Genetic basis for Antimicrobial Resistance

Question 1

Clinical Resistance:

Factors

Answer 1

Type of infecting bacteria & location in body
Distribution of AA in body and conc at site of infection
immune status of patient

Question 2

Mechanism of Antimicrobial resistance

Answer 2

Susceptible bacteria can acquire resistance to antimicrobials by either genetic mutation or by accepting antimicrobial resistant genes from other bacteria

Question 3

Resistance through mutation

DETAIL

Answer 3

Mutation is a change in DNA causing a change in gene product which is target for AA. For example, mutation in a gene of a bacteria may mean that fluoroquinolones may no longer be able to bind efficiently to the DNA gyrase and the DNA replication will occur as normal

Question 4

Resistance through destruction or inactivation:

DETAIL

Answer 4

Many bacteria possess genes which produce enzymes which chemically degrade AA which renders them useless against bacteria.

Question 5

Resistance through efflux:

DETAIL

Answer 5

Efflux pumps are channels that actively exports AA and other compounds outside the cell. Lethal activity of AA not allowed to occur

Question 6

Resistance through Genetic transfer:

3 methods

Answer 6

Genetic material can be transferred between bacteria.
Most often by conjugation, transformation & transduction.

Conjugation: mediated by circular DNA called plasmid which replicated independently of chromosome. Plasmids can carry genes which confer resistance to antimicrobials. When two bacterials cells are in close proximity, a hollow bridge-like structure called a pilus forms between them. As the plasmid duplicates, it can be transferred to the neighbouring cell, thus enabling susceptible bacteria to become resistant to a particulat AA.

Transformation: Genes transferred from one bacterium to another as naked DNA. When cells break apart, DNA can be released into surrounding environment, surrounding bacteria can scavenge the DNA and incorporate it into their own. This DNA may contain advantageous genes for resistance.

Transduction: Bacteria transferred from one bacterium to another inside a virus that infects bacteria. When a bacteriophage infects bacterium it takes over the genetic processes of the bacterium, producing more phage. Bacterial DNA may inadvertently be incorporated into new Phage DNA. Upon bacterial lysis, , these new phage go on to infect new bacteria, bringing genes from previously infected bacteria.
Also, insteading of replication and cell lysis, The phage DNA can be incorporated into the bacterial DNA to form a prophage and cell division can occur.

Question 7

Intrinsic resistance:

Answer 7

Innate ability of a bacterial species to resist activity to a particular AA through its inherent structural or functional characteristics.

Question 8

Acquired resistance:

Answer 8

Resistant strains emerging from previously sensitive strains usually after exposure to an agent.
Results from mutations in chromosomal genes, or from acquisition of plasmids, transposons or integrons
In clinical microbiology, it is mainly due to selective pressure during antimicrobial therapy.
Occurs in sensitive cells following alteration in existing genome or transfer of genetic information between cells.
1.) mutational resistance
2.) Transmissible resistance

Question 9

4 examples of organisms which are resistant to agents,, and the mechanism by which this is achieved.

1. ) Aerobic bacteria
2. ) Anaerobic bacteria
3. ) Gram +ve bacteria
4. ) Gram -ve bacteria

Answer 9

1. ) Metronidazole- Inability to reduce it to its active form
2. ) Aminoglycosides- lack of oxidative metabolism to drive uptake of aminoglycosides
3. ) Aztreonam (a beta-lactam)- Lack of penicillin binding protein that bind and are inhibited by this beta-lactam AA.
4. ) Vancomycin- Unable to penetrate outer membrane as it is too large to diffuse via porins in outer membrane to reach its targets.

Question 10

Factors affecting increase in antiobiotic resistance: 7

Answer 10

1. ) Increased use of a variety of AA
2. ) Clinical introduction of many closely related compounds which may have similar action and therefore have the microbes which have resistance to one will have resistance to another AA which operates by the same mechanism.
3. ) Increasing number of immunocomprimised patients increases mortality rate of the microbe.
4. ) Increased number of intensive care units
5. ) Agents used in animal husbandry
6. ) Agents applied to food crops
7. ) Agents used in household environment

Question 11

Resistance to more than one Agent:

Answer 11

Cross resistance: Resistance to a no. of different members of a group of chemically related agents which are all affected by same resistance mechanism
Multi-drug resistance: Resistance to several unrelated AA different resistance mechanisms. Genes conferring resistance to several unrelated AA may be transferred en bloc from cell to cell on plasmids which can give the appearance of cross resistance.

Question 12

Acquired resistance 1: Resistance determined by chromosomal genes;

Answer 12

Mutations occur spontaneously and randomly.
Deletion, substitution or addition of base pairs can alter the composition of the peptide product .
Mutations occur in all genes independently of presence/absence of AA but most are repaired.

Question 13

Plasmid borne resistance:

Answer 13

Bacteria have inhabited earth for 3.5 billion years.
Source of most antiobiotics have been developed from natural compounds produced by bacteria. These have evolved as weapons, used by bacteria to protect their ecological niche.
Bacteria have evolved complex DNA modifiying strategies to allow them to adapt.
Plasmids are extrachromosal genetic elements that replicate independently of bacterial chromosome.
Not essential for cells survival but give survival advantage in adverse conditions.
Plasmid borne resistance is more common than chromosomal resistance in clinically important bacteria. because of ability of its motility and as a vector of transposons. These factors all ensure survival of resistant cells and progeny under selective pressure.
SIGNIFICANTLY, bacteria can acquire resistance to several unrelated agents by a single genetic event,
When resistance genes are mobile, their spread is greatly accelerated. In E.coli and shigella, a greater percentage of the genome of plasmids encode resistance than the chromosome. AA resistant genes are overexpressed in plasmids.
plasmid encoded resistance significant in cephalosporins, fluoroquinolones, aminoglycosides.
Single plasmid can encode multiple resistances and bacteria can contain several plasmids.
K. pneumoniae plasmid pKP048 confers resistance to carbapenems, cephalosporins, FQNS, aminoglycosides, macrolides and mercury.