Actinomycosis

Definition/organism

Chronic suppurative disease, Actinomyces israelii — anaerobic BACTERIUM (not fungus, despite name/grouping with mycoses). Worldwide. Commensal in oral cavity/GIT/vagina. Infection always ENDOGENOUS (not person-to-person) — needs mucocutaneous breach or predisposing disease.

4 clinical types (by site)

Cervicofacial (60%, commonest, best prognosis) — via tonsils/carious teeth/periodontal disease/extraction trauma → “lumpy jaw” → abscess/sinuses → sulphur granules in pus
Thoracic — aspiration or extension from abdomen/liver → pneumonia-like → spreads to whole lung/pleura/ribs/vertebrae
Abdominal — appendix/caecum/liver; swallowed oral organisms or thoracic extension
Pelvic — IUCD complication

Morphology

Granuloma WITH CENTRAL SUPPURATION (abscess centrally + chronic inflammatory cells/giant cells/fibroblasts peripherally).

Sulphur granule (centre of abscess): radiating filaments (“ray fungus” — source of fungal misnomer) + hyaline eosinophilic CLUB-SHAPED tips (= host Ig, Splendore-Hoeppli phenomenon).

Stains: Gram-positive, NON-acid-fast filaments; GMS+ (fungal stain, another source of confusion).

Clinical relevance

Bacterial not fungal — despite name/ray-fungus/GMS+ — changes culture (anaerobic bacterial) + Rx (penicillin, not antifungal). Always endogenous → suspect with specific predisposing event (dental work=cervicofacial, aspiration=thoracic, appendix/liver path=abdominal, IUCD=pelvic). Club-shaped Ig-coated periphery = Splendore-Hoeppli phenomenon, seen around other slow-growing organisms too — don’t read it as fungal in isolation.

Definition and Organism

Actinomycosis is a chronic suppurative disease caused by the anaerobic, filamentous bacterium Actinomyces israelii. Despite the name suggesting a fungal disease — and despite the condition being conventionally taught alongside the mycoses — the causative organism is a true bacterium, not a fungus. It is worldwide in distribution and behaves as a commensal in the oral cavity, alimentary tract and vagina under normal conditions; infection is always endogenous, arising from the patient’s own commensal flora rather than person-to-person spread, when the organism gains access to normally sterile tissue through a break in mucocutaneous continuity or in the setting of some predisposing underlying disease.

Clinical Types

Classified by anatomic site of the lesion into four forms:

Cervicofacial actinomycosis — the commonest form (~60% of cases) and carries the best prognosis. Infection enters via tonsils, carious teeth, periodontal disease, or trauma following tooth extraction. A firm swelling develops in the lower jaw — the classic “lumpy jaw” — which over time breaks down into abscesses and draining sinuses discharging pus containing characteristic tiny yellow sulphur granules. Infection may extend into adjoining soft tissue and erode bone.
Thoracic actinomycosis — pulmonary infection from aspiration of oral organisms, or from direct extension of abdominal/hepatic disease; initially resembles pneumonia but progressively spreads to involve the whole lung, pleura, ribs and vertebrae.
Abdominal actinomycosis — commonly affects appendix, caecum and liver, resulting from swallowed oral organisms or extension from thoracic disease.
Pelvic actinomycosis — a recognised complication of intrauterine contraceptive devices (IUCDs).

Morphology

Irrespective of anatomic site, the microscopic appearance is consistent:

The inflammatory reaction is a granuloma with central suppuration — an abscess forms centrally, with chronic inflammatory cells, giant cells and fibroblasts at the periphery.
The centre of each abscess contains the bacterial colony, the “sulphur granule” — grossly a tiny yellow granule, microscopically characterised by radiating filaments (historically termed “ray fungus” for this radiating appearance, contributing to the fungal misnomer) with hyaline, eosinophilic, club-shaped peripheral ends that represent host-secreted immunoglobulin coating the colony.
Bacterial staining shows Gram-positive, non-acid-fast filaments, staining positively with Gomori methenamine silver (GMS) — a stain more usually associated with fungal identification, another source of diagnostic confusion with true mycoses.

Clinical Relevance

Actinomycosis is the clearest teaching example of a disease that is conventionally grouped with fungal infections — by name, by “ray fungus” morphology, and by GMS positivity — while actually being bacterial; correctly identifying it as such changes both the expected culture technique (anaerobic bacterial culture, not fungal) and antimicrobial choice (penicillin remains first-line, not antifungal therapy).
Because infection is always endogenous from commensal flora rather than acquired exogenously, actinomycosis should be suspected specifically in the context of a recognised predisposing local event — dental extraction/periodontal disease for cervicofacial disease, aspiration for thoracic disease, appendiceal or hepatic pathology for abdominal disease, and IUCD use for pelvic disease — rather than being treated as a random opportunistic infection.
The sulphur granule’s club-shaped, immunoglobulin-coated periphery (the Splendore–Hoeppli phenomenon) is a general host-response pattern seen around other slow-growing organisms as well, so recognising it on histology should prompt a search for the causative colony at the centre rather than being read as a fungal structure in isolation.

Diagram 1: Sulphur Granule — Microscopic Structure

Draw a low-power view of an abscess cavity with a single central colony (the sulphur granule) and its surrounding tissue reaction.

Centre: a rounded, basophilic mass representing the bacterial colony, with fine radiating filaments extending outward from it toward the periphery (the “ray fungus” appearance).
Periphery of the colony: club-shaped, hyaline, eosinophilic projections at the tips of the radiating filaments — this represents host immunoglobulin deposited around the colony (the Splendore–Hoeppli phenomenon).
Surrounding zone: a rim of neutrophils forming the abscess, outside which lies a zone of chronic inflammatory cells, giant cells and fibroblasts.

Labels required

The radiating filamentous structure at the centre must be labelled “bacterial colony (Gram-positive, non-acid-fast filaments),” not “fungal colony,” despite the ray-fungus appearance.
The club-shaped eosinophilic tips must be labelled specifically as host-derived (immunoglobulin), since mistaking these for a structural part of the organism itself is a common error.
The three-zone architecture (central colony → neutrophilic abscess → peripheral chronic inflammation/fibrosis) should be shown as concentric zones, mirroring the general granuloma-with-central-suppuration pattern that defines actinomycosis at any anatomic site.

Errors commonly made

Labelling the sulphur granule as a fungal structure because of its “ray fungus” name and its GMS positivity, rather than correctly identifying it as a bacterial colony.
Omitting the club-shaped peripheral projections, which are the single most recognisable feature of a sulphur granule in a spot-diagnosis image.