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The source(s) of microbes that influence spontaneous fermentation are somewhat debatable, especially when referencing [[Lambic]] producers in Belgium. It is often claimed that ''Brettanomyces'' living in the barrels is responsible for the secondary fermentation of spontaneous beers. However, James Howat of Black Project Spontaneous Ales reported conducting an experiment that showed a similar fermentation profile between a barrel fermented spontaneous beer and samples taken from the coolship and aged in glass flasks <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1977772602250944/?comment_id=1977929225568615&reply_comment_id=1978514285510109&comment_tracking=%7B%22tn%22%3A%22R%22%7D James Howat. Milk The Funk thread on the source of ''Brettanomyces'' in lambic. 02/05/2018.]</ref>. The spontaneous beers at Black Project were produced for a few years by placing a kettle on the roof outside of the brewery to collect microbes from the air and then racked directly to barrels, so there was no influence of microbes living within the brewery (Black Project now uses a coolship inside the brewery). This lead James to conduct this experiment to see how much the barrels were influencing the microbiome of the beers even after they were steamed to the [[Barrel#Sanitizing|point of possible pasteurization]]. Additionally, Brasserie-Brouwerij Cantillon (ref needed) and Oud Beersel <ref>[https://soundcloud.com/craftbeerbrew/podcast-episode-21-new-belgiums-wood-cellar-director-blender-lauren-limbach Lauren Limbach. Craft Beer and Brewing Magazine Podcast. Episode 21. 02/16/2018.]</ref> (~42 minutes in) are known to steam clean their barrels which might be enough to nearly [[Barrel#Sanitizing|sterilize them]].
Other sources for the microbes include the ceilings of the breweries which are often not cleaned in Belgian lambic breweries, and the coolship itself. Some breweries have opted to craft wooden overhangs to simulate having a relatively low ceiling above the coolship (see [[Coolship#Overhangs_and_Other_Innovations|Coolship]]). As the wort cools overnight, vapor rises and condenses on the wooden ceiling or overhang above, and then drips back down into the coolship carrying any microbes that might be living on the surface of the wood. The air also carries some amount of microbes, although it is rare to find wild ''Brettanomyces'' in the air (see [[Brettanomyces#Environment_and_Survival|''Brettanomyces'' envrionment]]). The sources for various microbes are unknown and potentially many, and probably vary from producer to producer. Additionally, no single source is likely to be required to have a successful spontaneous fermentation.
It is worth noting that it appears to be possible for spontaneous fermentations to not harbor lactic acid bacteria or ''Brettanomyces''. For example, Andy Mitchell from New Belgium Brewing had a homebrewed spontaneous fermentation analyzed by New Belgium's lab after a year of age, and they only found ''Saccharomyces cerevisiae'' var ''diastaticus'' living in the beer and no lactic acid bacteria or ''Brettanomyces''. Andy reported that the beer was also not sour and did not have any ''Brettanomyces'' character <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1879283728766499/ Andy Mitchell. Milk The Funk thread rearding homebrewed batch of spontaneously fermented beer with no ''Brettanomyces'' or LAB. 11/10/2017.]</ref>.