contaminated contact lens solution
Burkholderia
  • Aquatic environments
  • Aerobic
Vibrio
  • Humans and contaminated water
Bacteroides
  • Numerous
  • Account for 90% of all fecal flora
  • Responsible for the typical odor of stool
Bartonella
  • Cats are asymptomatic carriers
  • Bartonella quintana
  • Humans (the louse is usually the vector)
  • Trench fever

References:[45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64]

Branching filamentous bacteria

Genus Species Reservoir Characteristics and important virulence factors Diseases Antibiotic of choice
Actinomyces
  • Natural bacteria of the oral cavity and the intestine
High-dose penicillin G; alternatively tetracycline or erythromycin
Nocardia
  • Ubiquitous in soil worldwide

References:[65][66][67][68][69]

Atypical gram‑staining bacteria

Mycobacterium and mycoplasmataceae

Important species Reservoir Characteristics and important virulence factors Diseases Antibiotic of choice
Mycobacterium
  • Humans
  • Humans and armadillos
  • Ubiquitous
Mycoplasmataceae (Mycoplasma)
  • Mycoplasma pneumoniae
  • Humans are the only hosts
  • Ureaplasma urealyticum
  • May be part of the normal flora
  • Urocystitis

Spirochetes

Genus Important species Reservoir Characteristics and important virulence factors Diseases Antibiotic of choice
Treponema
  • Humans
  • Treponema vincentii
Borrelia
Leptospira
  • Animals (e.g., rats, cattle, sheep, and goats) are the natural hosts; incidental transmission to humans through direct contact to animals, or indirectly through contact with contaminated water or soil

References:[2][70][71][72][73][74][75]

Obligate intracellular bacteria

Genus Species Reservoir Characteristics and important virulence factors Diseases Antibiotic of choice

Chlamydiaceae (Chlamydia)

  • Humans are the only hosts
  • Gram-negative
  • Culture: difficult; requires tissue medium
  • Various pathogenic serotypes with different organ associations (see table in chlamydia infections)
  • Birds are the typical hosts
  • Microscopy: visible as inclusion bodies
  • Culture: difficult and dangerous (highly infectious)
  • Humans are the only known hosts
  • Microscopy: visible as inclusion bodies
  • Culture: difficult; requires tissue culture
Rickettsia
  • Weakly gram-negative bacteria
Coxiella
  • Cattle, sheep, and goats are the typical hosts
  • Gram-negative bacteria
  • Ability to survive in very harsh environments

References:[76][77][78][79][80][81]

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