DA : Direct agglutination.

Darkfield microscopy : Technique used to visualize very small microorganisms or their characteristics by a system that permits light to be reflected or refracted from surface of objects being viewed.

Debridement : Surgical or other removal of nonviable tissue.

Decontamination : Process of rendering an object or area safe for unprotected people by removing or making harmless biological or chemical agents.

Decubitus ulcer : A craterlike defect in skin and subcutaneous tissue caused by prolonged pressure on the area. This occurs primarily over bony prominences of the lower back and hips in individual who are unable to care for themselves well and unable to roll or move periodically; also known as pressure sore or bedsore.

Defined medium : A medium whose exact chemical composition is quantitatively known. Compare with complex medium.

Definitive host : Host in which the sexual reproduction of a parasite occurs.

Definitive identification : A valid identification of a microorganism to genus and species, regardless of the method(s) used to make the identification.

Degeneracy : In relation to the genetic code, the fact that more than one codon can code for the same amino acid.

Deletion : A removal of a portion of a gene.

Dematiaceous : Having structures that are brown to black; this is due to a melanotic pigment in the cell walls.

Denaturation : Irreversible destruction of a macromolecule, as for example the destruction of a protein by heat.

Denitrification : Conversion of nitrate into nitrogen gases under anaerobic conditions, resulting in loss of nitrogen from ecosystems.

Dental plaque : Bacterial cells encased in a matrix of extracellular polymers and salivary products, found on the teeth.

Denticle : Short, narrow projection bearing a conidium.

Deoxyribonucleic acid (DNA) : A polymer of nucleotides connected via a phosphate-deoxyribose sugar backbone; the genetic material of the cell.

Dermatitis : Inflammation of the skin (filariasis, schistosomiasis, infections with Strongyloides and hookworm larvae, leishmaniasis, Sarcoptes infections).

Dermatomal : Pertaining to the area of skin which is served by one sensory spinal nerve.

Dermatomycosis : Any fungus infection in the skin of animals and humans.

Dermatophyte : A fungus belonging to the genus Trichophyton, Microsporum, or Epidermophyton with the ability to obtain nutrients from keratin and infect skin, hair, or nails of humans or animals.

Dermatophytosis : An infection produced by a dermatophyte in the nails, hair, and skin.

Desiccation : Drying.

Desquamation : Shedding or scaling of skin or mucous membrane.

DFA : Direct fluorescent antibody test.

DGI : Disseminated gonococcal infection.

Diarrhea : Frequent passage of soft or liquid stool (no blood); may be caused by any parasite or infection normally found in any part of the intestine (Giardia lamblia, Isospora belli, Dientamoeba fragilis, Balantidium coli, Cryptosporidium parvum, Enterocytozoon bieneusi, possibly other microsporidia, Cyclospora sp., visceral leishmaniasis, Plasmodium falciparum malaria, tapeworms, trichinosis, schistosomiasis, hookworm).

DIC : Disseminated intravascular coagulation.

Dichotomous : Branching of hyphae into two equal branches that are each equal in diameter to the hypha from which they originated.

Dictyoconidium (pl. dictyoconidia) : A conidium with horizontal and vertical septa (a muriform spore).

Dideoxynucleotide : A nucleotide lacking the 3'-hydroxyl group on the deoxyribose sugar. Used in the Sanger method of DNA sequencing.

DIF : Direct immunofluorescence; diffuse interstitial fibrosis; dose increase factor.

Diff : differential blood count.

Differential medium : A medium which is used to differentiate different types of microorganisms based on their different colors or shapes of colonies. Examples of differential media are: Macconkey's agar and SS agar.

Differentiation : The modification of a cell in terms of structure and/or function occurring during the course of development.

Diluent : Fluid used to dilute a substance.

Dimorphic : Having two distinct morphological forms. In this guide, it refers to temperature-dependent changes in the organism on artificial culture media, i.e., fungi having a mold phase when cultured at 25-30 C and a yeast phase when cultured at 35-37C.

Dioecious : Having the male and female sexes of a species as separate individuals.

Diplococcus : A pair of cocci.

Diploid : Contains 2n chromosomes.

Diptheroid : A term used to describe gram-positive bacilli that morphologically resemble Corynebacterium diptheriae. C. diphtheriae and diptheroids often appear as relatively short rods that tend to form aggregates of cells, sometimes resembling Chinese letters or picket fences. Propionibacterium spp. are frequently (but not always) diptheroidal in appearance.

Direct Agglutination (DA) , is a general term for techniques which use agglutination (macroscopic clumping) of particulate reagents as an indicator of the presence of an antigen-antibody reaction. Examples: Hemagglutination, Latex Agglutination, and Coagglutination.

