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Flashcards in Bacterial Structure and Physiology Deck (153)
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How big are bacteria?

Generally: 1-2 um

The smallest bacteria overlap with viruses


What is included in a cell envelope?

Minimum: Cell membrane and cell wall 

Gram (+): Cell membrane and cell wall

Gram (-): Outer and inner membrane, thin cell wall, periplasm


What is the difference between the outer and inner membranes in Gram (-) bacteria?

Outer membrane: Contains lipopolysaccharides (LPS) instead of phospholipids

- Lipid A

- Core polysaccharide

- O-antigen

Inner membrane: a phospholopid bilayer similar to the cytoplasmic membrane of gram (+) bacteria


What are the functions of bacterial cell membranes?

  • Permeability barrier with transport mechanisms
  • Electron transport
  • Export of membrane and secreted proteins
  • Biosynthesis of cell wall components
  • Partition of newly replicated chromosome into daughter cells during cell division


What compound do most bacterial membranes NOT contain?


Exception: wall-less organisms (mycoplasmas)


What kinds of bacterial cell membranes contain sterols?


(These bacteria do not have cell walls)


Which bacteria have an external cytoplasmic membrane but no cell wall?



What are cocci?



What are bacilli?



What are coccibacilli?

Short rods that appear to be spherical


What are the three components of LPS?

  • Lipid A
  • Core polysaccharide
  • O antigen (or O-specific side chain)


What is Lipid A?

Where is it found?

What does it do?

Lipid A: a phosphorylated diglucosamine molecule with substituted fattey acids

Found in: Outer membrane of gram negative bacterial cell envelope; part of LPS

Function: contributes to the toxic effects of LPS


What is core polysaccharide?

A region attached to lipid A consisting of 7-9 sugar residues

(Ketodeoxyoctonate and heptose are two unusual sugar residues found there)


What is O-Antigen?

Where is it found?

What does it do?

O-Antigen: Repeating units of 3-5 sugar residues

Found in: Outer membrane of gram (-) cell envelope. Part of LPS. (However, not present in all gram (-) bacteria)

Function: Protective against complement-mediated lysis; it confers serum resistance

BUT: antibodies can recognize O-antigens, leading to clearance of the bacteria 


What component of LPS is toxic to humans? 

Lipid A


What is the unique component of bacterial cell walls?



Also known as murein


What are the functions of peptidoglycan?

  • Gives bacterial cells their shape
  • Protects cell against osmotic lysis


What is the basic structure of peptidoglycan?

Long polymers of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) w/ alternating L-and D- amino acids

The polymers are crosslinked to each other to form 3D sheets


What kinds of bacteria contain teichoic acid in their cell walls?

Gram positive


What is teichoic acid?

Where is it found?

What does it do?

Teichoic Acid: Long polymers of glycerol phosphate or ribitol phosphate. Contains sugars and amino acids

Where found: Embedded in cytoplasmic membranes of gram (+) bacteria. Extends through cell wall

Function: makes the bacteria sticky! Teichoic acids function as adhesins on the bacterial surface


Which enzymes are responsible for proper crosslinking of NAG/NAM chains?

Penicillin-binding proteins (PBPs)


What class of proteins are penicillin-binding proteins?



At low concentrations, how does LPS (aka endotoxin) affect the human host?

Effects that are protective of the host

  • Fever
  • Activates Complement
  • Stimulates B cells and macrophages
  • Activates acute phase response


Describe the crosslinking process

PBPs link the third amino acids of some tetrapeptides to the terminal amino acids of other tetrapeptides


What are penicillin binding proteins responsible for?

Crosslinking NAG/NAM chains to form 3D sheets


Describe the gram staining process

  1. Apply crystal violet 
  2. Apply iodine 
  3. Alcohol Wash
    (Crystal violet and iodine remain in gram (+) bacteria and wash out of gram (-) bacteria)
  4. Sarafin (couterstain; stains gram (+) bacteria red)


Which type of bacteria have a thick cell wall?



Which type of bacteria have a thin cell wall?



Which types bacteria are poorly visualized by Gram staining?

Mycobacteria, some spirochetes, and mycoplasma

Mycobacteria: cause TB, leprosy, other diseases

Spirochetes: Treponema pallidum causes syphilis


Why are myobacteria poorly visualized using Gram staining?

