Last updated on: 06.04.2021

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Bacteria are microscopic, single-celled organisms. They are among the earliest living things on Earth and live in the soil, seawater and deep inside the Earth's crust. Many bacteria live on and in the bodies of humans and animals - on the skin, in the respiratory tract, in the mouth, in the digestive tract, in the reproductive organs and in the urinary tract - without being pathogenic. Hundreds of different types of bacteria are normally found in the body, as well as several trillion individual bacteria. Most bacteria are found in the following places:

  • On the skin and on the teeth
  • In the area between teeth and gums
  • In the mucous membranes of the throat, intestines and vagina

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List of important human pathogenic bacteria

Acinobacter (see below Moraxacellaceae)


Bacillus (bacilli)




Bordetella (Bordetella pertussis and Bordetella parapertussis pathogens of whooping cough, Bordetella bronchiseptica)

Borrelia (causative agent of Lyme disease)

Brucella (see below Brucellosis)


Candidatus Neoehrlichia mikurensis

Campylobacter (C.coli, C.jenuni:causative agent of enteritis)

Chlamydiaceae (C.trachomatis; C.pneumoniae; C.psittaci - causative agent of trachoma, pneumonia, lymphogranuloma venereum)



Corynebacteria (Corynebacterium )

Coxiella (Coxiella burnetii pathogen of Q fever)

Cutibacterium (older name Propionibacteria)

Enterococci (facultative causative agents of urinary tract infections, sepsis and endocarditis)


Escherichia coli (facultative causes of various infections such as urinary tract infections, peritonitis or meningitis)

Enterohemorrhagic Escherichia coli (facultative cause of gastroenteritis, enterohemorrhagic colitis, complicating: hemolytic uremic syndrome (HUS))

Enteroaggregative Escherichia coli (facultative causative agent of diarrhoea and travellers' diarrhoea)

Francisella (causative agent of tularemia)


Gardnerella (main pathogen of bacterial vaginosis)

Haemophilus influenzae (causative agent of respiratory tract infections - sinusitis, bronchitis)

Helicobacter (Helicobacter pylori)

Klebsiella(Klebsiella pneumoniae, Klebsiella granulomatosis, Klebsiella oxytoca)



Leptospira (see below Spirochetes)

Listeria (Listeria)

Coagulase-negative Staphylococci


Mycobacterium (Mycobacteria/NTM=non-tuberculous mycobacteria; causative agents of tuberculosis, Buruli ulcers, leprosy)

Mycoplasmataceae(Mycoplasma and Ureaplasma; causative agents of pneumonia, non-gonorrheal genital infections)

Neisseriaceae(causative agents of meningitis, Waterhouse-Friderichsen syndrome, gonorrhoea)

Nocardia (morphological similarity to the actinomycetes; causative agent of nocardioses)



Proteus (catheter-associated urinary tract infections, diabetic wound infections, pneumonias)

Rickettsia (causative agent of rickettsial infections)


Serratia marcescens (facultative causative agent of urinary tract infections, sepsis, pneumonia, endocarditis, meningitis, osteomyelitis, especially in immunocompromised persons)

Shigella(shigella dysentery)



  • Staphylococcus aureus (invasive infections such as boils, carbuncles, bullous impetigo, wound infection, sinusitis, otitis media, sepsis, septic shock, endocarditis (after heart valve replacement), osteomyelitis, bacterial joint inflammation, pneumonia)
  • Staphylococcus epidermidis
  • Staphylococcus saprophyticus as opportunists in the normal flora of skin and mucous membranes
  • Staphylococcus intermedius (as commensal especially of the skin of the anal region, facultative pathogen in pyoderma, otitis externa, pyometra, wound infection)


  • Streptococcus pneumoniae (causative agent of pneumonia, sinusitis, otitis media, mastoiditis, bacterial endocarditis)
  • = pneumococci

  • Streptococcus pyogenes (causative agent of impetigo, erysipelas, phlegmon, sinusitis, otitis media, tonsillitis; scarlet fever; sepsis, septic shock, necrotizing fasciitis)

  • Oral streptococci (viridans group) Causes bacterial endocarditis - Endocarditis lenta

Ureaplasma (see below Mycoplasmataceae)

Yersinia(Y.enterocolitica; Y.pseudotuberculosis)


General information
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Staining: Bacteria can also be classified according to the colour they take on when certain chemical substances (staining agents) are added to them. Gram staining is a commonly used staining method. Some bacteria turn blue. These are called gram positive. Others turn red. These are called gram-negative. Gram-positive and gram-negative bacteria stain in different ways because their cell walls are different. These bacteria also cause different types of infections, and there are different types of antibiotics that are effective against them.

Forms: All bacteria can be assigned one of three basic forms. There are:

  • spherical (cocci)
  • rod-shaped (bacilli)
  • and spiral or helical bacteria (spirochetes).

Oxygen demand: Bacteria are also classified according to whether or not they require oxygen to live and thrive. Bacteria that require oxygen are called aerobes. Bacteria that have difficulty living or reproducing when they come in contact with oxygen are called anaerobes. Some bacteria, called facultative bacteria, can live and thrive with or without oxygen.

Hereditary factors: Special tests can be used to detect differences in the genetic makeup (genotype) of bacteria.

Microbiomes: The natural colonization of surfaces is called resident flora or the microbiome. The microbiomes of the skin, the oral cavity of the genitalia and the intestine differ quite considerably in their composition.

Human pathogenic bacteria: Only a few species of bacteria are human pathogenic. However, several bacteria can be used as biological weapons. These include anthrax, plague, and tularemia.

Bacterial infection: Bacterial infection of humans refers to the active or passive invasion of the human host by bacteria, their multiplication, and the response usually followed by the organism's defense after the incubation period. Bacteria can be ingested from the environment, for example through respiration or food. In humans, the mucous membranes of the respiratory tract and the digestive tract are particularly frequently affected. The organ with the highest incidence of bacterial infections is the skin itself, because of its capacity as a border organ.

Routes of infection: Bacteria can reach the host through various routes of infection, such as contamination of food and water, and in the case of aerogenic transmission as droplet infection through the respiratory air. In a weakened immune situation, the resident "normal flora" can lead to opportunistic disease.

Adhesion: In intact barriers (skin, mucous membranes), penetration into the host organism takes place actively via species-specific adhesion of the bacteria by means of so-called adhesins to receptors of their target structures. In the case of damaged barriers, invasion occurs passively.

Last updated on: 06.04.2021