Direct Fluorescent Antibody (DFA) , is the straightforward detection of antigens using fluorescently labeled antigen-specific antibody. Because detection of the antigen in a substrate of patient sample (cellular smear, fluid or patient-inoculated culture medium) is the goal, DFA is seldom quantitative.

Direct smear (stool) : Approximately 2-mg suspension of feces in water or saline for the purpose of examination for parasites; primary aim is to see motility.

Direct wet mount : A preparation from clinical material suspended in sterile saline or other liquid medium on a glass slide and covered with a coverslip; used for microscopic examination to detect microorganisms in clinical material and, in particular, to detect motility directly.

Disequilibrium : Unsteady balance.

Disinfectant : Agent that destroys or inhibits microorganisms that cause disease.

Disinfection : Killing of infectious agents outside the body by direct exposure to chemical or physical agents. High-level disinfection may kill all microorganisms with the exception of high numbers of bacterial spores; it requires extended exposure to ensure killing of most bacterial spores. It is achieved, after thorough detergent cleaning, by exposure to specific concentrations of certain disinfectants (e.g., 2% glutaraldehyde, 6% stabilized hydrogen peroxide and up to 1% peracetic acid) for at least 20 minutes. Intermediate-level disinfection does not kill spores; it can be achieved by pasteurization (75 deg. C. [167 deg. F.] for 30 minutes) or by appropriate treatment with EPA-approved disinfectants.

Disinfestation : Any physical or chemical process serving to destroy or remove undesired small animal forms, particularly arthropods or rodents, present upon the person, the clothing, or in the environment of an individual, or on domestic animals. (See Insecticide and Rodenticide.) Disinfestation includes delousing for infestation with Pediculus humanus, the human body louse. Synonyms include the terms disinsection and disinsectization when only insects are involved.

Disproportionation : The splitting of a chemical compound into two new compounds, one more oxidized and one more reduced than the original compound.

Disseminated intravascular coagulation (DIC) : A phenomenon which arises due to the depletion of clotting elements in the blood; caused by many disease processes; diffuse, severe hemorrhaging can occur; without treatment, it is often fatal.

Diurnal : Pertaining to the daylight portion of the 24-h day.

dl : Deciliter(s).

DNA : Deoxyribonucleic acid, the lipoprotein molecule that contains the genetic code for most living things.

DNA "dot-blot" Hybridization (DOT-BLOT) , is a rapid technique used to detect the presence of a specific DNA in a specimen. Dots, or spots of the DNA containing sample are placed onto a nitrocellulose membrane and fixed. This membrane is then hybridized to a radioactively labeled DNA segment of known sequence, specific for the pathogenic DNA being tested. If the pathogenic DNA is present in the specimen, complementary DNA sequences present on the membrane will hybridize, or anneal, producing a doublestranded DNA segment with the radioactive label incorporated into the molecule. The presence of radioactivity is detected by autoradiography.

DNA fingerprinting : Use of the techniques of genetic engineering to determine the origin of dDNA in a sample of tissue.

DNA library : A collection of cloned DNA fragments, which in total, contain genes from the entire genome of an organism; also called a gene library.

DNase : Deoxyribonuclease, an enzyme that depolymerizes DNA.

Domain 1 : The highest level of biological classification, superseding kingdoms. The three domains of biological organisms are the Bacteria, the Archaea, and the Eukarya.

Domain 2 : Used to describe a region of a protein having a distinct function.

Dorsal : Referring to the back or posterior aspect of a tissue or organ.

Dorsal root ganglion : Group of nerve cell bodies outside the spinal cord which convey sensory impulses to the brain.

DOT BLOT : DNA dot-blot hybridization.

Doubling time : The time needed for a population to double. See also generation time.

Downstream position : Refers to nucleic acid sequences on the 3'-side of a given site on the DNA or RNA molecule. Compare with upstream position.

DRGs : Diagnosis related groups; system introduced to control rising medical cost under which hospitals are reimbursed for costs of caring for patients based on the primary diagnosis of the patients and what is considered to be a fair value for treatment of usual patients with those diagnoses.

Droplet nucleus : A tiny aerosolized particle that, because of its lack of mass, may stay suspended in air for extended periods of time.

Duodenum : The proximal portion of the small intestine (Strongyloides stercoralis, Giardia lamblia).

Durham tube : Small tube placed in an inverted position below the surface of growth-supportive broth and filled completely with the broth solution. Gas produced by the organism displaces the broth in the Durham tube, and the resulting gas bubble is visual evidence of gas production.

Dx : Diagnosis.

Dysentery : Inflammation of the intestinal tract, particularly the colon, with frequent bloody stools (e.g., bacillary dysentery).

Dysgonic : Growing poorly (bacterial cultures).

Dyspepsia : Gastric indigestion (upset stomach) due to alterations of gastric function that are cause by various disorders of the stomach.

Dyspnea : Difficulty in breathing.

Dysuria : Painful or difficult urination.