Bacteria are found everywhere. They can also grow on various substrates. Are there any bacteria that can be used as a food source? For example mix x with water and add some of these bacteria. These then either turn x into something nutritious or can be consumed themself. Are there any potential candidates? Bonus points if the substrate bacteria use to grow is abundant.
The best example of bacteria-as-food is the genus Arthrospira, which is a kind of cyanobacteria whose edible biomass is known as spirulina.
From the comments:
I was rather thinking about something abundant but not nutritious themself. Are there any examples with wood / cellulose?
In fact, the traditional Filipino food called nata (examples of which include nata de coco and nata de piña) are basically cellulosic pellicles of the bacterium Komagataeibacter xylinus, produced by culturing the bacterium in the appropriate medium (coconut water for the former, and pineapple juice for the latter).
(This resembles the SCOBY that is used in making kombucha.)
While not used as a food per se, bacteria are used in production of many foods, and you eat masses of them when consuming the product. At a guess; in the 10's of millions per bite is my estimate.
One type that is commonly used in several sorts of foods are the Lactobacilli. These are commonly used to generate sour products, through their fermentation of sugars into lactic acid. They are used extensively in yogurt production (Lactobacillus acidophilus usually), as well as other fermented diary products (cheese, kefir, cream cheese, sour cream, etc.) sourdough breads, production of fermented pickles such as sauerkraut and kimchi.
In some cases, such as kimchi and sourdough, these are often the natural lactobacilli and other bacteria and yeasts that are on the material used to make the product, so no added culture. In the case of sourdough, the fermentation can also be a result of yeasts in addition to the lactobacilli.
Apart from the classical microbial food processes, there is also a more recent approach captured under the term "single cell protein" (https://en.wikipedia.org/wiki/Single-cell_protein). Basically microorganisms are grown on a suitable substrate, and the protein that makes up their cells is used as food. Typically it would be either a food supplement or used in animal feed.
Regarding bacteria, it's most interesting to look into autotrophs, which can fix carbon from CO2 with a source of energy, or methanotrophs, which grow on methane derived from natural gas or biogas.
Basically all the mammals which eat a diet comprising hard-to-digest plants like grass, scrub, etc. use bacteria exactly like this. So cows, horses, deer, camels, goats, koalas, and so on. They either use foregut fermentation (like cows) or hindgut fermentation (like horses). In either case microbes, including bacteria, break down hard-to-digest carbohydrates like cellulose into easy-to-digest simple sugars.
For some foregut fermenters like ruminants (e.g. cattle), gut microbes (including bacteria) are also an important protein source--the cow gut is split up into multiple sections, the first section (the rumen) is a comfortable place for microbes to live and break down cellulose but some of the microbes end up traveling with the food into another section where the microbes themselves are also directly digested by the cow.
Even humans have some bacteria which do this in a limited fashion, just not as efficiently as grazing animals.
Bonus side note: This does not include panda bears. Panda bears eat specific parts of bamboo which are easy to digest and high in protein already and their digestion functions much more like that of a carnivore.