Infectious Disease Pathology
Case 1 -
Mycetoma due to filamentous bacteria morphologically consistent with Streptomyces somaliensis.
Armed Forces Institute of Pathology
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The patient was a 15 year-old Samoan girl who moved to Hawaii at age 13. At that time she had a
febrile illness with cough and bloody sputum. She was found to have trichuriasis and hookworm infection.
She was treated with mebendazole, and her symptoms slowly resolved. She continued to have intermittent
cough, productive of a small amount of sputum, but was otherwise asymptomatic. She moved from Hawaii to
California and then to Georgia at age 14. Nine months prior to the most recent admission she was seen at
a local university hospital with bilateral pneumonia and a right pleural effusion. Her temperature was
101.7°F, and white count was 16,300. After 2 weeks of oral penicillin, her cough and pleural effusion
resolved, but general malaise continued. Six weeks later, she presented again with fever, chills and
cough. Chest x-ray showed resolving pneumonia with residual scarring of the bases bilaterally. Two
weeks prior to the most recent admission, she presented again with mild cough and no chest pain. Chest
x-ray showed bilateral fluffy infiltrates consistent with severe chronic lung disease and bronchiectasis
with superimposed pneumonia. Four days prior to admission, she developed left chest pain which became
worse on inspiration. On admission her temperature was 104°F. Her white count was 13,300 (86 segs, 2
bands, 9 lymphs, 3 monocytes), hematocrit 37 and hemoglobin was 12.4. She was anergic. Subsequently,
her white count increased to 49,100 (84 segs, 7 bands, 4 lymphs, 4 monos). Chest x-ray showed
opacification of the left lung field. She was treated with oxacillin because of a single blood culture
that was positive for coagulase-positive Staphylococcus species.
Bronchoscopy showed that the bronchial mucosa was inflamed and erythematous throughout her bronchial tree
bilaterally, with pus was draining from both lungs. Left thoracotomy was performed for decortication and
drainage of pleural effusion. Approximately 800cc of thick gelatinous yellow-green fluid with fragments
of tissue and pleura were removed. The lung was not biopsied because of severe bleeding. Representative
sections of the fragments of tissue and pleura removed during thoracentesis were submitted for histology.
Case 1 - Figure 1 -
Fragment of pleural tissue removed during left thoracotomy for decortication and drainage of pleural effusion showing necrosis and inflammation. (Hematoxylin and eosin, X10.)
Case 1 - Figure 2 -
Area of necrosis in a fragment of pleural tissue removed during thoracotomy. (Brown-Hopps tissue gram stain, X100.)
Diagnosis: Mycetoma due to filamentous bacteria morphologically consistent with Streptomyces somaliensis.
Sections stained with Brown Hopps tissue gram stain show necrotic inflamed tissue containing long
slender branching filamentous gram-positive bacteria that are also visible on H&E-stained sections.
Some of the bacteria are within small grains that have smooth borders. The bacteria stain poorly with
GMS and are not acid fast. Spores are seen on the tips of some filaments.
The major genera of actinomycetes that cause disease in humans include Nocardia,
Actinomyces, Rhodococcus, Gordonia, Tsukamurella, Actinomadura ,Tropheryma, and Streptomyces.
They are characterized by filamentous, partially branching bacteria that reproduce by either spore
formation or fragmentation. They have a cell wall composition similar to other bacteria , are all
gram-positive; and some, such as Nocardia spp. are partially acid fast.
They are susceptible to inhibition by antibacterials. Actinomycetes are widespread environmental
The first description of clinical disease caused by actinomyces appeared in 1878, and in 1888, Nocard
described Nocardia as a cause of bovine granulomatous disease. Nocardia was first isolated from a human
patient in 1890. This organism was designated as Nocardia asteroides in 1896. Waksman and Henrici classified and differentiated Nocardia and Actinomyces spp in 1943.
The first description of Actinomadura ("Madura foot") appeared in 1894, and the causative agent was
designated as "Streptothrix madurae". Streptomyces
somaliensis was first described (as "Indiella somaliensis") in 1906. The genera Tsukamurella and Gordonia are more recent additions
to actinomycetes (1988).
Cervicofacial actinomycosis due Actinomyces israelii occurs in patients
with poor dental hygiene or who have had recent oral surgery. It presents initially with perimandibular
soft-tissue swelling that eventually spreads into adjacent tissues if untreated. Fistula formation may
occur with the discharge of purulent material containing yellow "sulfur" granules. Other infections due
to actinomycetes also result in the formation of grains, and are thus classified as mycetomas.
Infections with grains are classified as "botryomycetoma" if
the agent is nonfilamentous bacteria or as mycetoma if the agent is filamentous organisms. If the
filamentous organisms are bacteria, it is called "Actinomycetoma". "Eumycotic mycetoma" is the term used
for mycetoma due to pigmented or nonpigmented fungi.
Certain species of the genera Nocardia, Streptomyces, Actinomyces,
Actinomadura, and Madurella cause actinomycetoma. The most common
anatomic location is the dorsal forefoot, the so-called Madura foot, but actinomycetoma can occur
anywhere on the body. Foot and lower leg infections are most common. Hand is the next most common
location. Trunk lesions are frequently caused by Nocardia species; whereas,
head and neck lesions are usually caused by Streptomyces somaliensis.
Bacteria from the environment enter through sites of local trauma. An initial neutrophilic
response is followed by granulomatous inflammation. The process spreads through facial planes and
sometimes involves the underlying bone. Hematogenous or lymphatic spread is uncommon.
