Priyanka Mishra Phosphate is a rich nutrient for bacteria. So most bacteria will be happy in PBS, but cant really say the same is true for lactic acid bacteria.

Priyanka Mishra Contamination can come from various sources. Bacillus spp spores are known to be difficult to be killed and can survive autoclaving (do increase autocl time to 20 min). contamination can occur during sampling of PBS for sterility test. do check the laminar flow as well even though you have a control plate.

Hypothetically not much should be able to grow in PBS because there's no carbon or nitrogen source, but there's a lot of things that could just hang out in it and survive in it for long periods of time - I'm guessing that's the case if it's been frozen. However, a lot of reagents (water especially) can be contaminated with trace amounts of organic compounds that will allow for the growth of microbes if they get in it, especially if it's stored at ambient temperatures.

Then there's also always the possibility that someone was tired and accidentally put a contaminated pipette tip full of bacteria in it.

What ive seen is that in rare cases after an autoclaved high conc PBS (phosphate-buffered saline) solution has been stored cold, it can result in precipitation which may create the impression of contamination.

Cold stored PBS ( ~ 4C // -20C) should not accommodate general lab microbial contaminants to grow. Firstly, essential C and N sources are lacking and the potential of psychrophilic microbes being present under general lab conditions is highly unlikely. It's more likely that the contaminant got into your Soln when it was being carried or handled or during the frequent contamination checks and remained stationary. PBS is stable indefinitely, I have a100 mM stock for about 14 months at 4C and it was sterile, no precipitation.

Don't lose sleep over it. A way to prevent this from happening in the future is to make small aliquots of PBS that are enough for your general use, completely cover them in a single layer of foil and autoclave. I am in a Bacillus lab, and the likelihood of spores being present on surfaces is quite high. Thus far i have had no contamination issues even with my nutrient-rich buffered media.Spray the foil (if you really worried about contamination, first with 10% bleach ) and then with 70% EtOH the foil and remove it before you want to use your PBS and it should be stable and sterile. Bleach and EtOH are not sporicidal w.r.t. Bacillus but for general microbes, it should be okay.

Try it out and let RG comments know ?

Microorganisms that could are halophiles...extremophiles. But I highly agree with Danvir Ramesar , could be just precipitation.

Hello Priyanka Mishra

I believe if the PBS is properly sterilized there should be no growth of any bacteria until it is inoculated with one, however if you still observed some bacteria that can be any kind of cross contamination.

Moreover if you see in the literature there are numerous bacteria that can survive for years in PBS as it can serve as a nutrient rich media, so we cannot say which specific bacteria can be present in your buffer!