more bacteria found in flora can use glycerol for PHA production e.g Pseudomonas,

Trace element are required by almost by many microbes and Glycerol can be used by many microbes. So there are chances of contamination.

You can use an addition medium component or alter growth conditions, which will favour growth of only your bacteria and PHA production. If you can provide name of your microbe, i might be able to suggest you some contion/component.

Ethanol production occurs in unsterile condition, low pH allows yeast growth in favour of most of the bacteria.

Thanks a lot musthafa , I am using cupriavidus necator for the production and alaa Kareem will the microaerophile condition still enhance the growth of other microbes

I've found a lot of environmental isolates capable of utilizing glycerol so I'd say there is a reasonable risk of contamination.

Can you maintain microaerophile condition. It should avoid growth of few microbes. cupriavidus necator can also use CO2/ H2/ chlorinated aromatic (chloroaromatic) compound/ organic compound for growth. You can use addition of littke conc of (in ppm ) to avoid growth of other microbes. may be addition of chlorinated aromatic compound.

I had been trying to maintain micro aerophilic condition by maintaining higher volume in shake flask wrapping the flask with parafilm so that no oxygen transfer happens 

I did think of passing nitrogen to an extent before incubating as cupriavidus is capable of producing hydrogen from the source which could be used but don't know how far it is feasible 

I am supplementing the medium with inorganic nitrogen rather than organic as it itself will enhance the growth of other microbes And in a paper I read that the microbe is copper tolerant

Musthafa Vohra , I have used sodium bicarbonate to maintain the co2 environment in the medium by which I am able to supress the growth of other microbes . being Microaerophilic , glycerol uptaking organism I hope this medium supplement would make the fermentation broth contaminant free

If your organism is microaerophilic, then naturally you will get a very low yield. Contamination is your technique problem. It has nothing to do with the presence or absence of glycerol in the medium.

Jai Ghosh , In that case they had reported 65-72% production in most of the literature and they have followed a sterile method of production unlike in my case.

Mustafa Vohra , Patrick Druggan I am able to culture the organism in the above said condition and I took microslide of the broth at different intervals and I could see coccoid rods as given in the literatures. Thanks for the support but I had to keep tracking the risk of contamination.

Thats good sivananth. But i would like to know one more thing. As per my knowledge PHA is produce in nutrient limiting condition, so which nutrient is limited in your medium? Also i think you can sparge a bit of CO2 in your medium. As per the condition given by you, it will be useful for your organism and will inhibit many aerophilic microbes. Anyways Best of luck for your experiments.

The controlled fermentation requires sterile conditions including the medium. In all these fermentations, when working under non sterile conditions the yield is low, to escape this situation you have to work in sterile conditions. This is not a proof that your low yield is due to the contamination, it could be due to other factors.

guys thanks for the timely replies , it is that there has been reports on high yield of production in sterile condition and similarly there has been  comparatively high yield when another organism was used in an unsterile condition with waste water as carbon source. I had been more concerned on maintaining a pure culture than the yield.  the statement on yield is just to putforth my observation.

Thank you patrick I will consider the same during my experiments.

I was able to grow colonies and I can see that they are white to off white and glistening and smooth , I read that the microbe is capable of lysing other microbes in the soil, which I feel would lead to the growth of only cupriavidus. please feel free to correct me if im wrong