Special lipids in their cell envelops prevent them from taking up crystal violet/iodine dye


Use acid-fast, Ziehl-Neelsen, or Kinyoun staining instead


After gram staining, an unidentified bacterium appears purple. 

What kind of bacterium is this? 

What is causing it to appear purple?

Gram positive bacteria appear purple after gram staining due to retention of crystal violet/iodine dye


After gram staining, an unidentified bacterium appears red. 

What kind of bacterium is this?

Why is it red?

Gram negative bacteria appear red because the take up the safarin counterstain 

(After the crystal violet/iodine has been washed away by alcohol)


What staining porcess can detect mycobacteria?

Acid-fast, Ziehl-Neelsen, or Kinyoun staining


Why are some spirochetes (such as Treponema pallidum) poorly visualized using Gram staining?

The cell walls are too thin to be seen by standard light microscopy


Use dark-field microscopy instead


Why are Mycoplasma spp. poorly visualized by Gram staining?

They lack cell walls


At high concentrations, how does LPS (aka endotoxin) affect the human host?

High concentrations of endotoxin cause septic shock (aka endotoxic shock or gram (-) shock) due to systemic spread of gram (-) bacteria

This is characterized by...

  • Hypotension
  • Circulatory collapse
  • Disseminated intravascular coagulation
  • Multi-organ failure
  • Death


What basic structures do the cell envelopes of Gram negative bacteria consist of?

Two lipid bilayer membranes - cytoplasmic (inner) and outer

Periplasm between the two membranes - consists of glucan structures (osmolarity) and thin peptidoglycan layer


How does endotoxin induce its effects?

Stimulates cytokine production by mononuclear cells


Describe the pathway inside the body that is activated by LPS

LPS stimulates cytokine production by...

  1. LPS binds to LPS-Binding Protein (LBP)
  2. LPS-LBP complex binds to CD14 receptors on monocytes and macrophages
  3. The LPS-LBP-CD14 activates TLR-4
  4. TLR-4 stimulates the release of pro-inflammatory cytokines IL-1 and TNF
  5. This causes:
    1. Fever
    2. Increased stickiness of endothelial cells (more leukocytes bind to the surface)
    3. Stimulation of leukotrienes and prostaglandins
      1. This causes a secondary wave of cytokine production, which can lead to concomitant effects and septic shock 


Where is LPS found?

Outer leaftlet of outer membrane of gram negative bacteria


What is the function/utility of an O antigenic chain?

Antigenic and allows serotyping


What is the function of long O-side chains?

Protect against complement-mediated lysis and cause bacteria to be serum resistant

But can still be recognized by specific antibodies and then cleared


What causes bacteria to be serum resistant?

Long O-side chains


But can still be recognized by specific antibodies and then cleared


What causes bacteria to be serum sensitive?

Lack of long O-side chains


But are susceptible to complement mediated lysis


Describe the characteristics of septic shock


  • Hypotension
  • Circulatory collapse
  • Diseminated intravascular coagulation
  • Multi-organ failure
  • Death


What causes septic shock?

High concentrations of LPS, typically due to gram (-) bacterial infections. 

Note: Overwhelming gram (+) and fungal infections can also cause septic shock


What are the external structures of bacteria?

Capsules, flagella, pili


What are capsules made of?


Layers of organic polymers (usually polysaccharides, but can be a peptide) that form hydrophilic gels surrounding the cells of some bacteria


What are the functions of capsules?

  • Prevent dessication 
  • Plauy a role in virulence of some pathogens by preventing engulfment by phagocytes


How do capsules prevent engulfment by phagocytes?

Prevent complement activation via the alternate pathway on the cell surface


What factor is important in reducing ability of phagocytes to engulf bacteria whose capsule prevents phagocytosis by phagocytes?

Absence/reduction of C3b bound to bacterial cell surface


In which bacteria is the capsule an important virulence determinant?

  • Streptococcus pneumoniae
  • Neisseria meningitidis
  • Haemophilus influenzae
  • Klebsiella pneumoniae
  • Streptococcus agalactiae
  • Some strains of Escherichia coli
  • Bacillus anthracis


What diseases are encapsulated bacteria important in causing?

  • Meningitis
  • Bacteremia in people without a functional spleen


What are K antigens?

Capsules of certain bacteria used for serological typing


For which bacteria are capsular vaccines available?

