Bacteria consume sugar, which produces acid as a waste-product, which erodes enamel. Gum disease is primarily an inflammatory response to the presence of external irritants like dental plaque (which contains bacteria as well).

So there's no specific gum disease bacteria? It's just the same bacteria that causes cavities?

According to Feng & Weinberg:

Approximately 500 bacterial species reside in the oral cavity. Of these, 415 species are estimated to be present in subgingival plaque. Specifically:

"A. actinomycetemcomitans has been strongly implicated in localized aggressive (juvenile) periodontitis Likewise, bacteria such as P. gingivalis, A. actinomycetemcomitans, Tannerella forsythia (formerly, Bacteroides forsythus), Treponema denticola, and Eikenella corrodens have been associated with chronic (adult) periodontitis."

Ref: http://www.perioimplantelsalvador.com/Informacion/bibliograf...