bbracken has your answer with the bacterial strains, and linuxboy explains why.
Lactobacillus subr. Helveticus (note, Helvetia is the latin name of Switzerland, which is officially named Confoederatio Helvitica) produces the nutty flavor you associate with long aged Alpine type cheeses, such as Gruyere. It is a long-period maturing culture that produces its flavor only very slowly.
Note though, it is not the only bacteria that can produce this nuttiness.
The culture I use, which I imported from Switzerland, doesn't have it, and it is used to make a very long term aged, extremely nutty cheese. This culture (MK 410 Lyo Kultur) contains
Lactobacillus delbrueckii subsp. lactis
Streptococcus salivarius subsp. thermophilus
Lactococcus lactis subsp. lactis