Microbiology Terms Starting With D
Microbiology Glossary: D
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Death Phase
/ DETH fayz / · Old English death; Greek phasis, appearance
Death Phase is the final stage of a bacterial growth curve in which the number of viable cells decreases exponentially as nutrient depletion, waste accumulation, and environmental deterioration exceed the rate of new cell production.
During this stage, the rate at which cells die from nutrient exhaustion, waste product toxicity, and pH change exceeds the rate of cell division, causing viable counts to decline logarithmically. Some bacterial species respond by forming endospores, metabolically dormant structures that can survive months or years without nutrients, or by entering a viable-but-non-culturable state in which cells remain alive but cannot grow on standard laboratory media. Bacillus species illustrate the endospore strategy, while Vibrio cholerae is a well-documented example of viable-but-non-culturable persistence in aquatic environments.
In clinical microbiology, the D-value quantifies how long a disinfectant or sterilization method takes to reduce a viable cell population by 90 percent, data that are foundational for validating medical device safety.
Clostridium botulinum endospores formed during death phase can persist in improperly processed canned foods for years and germinate to produce botulinum toxin, one of the most potent biological toxins known, with a lethal dose estimated at roughly 1 to 2 nanograms per kilogram of body weight.
Every cell in a culture dies at the same moment when death phase begins. Survival varies considerably within a population, and a small subpopulation of persisters can remain viable long after the majority have died.
A nutrient broth inoculated with Escherichia coli typically enters death phase after 24 to 48 hours as glucose becomes exhausted and acidic fermentation products accumulate. Viable counts can fall from roughly 10 to the 9th cells per milliliter during late stationary phase to 10 to the 6th cells per milliliter or fewer within days, a decline of three orders of magnitude.
Disinfection
/ dis-in-FEK-shun / · Latin dis (apart) + inficere (to taint)
Disinfection is the process of eliminating most pathogenic microorganisms on inanimate surfaces or objects using chemical or physical agents, without necessarily destroying all microbial life.
Disinfectants are grouped by the spectrum of organisms they destroy. High-level disinfectants such as glutaraldehyde eliminate all microorganisms except large numbers of bacterial endospores; intermediate-level disinfectants inactivate most vegetative bacteria, mycobacteria, and many viruses; low-level disinfectants reduce common bacteria and some viruses but leave mycobacteria and endospores intact. Earle H.
Spaulding introduced a classification system in 1968 that links the required disinfection level to the infection risk posed by a medical device, distinguishing critical, semi-critical, and non-critical items. A bronchoscope, for example, contacts mucous membranes and requires high-level disinfection between patients, whereas a blood pressure cuff requires only low-level disinfection because it touches only intact skin.
Prions, the misfolded proteins responsible for diseases such as variant Creutzfeldt-Jakob disease, resist all standard chemical disinfectants and even survive many autoclaving cycles, requiring specialized decontamination protocols that include prolonged exposure to sodium hydroxide or extended steam sterilization at 134 degrees Celsius.
Disinfection and sterilization mean the same thing. Sterilization destroys all viable microorganisms including endospores, while disinfection reduces pathogen numbers to a level considered safe for a defined purpose without guaranteeing complete elimination.
Hospitals commonly apply sodium hypochlorite solution at concentrations of 1,000 to 5,000 parts per million to hard surfaces contaminated with Clostridioides difficile spores. Standard quaternary ammonium disinfectants are ineffective against C. difficile spores, making the choice of agent critical in outbreak control within a measurable window of 24 to 48 hours.
Dysbiosis
/ dis-by-OH-sis / · Greek dys, bad; bios, life; -osis, condition
Dysbiosis is a disruption in the composition or function of a microbial community associated with a host, in which the normal balance of microorganisms shifts in ways that can impair host health.
A healthy human gut microbiome harbors roughly 100 trillion microbial cells representing hundreds of species, and this community remains relatively stable under normal conditions. Broad-spectrum antibiotic treatment can reduce microbial diversity within days, depleting short-chain fatty acid producers such as Faecalibacterium prausnitzii and creating ecological space for opportunistic organisms. The resulting shift can increase intestinal permeability, reduce production of butyrate and other metabolites that nourish colonocytes, and amplify mucosal inflammation.
Dysbiosis has been associated with conditions ranging from Clostridioides difficile infection to inflammatory bowel disease, though researchers continue to distinguish associations from proven causal relationships.
Fecal microbiota transplantation, which transfers stool from a healthy donor into a recipient's colon, resolves recurrent Clostridioides difficile infection in more than 80 percent of cases by restoring a diverse microbial community, providing direct evidence that microbial community composition can determine disease outcome.
Dysbiosis always means one harmful microbe has invaded and taken over. It more often involves the loss of beneficial species, a reduction in community diversity, or a shift in community-level metabolic output rather than the dominance of a single pathogen.
After broad-spectrum antibiotic treatment for an unrelated infection, a patient's gut microbial diversity can drop sharply within 48 hours. This dysbiosis can permit Clostridioides difficile to proliferate, since the competing anaerobes that normally suppress it have been eliminated; studies show diversity may take weeks to months to recover even after antibiotics are stopped.