Actinomyces: Introduction, Classification, Morphology, Pathogenecity, Lab

Actinomyces: Introduction, Classification, Morphology, Pathogenecity, Lab Diagnosis, Treatment and Keynotes

Introduction of Actinomyces

The above picture is showing Actinomyces in a Gram-stained smear of pus in a patient with odontogenic cervicofacial actinomycosis. It is also called a Ray-like fungus. Wolff and Israel described this organism in 1891.

Classification of Actinomyces

Domain: Bacteria
Phylum: Actinobacteria
Class: Actinobacteria
Order: Actinomycetales
Family: Actinomycetaceae
Genus: Actinomyces
Species: A. israeli

A. bovis

A. eriksoni

A. naeslundi

B. odontolyticus

A. viscisus

The main species of medical importance is Actinomyces israeli.

Morphology of Actinomyces

They are Gram-positive, nonmotile, non-sporing, non-acid -fast organisms, often growing in the form of primary mycelium which breaks up into coccid bodies and rods of uneven length. They grow preferably under anaerobic conditions in the presence ofCO₂ and produce acid from varieties of carbohydrates. Cell wall contains alanine, glutamic acid, lysine, ornithine, and in some species apart acid. Catalase negative except A. viscosus and they are also part of oral flora. They are also normal flora of the mouth, female genital tract, and are also found in the soil.

Pathogenicity of Actinomyces

Actinomyces cause a disease called Actinomycosis. The common types of actinomycosis are pulmonary, cervicofacial, and central nervous system actinomycosis. The mechanism of immune response in this disease remains unclear, but some factors, by altering this response, probably promote the disease. Risk factors of actinomycosis are Human immunodeficiency virus (HIV) infection, steroid use, infliximab treatment, lung, and renal transplantation, and acute leukemia during chemotherapy. It is a chronic granulomatous infection in which pus containing granules (sulfur granules) are discharged through the sinus (abnormal channel permitting the escape of pus), which open on the surface of the skin. The jaw is the usual site of infection often following tooth extraction

Laboratory Diagnosis of Actinomycosis

Specimens: It depends on the site of infection and the common specimens are Pus, sputum, Infected tissues (biopsy).

Granules Treatment before staining

Take tube. Wash the granules preferably with thioglycollate broth ( Distilled water or normal saline can also be used) and display them in a Petri dish to study the texture of granules.
Crush the granules by placing them between two- glass slides.
The granules are usually yellow (The color of sulfur) but can also be white or brown.
These are firm, round, large, and 0.1–1 mm in diameter.
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Microscopy

Gram’s stain: Gram-positive, thin branching, fragmented, coccobacillus form, and no spore. Clubs are Gram-negative.

Ziehl -Neelsen Stain

Braches are not acid fast.
Clubs are acid-fast.
Clubs are lipoid materials of host tissue origin, deposited around the bacteria filaments as a part of a defense attempt.

Culture

Blood agar/ chocolate agar

Thioglycollate broth: Incubate at 37°C for 2 weeks both under aerobic and anaerobic conditions.

Colony characteristic: On blood agar after 5-7days incubation, colonies are small, creamy, gray, white with rough nodular surface- Spidery colonies. Glistening and adhere to the medium while in thioglycollate broth colonies are breadcrumb below the surface of the medium.

Biochemical Reaction

A.israeli is catalase-negative, urea hydrolysis test negative, nitrate reduction test negative. It hydrolysis Aesculin, and ferments glucose, lactose, mannitol.

MALDI-TOF for Quicker Detection: The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) allows accurate identification at the genus level, but species identification remains uncertain.

Serology: Serological assays have been developed but need to be improved before they become clinically useful.

Histopathological Examination: H/E Staining

Molecular Test: Comparative 16S ribosomal RNA (rRNA) gene sequencing is important for species identification since molecular techniques such as 16S rRNA sequencing serve as the reference for identification.

Others

X-ray
CT scan

Treatment

Penicillin G or amoxicillin are considered drugs of choice for the treatment of actinomycosis. Other active antimicrobial agents are piperacillin-tazobactam, imipenem, and meropenem while oxacillin, cloxacillin, and cephalexin, a first-generation cephalosporin, fluoroquinolones (ciprofloxacin and moxifloxacin) inactive. As Actinomyces species do not produce beta-lactamases, it is not useful to combine amoxicillin with beta-lactam inhibitors ( clanuvic acid, sulbactam) such as clavulanic acid, except if co-pathogens such as Enterobacteriaceae are involved in the disease.

Keynotes on Actinomyces

Except for Actinomyces meyeri, which is small and nonbranching, all the other species are branching filamentous rods.
Actinomyces israelii forms a “molar tooth” colony on agar and grows as clumps within the broth, whereas A. odontolyticus forms rust-brown or red-colored colonies. Actinomyces are indole-negative bacteria.
Gram staining of pus and pathology of infected tissue is of great interest for the diagnosis of actinomycosis, as it is usually more sensitive than culture.
Yellowish sulfur granules are constituted by the conglomeration of bacteria trapped in biofilm.
Gram staining can additionally show Gram-positive filamentous branching bacteria at the periphery of the granule that is highly suggestive of actinomycosis.
Immunofluorescence techniques have poor sensitivity but are highly specific in the diagnosis.
The common phenotypic tests for identification are urease, catalase, fermentation of sugars, etc.
The swab specimen must be avoided.
In the case of odontogenic cervicofacial actinomycosis, prescription of oral antimicrobials is common before surgery since leading frequently to false-negative results of the cultures.

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