When the fermentation starts the main hurdles against microbiological spoilage of the sausage are the low bacteria count of the meat, the presence of nitrite and salt. Keep in mind that in time the sausage will be losing more and more moisture but the salt remains inside and the percentage of salt in a finished sausage will be higher. In about 48 hours lactic bacteria metabolize enough sugar to produce a sufficient amount of lactic acid to drop pH (increase acidity) of the sausage and this stabilizes the sausage making it more resistant to spoilage.