In 1945, in his Nobel lecture "Penicillin," Alexander Fleming warned against the use of sub therapeutic doses of antibiotics ‐ – "bought by anyone in the shops" without a prescription:

Here is a hypothetical illustration. Mr. X. has a sore throat. He buys some penicillin and gives himself, not enough to kill the staph aureus but enough to educate them to resist penicillin. He then infects his wife. Mrs. X gets pneumonia and is treated with penicillin. As the Staph Aureus are now resistant to penicillin the treatment fails. Mrs. X dies.

NOTE: The number of persons inappropriately prescribed antibiotics is a greater factor in the increasing rates of bacterial resistance.

ACQUISITION SPEED OF ANTIMICROBIAL RESISTANCE

Speaker: Avinash SinghResource Faculty: Dr. Narayan Raj Bhatttarai

OBJECTIVE

• MECHANISM OF ACQUISITION OF DRUG RESISTANCE

• DISSEMINATION

MECHANISMS OF ANTIBIOTIC RESISTANCE

MOLECULER GENETICS OF ANTIBIOTIC RESISTANCE

• Genetic variability is essential for microbial evolution.

Micro evolutionary change• Point mutation occur in a nucleotide base pair.

Macro evolutionary change• A second level of genomic variability in bacteria and results

in whole scale rearrangements of large segments of DNA.

• Such rearrangements may includes Inversions, duplications, Insertions , deletions or transposition of large sequences of DNA from one location of bacterial chromosome to another.

• The whole rearrangement of large segments of bacterial chromosomes are frequently created by specialized genetic elements known as transposons.

• A third level of genetic variability in bacteria is created by the acquisition of foreign DNA carried by plasmids, bacteriophages or transposable genetic elements.

AQUISITION OF ANTIBIOTIC RESISTANCE

• Antibiotic resistance is now a major clinical problem all over the world that proves to the success and speed of bacterial adaptation.

• Acquisition of genes is greatly aided by a variety of gene transfer systems, such as bacterial conjugative plasmids, transposable elements and integron systems.

LATERAL OR HORIZONTAL GENE TRANSFER

1. TRANSDUCTION• Occurs when bacteria-

specific viruses or bacteriophages transfer DNA between two closely related bacteria.

2. TRANSFORMATION

Is a process where parts of DNA are taken up by the bacteria from the external environment. This DNA is normally present in the external environment due to the death of another bacterium.

3. CONJUGATION

• Gene transfer from a donor to a recipient by direct physical contact between cells.

PLASMIDS

• Extrachromosomal, autonomous, circular DNA molecules found in bacterial cells.

• F plasmid

• R-plasmid

F PLASMID: It is a conjugative plasmid found in F+ (male) bacterial cells.

R-PLASMID: It is a conjugative factor in bacterial cells that is thought to be the main mechanism of the antibiotic resistant gene transfer.

R-plasmid:

Transferrable factor: Resistance transfer factor (RTF) – Spread of multiple drug resistance among bacteria.

Resistance determinant(R): Carrying resistant genes for each of the several drugs.

TRANSPOSONS

• Insertion of such transposons leads to the acquisition of new characteristics by the recipient DNA molecule.

• Thus, by transposition a segment of the DNA can be transferred from a molecule to another molecule that has no genetic homology with either the element or the DNA.

• Discrete segments of DNA that have ability to move around in a “cut and paste ” manner between chromosomal and extra chromosomal DNA.

• It is a new class of transposable element.

• It has the capability to move from the chromosome of one bacterium to another without becoming incorporated into a plasmid.

• It is found in aerobic and anaerobic gram positive organisms.

CONJUGATIVE TRANSPOSONS

1. Gene cassette

2. Integrase enzyme

3. Integron

DNA INTEGRATION ELEMENTS

DNA INTEGRATION ELEMENTS

• Integrons are transposons that can carry multiple gene clusters called gene cassettes that move as a unit from one piece of DNA to another. Integration elements differ structurally and functionally from transposons.

DNA INTEGRATION ELEMENTS

• An enzyme integrase enables these gene cassettes to integrate and accumulate within the integron. In this way, a number of different antibiotic resistance genes can be transferred as a unit from one bacterium to another.

DISSEMINATION

Introduction of antibiotics into the clinical medicine has created selection pressures that favored the dissemination of antibiotic resistance genes via mobile genetic elements.

Examples : Plasmids, Transposons.

Plasmids are well adapted to serve as agents of genetic evolution and resistance gene dissemination.

Plasmids may determine wide range of functions including virulence , metabolic capacities.

Plasmids are autonomous , self – reproducing genetic elements that require an origin for replication and regions that facilitate its maintenance in host bacteria.

PLASMIDS MAY BE INVOLVED IN DISSEMINATION OF ANTIBIOTIC RESISTANCE IN SEVERAL WAYS:

Clonal spread.

Plasmid transfer.

Plasmid spread.

Resistance gene mobilization by transposition from plasmid to chromosome.

TRANSPOSONS MAY BE INVOLVED IN DISSEMINATION OF ANTIBIOTIC RESISTANCE BY FOLLOWING STEPS:

• Transfer of resistance encoded by genes within the conjugative transposons.

• Mobilization of other replicons.

• Large, multidrug resistance composite elements.

• Resistance exchange by chromosomal recombination.

SUMMARY

REFERENCES

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