I don't know the magnification of your pictures but it looks like yeast. My supervisor brings down the hammer on people that get this type of contamination, because it's always of human origin. Ethanol doesn't always put down fungal contamination, neither does bleach or UV, unless the exposure is prolonged. I doubt your surfaces are to blame, it is almost always originating from lack of proper prep and sterilization of ones self. To avoid this I have often gone "over the top" with my clean-up before tissue culture work, spraying my arms up past the elbows with 70% ethanol, spraying both the inside and outside of my gloves as well as my lab coat sleeves, and spraying compulsively throughout any procedure (once ever 2-3 minutes). Not light mists either but enough to get things quite wet. And since I work without the face shield typical of most biohazard flow-hoods, I also wash my face, neck and chest with harsh soap, with an additional spray of ethanol to the lab coat neckline. This is the only way I know to avoid yeast contamination. You can try antifungals like cefotaxime or econazole, if even scrupulous cleaning does not solve the problem. Also does your lab share media, or do you use personal supplies?

I'm not sure that's contamination.

It rather resembles the aggregation of compounds from the culture medium.

This happens when the medium is a little bit too old, or alkaline, or that the conditions of incubation are too long.

In this last case, check the status of your incubator and its humidification, or reduce the incubation time.

no bar in the pics thus it is hard to see how big these cells are, hard to say they are bacteria or yeast. Did you do some staining? Based on the first image, it also looks like bacillus and looks some spore inside the cell

You need to specify the magnification, or insert a size bar. I can't even tell if it's Eukarya or Bacteria. If this is 200x magnification, it's probably yeast. If it's 1000x, it's Bacteria. Have you tried to stain them?

It could also be non-biological, like small crystals of salts that have precipitated. That happens if the medium has been mixed incorrectly or stored at the wrong temp. It is often necessary to add sterile-filtered salt solutions to cold media.

Yeasts, mold spores and mycobacterium can be very difficult contaminants.

Thanks all for the responses.

These images are light microscopy images at only 40x. I'm not convinced that it's bacteria as they appear quite static (compared to a few microscopy videos that I've seen of them - very mobile/vibrating in nature). We were worried that it might be bacterial in origin as some member of the lab has started bacterial work recently - although in a different lab space on a different floor. 

Today there is still no growth on the LB agar - we've concluded that its most probably not bacterial contamination. We've not done any staining procedures. 

If its yeast, aside from techniques describe by Jordan above for aseptic practices, is there anything else that we should/can do for decontamination/cleaning?

Huzaimi

It's crystals. It must be some insoluble salt that has precipitated.

The solution is to always mix nutrient media when it's cold. Never autoclave salt solutions, salts should be sterile filtered and added separately to autoclaved medium when it has cooled.

If you want to know what it is plate it out on some blood agar plates.  Once you know what it is then you can figure out where it came from.