Nocardia: Characteristics, Morphology, Culture, Pathogenesis

Nocardia is a genus of acid-alcohol resistant bacteria that are present in a wide variety of environments including soil, dust, water, and even decaying organic matter.

This genus was described in 1888 by Edmon Nocard, a French microbiologist and veterinarian, from an infection in a bovine. These types of bacteria are unknown in many respects, so characteristics and properties are constantly being discovered.

Culture of Nocardia asteoides. Source: Public Domain Files

This genus includes a total of approximately 30 species, of which about 11 can cause pathologies in humans. These include  Nocardia asteroides, Nocardia brasiliensis, Nocardia otitidiscaviarum, Nocardia transvalencis, Nocardia brevicatena and Nocardia veterana , among others.

Infections caused by bacteria of the genus Nocardia are generally opportunistic. This means that they affect individuals whose immune systems are weakened, such as those who are infected with HIV.

In a generic way, infections caused by some species of this genus are known as nocardiosis. Being a bacterial infection, the first line treatment is antibiotics.

However, these bacteria have been shown not to follow the same pattern in terms of resistance and susceptibility to antibiotics. Therefore, the therapy to be followed must be individualized according to the characteristics of the infectious strain.

Article index

  • one


  • two


  • 3

    General characteristics

  • 4


  • 5


    • 5.1

      Mechanism of infection

  • 6



The taxonomic classification of the genus Nocardia is as follows:

Domain: bacteria

Phylum: Actinobacteria

Order: Actinomycetales

Suborder: Corynebacterineae

Family: Nocardiaceae

Genus: Nocardia


Bacteria that belong to the genus Nocardia can initially present a configuration known as a “bead necklace.” As they grow, they take the form of elongated bacilli, with the appearance of rudimentary filamentous hyphae that branch. In the microscope they give the appearance of elongated threads. They have a diameter of approximately 0.5 – 1.2 microns.

In the cultures colonies of different color and appearance are appreciated. They can be brown, pink, tan, orange, white, or gray. The appearance ranges from chalky to velvety. The texture also varies, being able to appreciate smooth, irregular or granular colonies.

Its cell wall is characterized by the presence of Mesodiaminopimelic Acid (DAP) and by the carbohydrates galactose and arabinose. Likewise, they have mycolic acids as components of the membrane, which gives them the property of being acid-alcohol resistant.

General characteristics

They are strict aerobes

Bacteria belonging to the genus Nocardia must be in environments with a wide availability of oxygen, since they require this chemical element to carry out their different metabolic processes.

Synthesize the enzyme catalase

These bacteria synthesize the enzyme catalase. This allows them to decompose hydrogen peroxide (H 2 O 2 ) into its constituent elements: water (H 2 O) and oxygen (O 2 ). When this process occurs, it is possible to appreciate the presence of bubbles, an obvious sign of the release of oxygen.

They are slow growing

This genus of bacteria takes more than seven days to generate colonies when grown artificially in the laboratory. Cultures have been observed in which even 14 days have passed for colonies to become apparent.

They are alcohol – acid resistant

This implies that they are resistant to the typical discoloration process, intrinsic to traditional staining methods. This is due to the mycolic acids found in its cell wall.

The method by which these bacteria are stained is that of Kinyoun. This method uses a dye (carbol fuchsin) that has a high concentration of phenol, which stimulates the permeability of the dye through the mycolic acid layer. In this method, methylene blue is used as a contrast method.

They are mesophilic bacteria

Mesophilic organisms are those that, by definition, develop optimally at temperatures ranging from 15 to 35 ° C.

Taking into account that the bacteria belonging to the genus Nocardia have an optimal growth temperature of between 25 and 37 ° C, then it can be said that they are mesophilic.

Synthesize the enzyme urease

These bacteria synthesize the enzyme urease, which is responsible for catalyzing the chemical reaction in which urea is hydrolyzed to ammonia and carbon dioxide. This occurs according to the following reaction:

(NH 2 ) 2 CO + H 2 O ———————— CO 2 + 2NH 3

This is an important characteristic that allows to differentiate and discriminate bacteria in order to identify them at an experimental level.

Synthesizes the enzyme oxidase

Bacteria of the genus Nocardia synthesize the enzyme oxidase. This enzyme catalyzes the oxide-reduction reaction, using oxygen as an electron acceptor.


Species of the genus Nocardia are ubiquitous, that is, they are found throughout the globe. They are saprophytes, which means that they develop in decomposing organic matter, feeding on it.

They are also an important part of the bacterial flora of the soil and are close to water. 
Other sites where it has been identified in dust, sewage, air, insects, and some decaying vegetables.

They are chemoorganotrophic

Bacteria belonging to this genus are chemoorganotrophic. This indicates that they have a metabolism based on oxide reduction reactions, in order to obtain energy.


Bacteria of the genus Nocardia grow in all culture media, although they have a predilection for blood agar and Sabouraud agar. It should be kept within a temperature range of 25-37 ° C.

Culture of Nocardia farcinica. Source:

The culture grows slowly, reminding yourself that it may take up to 14 days for colonies to develop.


Among the different species that make up the genus Nocardia , there are some that are considered pathogenic for humans. These include  Nocardia asteroides, Nocardia brasiliensis, and Nocardia otitidiscaviarum.

Nocardia asteroides and Nocardia otitidiscaviarum are responsible for various pulmonary and disseminated infections. While Nocardia brasiliensis causes skin infections, especially in tropical places.

The most common form of transmission is through inhalation.

Mechanism of infection

In the species that cause lung disease, the bacterial particles are inhaled and reach the lung alveoli. Here they begin to proliferate in their branched hyphal form.

The mycolic acids found in the cell wall help to inhibit phagosome-lysosome activity, so that the bacterium manages to escape lysis and continues to reproduce. Through the bloodstream, the bacteria can even reach the central nervous system.

On the other hand, when the bacteria are skin inoculated, they begin to proliferate, generating abscesses and granulomas.


  1. Candel, F. González, J., Matesanz, M., Cinza, R., Cías, R., Candel, I., Pontes, J., Roca, V. and Picazo J. (2005, October). Bacteremic infection due to Nocardia otitidiscaviarum : a case study. Annals of Internal Medicine 22 (10)
  2. Nocardia asteroides . Obtained from:
  3. Nocardia ssp . Retrieved from:
  4. Nocardia . Retrieved from:
  5. Retrieved from:
  6. General properties of actinomycetes. Retrieved from:
  7. Wilson, J. (2012). Nocardiosis: Updates and Clinical Overview. Mayo Clinic Proceedings. 87 (4). 403 – 407

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