While someone was looking after my High Five insect cell suspension culture last week large spheres formed in the bottom of the flask (see attached pictures). The spheres are apparently uniform in size, and reasonably solid (apparently not fluffy or squishy looking). The High Five cells were apparently healthy and there was no sign of any mycelia/contaminating cells through the microscope. There was also nothing growing in the media bottle (though I don't think it tested by incubation at 28 degrees).
These spheres were not present in any other cell cultures split in the same laminar flow hood (SF21) or indeed in the same tissue culture lab (HEK), though they apparently were seen in the waste bottle.
The media is Express Five (serum free) supplemented with L-glutamine and pent-strep.
Despite the appearance of the spheres the cells were split as normal on Thursday, but on Friday the spheres were forming again, so the cell culture and media were discarded and everything has been cleaned.
Has anyone ever seen anything like this? Our best guess is that it is fungal, and I am considering whether or not to add fungizone (or similar) as a precaution next time I start up this culture.