Microbiological characteristics of natural mineral water

Natural, non-carbonated mineral water is, like every other natural water from a spring, never sterile. However, the microbial level is always very low. But after its bottling, the level rises rapidly and numbers of more than 10,000 to 100,000/ml can be reached. In principle 2 groups of bacteria of very different origin and properties can be found in the microbial flora of the bottled, non-carbonated mineral water. Allochthonous bacteria will get into the water by contamination from the containers, closures, air or the bottling machines. They are mostly transitory as they cannot grow in a substrate with an extremely low nutritive level and die off more or less rapidly. From the hygienic point of view the permanently contaminating flora with Pseudomonas aeruginosa as main representative is more serious. These special gram-negative bacteria are oligocarbotolerant and can therefore multiply in the mineral water of extremely low nutrient level after a certain adaptation. Their effective bacteriological control is possible by colony counting with incubation at +37 degrees C but only just after bottling. The autochthonous microbial flora consists of psychrotrophic and of distinctly oligocarbophilic, mainly gram-negative bacteria such as Achromobacter, Flavobacteria, Pseudomonas as well as gram-positive Arthrobacter-species. According to indirect experiences, this autochthonous microbial flora must be growing in the open system of the underground source and renew itself constantly. The bottling of the natural spring water implies a drastic environmental change from this open system into a closed one. Then the bacteria start multiplying more or less rapidly like in a batch culture. Main reason for this is the extension of the inners surface of the system. The multiplication of bacteria after bottling of a mineral water of extremely low nutrient level therefore is an entirely normal biological process. For this reason, limits of the aerobic colony count at +20 degrees C incubation for natural mineral water seem not to be justified.