Microbiology Terms Starting With R

Rabies is a fatal viral encephalitis caused by rabies lyssavirus, transmitted primarily through the saliva of infected mammals and characterized by progressive neurological deterioration that is almost universally lethal once clinical symptoms appear.

Rabies lyssavirus belongs to the family Rhabdoviridae and carries a negative-sense, single-stranded RNA genome. After entering a bite wound, the virus binds to nicotinic acetylcholine receptors at the neuromuscular junction and travels by retrograde axonal transport toward the central nervous system at a rate of roughly 50 to 100 millimeters per day, which explains why bites closer to the head carry shorter incubation periods. Once the virus reaches the brain, it causes acute encephalitis marked by agitation, hydrophobia, and autonomic instability.

Post-exposure prophylaxis, a series of vaccine doses combined with rabies immunoglobulin administered promptly after a high-risk exposure, is nearly 100 percent effective at preventing disease if started before symptoms develop.

Reservoir Host is an animal or environmental source that maintains a pathogen through long-term infection without developing serious disease, providing a persistent source from which the pathogen can spread to other species.

Reservoir hosts typically share a long evolutionary history with their pathogens, and this coevolution has produced immunological adaptations that limit disease. Bats, for example, tolerate a remarkable diversity of RNA viruses, including the ancestors of SARS-CoV-2, Ebola virus, and Marburg virus, partly because their constitutively active interferon signaling suppresses viral replication without triggering the damaging inflammatory responses seen in naive hosts. Rodents maintain Yersinia pestis, the plague bacterium, in flea-mediated transmission cycles across grassland ecosystems on multiple continents, sustaining the pathogen between human outbreaks.

When a pathogen spills over into a host species that lacks coevolved tolerance, such as humans encountering a bat coronavirus for the first time, the mismatch between viral replication rate and host immune response can produce severe or fatal disease.

Resistance Gene is a gene encoding a product that confers on the host bacterium the ability to survive antibiotic concentrations that would otherwise inhibit or kill susceptible strains.

Resistance genes encode diverse mechanisms, including enzymes that destroy antibiotics, proteins that pump drugs out of the cell, and altered targets that drugs no longer bind effectively. Beta-lactamases, for example, hydrolyze the beta-lactam ring of penicillins and cephalosporins, rendering these drugs inactive before they can inhibit cell wall synthesis. Many of these genes sit on mobile genetic elements such as plasmids, transposons, and integrons, which transfer between bacterial cells through conjugation, transformation, or transduction.

A single plasmid can carry multiple resistance genes simultaneously, allowing one horizontal transfer event to confer resistance to several unrelated drug classes at once.

Reverse Transcriptase is an enzyme encoded by retroviruses and some other mobile genetic elements that synthesizes a DNA strand using an RNA molecule as its template.

Reverse transcriptase in HIV (human immunodeficiency virus) copies the viral RNA genome into DNA at a rate of roughly 100 nucleotides per second, far slower than most cellular DNA polymerases. Critically, the enzyme lacks a proofreading domain, introducing approximately one error per 10,000 nucleotides copied; across a genome of about 9,700 nucleotides, nearly every new viral copy carries at least one mutation. This high error rate generates enormous genetic diversity within a single infected patient, accelerating the emergence of drug-resistant variants.

After synthesizing a complementary DNA strand to form an RNA-DNA hybrid, the enzyme’s built-in RNase H activity degrades the original RNA template, and reverse transcriptase then synthesizes the second DNA strand to complete a double-stranded DNA copy ready for integration into the host chromosome.

Rhizobium is a genus of soil bacteria that fixes atmospheric nitrogen inside root nodules formed through a symbiotic relationship with leguminous plants, converting nitrogen gas into ammonia that the host plant can absorb as a nutrient.

Atmospheric nitrogen gas makes up about 78% of air, yet plants cannot absorb it directly because the triple bond holding the two nitrogen atoms together requires more energy to break than most organisms can supply. Rhizobium bacteria produce nitrogenase, an enzyme complex that cleaves this bond and combines nitrogen with hydrogen to yield ammonia, a form plants readily take up. The plant supplies the bacteria with photosynthetically derived sugars in return, and the root nodule maintains very low oxygen concentrations using a protein called leghemoglobin, which protects the oxygen-sensitive nitrogenase from inactivation.

A single soybean (Glycine max) plant can host nodules fixing more than 100 kilograms of nitrogen per hectare per growing season when conditions are optimal, reducing the need for synthetic nitrogen fertilizer.

Rickettsia are obligate intracellular bacteria transmitted to humans primarily through the bites of infected arthropods such as ticks, fleas, and lice, and they cause diseases including Rocky Mountain spotted fever and epidemic typhus.

Unlike most bacteria that grow freely in nutrient broths, Rickettsia cannot replicate outside a living eukaryotic cell because they lack the metabolic machinery to produce their own ATP independently. After an infected tick or louse deposits the bacteria during feeding, Rickettsia enter host endothelial cells lining blood vessels, escape the phagosome before it fuses with a lysosome, and replicate directly in the cytoplasm. Rickettsia prowazekii, the agent of epidemic typhus, spreads between cells by hijacking the host cell’s actin polymerization machinery, propelling itself through the cytoplasm and into adjacent cells.

Rocky Mountain spotted fever, caused by Rickettsia rickettsii, kills roughly 20% of untreated patients, making early antibiotic treatment with doxycycline critical to survival.

RNA Virus is a virus that carries its genetic information as ribonucleic acid rather than DNA and replicates using RNA-dependent polymerases or reverse transcriptase.

Because RNA-dependent RNA polymerases introduce errors at roughly one mutation per genome per replication cycle, RNA virus populations exist as swarms of closely related but genetically distinct variants called quasispecies, which accelerates adaptation to new hosts and antiviral drugs. RNA viruses are classified by genome polarity: positive-sense genomes can be translated directly by host ribosomes, negative-sense genomes must first be copied into a complementary strand, and double-stranded RNA genomes are found in viruses such as rotaviruses. Retroviruses such as HIV carry a reverse transcriptase enzyme that converts their RNA genome into DNA, which then integrates into the host chromosome as a provirus.

Among the most consequential human pathogens are RNA viruses: influenza viruses, SARS-CoV-2, measles virus, poliovirus, and rabies lyssavirus all belong to this category.