Whenever i do double agar plate method for bacteriophage studies i get this kind of contamination in my plates. Is anyone familiar with this kind of colony?
Can't really determine that from just looking at a colony on a plate. If you really want to know what it is, you could take a sample and smear it on a microscope slide then stick it under the microscope. It may need some additional staining for exact determination, but a smear would at least tell you if it is bacteria or yeast. Most likely it's Staphilococcus aureus or Saccharomyces cerevisiae (skin bacteria or baker's yeast).
I agree with Petra, it is most likely S. aureus (I've never had the yeast contamination) - do they turn more yellow over time? If your extremely curious - try plating it on a plate with mannitol as the carbohydrate supply. S. aureus will grow on mannitol and turns more yellow with time, at least on those plates.
Do the colonies appear to be sinking into the agar? Like this: http://www.scielo.br/img/revistas/bjm/v46n3//1678-4405-bjm-46-20131289-gf01.jpg?
If it is similar to that photo - it might be an agar degrader, like Cytophaga (?)
The caracteristic of this colonies is like a Yeasts , they are big and can be a diferents colors (Yellow, orange, White) is a contamination of the agar. ( sorry for my english)
I'm afraid we'd need some more information to evaluate this. I think you may have some sort of contamination in your soft agar (small colonies) unless the lysis of your bacterial lawn is confluent? I see 3 types of things here: the large colonies seem like on top of the lawn and could be E.coli or yeast (although in our hands yeast contaminants were most often pink!). The small ones and those with some sort of faint circle around them are in the lawn. So whats supposed to be in the lawn and what's the phage and at which supposed concentration?
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