Multiple resistance?

John R. McQuiston zje8 at
Wed Sep 29 06:38:10 EST 1999

I guess the first question would be, are your antibiotic plates still 
effective.  Amp plates are usually only good for a few weeks when stored at 
4C.  You need to test them with a known strain that is sensitive to the 
antibiotics.  This would be my first guess.
Second.  You need to find a single colony isolate on non-selective media and 
show that it is resistant to all the antibiotics to say you have 
multi-resistant bugs.  What you may have is one amp resistant, one strep 
resistant....  and each grows independantly on the plates they are resistant 
to.  This does not mean multiresistant single organisms.  
To show that the resistance is plasmid mediated you have to do some molecular 
work.  I doubt what you have is one strain of multiresistant bacteria but it's 
First get your controls working (pos. and neg.) then take a single clony and 
plate it on each of the different plates in quadrants with the controls on the 
same plate.  This will tell you more.  Or you can take a single colony and 
plate it and get hold of some antibiotic containing disks and look at zones of 
Don't give up, just get your experiment in order.

John R. McQuiston
Foodborne Diseases Branch
CDC Atlanta

In article <120086A01BE at>, g98C1723 at says...
> Recently ( 2 days ago ) , in an experiment to isolate any bacteria( 
>that show antibiotic resistance) from a processed meat product( sold 
>at a local butcher--> mince ) , My results thus far , have been 
>subjected to alot of scrutiny from my Colleagues. This is my problem. 
>After a suggested incubation time of 90 minutes ( ie , 1 gram of 
>mince in luria broth ) , i plated out  serial dilutions of the 
>sample on the following plates : La(luria agar) 80ug/ml Amp( 
>Ampicillin ) , LA 60ug/ml Strep , LA 30ug/ml Amp , LA 25ug/ml Tet , 
>LA 20ug/ml Strep , LA 10 ug / ml Tet , and as a control i used Luria 
>        After a 24hr incubation , all my plates ( with the exception 
>of Streptomycin serial dilution @ 100 , 1000 ) had growth on them . 
>This has serious implications as you all know. Does it imply that 
>the local butcher is selling meat containing bacteria that show 
>multiple antibiotic resistance ? But the again where has the 
>contamination arisen from ( should i be thinking on the lines of 
>genetics.. ?) And lastly ,if it is genetics that i must look at , 
>would i be able to distinguish between plasmid and chromosomal 
>encoded antibiotic resistance ?
> Thanks in Advance ?
>Abdus Samad
>U Labs
>South Africa
>Dept Micro&Biochem

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