Zygomycetes

Zygomycetes: General Considerations

• Found widely in the environment: soil, dung, vegetable matter
• Humans usually encounter through inhalation or ingestion
• Associated with sinus and pulmonary disease, especially in patients with predisposition: immunosuppression, diabetes, burns, malnourishment, intravenous drug abuse
• Along with Aspergillus and Candida are sometimes considered the classic fungal opportunists
• Proclivity to invade blood vessels
• Hyphae are typically broad, ranging from 6-15μ in width
• Grows rapidly on agar in gray-white, brown, or gray-brown cottony or wooly colonies without distinct margins
• Zygomycetes are nearly nonseptate, but septae may be seen on sporangiophores subjacent to the sporangia and in other locations in older cultures
• Columella and rhizoids important in genus identification as is the presence or absence of branched sporangiophores. Other distinguishing characteristics include the shape of the apophysis and the size and shape of the sporangia

Zygomycetes: Direct Examination

• Irregular
• Hyphae broad, ribbon-like, ranging from 6-50 microns in width
• Usually devoid of septae, with rare exceptions
• Proclivity to invade blood vessels
• The following three images are from the autopsy of a young immunocompromised woman with disseminated zygomycosis; premortem respiratory culture revealed Rhizopus species. Note the broad, generally aseptate hyphae. Zygomycetes may occasionally demonstrate septae, as shown in the Figure 3, below.

FIG. 1. This is a hematoxylin and eosin stained section of lung. The pleural surface is effaced by fungal hyphae. In this photomicrograph the pleural space on the left contains blood and inflammatory cells; thickened alveoli are on the right.

FIG. 2. This is a high power view of hyphae. Note the breadth of the hyphae and lack of septae in this photomicrograph.

FIG. 3. Septae may rarely be present both in tissue sections (as seen here) and in culture, particularly as cultures age.

Common Zygomycetes:

• Mucor: no rhizoids, sporangiophores tend to branch
• Rhizopus: large rhizoids on stolon immediately adjacent to sporangiophore ("nodal" position)
• Rhizomucor: resemble Mucor, but may have poorly developed rhizoids; thermatophiles, grow up to 60ºC

Mucor:

• A common contaminant, but may cause invasive disease.
• Colonies are fast-growing and resemble white-to-gray cotton candy, darkening with time.
• The reverse is light-colored to white.
• Hyphae are wide, like other Zygomycetes, usually approximately 6-15μ.
• No rhizoids are formed.
• Sporangiophores are long, often branch, and bear large (50-300μ), round sporangia.

Rhizopus:

• Although this genera can cause disease, they are also commonly found as a contaminant.
• Colonies grow rapidly and resemble cotton candy. Colonies darken with age, becoming gray or yellow-brown. The reverse is white.
• Mycelia are marked by numerous stolons connecting groups of long sporangiophores.
• Sporangiophores are usually unbranched, long, and terminate in a columella and a dark round sporangium containing oval colorless to brown spores.
• Stolons bear large rhizoids which are found immediately adjacent to the sporangiophore in the nodal position.
• Columella and sporangium collapse easily after discharging spores.

FIG. 1. Rhizopus in slide culture

Rhizomucor:

• Colonies are fluffy and gray, browning with time. The reverse is white or light-colored.
• Microscopic morphologically is intermediate between Rhizopus and Mucor; sporangiophores are often branched and rudimentary, small rhizoids are remote from the base of the sporangiophore (i.e. rhizoids are said to be "internodal.")
• Sporangia are often smaller than those of other zygomycetes and range from 60-100μ; some overlap in size still exists, however.

FIG. 1. Rhizomucor in slide culture. Note the single, rudimentary rhizoid arising nearby--but not adjacent to--the sporangiophore.

Absidia:

• Colonies mature rapidly and resemble coarse, gray wool or cotton candy. The reverse is white or light gray.
• Frequently considered a contaminant in cultures, Absidia species are similar in microscopic appearance to Rhizopus, but rhizoids are internodal.
• Rhizoids may be difficult to find, as in the case illustrated below.
• Sporangia are slightly elongated spheres ranging from 20-120μ in diameter. Sporangiospores are round to oval and measure 3-4.5μ.

FIG. 1. Low power photomicrograph of Absidia in slide culture. Internodal rhizoids are not apparent here.

FIG. 2. Absidia in slide culture. A cup-shaped columella (long arrow) supports a spherical sporangium (short arrow). Note also the pale tan sporangiospores (arrowhead)recently released from a sporangium.

Cunninghamella:

• Frequently considered a contaminant in cultures, Cunninghamella species have broad, almost aseptate hyphae and long, branched sporangiophores that end in swollen vesicles that are covered with spine-like denticles.
• Attached to the denticles are round-oval sporangiolum that range in size from 6-14μ.

FIG. 1. High power photomicrograph of Cunninghamella in slide culture. The swollen vesicle is located at the end of a sporangium and is covered by denticles and attached, round sporangiolum (arrowhead).

Saksenaea:

• An infrequent cause of zygomycosis, most often encountered in the setting of traumatic implantation of fungus.
• Colonies mature rapidly and are white, cottony, and have a white reverse.
• Sporulation does not occur in routine culture, but requires a modified water culture.
• Sporangiophores are distinctive, flask-shaped, and 24-64μ long.
• Rhizoids often form near the base of the sporangiophore, are dichotomous, and darken as the culture ages.

FIG. 1. High power photomicrograph of Saksenaea from water culture. This specimen was cultured from abdominal wounds received by a man in a motorcycle accident; radical debridement was required in an attempt to control the infection. The characteristic sporangiophore dominates this image, with darkening rhizoid near the base of the sporangiophore, at the upper left.