Microbiology Terms Starting With I
Microbiology Glossary: I
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Immunoevasion
/ im-yoo-noh-ee-VAY-zhun / · From Latin immunis, meaning exempt, and evadere, meaning to escape.
Immunoevasion is the collection of molecular and cellular strategies by which pathogens avoid, subvert, or manipulate host immune responses to survive and replicate within a host.
Pathogens have evolved diverse mechanisms to escape immune detection and destruction. Antigenic variation, seen in influenza viruses and Trypanosoma brucei, continuously alters surface proteins so that previously generated antibodies no longer bind effectively. Some bacteria produce polysaccharide capsules that physically block complement deposition and phagocyte recognition, while Mycobacterium tuberculosis survives inside macrophages by arresting phagosome maturation and preventing fusion with lysosomes.
Certain herpesviruses downregulate MHC class I molecules on infected cells, reducing cytotoxic T lymphocyte recognition, and Staphylococcus aureus secretes protein A, which binds the Fc region of IgG antibodies in reverse orientation and blocks opsonization. Each of these strategies reflects millions of years of co-evolution between pathogen and host immune system.
Neisseria gonorrhoeae shuffles gene segments encoding its pilus protein through a recombination mechanism that can generate thousands of distinct antigenic variants from a single genome, which is a key reason no licensed vaccine against gonorrhea exists despite decades of research.
Immunoevasion is the same as immunosuppression. Immunoevasion describes pathogen-specific tactics to avoid immune recognition, whereas immunosuppression is a broader reduction of immune activity that can result from disease, drugs, or genetic conditions regardless of any pathogen strategy.
Plasmodium falciparum, the parasite responsible for the most severe form of malaria, expresses approximately 60 antigenically distinct variants of the surface protein PfEMP1 on infected red blood cells. The parasite switches expression to a new variant roughly every 7 to 14 days to stay ahead of the antibody response the host mounts against the previous variant, a strategy that makes natural immunity slow to develop and has so far defeated every candidate malaria vaccine tested in large trials.
Infection
/ in-FEK-shun / · Latin infectio, from inficere, to taint
Infection is the invasion and multiplication of a pathogenic microorganism, including bacteria, viruses, fungi, or parasites, within a host organism's tissues, with outcomes ranging from no symptoms to severe disease depending on pathogen virulence and host immune status.
When a pathogen breaches host defenses and begins replicating in tissues, the host immune system mounts a response that may itself cause many of the recognizable signs of illness, such as fever, inflammation, and fatigue. Some infections remain subclinical, meaning the pathogen multiplies without producing noticeable symptoms, as occurs in the majority of people infected with Mycobacterium tuberculosis, where roughly 90 percent never develop active disease. The outcome of any infection reflects a balance between pathogen load and virulence factors on one side and innate and adaptive immune defenses on the other.
Transmission routes vary by pathogen and include direct contact, respiratory droplets, fecal-oral spread, vector bites, and contaminated food or water.
Poliovirus infects the vast majority of people without causing paralysis. Fewer than 1 percent of poliovirus infections result in paralytic disease, yet the virus can spread silently through a community via the fecal-oral route for weeks before any paralytic case is detected.
How To Become An Infectious Disease Specialist? →Infection and disease are the same thing. Disease occurs only when infection damages normal body function enough to produce symptoms or measurable physiological disruption, and many infections never reach that threshold.
Salmonella enterica serovar Typhimurium infects the human small intestine after ingestion of contaminated food, attaching to and invading intestinal epithelial cells within 6 to 48 hours. The bacteria multiply intracellularly and trigger an inflammatory response that produces diarrhea, fever, and abdominal cramping lasting 4 to 7 days in otherwise healthy adults.
Fun Facts About Digestive System →Inoculation
/ ih-nok-yoo-LAY-shun / · Latin inoculare, to graft; oculus, bud
Inoculation is the deliberate introduction of a microorganism, microbial material, or antigenic substance into a culture medium, living organism, or test system to initiate growth, establish infection, or stimulate an immune response under controlled conditions.
In a clinical or research microbiology laboratory, inoculation typically involves transferring a measured quantity of cells or sample onto solid agar or into liquid broth using sterile loops, swabs, or pipettes to prevent contamination by unwanted organisms. The size of the inoculum matters: too few cells may fail to establish growth, while too many can overwhelm a medium or obscure colony morphology. In medicine, inoculation with attenuated or killed pathogens, or with purified antigens, primes the adaptive immune system to produce memory cells without causing disease.
Edward Jenner performed one of the earliest documented deliberate inoculations in 1796, introducing cowpox material into a boy named James Phipps to protect him against smallpox.
The practice of variolation, deliberately inoculating people with material from smallpox pustules to induce milder protective infection, was documented in China as early as the 10th century and was introduced to England in 1721 by Lady Mary Wortley Montagu after she observed the practice in Ottoman Turkey.
Inoculation always introduces disease into a host. Inoculation describes the controlled introduction of microbial or antigenic material for purposes that include initiating laboratory cultures, testing antimicrobial susceptibility, and stimulating protective immunity, none of which require causing disease.
A clinical microbiologist inoculates a blood agar plate by streaking a patient's throat swab in a four-quadrant pattern to isolate individual colonies. After 24 hours of incubation at 37°C, beta-hemolytic colonies of Streptococcus pyogenes appear as distinct zones of clearing around each colony, allowing identification and antibiotic susceptibility testing.
Intracellular Pathogen
/ in-truh-SELL-yoo-ler PATH-oh-jen / · From Latin intra, meaning within, cellula, meaning small room, Greek pathos, meaning suffering, and genes, meaning born.
Intracellular Pathogen is a disease-causing microorganism that invades and replicates inside host cells, using the intracellular environment to evade immune defenses that cannot easily reach targets hidden within living tissue.
Obligate intracellular pathogens such as Chlamydia trachomatis and Rickettsia prowazekii cannot replicate outside a host cell because they lack the metabolic machinery for independent reproduction, while facultative intracellular pathogens such as Salmonella enterica and Mycobacterium tuberculosis can survive both inside and outside cells. Once inside, pathogens may reside within membrane-bound vacuoles, as M. tuberculosis does by arresting phagosome maturation, or escape into the cytoplasm, as Listeria monocytogenes does within minutes of uptake.
Antibiotics targeting intracellular pathogens must penetrate the host cell membrane and, in some cases, reach acidic or modified compartments where drug activity may be reduced.
Listeria monocytogenes polymerizes host actin filaments at one pole of its cell surface, generating a propulsive "comet tail" that drives the bacterium directly through the host cell membrane into an adjacent cell. This actin-based motility lets the pathogen spread from cell to cell without ever entering the extracellular space where antibodies and complement could act on it.
Intracellular pathogens infect only immune cells such as macrophages. Many intracellular pathogens preferentially target epithelial cells, endothelial cells, hepatocytes, or neurons depending on the species, and some, like Plasmodium falciparum, invade red blood cells, which are not immune cells at all.
Plasmodium falciparum invades human red blood cells, where it digests hemoglobin and multiplies asexually over a 48-hour cycle. Each infected cell ruptures to release 16 to 32 new merozoites, and the synchrony of these rupture events produces the cyclical fever spikes characteristic of falciparum malaria.