L form (L-phase, L-phase variant) : L-forms are bacterial spheroplasts or protoplasts originating from normal bacteria following partial (spheroplasts) or complete (protoplasts) removal of the cell wall. The formation of L-forms can be either spontaneous (occurring during certain phase of growth) or artificial (suppression of the rigid cell wall by stimuli such as enzymes, heat-shock, or special L-form induction medium, etc.) Bacterial genera from which L-forms have been derived include: Agrobacterium, Bacillus, Bacterodes, Bartonella, Bordetella, Brucella, Clostridium, Corynebacterium, Erysipelothrix, Escherichia, Flavobacterium, Haemophilus, Listeria, Neisseria, Proteus, Pseudomonas, Salmonella, Sarcina, Serratia, Shigella, Staphylococcus, Streptobacillus, Streptococcus, and Vibrio.

LA : Latex agglutination.

Lactic acid bacteria : A group of gram-positive, non-sporing bacteria which carry out a lactic acid fermentation of sugars. It includes species of Lactobacillus, Leuconostoc, Pediococcus and Streptococcus.

Lactic acid fermentation : A type of fermentation carried out by lactic acid bacteria in which sugar (e.g. lactose, glucose, pentose) are converted either entirely (or almost entirely) to lactic acid (homolactic fermentation) or to a mixture of lactic acid and other products (heterolactic fermentation).

Lag phase : Period of slow microbial growth that occurs following inoculation of the culture media.

Lagochilascris minor : An unusual nematode parasite of humans (ingestion of infective eggs from the environment or ingestion of infective larvae in the tissues of an intermediate host). In most cases, the worms are located in the soft tissues of the neck and throat, tonsils, mastoids, and paranasal sinuses (chronic or recurrent abscesses). Fatal encephalopathy (one case) has also been reported. All cases were in the tropical regions of the Western hemisphere.

Laked blood : Hemolyzed blood; hemolysis may be effected in various ways, but alternate freezing and thawing is a simple method.

Laked kanamycin-vancomycin agar (LKV) : This medium contains laked blood and the antibiotics kanamycin and vancomycin. LKV selects for obligate anaerobes and microaerotolerant organisms that are gram-negative bacilli.

Laminar flow : Nonturbulent flow of air in layers (flowing in a vertical direction in the case of a biosafety hood).

Lanose : Woolly.

Larva migrans, cutaneous/visceral/ocular : Disease characterized by thin, red, convoluted papular or vesicular lines of eruption that extend at one end while fading at the other (dog or cat hookworm). Visceral

Laryngitis : Inflammation of the larynx (voice box).

Laryngotracheobronchitis : Inflammation of the larynx and larger airways.

Latent : Not manifest; potential.

Latent virus : A virus present in a cell, yet not causing any detectable effect.

Latex Agglutination (LA) , also known as latex particle agglutination, for detection of antibodies is identical to Hemagglutination (HA) principle, but the substitution of smaller, antigen-coated latex particles for erythocrytes results in improved sensitivity and reagent longevity. Alternatively, antibodies can be absorbed to the latex particles (under appropriate ionic and pH conditions) by binding to the Fc region of antibodies, leaving the Fab region free to interact with antigens present in the applied specimens. This phenomenon has made LA a popular technique for detecting antigens as well.

lb : Pound.

Leaching : Removal of valuable metals from ores by microbial action.

LD : Loading dose.

LDH : Lactate dehydrogenase.

LE : Lupus erthyematosus.

Lecithin : One of a group of phosphoglycerides found in various plant and animal substances, including egg yolk, nerve tissue, semen, and cell membranes.

Lecithinase : An enzyme that catalyzes the breakdown of lecithin; also called phospholipase. Lecithinase production is a useful criterion for identification of various gram-positive and gram-negative bacteria that produce it.

Lectin : Naturally produced proteins or glycoproteins that can bind with carbohydrates or sugars to form stable complexes.

Leishman-Donovan (L-D) body : Small, round intracellular form (called amastigote or leishmanial stage) of Leishamania species and Trypanosoma cruzi.

Leptomonad : Old term for the promastigote stage, which is long and slender and found in the insect vector for Leishmania spp.; it is also recovered in artificial culture media (Novy, MacNeal, and Nicolle medium).

LET : Leukocyte esterase test.

Lethargy : Drowsiness or decreased responsiveness.

