Measurement of Bacterial Concentration Using a Portable Sensor System With a Combined Electrical-Optical Approach

Bacterial concentration, an important parameter for different applications, is usually measured by means of the plate count technique (PCT), a laboratory method that is reliable but also time consuming (24-72 h) and not easily implementable in automatic form. This paper investigates two alternative methods, based on electrical and optical measurements, suitable to realize mobile, low-cost and user friendly instruments. A comparative analysis for both methods is performed with enriching medium solutions and milk samples inoculated with four different bacterial species (Escherichia coli, Lactobacillus plantarum, Pseudomonas aeruginosa, and Staphylococcus aureus).The results show that both methods can detect the bacterial concentration more easily and in shorter time than PCT (3-14 h) and, in many cases, the optical approach provides a faster response than the electrical one (30 - 240 min) and higher accuracy ($\text{R}^{2} >0.9$ ). Furthermore, it is found that, in some cases, the combined use of optical and electrical measurements allows to determine the type of bacterial species present in the sample. Both such methods have been implemented in the form of a portable and automatic sensor system, suitable for in-situ measurements with no need of trained personnel.