Medically Significant Molds

Fungi constitute an extremely diverse group of organisms and are generally classified as molds or yeasts. Some have been recognized as classic pathogens, whereas others are recognized only as environmental saprobes, living on nonliving material. Fungi can cause mild infections, trigger allergic reactions, including asthma, and produce serious life-threatening disease. With the advent of chemotherapy, radiation therapy, and diseases such as acquired immunodeficiency syndrome (AIDS) that affect the immune system, the line between pathogen and saprobe has been blurred. The isolation of all organisms, especially in the immunocompromised patient, must initially be considered a significant finding and evaluated in light of the patient’s history and physical examination results.

General Characteristics

The characteristics of the mold differ from those of plants or bacteria. Molds are eukaryotic; they possess a true nucleus, with a nuclear membrane and mitochondria. Bacteria are prokaryotic, lacking these structures. Unlike plants, fungi lack chlorophyll and must absorb nutrients from the environment. In addition, mold cell walls are made of chitin, whereas those of plants contain cellulose. Most molds are obligate aerobes that grow best at a neutral pH, although they tolerate a wide pH range. Moisture is necessary for growth, but spores and conidia survive in dry conditions for extended lengths of time.

Yeasts Versus Molds

Yeasts are single vegetative cells that typically form a smooth, creamy, bacterial-like colony without aerial hyphae. Because their macroscopic and microscopic morphologies are similar, identification of yeasts is based primarily on biochemical testing. Yeasts reproduce by budding or fission. Budding involves maturation of the bud to an independent blastoconidium (daughter cell). This process involves lysis of the yeast cell wall so that a blastoconidium can form. As this structure enlarges, the nucleus of the parent cell undergoes mitosis. Once the new nucleus is passed into the daughter cell, a septum forms and the daughter cell breaks free. During fission, two cells of equal size are formed. These cells continue to grow from the tips of the cell and divide only after a medial fission is formed.

Most molds have a fuzzy or woolly appearance because of the formation of mycelia. The mycelia are made up of many long strands of tube-like structures called hyphae, which are either aerial or vegetative. Aerial mycelia extend above the surface of the colony and are responsible for the fuzzy appearance. In addition, aerial mycelia support the reproductive structures that produce conidia. Conidia, in many cases, are used to identify different fungal genera. The vegetative mycelia extend downward into the medium to absorb nutrients.

The microscopic appearance often aids in the identification of molds. In some species, antler, racquet, rhizoids, or spiral hyphae are formed. Antler hyphae have swollen, branching tips that resemble moose antlers. Racquet hyphae contain enlarged, club-shaped areas. Spiral hyphae are tightly coiled. Rhizoids, rootlike structures, might be seen in some of the Zygomycetes, and their presence and placement can assist with identification. Frequently, when fungal hyphae are being described, they are referred to as septate or sparsely septate. Septate hyphae show frequent cross-walls occurring perpendicularly to the outer walls of the hyphae, whereas sparsely septate hyphae have few cross-walls at irregular intervals. The term aseptate, meaning the absence of septations, has historically been used to describe the hyphae of the Zygomycetes. Microscopic examination of hyphae associated with the Zygomycetes often reveals occasional septations; therefore these hyphae are more correctly termed sparsely septate as opposed to aseptate