I cultured MRSA in LB media to make it grow to 10^8/mL and added 1 mg/mL Vancomycin hydrochloride. Then cultured the MRSA for 1h, 6h. At each timepoint, I did agar plate spread to count the CFU change. I found the growth of MRSA on the agar can be inhibited in *10 dilution plate. However, the MRSA spread a lot in the plate with dilution of *1000. The amount of total MRSA did not change before and after Vancomycin added.
So does it mean the vancomycin cannot kill MRSA?
My understanding is that some MRSA strains are resistant to vancomycin but many should remain sensitive. So it may depend on the strain.
However I am struggling to understand the experiment you describe
Thank you for your information. The experiment I did is as follow: MRSA bacteria is seeded in the LB media and cultured under 37 degreeC water bath for 4 h to make the amount of MRSA reach 10^8/mL. Then I added vancomycin hydrochloride powder to the bacteria solution to make the drug concentration as 1mg/mL. Then the bacteria solution is continued for incubation in 37 degreeC water bath. After 6 h incubation, I collect the bacteria solution and dilute it to 10 times and 1000 times and spread on the agar plate. After incubation the agar overnight at 37 degreeC, I found the MRSA in the 10 times dilution agar did not grow well. You can see from the figure attached. However, the MRSA can still grow very well in the 1000 times dilution agar. Besides, the total amount of MRSA did not change based on this when compare with the amount before vancomycin added.
It means that the almost all the MRSA can be suvival under 1 mg/mL vancomycin, which is abnormal.
First of all you are interpreting the plates incorrectly. At the 10-fold dilution you have so many cells that it forms a lawn and you are not able to see single colonies. It is growing fine. Even at the 1000 fold dilution you have too many colonies to really be able to accurately assess. You need to make more dilutions, probably 10,000 and 100,000 (10e4 and 10e5) dilutions so you can really see single colonies well.
However you should see whether your cells are perhaps vancomycin resistant. Prepare tubes with and without vancomycin and inoculate both with a small amount of a culture and monitor the growth over many hours. If both tubes grow at a comparable rate then your strain may be vancomycin resistant to begin with, some MRSA are. Or if you have vancomycin plates you can streak your strain on such plates to see if you get colony formation.
Vancomycin is the magic drug for MRSA, but there are strains of them that I developed resistance against. In my work on MRSA, there were such strains, I hope the following links are useful, especially the presentation because it contains results of isolates resistance to antibiotics, including vancomycin.
Thesis Isolation and identification of methicillin resistant staphy...
Article Antibacterial effect of ozonated water against methicillin-r...
Presentation Prevalence of Methicillin -Resistant Staphylococcus aureus (MRSA)
This is an odd experiment (why 1mg/mL vancomycin? the MIC is 1mg/L). The results, ie survival of MRSA in presence of 1000xMIC, are probably because of a) the very high bacterial inoculum, and b) the short duration of exposure (antibiotics, especially time-dependent ones, do not kill rapidly). The appearance of colonies at x1000 dilution but not at x10 is probably because you are diluting the vancomycin as well as the bacterial suspension. Also, it is not correct procedure to add vancomycin powder directly to a culture, and perhaps some of it did not dissolve. Check the vancomycin MIC of the inoculum before and after exposure to see if you have created resistance
in one of my researches on bacterial susceptibility of antimicrobials against resistant bacteria infection , vancomycin showed 100% susceptibility
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