M

Macromolecule: A very large and complex molecule usually associated with cell function and includes proteins, lipids, nucleic acids, and polysaccharides, that plays a role in tranfer of genetic information.
Macrophage: A large noncirculating phagocytic cell associated primarily with the reticuloendothelial system.
Mad cow disease: A slow viral disease of the brain affecting cows, but can be transmitted to people by eating contaminated beef.
Malaria: A mosquito-borne parasitic disease marked by recurring fever. It is the best known protozoal disease caused by one of four species of Plasmodium: P. falciparum, P. vivax, P. ovale, and P. malariae. Each are carried only in the Anopheles mosquito.
Mantoux test: The intradermal injection of antigen, used to measure hypersensitivity to the tubercle bacillus (TB).
Marburg/Ebola filoviruses: Viruses having heavy mortality rates, with 25% for Marburg and 90% for Ebola. These viruses first gained some notariety in 1967. In Marburg, Germany, three employees of a vaccine factory complained of flu-like symptoms. By the next day, their conditions had deteriorated considerably. Over the next several weeks, they displayed the following symptoms: acute viral blood poisoning, bizarre rashes, severe diarrhea, bloody vomiting, hemorrhages from every orifice, and the peeling away of their flesh. Throughout Germany and Yugoslavia, pharmaceutical workers and the health care professionals who treated them succumbed to the same symptoms. Several died, while others suffered liver damage, impotence, and psychosis. Investigators linked this mysterious outbreak to a contaminated shipment of monkeys from Uganda that had been brought in for experimental purposes. Scientists were unable to discover the source of the viruses or anything else about them, although they did discover that methyl alcohol knocked it out. By the summer of 1976, panic was sweeping Zaire and the Sudan as a virus similar to Marburg, only much worse, was claiming lives. Outside investigators isolated a virus that had, ironically, the shape of a question mark. It was named Ebola after a nearby river that was the starting point for the outbreak. The two viruses were closely linked and equally mysterious.
Mast cells: Cells primarily found in the respiratory and intestinal tracts to which IgE antibodies attach and which release histamine and other substances.
Mastigophora (flagellates): Protozoa that move using whiplike flagella to propel them like swimmers. They are responsible for the diseases trichomoniasis and giardiasis, as well as the more serious diseases of trypanosomiasis and leishmaniasis.
Measles (rubeola): A highly contagious disease caused by a paramyxovirus. Occasionally, a rare degenerative neurological disease (SSPE) develops in a few children or adolescents several years after a measles infection.
Medium: The environment in which a microbe grows.
Meninges: Three layers of membrane that protect the brain and spinal cord. Meningitis, an inflammation of this area, can be either septic (bacterial) or aseptic (nonbacterial -- often caused by viruses).
Meningitis: An inflammation of the lining of the brain or spinal cord, the meninges. The term is often used to describe a disease involving those areas. Forms of meningitis include: Spinal (an inflammation solely of the spinal cord without involving the brain); Aseptic or Viral (if the cerebrospinal fluid does not yield a positive microbial culture and often follows an attack of mumps or one of the picornaviruses), Cocci (a form of coccidioidomycosis that attacks the lining of the brain), Bacterial (when specific bacteria, from other diseased areas of the body, migrate to the meninges). Three pathogens are responsible for bacterial meningitis: Neisseria meningitidis (meningococcus), Streptococcus pneumoniae (pneumococcus), and Haemophilus influenza of the Pittman type B (accounts for about 80% of all the meningitis cases). Newborns are particularly vulnerable to meningitis caused by the Lancefield group B streptococci, coliform bacteria, and Listeria monocytogenes. Meningitis caused by H. influenzae typically occurs in those six months to two years of age, with more than two-thirds of all cases involving children under five years of age. Older children and young adults are more prone to the bacterium Neisseria meningitidis, especially if they live in overcrowded conditions. People of any age can be victims of meningitis caused by S. pneumoniae.
Merozoite: A malaria trophozoite found in infected red blood or liver cells.
Mesophiles: Organisms that grow best when it is warm, at 20°C to 50°C (68°F to 122°F).
Metabolic: Relating to the chemical processes occurring within a cell and often associated with energy-producing processes.
Metabolism: The involvement all the biochemical anabolic and catabolic reactions in a cell.
Metazoa: The division of the animal kingdom that includes the multicellular organisms, that is, all animals except the Protozoa.
Methanogen: A methane-producing prokaryote and a member of the Archaea.
Methanotroph: An organism capable of oxidizing methane.
Methicillin-Resistance: A penicillinase-resistant antibiotic called methicillin. Methicillin-resistant organisms create a difficult treatment problem because of the limited number of antibiotics that can be selected from for use. One commonly known methicillin resistant organism is MRSA (methicillin-resistant Staphylococcus aureus).
