Yeast, molds, and cell types in 2 benchtop guides

Another potentially alarming pathogen: Aspergillus terreus, the mold that produces that attractive cookie-like colony mentioned earlier. “It’s being seen increasingly as a hospital-acquired infection,” Dr. Love says. “In some hospitals, it’s been found to colonize water systems. The issue with Aspergillus terreus is that if it causes an infection, it may have the ability to produce conidia directly from hyphae inside the body. This is in distinction to the rest of the species of Aspergillus, which produce fruiting heads with conidia only when there’s air contact as in molds growing on plated media or sometimes in pulmonary or sinus mycetomas.” While Aspergillus terreus also produces fruiting heads with conidia when in contact with air, conidia growing directly from hyphae inside the body can then circulate in the blood. As a result, Aspergillus terreus is the only species of Aspergillus in which positive blood cultures are readily produced in patients. “This ability to produce conidia inside the body is a pathogenic mechanism that’s very alarming,” Dr. Love says.

The guide also contains many discussions of far less dangerous organisms, such as Aspergillus versicolor. A rapidly growing mold, Aspergillus versicolor can sometimes be confused with Penicillium. That’s because, as Dr. Love explains, Aspergillus versicolor differs from other Aspergillus species in that some of its fruiting heads have conidial heads so reduced in size that they’re almost absent: “In these cases, the structure looks just like Penicillium, which does not have fruiting heads.”

So how to make the correct identification? Start with the book’s color photographs, Dr. Love suggests. “For Aspergillus versicolor, you want to look for a green or tan downy colony,” he says, “though in very mature colonies, there is a wine-red exudate that may be produced. Then look at the microscopic morphology. You want to see long, smooth conidiophores supporting vesicles. One-half or more of vesicle surfaces are covered by double rows of cells that support chains of conidia.”

Of course, the guide also offers much information about yeast and yeast-like fungi. Among the former is Cryptococcus neoformans, which produces round, budding yeast without hyphae, and which can produce systemic infection in immunocompromised patients. “I saw it extensively in the HIV epidemic in the 1980s, before effective retrovirals were available,” says Dr. Love. “What fascinates me about Cryptococcus neoformans is that it can show a wide range of sizes. In tissue and body fluids, it can grow as small as two to three microns in diameter, or as large as, say, 25 micrometers or more in diameter. In the mid-1980s, I did an autopsy on a patient who had a brain lesion due to Cryptococcus neoformans, and the yeast cells—excluding the capsule—were 30 micrometers or more in diameter. But when I grew the Cryptococcus neoformans in the microbiology laboratory, they were only about three to six micrometers in diameter. That yeast has always been fascinating to me.”

Dr. Love believes that the mycology guide will continue to be useful even as more and more laboratories embrace molecular techniques for identification. While “ultimately, molecular technique will predominate,” in his view “having a command of the morphology will continue to be important because you can select the most appropriate, least expensive molecular method if you are guided by morphology.”

Somewhat smaller in size, but just as comprehensive, is the Body Fluids Benchtop Reference Guide, which Dr. Etzell created with Martha R. Clarke, MD; George Girgis, MT(ASCP); Alice L. Werner, MD; Anna K. Wong, MD; and Tracy I. George, MD, all members of the CAP’s Hematology/Clinical Microscopy Resource Committee.

“We included the most common cell types that are seen in a variety of body fluids as well as some of the cell types that can be difficult to distinguish,” says Dr. Etzell. “In addition, we’ve included some examples of malignant cells that can occur in body fluids.”

The guide organizes body fluids into the following categories: erythroid series, lymphoid series, myeloid series, mononuclear phagocytic series, lining cells, miscellaneous cells, crystals, and microorganisms. A section called “Miscellaneous Findings” includes a discussion of mitotic figures, which, as the book notes, can be difficult to distinguish from degenerating cells.

“Mitotic figures tend to have some structure to them,” Dr. Etzell explains. “In the photograph in the book, you can see some linear structures within the mitotic figure. But sometimes, if the mitotic figure is more condensed, it can look more like degenerating nuclear chromatin. Both can have a very deep purple homogeneous structure. As a cell degenerates, the nucleus degrades as well, and it may not always retain the round shape; there may be some irregularities to the nuclear membrane.”

Also in the “Miscellaneous Findings” section is a discussion of starch granules, which occasionally show up as a contaminant from the powder on medical gloves. As the guide notes, these granules typically have a diameter four to six times that of a red blood cell, and appear irregularly round with a central slit or indentation. “I’d say it’s less than 10 percent of the time that we see them, but we certainly do see them,” Dr. Etzell says, “and it’s important not to confuse them with a cellular type or a true crystal.”

One important distinction she hopes the guide will help users make is that between blast cells and normal lymphocytes. “The presence of blasts in body fluid often means involvement by acute leukemia or another hematopoietic neoplasm,” she explains. “The photomicrograph in the guide nicely illustrates blasts, and illustrates that they’re larger than normal lymphocytes. They do have more dispersed chromatin with a nucleolus, and this is certainly a very important cell type for technologists and pathologists to be able to reliably identify, since involvement of fluids by leukemia or other hematopoietic neoplasms is diagnostically quite important.”

She hopes, too, that the guide will prove useful to users who are attempting to distinguish germinal matrix cells from malignant cells. “Germinal matrix cells,” she reminds readers, “typically will occur in neonates, and they have many features that resemble those of malignant cells—including high nuclear-to-cytoplasmic ratio, some nuclear irregularities, and finely dispersed chromatin. So, as with most things that we do in the clinical lab, it’s important to correlate the patient’s age and history with what we’re seeing on the slide to ensure that we don’t confuse something like germinal matrix cells with a malignancy.”

Anne Ford is a writer in Evanston, Ill. To order the reference guides, call 800-323-4040 or 847-832-7000 option 1. Each of the two benchtop reference guides is $78.