Section O

O₂: Oxygen; both eyes.

O & P: Ova and parasites.

O-F: Oxidation-fermentation medium.

Obligate: An adjective referring to an environmental factor (for example, oxygen) which is always required for growth, for example, obligate aerobe. Compare with facultative.

Obligate aerobe: An organism which can grow only in the presence of oxygen (O₂).

Obligate anaerobe: An organism growing only in an anaerobic environment, not in a microaerophilic environment, a CO₂ incubator, or air.

Obligate parasite: Parasite that must always live in contact with the host.

Occult blood: Blood present in very small amount; usually detectable by chemical means; specimen is most often stool; may or may not be related to parasitic infection.

Octal numbers: Numbers employed in computer data bases to identify biochemical profiles of organisms and thus their identification.

Oil immersion microscopy: Use the immersion oil to fill the space between the slide being studied and the special objective of the microscope; this keeps the light rays from dispersing and provides good resolution at high magnification (total magnification of 1000x).

Oligonucleotide: A short nucleic acid molecule, either obtained from an organism or synthesized chemically.

Oligotrophic: Describing a body of water in which nutrients are in low supply.

Onchocercoma: Nodule containing adult worms (onchocerciasis).

Oncogene: A gene whose expression causes formation of a tumor.

Oncogenic: Possessing the potential to cause normal cells to become malignant; causing cancer.

Oncosphere: Spherical, six-hooked tapeworm larva within the egg shell (Taenia and Hymenolepis spp.).

ONPG: O-nitrophenol-B-galactopyranoside (B-galactosidase test).

Open reading frame (ORF): The entire length of a DNA molecule that starts with a start codon and ends with a stop codon.

Operator: A specific region of the DNA at the initial end of a gene, where the repressor protein binds and blocks mRNA synthesis.

Operculated ova: Ova possessing a cap or lid (trap door) at one end through which the larva escapes (Diphyllobothrium, Clonorchis, and Paragonimus spp.).

Operculum: A lidlike structure on one end of the egg shell through which the larval form escape (Diphyllobothrium, Clonorchis, and Paragonimus spp.).

Operon: A cluster of genes whose expression is controlled by a single operator. Typical of prokaryotic cells.

Opisthotonic: Spastic state in which the head and heels are bent backward and the torso extends outward.

Opportunistic infection: An infection caused by an organism capable of causing disease only in individuals whose resistance to infection is lowered.

Opportunistic pathogens: Microorganisms that, under ordinary circumstances, cause no harm but can cause disease under certain conditions (e.g. after immunosuppressive therapy or when the organisms gain access to a usually sterile body site).

Opsonic: Pertaining to an agent (typically an antibody) that, when bound to an antigen such as bacterial proteins, enhances the ingestion of the antigen by white blood cells.

Opsonization: Promotion of phagocytosis by a specific antibody in combination with complement.

Opsonize: To facilitate destruction of pathogens by phagocytic ingestion or lysis by complement through the action of adherent antibodies.

OPV: Oral polio vaccine.

Orchitis: Inflammation of a testis; may be accompanied by swelling, pain, fever (filariasis).

Organelle: A membrane-enclosed body specialized for carrying out certain functions; found only in eukaryotic cells.

Organomegaly: Abnormal enlargement of the organs; visceromegaly.

Organotroph: In reference to energy source (electron donor) - An organism which obtains energy by the metabolism of organic substrates (as electron donors). There are two type of organotrophs: chemoorganotroph and photoorganotroph. Compare with autotroph, lithotroph, heterotroph, phototroph.

Oropharyngeal: Pertaining to the oral and pharyngeal cavities.

Orthostatic hypotension: Decreased blood pressure caused by standing erect; often seen in patients who are dehydrated.

OSHA: Occupational Safety and Health Administration.

Osmosis: Diffusion of water through a membrane from a region of low solute concentration to one of higher concentration.

Osteomyelitis: Inflammation of the bone and the marrow.

Ostiole: A mouth or opening.

OTC: Over-the-counter.

Otitis: Inflammation of the ear from a variety of causes, including bacterial infection; otitis media; inflammation of the middle ear.

Ototoxic: Refers to a substance that has a toxic effect on the ear; some antibiotics, for example, have this property.

Outbreak: The occurrence of a large number of cases of a disease in a short period of time.

Oxic: Containing oxygen; aerobic. Usually used in reference to a microbial habitat.

Oxidase test: Determines the organism's ability to produce cytochrome oxidase.

Oxidation: A metabolic pathway of the microorganism that involves use of oxygen as a terminal electron acceptor. This type of reaction occurs in air.

Oxidation-reduction potential: Electromotive force exerted by a nonreacting electrode in a solution containing the oxidized and reduced forms of a chemical, relative to a standard hydrogen electrode; the more negative the value, the more anaerobic conditions are.

Oxidation-reduction (redox) reaction: A coupled pair of reactions, in which one compound becomes oxidized, while another becomes reduced and takes up the electrons released in the oxidation reaction.

Oxidative phosphorylation: The non-phototrophic production of ATP at the expense of a proton motive force (PMF) formed by electron transport. Also electron-transport phosphorylation.

Oxygenic: Able to produce oxygen. (Contrast with anoxygenic.)

Oxidative phosphorylation: The non-phototrophic production of ATP at the expense of a proton motive force (PMF) formed by electron transport. Also electron-transport phosphyrylation.

Oxygenic photosynthesis: Use of light energy to synthesize ATP and NADPH by noncyclic photophosphorylation with the production of oxygen from water.

oz: Ounce.

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