Streptococcus pneumoniae, Haemophilus influenzae B, Neisseria meningitides A & C


What is the function of flagella?

Movement (chemotaxis)


What are flagella composed of?

Filament (made of multiple subunits of flagellin protein) attached to a flexible hook that connects basal body/apparatus in bacterial envelop


What is the function of the basal body of the flagella?

Motor that drives energy-dependent rotation of the filament


What are flagella designated for serotyping?

H antigens


The presence of flagella can be used to serologically distinguish and classify which bacteria?

Gram-negative enteric bacteria


Ex: Escherichia coli O157:H7 has type 7 flagella and type 157 O antigen


What is chemotaxis?

What component of bacterial structure used for chemotaxis?

What two sorts of movements does chemotaxis include?

Movement toward attractants and away from repellants

Uses flagella

Running (counterclockwise rotation of flagella) and tumbling (clockwise rotation of flagella)


What causes bacterial running?

Counterclockwise rotation of flagella


What causes tumbling of bacteria?

Clockwise rotation of flagella


Which two flagella interactions with host cells can lead to inflammatory responses?

  • Binding to external toll-like receptor 5 (TLR-5)
  • Binding to internal receptor NLRC4 (inflammasome)


What are pili/fimbraie?


Long thin filamentous structures found in the surface of bacterial cell



What are pili/fimbraie composed of?

Composed mainly of subunits of pilin


What is the function of pili/fimbraie?

How is this accomplished?

Mediating adherence of certain bacteria to cells and tissues of human host

Pili bind receptors consisting of surgar residues on glycolipids or glycoproteins in the host cell membrane


What factor related to pili accounts for the tissue specificity of many bacteria?

There are many different types of pili, each recognizing a different cell receptor


What is phase variation?

When certain pathogenic bacteria turn on/off production of particular surface proteins


What is antigenic variation?

Which group of bacteria utilizes antigenic variation?

What is the purpose of antigenic variation?

When bacteria make a surface protein but change its antigenic form

Neisseria spp. changes antigenic nature of pili by changing the type of pilin that is produced

Prevents detection of organism by host immune system


What does the bacterial cytoplasm consist of?

Proteins and RNA


What is the main biosynthetic activity of growing bacterial cells?

Protein synthesis

(cytoplasm loaded with ribosomes)


Describe bacterial ribosomes 

What subunits do they include?

70S ribonucleoprotein structures

50S subunit (includes 23S, 5S, 34 proteins)

30S subunit (includes 16S and 21 proteins)


Why are bacterial ribosomes an important target for antibiotics that inhibit bacterial protein synthesis?

Bacterial ribosomes (70S composed of one 50S and one 30S subunit) differ from eukaryotic ribosomes (80S composed of one 60S and one 40S subunit)


The gene sequence of which portion of bacterial ribosomes can be used to identify bacteria?

Which subunit is it found in?

Why can this be done?

16s rRNA gene sequence 

Found in 30S subunit

The sequences of some regions of 16S rRNA gene are unique to various bacterial genera and species


How do broad-range PCR primers amplify the rRNA gene of previously unknown/unculturable bacteria from human tissue specimens?

They recognize 16S rRNA gene sequences common to all members of the bacterial domain


Describe the DNA of a typical bacterium

One double stranded circular molecule of 1-8 million base pairs (nucleoid)

Not surrounded by a membrane

Packed into cytoplasm in an irregular way 


What are plasmids?

Why are they medically important?

Extrachromosomal DNA present as small circular molecules

Genes specify proteins that act as virulence factors or allow the organisms to resist antibiotics or heavy metals


What is the process bacteria use to multiply?

Describe the main steps involved

Asexual reproduction

  1. Grow (after DNA replication)
  2. Divide the cell at the middle into two daughter cells
  3. Each daughter cell acquires one copy of chromosome at cell division


Describe the main steps of bacterial DNA replication

  1. DNA polymerase replicates circular chromosome starting at oriC
  2. DNA gyrases separate the initially linked daughter chromosomes


In what ways does bacterial transcription differ from eukaryotic transcription?

  • Bacterial RNA polymerase is different
  • Genes may be organized into operons (clusters of genes all transcribed from a single promoter onto a single mRNA molecule)
  • Occurs in cytoplasm
  • Performed concurrently w/ translation


Why is bacterial RNA polymerase a useful target for antibiotics?