Leukemia : General term used to describe a group of malignancies of either lymphoid or hematopoietic origin; progressive proliferation of abnormal leukocytes can be found in the blood, the hematopoietic tissues, and other organs.

Leukocidin : A substance able to destroy phagocytes.

Leukocyte : A white blood cell, usually a phagocyte.

Leukocytosis : Elevated white blood cell count.

Leukopenia : Low white blood cell count.

L form : Cell wall-deficient form of a bacterium.

LGV : Lymphogranuloma venereum; the name for certain strains of Chlamydia trachomatis that cause a systemically expresses sexually transmitted disease.

Lichen : A fungus and an alga (or a cyanobacterium) living in symbiotic association.

Limulus amebocyte lysate : Product of white blood cells of the horseshoe crab used in an assay for endotoxin.

LIP : Lymphoid interstitial pneumonitis.

Lipase : Any enzyme that catalyzes the breakdown of triglycerides and phospholipids. Lipases occur in milk, the pancreas, adipose tissue, the stomach, and various other tissues.

Lipid : Water-insoluble organic molecules important in structure of the cytoplasmic membrane and (in some organisms) the cell wall. See also phospholipid.

Lipopolysaccharide : Carbohydrate-lipid complex; integral substance in gram-negative cell walls. Also known as "endotoxin."

Liposome : Small closed vesicle consisting of a single lipid bilayer.

Lithotroph : In reference to energy source (electron donor) - An organism which uses an inorganic substrate (such as ammonia, hydrogen) as an electron donor in energy metabolism. There are two types of lithotroph: chemolithotroph and photolithotroph. Compare with autotroph, heterotroph, organotroph, and phototroph.

LM : Light microscopy.

LMC : Lymphocyte microcytotoxicity.

Loeffler syndrome : Transient pulmonary infiltration; day-to-day clearing in 3 to 14 days; associated with marked peripheral eosinophilia (Ascaris lumbricoides).

Logarithmic phase : Period of maximal growth rate of a microorganism in a culture medium.

Lophotrichous : Having a tuft of polar flagella.

Low passage cell line : Cell culture used for virus isolation that is sensitive to the virus only after the cells have been subcultured (passaged) between 20 and 50 times; further passages result in cell with lower affinity for the virus.

Lower respiratory tract : Trachea, bronchi, and lungs.

LP : Lumbar puncture.

LPA : Latex particle agglutination.

LPS : Lipopolysaccharide; see endotoxin.

Lumbar puncture : A procedure that is used to obtain cerebrospinal fluid for diagnostic purposes.

Lumen : Cavity of hollow, tubular organ, such as the intestine or blood vessels.

Luminescence : Production of light.

Lyme disease : A tick-borne disease which affects humans and other animals. Lyme disease was named after a place called Lyme in Connecticut, USA, where this disease was first recognized and has rapidly become the most prevalent tick-borne disease in the United States. Lyme disease is caused by a spirochete named Borellia burgdorferi which is carried in and transmitted to humans by the deer tick called Ixodes dammini although it can also be spread by other ticks such as the common dog (wood) ticks. The initial symptoms of Lyme disease include an acute headache, backache, chills, and fatigue and often a large rash at the biting site. Lyme disease can be easily treated with antibiotics such as tetracycline or penicillin. However delayed or improper treatment may lead to the infection of the central nervous system by the spirochete and result in visual disturbances, facial paralysis or seizures.

Lymph : Plasma and white blood cells that bathe the tissue cells.

Lymphadenitis : Inflammation of the lymphatic nodes (filariasis).

Lymphangitis : Inflammation of the lymphatic vessels (filariasis).

Lymphocyte : A white blood cell involved in antibody formation or cellular immune responses.

Lymphocytosis : Absolute or relative increase of lymphocytes in the blood.

Lymphokine : Substance secreted from T lymphocytes which stimulates the activity of other cells.

Lymph varices : Dilated lymph vessels secondary to lymphatic blockage (filariasis).

Lyomphoma : Cancer of the lymph cells or their precursors.

Lymphoproliferative disorders : Any one group of cancers involving cells from which white blood cells or platelets are derived; includes lymphomas, leukemias, and multiple myeloma, among others.

Lysin : An antibody that induces lysis.

Lysis : Disintegration or dissolution of bacteria or cells.

Lysogen : A prokaryote containing a prophage. See also temperate virus.

Lysogeny : Process by which a viral genome is integrated into that of its host bacterium.

Lysosome : A cell organelle containing digestive enzymes.