After manufacture, a range of halotolerant yeasts (often of genera including Debaryomyces, Saccharomyces and Candida) together with Geotrichum candidum become established on the cheese surface where they metabolise lactate causing an increase in pH. This change in the environment favours the growth of Gram-positive organisms such as Micrococcus, Staphylococcus, Corynebacterium, Arthrobacter, Microbacterium and Brevibacterium. (Older sources generally mention Brevibacterium linens as the principal smear organism but more recent work has shown that this is only one organism amongst many.) The surface microflora may reach 10^11 cfu/cm^2 and has a huge effect on the development of cheese flavour. Although enzymes essentially do not diffuse through cheese curd, small volatile flavour compounds produced by the surface smear do move into the cheese. During ripening, the cheese surface is "smeared" (traditionally by hand using a cloth soaked in brine) to discourage bacteria growing as colonies and to favour a more uniform surface. The bacterial surface microflora is extremely complex; the figure below is a dendrogram showing strain identification (by RAPD-PCR) of isolates from a smear-ripened cheese.
Most smear-ripened cheeses orginated in northern Europe and include varieties such as Limburger, Tilsit, Pont l'Eveque, Port du Salut, Trappist, Livarot, Munster, Havarti and a whole host of others, including the delightfully named UK variety, "Stinking Bishop"! As the latter most name suggests, smear-ripened cheeses often have a tremendously pungent aroma that is commonly likened to that of smelly socks. Strange (and unappetising) as it may seem, this is actually a perceptive description as the bacterial microflora that dominates the surface of these cheeses is actually very similar to that of human and animal skin...
Brennan, N.M., Cogan, T.M., Loessner, M. and Scherer, S. (2004). Bacterial surface-ripened cheeses, in, Cheese: Chemistry, Physics and Microbiology, Volume 2 Major Cheese Groups, 2nd Edn., P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P. Guinee (eds), Elsevier, Amsterdam, pp. 119-225.