Microbiology Terms Starting With N

Narrow-Spectrum Antibiotic is an antimicrobial drug that kills or inhibits the growth of only a limited number of closely related bacterial species, leaving most of the normal microbial flora undisturbed.

Clinicians prefer narrow-spectrum antibiotics when laboratory culture and sensitivity testing have identified the causative pathogen, because targeting only the responsible organism minimizes disruption to the patient’s normal microbiota. Penicillin V, for example, targets gram-positive bacteria such as Streptococcus pyogenes by inhibiting cell wall synthesis, while leaving gram-negative organisms largely unaffected. Selective pressure for resistance also tends to be lower with narrow-spectrum agents because fewer bacterial species are exposed to the drug.

Isoniazid exemplifies extreme selectivity, targeting Mycobacterium tuberculosis by blocking mycolic acid synthesis, a pathway found only in mycobacteria.

Neurotropic Virus is a virus that preferentially infects cells of the nervous system, including neurons and glial cells of the brain, spinal cord, and peripheral nerves.

Neurotropic viruses reach nerve tissue through multiple routes, including direct inoculation at bite sites, hematogenous spread via the bloodstream, and retrograde axonal transport along nerve fibers. Infection severity depends on which neural cells are targeted and the host immune response, ranging from mild meningitis to fatal encephalitis. Rabies virus exemplifies retrograde axonal movement, traveling from muscle tissue toward the central nervous system at approximately 3 millimeters per day, which explains incubation periods of weeks to months depending on the distance between the bite site and the brain.

Japanese encephalitis virus, transmitted by Culex mosquitoes across Asia, infects neurons of the thalamus and brainstem and causes fatal encephalitis in roughly 20 to 30 percent of symptomatic cases.

Nitrification is the two-step microbial process by which specialized bacteria and archaea oxidize ammonia first to nitrite and then to nitrate, converting nitrogen from decomposed organic matter into a form that plants can absorb through their roots.

Nitrogen locked in proteins and nucleic acids is released as ammonia when organisms die and decompose, but most plants cannot absorb ammonia directly from soil. Nitrifying bacteria such as Nitrosomonas europaea carry out the first oxidation step, converting ammonia to nitrite, while Nitrobacter winogradskyi and related genera convert nitrite to nitrate in the second step. Both groups are obligate chemolithotrophs, meaning they derive all their energy from these inorganic oxidation reactions rather than from organic carbon.

Nitrification is sensitive to soil pH, proceeding most rapidly between pH 6.5 and 8.0 and slowing sharply in acidic soils below pH 5.5.

Nitrogen Fixation is the microbial conversion of atmospheric nitrogen gas into ammonia by nitrogenase-containing bacteria or archaea, making nitrogen biologically available to plants and food webs.

Atmospheric nitrogen gas is abundant but chemically inert because its two nitrogen atoms are joined by a strong triple bond that plants and animals cannot break. Nitrogen-fixing microbes use the nitrogenase enzyme complex to reduce N2 to ammonia, a reaction that consumes at least 16 ATP molecules per molecule of nitrogen gas reduced. Free-living fixers include cyanobacteria, Azotobacter, and some Clostridium species, while symbiotic fixers such as Rhizobium live in legume root nodules.

Because nitrogenase is oxygen-sensitive, many nitrogen-fixing organisms protect the enzyme through low-oxygen compartments, respiratory oxygen consumption, heterocysts, or leghemoglobin-buffered nodules.

Normal Flora is the traditional term for the resident microorganisms that colonize healthy body surfaces without usually causing disease at their normal anatomical sites.

Normal flora occupy the skin, mouth, intestines, respiratory tract, and urogenital tract, with each site selecting for organisms suited to its pH, oxygen level, moisture, nutrients, and immune defenses. These microbes can protect the host by occupying attachment sites, consuming nutrients that pathogens would use, producing bacteriocins, and stimulating local immune maturation. The term is older and somewhat imprecise because these communities include bacteria, archaea, fungi, and viruses, so many scientists now prefer resident microbiota or normal microbiota.

Disruption by antibiotics, immune suppression, or medical devices can turn harmless residents such as Staphylococcus epidermidis or Candida albicans into opportunistic pathogens.