Likeliest cause of mold in fruit fly cultures?

[katamari]

For the first time since keeping/buying fruit fly cultures, a batch that I ordered from PetCo (direct shipped to me) developed mold in them a few days after arriving.

I have never had mold appear in my cultures from them before, and never in any of the cultures I made at home either, and the environmental conditions haven't changed much other than gotten colder.

I assumed it was contaminated on their end (they were still sealed, I never opened them, and it was like 3 days after they arrived), and Petco was happy to send replacements.

But.. in the week since then, I've had mold develop in my most recent homemade culture too - one seeded on Jan 9th (before the tainted order showed up). None of my older cultures still around from Dec 12th and on have mold. Just the Jan 9th one. What's worse, the replacements from Petco look to be developing it again (and I've kept them apart from the others upon arrival just in case).

Could the mold have jumped from one culture container to another through the breathable lids? And regardless, is there anything I can do to minimize/prevent mold in my homemade cultures?

They're typically kept around 60-65° in a poorly insulated garage. I wouldnt expect humidity to be the issue. I can provide more pics of cultures if that would help.

Also my homemade cultures have all been made using standard NEHERP FF culture kit & media, if that matters.

Thanks in advance!


A pic of one of the first batch as soon as I noticed it

 

[DaveM]

Fly labs always get mold, and how much mold you get is one of the main deciding factors on when to move the flies to new vials.
For labs, the food medium gets heated in advance to kill some microorganisms initially present in the media, and then it's kept sealed before use to reduce colonization.

But as soon as you transfer flies to the vial, you are transferring mold spores; they come with the flies. Cultures last longer when kept slightly cooler than room temperature (15°C), though that slows the growth of new flies as well.
Some media gets antifungal additives, (common with the blue, high school science class fly food) but that's frowned upon in real research due to potentially confounding effects on experiments.
Another thing you can do is add beneficial fungus: yeast. The yeast competes (partially successfully) with mold and can extend the life of your cultures. Sprinkle bread yeast or brewer's yeast (doesn't matter which) into the vial. Plus, as an added bonus, flies love to eat yeast.

Edit: As an extra note, it's easy to make your own fly medium with cornflour / corn starch and molasses.

 

[katamari]

Fly labs always get mold, and how much mold you get is one of the main deciding factors on when to move the flies to new vials.
For labs, the food medium gets heated in advance to kill some microorganisms initially present in the media, and then it's kept sealed before use to reduce colonization.

But as soon as you transfer flies to the vial, you are transferring mold spores; they come with the flies. Cultures last longer when kept slightly cooler than room temperature (15°C), though that slows the growth of new flies as well.
Some media gets antifungal additives, (common with the blue, high school science class fly food) but that's frowned upon in real research due to potentially confounding effects on experiments.
Another thing you can do is add beneficial fungus: yeast. The yeast competes (partially successfully) with mold and can extend the life of your cultures. Sprinkle bread yeast or brewer's yeast (doesn't matter which) into the vial. Plus, as an added bonus, flies love to eat yeast.

Edit: As an extra note, it's easy to make your own fly medium with cornflour / corn starch and molasses.

Thank you for the great info! I already add yeast to my own cultures, and I don't open the containers before necessary, so I guess I just got (un)lucky. Good to know this isn't an uncommon thing - guess I missed that in my initial research.

Would slightly increasing the amount of yeast I use per culture help? I admit I'm not very exacting in the amount of yeast I add each time, just eyeballing 10-20ish grains.

I plan to make my own culture medium by hand but the NEHERP kit was convenient to get me started in the beginning.

 

[DaveM]

The fly labs work fast with hundreds of different stocks. They weren't precise with the amount of yeast (just had a big shaker and would dust over a big tray of vials before adding flies), and I think the yeast got a big head start on the mold. The mold doesn't grow well in a acidic environment, whereas that doesn't matter to the yeast. I'm trying to think, and I do think they added ascorbic acid to the medium also as a preservative, maybe helped a little. It was a long time ago for me, just a ~6 month temporary project that I had, and the technicians did most of the work.

It's different, really, in a fly lab, than for keeping feeder insects. We would switch vials every 3 - 6 weeks for lines that we were actively crossing (and incubating at 25°C). For feeder insects, I guess you just want to maintain them as efficiently as possible long term. The larvae eat and destroy mold spores on the surface of the medium, but mold would eventually grow on the sides of the vials. There's probably some trick that could help you that is not coming to my mind right now. I'll email a real fly person (Drosophila researcher; she's not really part fly!) friend and let you know if she has any tricks. Good luck to you.

 

[viper69]

Mold in cultures is common

 

[katamari]

The fly labs work fast with hundreds of different stocks. They weren't precise with the amount of yeast (just had a big shaker and would dust over a big tray of vials before adding flies), and I think the yeast got a big head start on the mold. The mold doesn't grow well in a acidic environment, whereas that doesn't matter to the yeast. I'm trying to think, and I do think they added ascorbic acid to the medium also as a preservative, maybe helped a little. It was a long time ago for me, just a ~6 month temporary project that I had, and the technicians did most of the work.

It's different, really, in a fly lab, than for keeping feeder insects. We would switch vials every 3 - 6 weeks for lines that we were actively crossing (and incubating at 25°C). For feeder insects, I guess you just want to maintain them as efficiently as possible long term. The larvae eat and destroy mold spores on the surface of the medium, but mold would eventually grow on the sides of the vials. There's probably some trick that could help you that is not coming to my mind right now. I'll email a real fly person (Drosophila researcher; she's not really part fly!) friend and let you know if she has any tricks. Good luck to you.

Ok next culture instead of adding flies the same day I prep the medium, I'll try adding a bit more yeast and giving it a couple days for the yeast to get going before I add flies.

Also in the NEHERP kit instructions is to spritz the culture with white vinegar, which I've been doing.

 

[Tentacle Toast]

Spores

 

[Introvertebrate]

Ok next culture instead of adding flies the same day I prep the medium, I'll try adding a bit more yeast and giving it a couple days for the yeast to get going before I add flies.

Also in the NEHERP kit instructions is to spritz the culture with white vinegar, which I've been doing.

I used to add a pinch of Methyl Paraben to cultures. I would think that the NEHERP medium already includes a mold inhibitor. I'd bring my medium to a boil before pouring it into the container. Then I'd add the excelsior and zap the whole thing in the microwave one last time. Any longer than 30 seconds and it'll boil over. I'd let that return to room temperature before adding yeast and flies. Obviously, the flies themselves might introduce the mold, but it least you're off to a clean start.