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Bacteria overview

Last updated: November 5, 2020

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 article. An overview of the basics of bacteriology – including diagnosis and genetics – may be found in the article 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 [3]

All staphylococci are catalase-positive.

Pathogen Reservoir Bacterial culture Virulence factors and resistances

Diseases

Antibiotic of choice
Coagulase-positive staphylococcus (CoPS)
Staphylococcus aureus
  • Skin
  • Colonizes nares (most commonly ), ears, pharynx, axilla, hands, groin, and perineum [4]
Coagulase-negative staphylococci (CoNS)
Staphylococcus epidermidis
  • Natural skin flora
  • Urease producer
  • Adherent biofilm production
Staphylococcus saprophyticus
  • Natural flora of the female genital tract and perineum
  • Grape‑like cluster arrangement
  • Novobiocin-resistant
  • Urease producer

NO StRESs: NOvobiocin sensitivity patterns, Saprophyticus (Resistant), Epidermidis (Sensitive)

Streptococcus [3]

Pathogen Reservoir Bacterial culture Virulence factors and resistances Diseases Antibiotic of choice
Alpha-hemolysis (partial hemolysis)

Streptococcus pneumoniae (Pneumococci) [6]

  • Nasopharynx

Viridans streptococci (e.g., S. mutans, S. mitis, and S. sanguinis)

  • Oropharynx
Beta-hemolysis (complete hemolysis)
Streptococcus pyogenes (Group A Streptococcus) [7][8]
  • Nasopharynx
  • Humans are the sole host
Streptococcus agalactiae (Group B Streptococcus)
  • Gastrointestinal and genital tract
  • Capsule

Streptococcus gallolyticus [11]

Streptococcus anginosus [4][13]
  • Oropharynx, gastrointestinal, and genitourinary tract

B is for Babies: group B streptococci primarily affect neonates.

PaSsVORd: Pnemunoniae is Sensitive and Viridans is Optochin-Resistant.

BBRAS: Bacitracin sensitivity pattern of group B streptococci (Resistant) and for group A streptococci (Sensitive).

Sanguinis vessels lead to your heart: S. sanguinis can cause endocarditis.

Other gram-positive cocci [3]

Peptostreptococci and enterococci are catalase-negative.

Pathogen Reservoir Bacterial culture Diseases Antibiotic of choice
Peptostreptococcus (e.g., P. anaerobius, P. magnus)
  • Mucocutaneous surfaces
Enterococcus (e.g., E. faecium and E. faecalis) [14]

Bovis in the blood, cancer in the gut: S. bovis is associated with colon cancer.

Entero (intestine), faecalis (feces), strepto (twisted), and coccus (berry): the etymology of enterocci describes its characteristics.

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:[15][16][17][18][19][20][21][22][23][24]

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:[17][25][26][27][28][29][30][31][32]

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:[33][34][35][36][37][38]

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:[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58]

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:[59][60][61][62][63]

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
  • Ticks or mammals
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][64][65][66][67][68][69]

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
  • Arthropods (transmission to humans via ticks or mites)
  • Weakly gram-negative bacteria
Coxiella
  • Cattle, sheep, and goats are the typical hosts
  • Gram-negative bacteria
  • Ability to survive in very harsh environments

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

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