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.