Exam 3 Microb

Virus

A genetic element containing either RNA or DNA surrounded by a protein capsid that replicates only inside host cells.

Virion

The extracellular form of a virus that allows it to travel from one host cell to another.

Virus size range

Typically 0.02 to 0.3 μm (20–300 nanometers).

Viral genome types

Consist of DNA or RNA, single-stranded or double-stranded, and can be circular or linear.

Capsid

The protein shell that surrounds the genome of a virus particle.

Capsomere

The individual protein molecule subunits that make up a virus capsid.

Naked virus

A virus consisting of only a nucleocapsid (nucleic acid and protein) with no further outer layers.

Enveloped virus

A virus having a lipoprotein membrane (derived from host membrane) surrounding the nucleocapsid.

Complex virus

A virus consisting of several parts with different shapes, such as head-plus-tail bacteriophages like T4.

Helical virus

A rod-shaped virus (like TMV) where length is determined by nucleic acid length and width by capsomere packaging.

Icosahedral virus

A roughly spherical virus with 20 triangular faces and 12 vertices; the most efficient subunit arrangement.

Bacteriophages

Viruses that specifically infect bacterial cells.

Virulent (lytic) infection

A type of infection where the virus redirects host metabolism to replicate and ultimately destroys the host.

Lysogenic infection

A state where the viral genome becomes part of the host genome and is replicated in step with it.

Lysozyme role

Small quantities make entry pores in peptidoglycan; large quantities lyse the host cell for virion release.

Reverse transcriptase

The retroviral enzyme that produces DNA from an RNA template.

Neuraminidase

Envelope proteins that destroy animal cell connective tissue to liberate mature virions.

Viral host types

Includes bacteriophages (bacteria), archaeal viruses, animal viruses, and plant viruses.

Cultivation methods

Bacteria use liquid or agar lawns; animal viruses are grown in tissue cultures from animal organs.

5 phases of viral replication

Attachment, Penetration (entry), Synthesis, Assembly/Maturation, and Release.

One-step growth curve

A growth pattern showing no increase in extracellular virions until the host cells burst and release them.

Latent period

The time during a viral infection that includes the eclipse and maturation phases.

Eclipse period

The phase after infection where the genome is replicated/translated but the virus is no longer infectious if the cell breaks.

Burst size

The number of infectious virions released per host cell.

Attachment specificity

Controlled by interactions between virion surface proteins and specific host cell components called receptors.

Viral receptors

Host surface components (proteins, LPS, flagella, pili) that the virus recognizes to initiate attachment.

Permissive cell

A cell that supports the complete replication cycle of a virus.

Bacteriophage T4 infection

Tail fibers bind LPS, tail pins contact wall, lysozyme forms pore, sheath contracts, DNA is injected.

Restriction-modification systems

Bacterial defense using enzymes to cleave foreign dsDNA while protecting host DNA via methylation.

Restriction endonucleases

Enzymes that cleave foreign DNA at specific sites to prevent viral invasion.

DNA protection (T4)

Substituting 5-hydroxymethylcytosine for cytosine and glucosylating it to resist host restriction enzymes.

Early proteins

Proteins synthesized soon after infection (e.g., replisome enzymes) before viral genome replication.

Late proteins

Structural and release proteins (e.g., head/tail proteins, lysozyme) synthesized later in the infection.

Plus (+) vs. minus (-) RNA

Plus (+) has the same sequence as mRNA; Minus (-) is complementary to the mRNA sequence.

Positive-strand RNA virus

An RNA virus with a genome of the plus configuration.

Negative-strand RNA virus

An RNA virus with a genome of the minus configuration.

Baltimore classification

A system classifying viruses based on the relationship of the viral genome to its mRNA.

Lysogeny

State where the viral genome (prophage) is replicated in synchrony with the host chromosome without killing the host.

Prophage

The lysogenic form of a bacteriophage that is integrated into the host genome or exists as a plasmid.

Lysogen

A bacterium that contains a prophage.

Bacteriophage Lambda

dsDNA head-and-tail virus; linear genome circularizes at cos sites and integrates at att sites.

Rolling circle replication

A mechanism where one DNA strand is nicked and "unrolled" to serve as a template for concatemers.

Temperate vs. virulent phage

Virulent phages always kill the host; temperate phages can establish lysogeny or enter the lytic cycle.

Animal virus entry

Unlike bacteriophages, the entire virion typically enters the host cell via fusion or endocytosis.

Animal infection consequences

Can be lytic (lysis), persistent (slow budding), latent (inactive), or transformation (tumor formation).

Retroviruses

Enveloped animal viruses with an RNA genome that replicate through a DNA intermediate.

