Microbiology Terms Starting With L
Microbiology Glossary: L
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Lag Phase
/ LAG FAYZ / · Swedish lag (slow) + Latin phasis (appearance)
Lag Phase is the initial period of the bacterial growth curve during which cells adapt to a new environment by synthesizing enzymes, cofactors, and other molecules needed for replication, with little or no net increase in cell number.
During lag phase, bacteria are metabolically active but not yet dividing at a measurable rate. Cells upregulate biosynthetic genes, repair DNA damage accumulated during storage or stress, and build up intracellular pools of ATP, ribosomes, and precursor molecules required for rapid replication. Duration of the lag phase depends on the physiological state of the inoculum, the composition of the growth medium, and environmental conditions such as temperature and pH; cells transferred from a rich medium into a chemically identical fresh medium show a shorter lag than cells moved from a nutrient-poor or cold environment.
In food safety, understanding lag phase duration is important because pathogens such as Listeria monocytogenes can extend their lag phase at refrigeration temperatures, slowing the onset of dangerous population growth.
When Escherichia coli is shifted from glucose-containing medium to lactose-containing medium, it undergoes a secondary lag phase called diauxic lag while it synthesizes the enzymes of the lac operon. This phenomenon, described by Jacques Monod in the 1940s, was central to his Nobel Prize-winning work on gene regulation.
Are Enzymes Proteins? →No bacterial activity occurs during lag phase. Cells in lag phase can be among the most metabolically active in the entire growth cycle, consuming nutrients and synthesizing macromolecules at high rates even though population size has not yet increased.
Escherichia coli transferred from a stationary-phase culture stored at 4°C into fresh Luria-Bertani broth at 37°C typically shows a lag phase of 30 to 90 minutes before exponential division begins. During this interval, ribosome content per cell increases several-fold as the bacteria rebuild the translational machinery needed to sustain rapid growth.
Lignin Degradation
/ LIG-nin deg-rah-DAY-shun / · Scientific term used in microbial ecology.
Lignin Degradation is the enzymatic breakdown of lignin, a phenylpropanoid polymer that rigidifies plant cell walls, carried out primarily by wood-rotting fungi and certain bacteria that secrete oxidative enzymes capable of cleaving the polymer's ether and carbon-carbon bonds.
White-rot fungi such as Phanerochaete chrysosporium produce three main oxidative enzymes, lignin peroxidase, manganese peroxidase, and laccase, that generate free radicals attacking lignin non-specifically rather than through a defined active site, allowing degradation of a polymer too irregular for conventional substrate-specific enzymes. This oxidative mechanism is energetically expensive, requiring the fungus to invest carbohydrate carbon to generate the necessary reducing equivalents and hydrogen peroxide. Brown-rot fungi, by contrast, modify lignin chemically without fully mineralizing it, leaving a brown, crumbly residue of altered polymer in decayed wood.
Globally, lignin degradation returns an estimated 2.4 billion tons of carbon per year from dead plant material to the atmosphere and soil, making it one of the largest fluxes in the terrestrial carbon cycle.
Certain soil bacteria in the genus Nocardia and the actinomycete Streptomyces viridosporus can partially degrade lignin using peroxidase-like enzymes, but no bacterium studied so far mineralizes lignin as completely as white-rot basidiomycetes. This gap has driven research into engineering bacterial lignin-degrading pathways for industrial lignocellulose bioconversion.
Are Enzymes Proteins? →Dead wood disappears mainly through physical weathering and rainfall. Fungal enzymatic degradation, particularly by white-rot basidiomycetes, drives the majority of chemical lignin breakdown in forest ecosystems, with physical weathering playing a secondary role.
Explore Mycology →Phanerochaete chrysosporium colonizing fallen hardwood logs produces visible white mycelial fans across the wood surface while its peroxidases degrade lignin within the cell walls. Within weeks of colonization under warm, moist conditions, wood density can decrease by more than 30 percent as lignin and cellulose are progressively mineralized.
Lipopolysaccharide
/ LIP-oh-pol-ee-SAK-ah-ryd / · Scientific term used in bacteriology.
Lipopolysaccharide is a large amphipathic molecule anchored in the outer membrane of Gram-negative bacteria that consists of a lipid A anchor, a core oligosaccharide, and a variable O-polysaccharide chain, and that triggers potent innate immune responses in infected animals.
Lipid A, the membrane-anchored portion of lipopolysaccharide, contains two glucosamine sugars carrying six fatty acid chains and phosphate groups that bind Toll-like receptor 4 on mammalian immune cells, activating nuclear factor kappa-B signaling and driving production of interleukin-6 and tumor necrosis factor-alpha. Concentrations as low as 1 to 10 nanograms per kilogram of body weight can induce fever and systemic inflammation in humans, and higher doses trigger septic shock by causing uncontrolled cytokine release. Smooth lipopolysaccharide, produced by pathogens such as Salmonella enterica and Shigella flexneri, carries long O-polysaccharide chains that shield the bacterial surface from complement deposition and antibody binding, enhancing immune evasion.
Rough mutants lacking the O-chain, such as certain laboratory strains of Escherichia coli, are far more susceptible to complement-mediated killing than their smooth counterparts.