Actinomycetes can cause deep or disseminated infection, especially in immunocompromised hosts.
Central nervous system actinomycosis occurs as a result of hematogenous spread from a distant site, such
as lungs, abdomen, or pelvis, or from direct extension from cervicofacial actinomycosis. Thoracic actinomycosis occurs as
a result of aspiration of oropharyngeal secretions, esophageal perforation, direct spread from an
actinomycotic process of the neck or abdomen, or hematogenous spread from a distant lesion. Pulmonary
infection, if untreated, can spread to the pleura, pericardium, and chest wall, ultimately leading to the
formation of sinuses that discharge sulfur granules. Patients with abdominal actinomycetosis usually
have a history of colonic perforation as a result of surgery, appendicitis, diverticulitis, trauma or
ingestion of foreign bodies. A frequent presentation is a slowly growing tumor in the ileocecal region.
Involvement of any abdominal organ, including the abdominal wall, can occur by direct spread, with
eventual formation of draining sinuses. Actinomycosis of the pelvis most commonly occurs in association
with a longstanding intrauterine contraceptive device.Whipple disease is characterized by the presence of
the Tropheryma whippleii organisms within tissue macrophages. The classic
presentation is intestine infection, often causing malabsorption. The infection may also infect joints,
the central nervous system, or the cardiovascular system. The bacteria ingested by tissue macrophages
are PAS-positive in tissue sections. Definitive diagnosis is made by electron microscopy or PCR.
Actinomycetes have characteristic morphologic features in tissue sections. They are branching
filaments, 1 mm in diameter and may be beaded or fragmented to form what appear to be chains or clusters
of bacilli or coccobacilli. They are gram-positive on sections stained with either Brown-Brenn or
Brown-Hopps. On sections stained with modified acid fast stains (Fite-Faraco or Coates-Fite) Nocardia species are positive and Actinomyces species
are negative. In some infections, granules or grains form. Grains are aggregates of bacteria bordered
by Splendore-Hoeppli material. In some cases, a presumptive identification may be made from the
morphology of the grains. The grains of Actinomyces species stain intensely
with H&E and have short radiating clubs around the periphery. Actinomadura
madurae grains are large (1-5 mm) and have fringes that radiate out up to 50 micrometers. The
grains of Actinomadura pelletieri stainred or pink on H&E sections. The
grains of Streptomyces somaliensis stain poorly with H&E and have smooth
surfaces with no radiations. Morphologic features of the bacterial filaments themselves are also useful
in making an identification. Streptomyces somaliensis forms spores at the
tips of the filaments, whereas the Actinomyces species do not form spores.
Aggregates of the bacteria in the form of "sulfur granules" may be seen in direct examination of
specimens submitted for culture. As in tissue sections, the actinomycetes are thin, gram-positive,
filamentous, branched bacteria. Nocardia spp are partially acid fast with
modified Kinyoun stain. Rhodococcus equi may be coccoid or coccobacillary;
non-equi species may be more filamentous.
For isolation of the organisms in culture, samples should be collected prior to treatment.
Actinomycetes are usually facultatively anaerobic, but may be microaerophilic, aerobic, or anaerobic.
Growth is supported by most non-selective bacterial, mycobacterial and fungal media. Because they are
slow growing, media should be incubated for 2 to 3 weeks. Specimens collected from non-sterile sites may
require selective enrichment to isolate actinomycetes, especially Nocardia.
Nocardia isolated from culture are partially acid-fast and have a
beaded appearance. The acid-fastness may be inconsistent, however, most notable with organisms grown on
mycobacterial media. Aerial hyphae may be seen in Nocardia or Streptomyces. Cultures Streptomyces may produce
coccoid forms that may be acid fast. Rhodococcus may produce coccobacilli
with a "zigzag" appearance and is weakly acid-fast. Antimicrobial susceptibility testing is performed by
broth microdilution, E-test, radiometric broth, or agar dilution methods. Optimal treatment depends on
identification of the causative organism. The sensitivity of culture is limited, and many species are
indistinguishable based on histologic and cytochemical features. Treatment is therefore often empiric.
Limited studies have been performed demonstrating the development and use of immunohistochemistry to
detect and identify actinomycetes. Antibodies are not commercially available. Most studies primarily
used aspirates and smears; few studies used formalin-fixed, paraffin embedded tissue sections. The
target antigens are cell wall and cytoplasmic polysaccharides. A limited spectrum of antibodies has been
studied, most have been against A. israelii, with cross-reactivity shown
among related organisms. Immunohistochemistry is not currently practical for routine diagnostic
In situ hybridization is in use for differentiation of several bacterial, fungal and viral pathogens
in tissue. The advantages are rapid turn-around-time, high specificity, improved sensitivity,
amenability to automation, and long reagent shelf-life. In one study, 26 cases (13 Nocardia and 13 Actinomyces) were assayed with probes
designed against the 16S rRNA genes of Nocardia and common Actinomyces spp. The results of in situ hybridization showed 100% correlation
with histochemical stains, morphology, and culture. Although staining was weak in a few cases, there was
no evidence of cross-reactivity or non-specific staining. In situ hybridization is a potential
adjunctive value to aid in differentiation of Nocardia and Actinomyces spp.
Most molecular amplification techniques are aimed at the 16S rRNA gene and variously employ PCR,
Restriction endonuclease analysis, sequencing or PCR-RFLP. The HSP gene has also been targeted. The
advantages of molecular techniques are a high degree of specificity and rapid turn-around-time compared
to conventional methods. These techniques are difficult to adapt for use with tissue sections.
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