Bacterial RNA polymerase differs significantly from eukaryotic RNA polymerase


What is an operon?

A cluster of genes that are all transcribed from a single promoter onto a single mRNA molecule


Where do transcription and translation occur in bacteria?

Both in the cytoplasm


Describe the temporal relationship between transcription and translation in bacteria

Transcription and translation can occur concurrently in bacteria


What is a spore?

A dormant form of bacterial cell that can survive for many years

Under favorable conditions, a spore germinates and forms vegetative cells that begin to grow


What stage do the Gram-positive bacilli have to be in for spores to develop?

Vegetative (dividing cells)


Which sorts of bacteria can form spores?

Which bacteria specifically?

Some Gram-positive bacilli

Clostridium and Bacillus


Why are spores hard to kill?

Spores are resistant to heat, UV radiation, drying, and chemical agents


How do many bacteria acquire iron from host molecules?

Secrete siderophopres (low molecular weight molecules) that remove iron from host molecules and allow it to be taken up by bacterial cells


(Exceptions: Neisseria meningitidis and Neisseria gonorrhoeae produce surface receptors that bind to lactoferrin and transferrin, then remove and internalize iron)


What are the two mechanisms bacteria can generate energy?

Fermentation and respiration


Describe the characteristics of fermentation

Generates energy by substrate-level phosphorylation

Organic compounds, such as glucose, service as electron donors and acceptors


Do not require oxygen

Produces endproducts, such as acids, that accumulate in culture medium


Describe the characteristics of respiration

Efficient generator of energy

Final electron acceptor is O2 for aerobic respiration and NO3- for anaerobic respiration

  1. Electrons transported through a chain of carriers in plasma membrane to the final electron acceptor
  2. Protons pumped from cytosol to external side of plasma membrane creating a protonmotive force
  3. Protonmotive force harvested by ATP synthase to generate ATP from ADP (chemiosmosis)



What is the protonmotive force in respiration utilized for?

  • Chemiosmosis: generating ATP from ADP via ATP synthase
  • Flagellar rotation
  • Uptake of some small molecules into the cell


Which bacteria contain cytochrome c?

What test can detect cytochrome c? What does it measure?

Pseudomonas aueroginosa and Neisseria spp.

Oxidase test: measures ability of bacteria to oxidize and change color of N,N-dimethyl-p-phenylenediamine


What are oxidase positive bacteria?

Bacteria containing cytochrome C


Which reactive and toxic substances does oxygen give rise to?

  • Hydrogen peroxide (H2O2)
  • Superoxide anion (O2-)


What is the function of superoxide dismutase?

Degrades superoxide 

2O2- + 2H+ -> H2O2 + O2


What is the function of catalase?

Degrades hydrogen peroxide to water and oxygen

H2O2 + H2O2 -> 2H2O + O2


What are the characteristics of a strict aerobe?

  • Grows with oxygen 
  • Does not grow without oxygen (cannot ferment)
  • Contains superoxide dismutase and catalase


What are the characteristics of a strict anaerobe?

  • Killed by oxygen
  • Grows in absence of oxygen (ferments)
  • Lacks superoxide dismutase and catalase


What are the characteristics of a facultative anaerobe?

  • Respires with oxygen
  • Ferments in absence of oxygen
  • Contains superoxide dismutase and catalase


What are the characteristics of an indifferent (aerotolerant anaerobe)?

  • Ferments in presence or absence of oxygen
  • Contains superoxide dismutase and catalase


What methods can be used to identify bacterial pathogens?

  • Staining of clinical specimens or after growth of bacterium on lab medium
  • Determining metabolic and biochemical characteristics and antibiotic resistance after growing the organism
  • Antibody tests in patient or laboratory assays
  • PCR
  • Whole genome sequencing


How fast is bacterial metabolism compared with human metabolism? 

Bacterial metabolism is 10-100x faster than human metabolism


Describe the lag phase of bacterial growth

Bacteria are adapting to their new environment

They are making enzymes and intermediates to utilize the nutrients in the medium

They aren't significantly increaseing in number



Describe the exponential phase of bacterial growth

Bacterial are rapidly dividing via binary fission

They are increasing in number very quickly


Describe the stationary phase of bacterial growth

Numbers are constant or slightly decreasing

Growth is limited by nutrient exhaustion or toxic product accumulation


What is the minimum inhibitory concentration (MIC)?