MHC (Major Histocompatibility Complex) or HLA (Human Leucocyte Antigen): Molecules present on the surface of all cells (class 1) or only on certain immune cells (class 2) in humans. They give the cells of each individual a unique label, identical twins having the same set. Grafts from one individual to an unrelated one are rejected because the foreign MHC molecules are recognized by the immune system. Their function is to display antigens on the cell surface, either to start off the immune response (class 2) or to tell cytotoxic T lymphocytes that the cell is infected (class 1). Different MHC molecules prefer to take on different peptide antigens and to cope with the vast range of peptides from microbes. It is best to have as wide a range of MHC molecules as possible. Greater variety in MHC genes of humans ensures that there will always be a few of us who can resist any terrible new disease that arises. Too much variation, however, gives the immune system a trigger-happy sensitivity that can lead to autoimmunity. A suitable balance is needed.
Microaerophile: An organism that is damaged by atmospheric concentrations of oxygen, but that requires some oxygen, typically 2-5%, to be present to grow. Microaerophilic refers to a microorganism that grows best at reduced oxygen concentrations.
Microbial contamination: The addition of viable bacteria to a previously microbial-free environment, or the addition of unwanted organisms to an environment that contains known or desirable microorganisms.
Microorganism: Any living organism too small to be viewed with the naked eye. It includes bacteria, viruses, protists, and some algae and fungi.
Miliary Tuberculosis: A condition in which the TB bacilli are widely disseminated throughout the lung, with each serving as the focus for the development of a tubercle.
Minimum Inhibitory Concentration (MIC): is the lowest concentration of an antimicrobial agent that stops the growth of microorganisms.
Mobiluncus: A genus of a Gram-negative anaerobic bacilli present in the normal vaginal flora of most females. It is becoming more significant in cases of vaginitis, but culturing the organism has been difficult because of its extreme sensitivity to oxygen.
Mold: A multicellular filamentous fungus.
Molecule: The smallest complete unit of a substance that can exist independently and still retain the characteristic properties of the substance. Molecule is the smallest unit of substance made up of different atoms. It can be very small as in the case of water with three atoms or table salt with two atoms or very large as in the cases of protein and DNA, consisting of thousands of atoms.
Monkeypox: An infection of Central African monkeys caused by a virus closely related to smallpox. Humans in close contact with monkeys have been infected, suffering a smallpox-like disease, but it fails to spread through more than a few person to person transfers.
Monoclonal antibodies: Genetically engineered antibodies specific for one particular antigen.
Monocytes: Mononuclear phagocytic leukocytes called macrophages after they migrate into tissues. They are the largest white blood cells and are two to four times the size of red blood cells.
Mononucleosis: An infectious disease that manifests a high number of monocytes in the blood, enlarged lymph nodes, prolonged fever, appetite loss, and malaise most often seen in young adults; glandular disease caused by the Epstein-Barr virus. Also called glandular fever.
Moraxella species: Gram-negative coccobacilli that are nonmotile and not able to ferment glucose. There is an ongoing disagreement as to proper classification of this species. The clinical significance is also in question, but it has been isolated from a variety of illnesses, including conjunctivitis, septic arthritis, bacteremia, and urethritis. Some feel they are only opportunists, but other think they are significant contributors. In either case, they are susceptible to penicillin -- for the moment.
Mosquitoes: Insects that reproduce in still water. The female feeds on blood and transmits viral and parasitic diseases to animals and man.
Motility: The property of deliberate movement.
M-protein: A major virulence factor of streptococci. Located on the surface of the cell, this antigen can be used to separate group A streptococci into over sixty serotypes.
MRI (Magnetic Resonance Imaging): A method of imaging the body using a strong magnetic field.
Mucosa: Another term for mucus-secreting membranes that line the body cavities and canals that connect with external air, e.g., mouth, vagina, etc.
Mucus: A thick secretion produced by cells that covers mucous membranes. Mucus contains a polysaccharide called mucin and a variety of cells and salts.
Multiresistant: Referring to the resistance of bacteri to a variety of antibiotics with different mechanisms of antimicrobial action.
Mumps: A communicable disease caused by a paramyxovirus. It mainly attacks one or more of the parotid glands located in front of the ears, but other glands are also usually affected. In children, it is relatively a minor disease, but it can be more serious in adults.
Mushrooms: Filamentous fungi that produce large, often edible, structures, called fruiting bodies.
Mutation: A chemical change in DNA causing a change in genetic character, which is inherited unless the mutation is lethal. An organism that has undergone such a change is called a mutant.
Mycelia (singular mycelium): A mass of threadlike tubes of molds and streptomycete bacteria that form the vegetative parts of a fungus.
Mycobacterium: A class of pathogens that comes in between bacteria and viruses, producing two ancient and well-known diseases -- leprosy and tuberculosis. AIDS can also be added to the list, even though it is officially being labelled as a disease caused by a virus.
Mycology: The study of fungi.
Mycorrhiza: The symbiotic association between a fungus and the roots of a plant.
Mycosis: A fungal infection. Systemic mycoses includes such infections as coccidioidomycosis, blastomycosis (seen mostly in South America), and paracoccidioidomycosis, an infection produced by Paracoccidioides brasiliensis.
Myeloma: A cancer of the bone marrow.
Myoclonic jerk: A sharp involuntary movement produced when brain function is momentarily impaired.
Myonecrosis: The death and destruction of muscle cells. It is a serious clinical condition characteristic of gas gangrene and fatal without proper rapid treatment.