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Discussion Starter · #1 ·
Bacterial strain X is resistant to Ampicillin and sensitive to Gentamycin. Bacterial strain Y is resistane to gentamycin and sensitive to Ampicillin. Bacterial strain X and Y are grown in mixed culture in medium without antibiotics, then the culture is plated on medium containing both ampicillin and gentamycin. Bacterial colonies grow on the plate. In a second experiment DNase is added to a mixed culture medium. When this mixed culture is plated on medium containning both antibiotics ,no colonies grow. Assuming that bacterial cells are impermeable to DNase, which of the following processes best explain these observations?

A- Conjugation
B- Mutation
C- Transduction
D- Transformation
E- Transposition
 

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Transposons are segments of DNA that can jump form bacterial cell to bacterial cell and attribute to the receiver a property of the donor. In the present case, apparently strain X bacteria contain transposons that incur resistance to Ampicillin, whereas strain Y bacteria contain transposons that incur resistance to Gentamicin. When cultured together, these strains "protect" each other, through exchanging the respective transposons, this is the reason why they both develop colonies, despite the presence of antibiotics. However, when DNase is introduced, transposons are degraded, thus antibiotic resistance cannot be transmitted from the resistant strain to the non-resistant one. This results to inhibition of development of both strains, because each one is susceptible to at least one antibiotic.

P/S: it is gentamIcin, not gentamYcin; mYcin indicates that the antibiotic derives from StreptomYces spp, whereas mIcin is related to MIcrospora spp.
 

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good explanation, man !

Transposons are segments of DNA that can jump form bacterial cell to bacterial cell and attribute to the receiver a property of the donor. In the present case, apparently strain X bacteria contain transposons that incur resistance to Ampicillin, whereas strain Y bacteria contain transposons that incur resistance to Gentamicin. When cultured together, these strains "protect" each other, through exchanging the respective transposons, this is the reason why they both develop colonies, despite the presence of antibiotics. However, when DNase is introduced, transposons are degraded, thus antibiotic resistance cannot be transmitted from the resistant strain to the non-resistant one. This results to inhibition of development of both strains, because each one is susceptible to at least one antibiotic.

P/S: it is gentamIcin, not gentamYcin; mYcin indicates that the antibiotic derives from StreptomYces spp, whereas mIcin is related to MIcrospora spp.
good explanation, man !
 

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Discussion Starter · #4 ·
bacterial drug resistance

you tried but that is not the ans. the ans is D. This the explanation.

a) conjugation is bacterial sex. in conjugation DNA is transferred directly by cell-to-cell contact through sex pili. so in this case DNA is secured by cell wall and membrane from action of DNAse.

b) mutation can occur. but it is very rare to bacteria obtain antibiotic resistance due to mutation...

c) transduction occurs when a virus that infects bacteria, bacteriophage, carries a piece of bacterial DNA from one bacterium to another.. in this case DNA also secured by virus capsid or envelope..

e) transposition... transposons are mobile geneticx elements. they insert into the DNA of phages, plasmids, and bacterial chromosomes. they do not replicate independently but are copied during their host's DNA tracscription. transposons can move drug resistance to the plasmids... but all these occur inside of the bacteria, and DNA is secured by bacterial membrane and cell wall, from action of DNAse..

D) TRANSFORMATION. naked DNA fragments from one bacterium, released during cell lysis, bind to the cell wall of anothe bacterium. the recipient bacterium must be competent, which means that it has structures on its cell wall that can bind the DNA and take it up intracellularly. Recipient competent bacteria are usually of the same species as the donor. the DNA that has been brought in can then incorporate itself into the recipient's genome if there is enough homology between strands (another reason why this transfer can only occur between closely related bacteria).
in this case when two strains of bacteria grow together in one medium, they exchange antibiotic resistance by means TRANSFORMATION and become both resistant to both antibiotics (gentamicin and ampicillin)... if two strains were cultured in the medium containing DNAse, transformation can't occur, b/c DNAse cleaves DNA fragments outside the cells, so both strain remain susceptible to another antibiotic, and both strain can't grow in medium containing both antibiotics..
they discribe two experiments... in first two bacterial strains X and Y were grown in mixed culture in medium without antibiotics, without DNAse....
in the second experiment they added DNAse...
in first experiment (without DNAse) transformation occured and bacterial strains obtained cross resistance to antibiotic to which they were before susceptible...
in the second experiment (with DNAse) no transformation occure and both strains were susceptible in medium containing both antibiotics.
 

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what is origin of this q? does not seem wellwritten...

...or i might be missing sth here,,,

for transformation to occur as you know, there should be free -naked- DNA. my q is;
if there is no DNSase used in first experiment, how these DNA fragments are available there to be able to transfer?? should not bacteria first be lysised?

and other thing confusing me, both bacteria are secretive to a drug, so how can they grow at all in presence of drug at first place?

the only thing coming to my mind is; first that they were both in the medium with that drugs, SOME died and SOME were lucky -for some reason- to not die, then the alive one now have both drug resistance genum...

any idease?
 

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Clearing your doubts

...or i might be missing sth here,,,

for transformation to occur as you know, there should be free -naked- DNA. my q is;
if there is no DNSase used in first experiment, how these DNA fragments are available there to be able to transfer?? should not bacteria first be lysised?

and other thing confusing me, both bacteria are sensitive to a drug, so how can they grow at all in presence of drug at first place?

the only thing coming to my mind is; first that they were both in the medium with that drugs, SOME died and SOME were lucky -for some reason- to not die, then the alive one now have both drug resistance genum...

any idease?
- The free DNA was available because the bacteria were lysed by the antibiotic :)
- The bacteria were able to grow despite being sensitive to antiobiotics because they were transformed :)
 

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DNase = deoxyribonuclease

DNase = deoxyribonuclease see pg 126 FA 2013
The answer is D

:)

Bacterial strain X is resistant to Ampicillin and sensitive to Gentamycin. Bacterial strain Y is resistane to gentamycin and sensitive to Ampicillin. Bacterial strain X and Y are grown in mixed culture in medium without antibiotics, then the culture is plated on medium containing both ampicillin and gentamycin. Bacterial colonies grow on the plate. In a second experiment DNase is added to a mixed culture medium. When this mixed culture is plated on medium containning both antibiotics ,no colonies grow. Assuming that bacterial cells are impermeable to DNase, which of the following processes best explain these observations?

A- Conjugation
B- Mutation
C- Transduction
D- Transformation
E- Transposition
 
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