The least amount of antibiotic that prevents bacterial growth

No new colonies can form


WhaAddt is the minimum bactericidal concentration (MBC)?

The least amount of antibiotic required to kill a predetermined portion of an inoculum (usually 99.9%) in a given amount of time


When would you want to use a selective media to grow bacteria?

When you are trying to culture a slower-growing bacteria

Example: you take a sputum sample and want to culture a slower-growing bacteria. You don't want the faster growing bacteria to out-compete and obscure the slow grower. You use a media that is selective for the slower bacteria


Why would you want to use sheep-blood agar?

To distinquish heolytic from non-hemolytic bacteria


Why would you use MacConkey agar?

MacConkey agar is selective for gram (-) rods

It is also differential for lactose fermentation


When would you want to use Bordet-Genou agar to grow bacteria?

When you are suspicious for Bordetella Pertussis


What is a transposable element?

  • A DNA fragment that mediates its own movement from one location on a chromosome or plasmid to another
    • A "jumping gene"
  • These are important in the transfer of antibiotic resistance or creating mutation
  • 2 types: Insertion Sequences (IS) and Transposons


What is a siderophore?

Siderophores remove iron from trensferrin and lactoferrin so it can be taken up by bacterial cells


What is an insertion sequence?

What is its structure?

The simplest transposable element


  • Inverted repeated base sequences (15-40 BP) at ends
  • Genes encoding tranposases in the middle


What are transposases?

Enzymes that mediate tranposition

  • Recognize inverted repeats
  • Catalyze the cutting and resealing of DNA
  • Allow movement of IS elements from one location to another


What is a transposon?

Larger transposable element

  • Carries extraneous genes in addition to transposition genes
    • May encode antibiotic resistance or virulence determinants
  • Some consist of extraneous genes flanked by IS elements
    • IS element = inverted repeats w/tranposase gene between
  • Others just have inverted repeats of 30-40 BPs at each end


What is the difference between an insertion sequence and a transposon?

Insertion sequences (IS) only contain genes for tranposases

Transposons contain genes for transposase AND other genes that may encode antibiotic resistance or virulence determinants. 

Note: Transposons may contain IS at each end


What are plasmids?

  • Small, extrachromosomal, circular, double stranded DNA (basically a bably bacterial chromosome)
  • Replicates independently of host chromosome
    • They have their own initiations sites for DNA polymerase: oriP
  • May carry and disseminate antibiotic resistance or virulance determinant genes
  • 2 Types: nonconjugative and conjugative


What is a nonconjugative plasmid?

A plasmid that cannot mediate its own transfer. 

Usually smaller


What is a conjugative plasmid?

A plasmid that can mediate its own transfer

- Usually larger


What is an R plasmid?

Why is it significant?

An R plasmid is a conjugative plasmid that carries genes for antibiotic resistance

If a bacterium acquires the R plasmid, it will gain antibiotic resistance


What virulence determinants of Escherichia coli are encoded by plasmids?

Enterotoxins (LT and ST)


What virulence determinants of Shigella spp are encoded by plasmids?

Type III secretion system that facilitates the invasion of epithelial cells


What virulence determinants of Clostridium tetani are encoded by plasmids?

Tetanus toxin


What is a bacteriophage?

A virus that infects bacteria. Also called a phage

2 types: Virulent and Temperate


How do bacteriophages infect bacteria?


  • The phage binds to specific receptors on the bacterium's surface
  • The phage injects DNA or RNA
  • The phage uses the host's replication machinery to replicate its nucleic acids


What is a virulent phage?

A virulent phage causes a lytic infection

The injected DNA or RNA takes over the replication system of the host and produces many new virons within the infected cell.

This causes the host bacterium to lyse, spilling the new virons out 


What is a temperate phage?

Can cause a lytic or lysogenic infection

Lytic = host cell lyses

Lysogenic = quiescent;

  • The infected bacterium grows and divides
  • Passes on the phage (called prophage in host) to progeny.
    • If the bacterium is stressed, more prophage DNA may be induced which can cause a lytic infection
    • The prophage DNA may be integrated into the host chromosome
    • The phrophage DNA may change the characteristics of the host in phage conversion


What is phage conversion?