Pharmaceutical manufacturers test injectable drugs and medical devices for lipopolysaccharide contamination using the Limulus amebocyte lysate assay, which exploits a clotting reaction in the blood cells of the horseshoe crab (Limulus polyphemus). This assay can detect lipopolysaccharide at concentrations below 0.01 endotoxin units per milliliter, a sensitivity no chemical test has matched.
Lipopolysaccharide is present in all bacteria. It is a defining outer membrane component of Gram-negative bacteria specifically; Gram-positive bacteria lack an outer membrane entirely and carry teichoic acids instead.
Neisseria meningitidis, a Gram-negative bacterium that colonizes the human nasopharynx, produces a lipopolysaccharide variant called lipooligosaccharide that lacks the long O-polysaccharide chain. When N. meningitidis enters the bloodstream, lipooligosaccharide release triggers rapid cytokine production that can progress to septic shock and death within 24 hours if untreated.
E-coli →Log Phase
/ LOG FAYZ / · Logarithm; Old French phase
Log Phase is the period of microbial growth during which cell numbers increase exponentially because each cell divides at a constant maximum rate, resulting in a predictable population doubling time under favorable conditions.
During log phase, cells actively synthesize DNA, RNA, proteins, membranes, and cell wall material while nutrients remain sufficient and waste products have not yet accumulated to inhibitory levels. Generation time varies from about 20 minutes for Escherichia coli under optimal conditions to several hours for slow-growing organisms such as Mycobacterium tuberculosis. Log-phase cells are often more sensitive to antibiotics that target cell wall synthesis, DNA replication, or protein synthesis because those processes are highly active.
Researchers routinely harvest log-phase cultures for experiments requiring uniform, metabolically active cells, since cells in other growth phases behave less predictably.
The log phase concept was formalized through quantitative growth studies in the early twentieth century, and Jacques Monod's 1949 work on bacterial growth kinetics in defined media helped establish the mathematical framework still used to model exponential growth today.
Cell Cycle →Log phase means cells are only logging data. The term "log" refers to logarithmic, meaning exponential, increase in cell number.
A fresh culture of Bacillus subtilis inoculated into rich broth typically enters log phase within one to two hours after an initial lag period. During log phase, the population can double every 25 to 35 minutes under optimal temperature and nutrient conditions, increasing cell counts from roughly one million to one billion cells per milliliter within a few hours.
Lysogenic Cycle
/ ly-soh-JEN-ik SY-kul / · Greek lysis, loosening; gennan, to produce; kyklos, circle
Lysogenic Cycle is a mode of viral replication in which a bacteriophage integrates its genome into the host bacterial chromosome as a prophage, replicating silently with the host through successive cell divisions without immediately killing it.
In the lysogenic cycle, a bacteriophage injects its DNA, which integrates into the bacterial chromosome as a prophage. Every time the bacterium divides, the prophage is copied along with the host DNA. The virus can remain dormant for many generations, but stress from UV radiation, DNA damage, or starvation can trigger the prophage to excise, enter the lytic cycle, make new virus particles, and burst out of the cell.
Lambda phage (Enterobacteria phage lambda) is the best-studied example, and its genetic switch between lysogeny and lysis has been analyzed in detail since the 1950s.
Lysogenic conversion can alter the behavior of the host bacterium in medically significant ways. The bacterium Corynebacterium diphtheriae produces its lethal diphtheria toxin only when it carries a specific lysogenic phage called corynephage beta, meaning the phage, not the bacterium's own genome, encodes the toxin.
Phage infection always kills the bacterium immediately. Lysogenic phages can be copied silently alongside host DNA for many generations before any lytic activity occurs.
Lambda phage (Enterobacteria phage lambda) can enter a lysogenic state in Escherichia coli, integrating its roughly 48,500 base-pair genome at a specific attachment site on the bacterial chromosome. The prophage persists stably through hundreds of bacterial generations until an inducing signal, such as UV radiation damage, triggers excision and entry into the lytic cycle.
E-coli →Lytic Cycle
/ LIT-ik SY-kul / · Greek lysis, loosening; Greek kyklos, circle
Lytic Cycle is the mode of viral replication in which a bacteriophage hijacks the host cell's biosynthetic machinery, produces new viral particles, and then lyses the cell to release progeny virions, killing the host in the process.
The lytic cycle proceeds through five stages: attachment, penetration, biosynthesis, assembly, and release. During attachment, the phage binds specific receptor proteins on the host cell surface with high specificity, which is why a given phage infects only certain bacterial strains. After genome entry, the host’s ribosomes and enzymes are redirected to produce viral proteins and replicate viral nucleic acid, then new phage particles are assembled and the cell is ruptured.
T4 bacteriophage completes its entire lytic cycle in Escherichia coli in approximately 25 minutes at 37 degrees Celsius, releasing around 100 to 200 new phage particles per lysed cell.
T4 phage was central to the famous Hershey-Chase experiment of 1952, in which Alfred Hershey and Martha Chase used radioactive isotopes to confirm that DNA, not protein, carries genetic information during phage infection.
Every viral infection immediately bursts the host cell. Some viruses can enter latent or lysogenic states instead, persisting within the host for extended periods without causing immediate cell death.
T4 bacteriophage follows a lytic cycle in Escherichia coli, attaching to lipopolysaccharide and protein receptors on the bacterial outer membrane. The entire cycle from attachment to cell lysis takes roughly 25 minutes, and each lysed bacterium releases approximately 100 to 200 new phage particles into the surrounding medium.
E-coli →