Bacterial Drug Resistance - USMLE Forums
USMLE Forums Logo
USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
Home
USMLE Articles
USMLE News
USMLE Polls
USMLE Books
USMLE Apps
Go Back   USMLE Forums > USMLE Step 1 Forum

USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 04-22-2010
USMLE Forums Veteran
 
Steps History: 1 + CK
Posts: 234
Threads: 63
Thanked 128 Times in 63 Posts
Reputation: 158
Bacteria Bacterial Drug Resistance

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
Reply With Quote Quick reply to this message
The above post was thanked by:
Taiwan_Guy (05-16-2011)



  #2  
Old 04-23-2010
ath.pantelis's Avatar
USMLE Forums Addict
 
Steps History: 1+CK+CS
Posts: 187
Threads: 4
Thanked 372 Times in 122 Posts
Reputation: 402
Default

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.
Reply With Quote Quick reply to this message
The above post was thanked by:
Seetal (04-23-2010), smile135 (04-24-2010), Taiwan_Guy (05-16-2011)
  #3  
Old 04-23-2010
USMLE Forums Scout
 
Steps History: Not yet
Posts: 77
Threads: 25
Thanked 108 Times in 29 Posts
Reputation: 118
Default good explanation, man !

Quote:
Originally Posted by ath.pantelis View Post
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 !
Reply With Quote Quick reply to this message
 
  #4  
Old 04-23-2010
USMLE Forums Veteran
 
Steps History: 1 + CK
Posts: 234
Threads: 63
Thanked 128 Times in 63 Posts
Reputation: 158
Default 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.
Reply With Quote Quick reply to this message
The above post was thanked by:
aktorque (03-08-2011), aspiringdoctor (04-21-2013), aungawa (04-24-2010), Dr. Mexito (03-26-2012), erickven (05-15-2016), FreakingOut (04-23-2010), Insomniac (09-21-2013), Janakpriya (09-04-2013), nano (05-02-2010), Sabio (04-23-2010), smile135 (04-24-2010), Sparco (01-05-2011), Taiwan_Guy (05-16-2011)
  #5  
Old 04-23-2010
ath.pantelis's Avatar
USMLE Forums Addict
 
Steps History: 1+CK+CS
Posts: 187
Threads: 4
Thanked 372 Times in 122 Posts
Reputation: 402
Default

In other words, right thought, wrong answer... grrrrrrrr! I'm this close every time!
Reply With Quote Quick reply to this message
  #6  
Old 01-04-2011
USMLE Forums Scout
 
Steps History: ---
Posts: 82
Threads: 22
Thanked 33 Times in 14 Posts
Reputation: 36
Default 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?
Reply With Quote Quick reply to this message
  #7  
Old 01-04-2011
kieffer's Avatar
USMLE Forums Scout
 
Steps History: Step 1 Only
Posts: 16
Threads: 0
Thanked 12 Times in 6 Posts
Reputation: 22
Arrow Clearing your doubts

Quote:
Originally Posted by hana View Post
...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
Reply With Quote Quick reply to this message
The above post was thanked by:
Evergreen (03-11-2013)
  #8  
Old 09-21-2013
Jacqattack2015's Avatar
USMLE Forums Scout
 
Steps History: 1+CK+CS
Posts: 11
Threads: 4
Thanked 45 Times in 5 Posts
Reputation: 55
Default DNase = deoxyribonuclease

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



Quote:
Originally Posted by achistikbenny View Post
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
Reply With Quote Quick reply to this message



Reply

Tags
Microbiology-, Step-1-Questions

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

Favorite USMLE Books
What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
Where you live. Leave blank if you don't want to tell.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Bacterial causes of membranoproliferative GN schiwei USMLE Step 1 Forum 1 11-28-2010 04:47 PM
question - lithium toxicity drug-drug interaction mtoi USMLE Step 1 Forum 5 10-31-2010 12:48 PM
Drug resistance? dheshah USMLE Step 1 Forum 1 09-07-2010 10:37 AM
Calculating the Pulmonary Vascular Resistance! doctorF USMLE Step 1 Forum 3 07-12-2010 02:04 PM
loss of ampicillin resistance in E. coli Seetal USMLE Step 1 Forum 4 04-27-2010 01:10 AM

RSS Feed
Find Us on Facebook
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

USMLE® & other trade marks belong to their respective owners, read full disclaimer
USMLE Forums created under Creative Commons 3.0 License. (2009-2014)