Potential of an electronic nose for the early detection and differentiation between Streptomyces in potable water

Abstract

Much attention has been focused on the production of musty-aroma compounds such as geosmin and their impact on the quality of fresh water and water-cultured raised fish and sea-food. At present, there are no efficient means of removing these off-flavours from water. Therefore, the rapid detection of geosmin-producing microorganisms, in particular the genus Streptomyces, at early stages of differentiation, is still the best option in preventing deterioration of water quality. We investigated the potential of an electronic nose consisting of an array of 14 conducting polymer sensors for the rapid and early detection of Streptomyces spores in reverse osmosis and tap water. Geosmin solutions in water at different concentrations were also prepared for headspace analysis in order to study the e-nose potential for detecting geosmin production in this environment. Normalised divergence data were analysed using principal component analysis (PCA) and discriminant function analysis (DFA). Data indicates that an e-nose could be employed to detect and monitor early activity of Streptomyces in water at different stages of differentiation, as well as to discriminate between different species based on their volatile production patterns. It also suggests that it could be used for monitoring geosmin production in water and possibly set threshold odour levels, as a routine task for specific water-screening purposes.

Introduction

Off-flavours and odours in potable water and aquaculture-raised fish and sea-food are a major source of consumer complaints all over the world [1]. Taste and odour occurrences are usually caused by the presence of trace organic compounds such as geosmin [2]. This volatile has been isolated and identified as having a muddy, musty odour discernible by the human nose when present at concentrations above 4–20 mg L⁻¹ [3]. Actinomycetes, particularly from the genus Streptomyces, are known to be the mainly responsible organisms for these occurrences in the water supply system. They produce geosmin as a secondary metabolite at a certain stage of differentiation prior to sporulation and therefore, the presence of spores in water can be correlated with geosmin production [4]. Unfortunately, at present, there are no efficient means of removing such off-flavours and odours from water, although activated carbon is still considered the best solution for their attenuation [1]. Therefore, the rapid detection of geosmin-producing microorganisms at early stages of differentiation is still the best option in preventing deterioration of water quality.

Gas chromatography (GC) and gas chromatography–mass spectrometry (GC/MS) are traditionally used to identify and quantify these compounds but although they are effective, reliable and low cost, they can be time consuming, particularly if many replicates are necessary. Other current methods for detection and quantification at similar threshold levels require large sample volumes and intensive sample concentration procedures [5], [6].

Rapid developments in sensor technology allowed the production of devices known as electronic noses, which represent an attempt to mimic the human sense of smell, with the same sensitivity and accuracy [7]. The basic principle of electronic nose technology involves exposing a range of non-specific sensors to volatile compounds, resulting in a change in conductivity of the sensor due to binding between that and the volatile [8]. Signals are detected and amplified by a software package system and may then be interpreted using a variety of methods such as principal component analysis (PCA) and discriminant function analysis (DFA). The pattern of the overall response generated by the sensor array is then used to characterise the odour. The obtained data are mainly comparative, since different samples or treatments may be characterised and discriminated based on their volatile production patterns [9].

E-nose technology has been widely employed for early detection of microorganisms causing food spoilage [10] and for biomedical purposes [11]. Some reports suggest that the application of e-nose technology for environmental diagnostics has been limited because of the inherent variability of environmental samples. Nevertheless, it has been successfully employed for detecting cyanobacteria in water [12] as well as heavy metals and pesticides [13]. Due to the growing interest in environmental monitoring, the early detection of microbial activity in producing volatile compounds such as geosmin is of increasing economic importance.

We have examined the potential of using an array of 14 conducting polymer sensors to detect and monitor early activity of Streptomyces in potable water prior to visible growth. Moreover, we tested the e-nose potential for differentiating between Streptomyces aureofaciens and Streptomyces griseus based on their volatile production patterns and for detecting and monitoring geosmin production in this environment.