Phage conversion is associated with infection by a temperate phage

  • It confers new properties to the host, including virulence determinants, toxins, and/or antibiotic resistance
  • It occurs when phage DNA that is integrated into the host chromosome takes part of the host DNA when is excised
  • The excised phage carries some of this host DNA when it integrates into a new host chromosome
    • The new host may now have additional characteristics


What is transformation?

Pieces of naked DNA from lysed bacterial cells are taken up by other bacterial cells and incorporated into their chromosomes


What are "naturally competent bacteria?"

Bacteria that are natrually able to take up DNA in transformation


Give 4 examples of naturally competent bacteria

Streptococcus pneumoniae

Bacillus spp

Haemophilus influenzae

Nisseria spp


What are artificially competent bacteria?

Why are they important?

Artificially competent bacteria are treated to induce transformation

They are important in lab research! E. Coli can be induced to become competent, which allows them to express genes that are important for research


Give an example of an artificially competent bacteria

E. Coli

Competence can be induced in E. Coli, allowing them to uptake DNA an express genes to create clones for research


What is transduction?

The mechanism that bacteriophages use for DNA transfer

2 types of transduction:

  • Generalized
  • Specialized


Describe generalized transduction

Host DNA is nonspecifically transferred from cell to cell

  • Bacteriophages produce caspids that can package any fragment of DNA
  • If host DNA fragments are present, they are packaged and injected into neighboring cells upon infection
  • Can result in transfer of virulence determinant and/or antibiotic resistance genes


Describe specialized transduction

Carried out by phages that excise and package phage DNA that is integrated into the host cell chromosome

  • Sometimes the phage will excise some of the host DNA that is adjacent to the phage DNA
  • This host DNA now travels along with the phage DNA
  • The amount and parts of host DNA that can be transferred are limited becasue the phage DNA only integrates into the host chromosome at specific places


Describe conjugation

Bacterial sex!

  • Cell to cell contact
  • Unidirectional transfer if DNA from donor to recipient
  • Mediated by conjugative plasmids and ICEs


Describe conjugation of the F Plasmid

The F plasmid encodes conjugation machinery

  • The F plasmid induces a sex pilus in the donor bacterial cell
  • The sex pilus allows the donor to contact and bind to the recepient bacterial cell
  • A copy of the F plasmid is transferred to the recipient via the sex pilus
    • If the F plasmid has been integrated into the host chromosome, additional genes may travel along with it when it is excised




List the phases of bacterial growth

  1. Lag phase: the bacterium is adpating to its new environment and making enzymes and intermediates needed to survive
  2. Exponential phase: The bacteria are rapidly dividing via binary fission
  3. Stationary phase: The bacteria are at a steady state or slightly decreasing; their replication is limited by nutrient availability or toxic product accumulation


How do plasmids and bacteriophages contribute to the virulence of certain pathogens?

Both may carry virulence genes that will be expressed by the bacteria that contain them


Can lytic bacteriophages every be useful to humans? How?

Lytic phages can be useful in treating certain bacterial infections in humans

The lytic phage infects the bacterium and causes it to lyse, thus killing the bacteria and potentially eradicating the bacterial infection


Would a beta-lactam antibiotic be useful against gram-positive or gram-negative bacteria?

Both; Both types of bacteria can be treated by beta-lactam antibiotics becasue both have peptidoglycan in their cell walls (although the layer is thinner in gram (-) bacteria)


What kinds of bacteria contain superoxide dismutase and catalase?

Obligate aerobes, facultative anerobes, aerotolerant anaerobe


What kinds of bacteria (in general) would not contain superoxide dismutase and catalase?

Obligate anaerobes


Which genetic exchange mechanism utilizes a sex pilus?



Which genetic exchange mechanism involves the uptake of naked DNA?



What is an integron?


"Antibiotic resistance machines"

A genetic entity that captures exogenous gene cassettes and ensures their expression.

They are not mobile on their own, but they can be inserted into mobile elements, such as plasmids or transposons



What is the structure of an integron?

3 core components:

  • Outward-oriented promoter
  • Primary recombination site downstream of promoter
  • A gene encoding an integrase (an integrase can insert gene cassettes into the integron)


When a gene cassette is insterted into an integron, the recombination site re-forms. More cassettes can be inserted; the cassettes are expresed as operons


What is a gene cassette?

A discrete unit of a singel gene

Most encode antibiotic resistance genes