Three types of bacteria are used in the production of Swiss cheese: "Streptococcus salivarius" subspecies "thermophilus" (also known as "Streptococcus thermophilus"), "Lactobacillus" ("Lactobacillus helveticus" or "Lactobacillus delbrueckii" subspecies "bulgaricus"), and "Propionibacterium" ("Propionibacterium freudenreichii" subspecies "shermani"). In a late stage of cheese production, the propionibacteria consume the lactic acid excreted by the other bacteria and release acetate, propionic acid, and carbon dioxide gas. The carbon dioxide slowly forms the bubbles that develop the "eyes". The acetate and propionic acid give Swiss its nutty and sweet flavor. A hypothesis proposed by Swiss researchers in 2015 notes that particulate matter may also play a role in the holes' development and that modern sanitation eliminated debris such as hay dust in the milk played a role in reduced hole size in Swiss cheeses, or even "blind cheese". Historically, the holes were seen as a sign of imperfection and cheese makers originally tried to avoid them by pressing during production -the holes only became an identifier of the cheese in modern times.