Retroviral enzymes

Includes reverse transcriptase, integrase, and a viral-specific protease carried within the virion.

Retrovirus genes

gag (structural proteins), pol (reverse transcriptase and integrase), and env (envelope proteins).

Retrovirus replication

1. Entry, 2. Uncoating, 3. Reverse transcription, 4. DNA entry to nucleus, 5. Integration, 6. Transcription, 7. Translation, 8. Assembly/Budding.

Defective viruses

Viruses unable to replicate or infect without the assistance of a helper virus.

Viroids

Infectious, small (246–399 bp), circular ssRNA molecules that lack a protein coat and cause plant diseases.

Prions

Infectious agents consisting entirely of protein with no DNA or RNA.

PrPC vs. PrPSc

PrPC is the native cellular form; PrPSc is the pathogenic misfolded form that causes disease.

Prion disease mechanisms

Can be infectious (transmitted), sporadic (random misfolding), or inherited (Prnp gene mutations).

Transmissible spongiform encephalopathies

Collective name for animal prion diseases (e.g., mad cow) characterized by "spongy" loss of neural tissue.

Class 1 (dsDNA)

Double-stranded DNA → transcribed into mRNA

Class 2 (ssDNA)

Single-stranded DNA → must become dsDNA → then mRNA

Class 3 (dsRNA)

Double-stranded RNA → use (–) strand to make mRNA

Class 4 (+ssRNA)

Positive-sense RNA = already mRNA → directly translated

Class 5 (–ssRNA)

Negative-sense RNA → must be converted to +RNA

Class 6 (+ssRNA retrovirus)

RNA → DNA (reverse transcription) → mRNA

Class 7 (dsDNA reverse)

DNA → RNA → DNA → mRNA

Positive (+) strand

Same sequence as mRNA → can be translated

Negative (–) strand

Complementary to mRNA → must be converted

Reverse transcriptase

Enzyme that makes DNA from RNA

Class 4 key feature

Genome acts directly as mRNA

Growth

An increase in the number of cells

. Binary fission

A cell division process where two cells arise from one

. Generation time

The time required for a population of microbial cells to double in number

. Septum

The partition that forms between dividing cells during binary fission, resulting from the inward growth of the cytoplasmic membrane and cell wall

. Biofilm

An attached polysaccharide matrix containing embedded bacterial cells

. Planktonic growth

Growth of microorganisms in suspension within a liquid medium

. Exponential growth

A pattern of growth where the number of cells doubles within a constant time interval

. Batch culture

A closed-system microbial culture of a fixed volume

. Lag phase

The interval before growth begins after a microbial culture is inoculated into fresh medium

. Stationary phase

The growth phase where there is no net increase or decrease in cell number because the growth rate is zero

. Chemostat

A continuous culture device that allows for the independent control of growth rate and cell density

. Defined media

Culture media prepared by adding precise amounts of pure inorganic or organic chemicals to distilled water

. Complex media

Culture media made from digests of microbial, animal, or plant products where the exact nutritional composition is unknown

. Aseptic technique

A series of steps taken to prevent contamination of laboratory cultures and media

. Viable cell

A cell that is able to divide and form offspring

. Great Plate Count Anomaly

The phenomenon where direct microscopic counts of natural samples reveal far more organisms than those recoverable on a single culture medium

. Turbidity

A measurement of cell mass based on the fact that cell suspensions look cloudy because cells scatter light

. Psychrophile

An organism with an optimal growth temperature of 15°C or lower

. Mesophile

An organism that grows best at temperatures between 20°C and 40°C

. Thermophile

An organism with a growth temperature optimum between 45°C and 80°C

. Hyperthermophile

An organism with an optimal growth temperature above 80°C

. Acidophile

An organism that grows best at low pH, typically below 5.5

. Alkaliphile

An organism with an optimal growth pH of 8 or higher

. Halophile

An organism that requires NaCl for growth

. Obligate aerobe

An organism that requires O2 for respiration

. Obligate anaerobe

An organism that cannot use O2 and is inhibited or killed by its presence

. Facultative organism

An organism that can grow in either the presence or absence of O2

. Sterilization

The killing or removal of all living organisms and viruses

. Autoclave

A sealed heating device that uses steam under pressure to kill microorganisms

. Pasteurization

The use of controlled heat to reduce the microbial load in heat-sensitive liquids

. Bacteriostatic agent

An agent that inhibits bacterial growth but does not kill the cells

. Bactericidal agent

An agent that kills bacteria

. Minimum inhibitory concentration (MIC)

The smallest amount of an agent needed to inhibit the growth of a microorganism