Impact of the biotic and abiotic components of low mineralized natural mineral waters on the growth of pathogenic bacteria of human origin: A key to self-control of spa water quality

In order to assess the role of the biotic and abiotic components of natural mineral water (NMW) in the spread of allochthonous bacteria in non-thermal spa facilities, we examined the effect of low mineralized NMWs on the growth of several commensal and pathogenic human strains. These NMWs were collected from two Spanish spring spas and had different microbiological characteristics.Microorganisms were exposed to untreated, filtered and autoclaved NMWs at the temperatures of 22 °C and 37 °C for 2 days, mimicking the early stage of starvation. Starvation stress was controlled by the effect identified after incubation in saline. Changes in culturability after exposure were used as a measure of the water’s antibacterial effect. The specific biotic and abiotic effect of NMWs on the suppression of bacterial growth was estimated after excluding the bacterial response to starvation stress, characteristic of this natural oligotrophic environment.The incubation temperature strongly modulated both the consequences of starvation and the impact of the natural biotic and abiotic components of NMWs on the growth of commensal and pathogenic bacteria. A temperature of 22 °C conferred cross-protection of microorganisms to starvation and NMW abiotic stress, whereas a temperature of 37 °C decreased the tolerance to both, and had a negative influence on the abundance and diversity of NMWs microflora. This temperature-dependent behaviour of the allochthonous and autochthonous bacteria explained the different culturability of microorganisms after exposure to untreated NMWs at 22 °C (≈1–2.4 log colony forming units per ml -CFU/ml- mean reduction) and at 37 °C (≈1.8–3.2 log CFU/ml mean reduction).Discarding the effect of starvation, we estimated that the antibacterial effect of NMWs at the temperature of 22 °C was mainly driven by the microecosystem of NMWs, which explained ≥95% of NMW response. In contrast, at optimal temperatures for the growth of commensal and pathogenic microorganisms, ≥60% of the antibacterial response of NMWs was associated with the abiotic components of NMWs.The biotic and abiotic components of NMWs self-preserve the quality of water, preventing the progression of human pathogenic organisms that can occasionally cause water colonization. The influence of the intrinsic components of NMWs on the suppression of microbial growth is strongly modulated by environmental temperature.