Dermatophytes

The dermatophytes, which infect only the skin, hair, or nails, secrete extracellular enzymes that likely aid in the colonization of keratinous tissues,15 These extracellular enzymes thaL may be associated with virulence include keraiinase, elasiase, and lipase.78

general considerations for the laboratory diagnosis of fungal infections

COLLECTION, TRANSPORT, AND CULTURING OF CLINICAL SPECIMENS

The diagnosis of fungal infections is dependent entirely on the selection and collection of an appropriate clinical specimen for culture. Many fungal infections are similar clinically to mycobacterial infections, and often the same specimen is cultured for both fungi and mycobacteria. Many infections have a primary focus in the lungs; respiratory tract secretions are almost always included among the specimens selected for culture. It should be emphasized that dissemination to distant body sites may occur, and fungi may be recovered from nonrespiratory sites. The proper collection of specimens and their rapid transport to the clinical laboratory are of major importance for the recovery of fungi. In many instances; specimens not only contain the etiologic agent but also contain contaminating bacteria or fungi that will rapidly overgrow some of the slower-growing pathogenic fungi. It is common for many yeasts, such as Candida spp., to be recovered on routine bacteriology media and fungal culture media. A few speafic comments concerning, specimen collection and culturing are induded in this chapter.

Respiratory Tract Secretions

Respiratory tract secretions (sputum, induced sputum, bronchial washings, bronchoalveolar lavage, and tracheal aspirations) are perhaps the most commonly submitted specimens for fungal culture. To ensure the optimal recovery of fungi and prevent overgrowth byi contaminants, antibacterial antibiotics should be in." duded in the battery of media to be used. Cydohexb mide, an antifungal agent that prevents overgrowth by rapidly growing molds, should be induded in at least one of the culture media used. As much specimen as possible (0.5 mL) should be used to inoculate each medium.

Cerebrospinal Fluid

Cerebrospinal fluid (CSF) collected for culture should be filtered through a 0.45-|un membrane filter attached i a sterile syringe. After filtration, the filter is removed1 and is placed onto the surface of an appropriate culture medium, with the inoculum side down. Cultures should be examined daily and the filter moved to another location every other day. If less than 1.0 ml of specimen is submitted for culture, it should be centrifuged and 1' drop aliquots of the sediment should be placed onto several areas on the agar surface. Media used for the recovery of fungi from CSF should contain no antibacterial or antifungal agents. Once submitted to the laboratory, CSF specimens should be processed promptly. If prompt processing is not possible, samples should ie kept at room temperature or placed in a 30° C incu-"bator, because most organisms will continue to replicate in this environment.

Blood

Disseminated fungal infections are more prevalent than previously recognized, and blood cultures provide an _a£curate method for determining their etiology in many instances. Only a few manual fungal blood culture systems have been available over past years, and most are not used routinely by clinical microbiology laboratories. gXirrently, several automated blood culture systems, including the BACTEC (Becton Dickinson, Sparks, Md), BacT/ALERT (bioMérieux, Durham, NC), and ESP (Trek Diagnostics, Westlake, Ohio), are adequate systems for ihe recovery of yeasts. However, those laboratories that have a high incidence of dimorphic fungi recovered from blood are encouraged to use the lysis-centri-iugation system, the Isolator (Wampole Laboratories, Princeton, NJ). The Isolator has been shown to be optimal for the recovery of H. capsuhtum and other filamentous fungi.1213 Using this system, red blood cells and white blood cells, which may contain the microorganisms, are lysed, and centrifugation concentrates the organisms before culturing. The concentrate is inoculated onto the surface of appropriate culture media, and most fungi are detected within the first 4 days of incubation. However, occasional isolates of if. capsuhtum may require approximately 10 to 14 days for recovery. The optimal temperature for fungal blood cultures is 30° C, and the suggested incubation time is 21 days.

Hair, Skin, and Nail Scrapings

Specimens of hair, skin scrapings or biopsies, and nail clippings are usually submitted for dermatophyte culture and are contaminated with bacteria and/or rapidly growing fungi. Samples collected from lesions may be obtained by scraping the skin or nails with a scalpel blade or microscope slide; infected hairs are removed by plucking them with forceps. These specimens should be placed in a sterile container; they should not be refrigerated. Mycosel agar, which contains chloramphenicol and cycloheximide, is satisfactory for the recovery of dermatophytes. Cultures should be incubated for a minimum of 21 days at 30° C before being reported as negative.

Urine

Urine samples collected for fungal culture should be processed as soon after collection as possible. Twenty-

four-hour urine samples are unacceptable for culture. The usefulness of quantitative cultures is undetermined, and it should not be done. All urine samples should be centrifuged and the sediment cultured using a loop to provide adequate isolation of colonies. Because urine is often contaminated with gram-negative bacteria, it is necessary to use media containing antibacterial agents to ensure the recovery of fungi.

Tissue, Bone Marrow, and Sterile Body Fluids

Ail tissues should be processed before culturing by mincing or grinding or placement in a Stomacher (Tekmar, Cincinnati, Ohio). The Stomacher expresses the cytoplasmic contents of cells by pressure exerted from the action of rapidly moving metal paddles against the tissue in a broth suspension. Large portions of tissue should be cut into smaller pieces if the Stomacher is to be used for processing, After processing, at least 1.0 mL of specimen should be spread onto the surface of appropriate culture media, and incubation should be at 30° C for 21 days; cultures incubation may be extended if there is a high clinical suspicion of a mycotic disease. If tissue was submitted, it is important to be sure that portions of the tissue are inoculated onto the agar surface (i.e., not just the broth used to assist in the dissolution of the tissue in the Stomacher).

Bone marrow may be placed directly onto the surface of appropriate culture media and incubated in the manner previously mentioned. Sterile body fluids should be concentrated by centrifugation before culturing, and at least 1 mL of specimen should be placed onto the surface of appropriate culture media. An alternative is to place bone marrow and other body fluids in an Isolator tube and process it as a blood culture. All specimens should be cultured as soon as they are received by the laboratory to ensure the recovery of fungi from these important sources.

Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

Get My Free Ebook


Post a comment