Fluorometric detection of phages in liquid media: Application to turbid samples

During the last years there has been a growing interest in the development of methods for phage detection and quantification in environmental, public health and industrial sectors. Good methods of phage monitoring contribute to progress in phage therapies, biocontrol and food safety studies. They have also been used to indicate the possible presence of microbiological hazards in drinking and recreational waters, and are an essential tool to prevent failure of microbe-based industrial bioreactors. Many of the sophisticated methods that have emerged to cover these needs are strongly hampered by the presence of turbidity in the samples, that results in decreased sensitivity. To avoid this, time consuming pretreatment steps must often be included that increase the overall complexity of the assays and the time required to perform them. With this in mind, we have explored an alternative method that fulfills the criteria of being simple, rapid and inexpensive and can be used to perform analysis in turbid media without any pretreatment steps. In this paper we develop a method that monitors lysis of an indicator culture when exposed to samples containing the target phage. The method is based on the properties of resazurin, a redox dye that becomes fluorescent when reduced by an active microbial culture. We analyzed the fluorescence kinetics of non-turbid phage-infected bacterial cultures as a function of both, phage abundance and initial cell concentration. For this purpose, different phage/host combinations were used and then, the addition of resazurin at different times (0, 30 and 60 min) was carefully evaluated for each phage/host combination, thus providing data for 168 combinations in total. Next, selected phage/host combinations were tested over 4 different turbidity models: 0, 1000, 2000 Nephelometric Turbidity Units (NTU) as well as in milk. The data obtained provided information about the duration of the assay and sensitivity thresholds in matrices with different turbidity grades. The results obtained indicate that the method can detect as few as 10 phage particles per assay volume within 3.5 h. If sensitivity is not an issue and the threshold of detection is increased to 10⁷ phages the assay is considerably shortened, providing reliable results in only 40 min. Overall, the detection approach proposed in this work provides a simple, rapid and inexpensive solution that compares favorably, in terms of performance, with other high-end methods.