Abstract
Abstract. The detection of bacterial concentrations in metalworking fluids (MWFs), oil-in-water emulsions used in the cutting industries for cooling and lubrication, is important in order to extend the product life-cycle and plan its disposal according to regulations and legislations. The standard method of measuring culturable bacterial concentration is the plate count technique (PCT) that, however, has long response times and is not suitable for automatic implementation outside a laboratory. In this paper a portable sensor system that measures the bacterial concentration in liquid and semi-liquid media exploiting impedance microbiology is presented and tested for the application of MWF microbial monitoring. A set of MWF samples, taken from metalworking plants, have been tested and good agreement has been found between the system response and that of the PCT. The proposed system allows automated bacterial concentration measurements with shorter response times than the PCT (4 to 24 h vs. 24 to 72 h) and is suitable for in-the-field MWF monitoring.
Highlights
Results from the sensor system, i.e. measured detection time (DT) for two electrical parameters (Rs and Q) and three different enriching media, have been compared with the bacterial concentration obtained with the plate count technique (PCT)
DT values have been registered for any experimental condition and compared to bacterial concentration measured by the PCT
The calculated value of TGEN is in the range 17 to 25 min for all tested media and monitored parameters, with Luria Bertani (LB) giving the best results in terms of sensor response time
Summary
Bacterial concentration detection and monitoring are important in different fields (Bahadir and Sezginturk, 2015), such as food quality assurance (Fakruddin et al, 2013; Calix-Lara et al, 2014), clinical analysis (Singh et al, 2014) and environmental monitoring (Rizzo et al, 2013), in order to (a) ensure that the total bacterial concentration does not exceed thresholds set by national and international regulations, and (b) guarantee the absence of particular pathogens, such as Salmonella typhimurium and Escerichia coli O157:H7, representing a threat to human health even at very low concentrations. The standard method to measure culturable bacterial concentration is the plate count technique (PCT) (Grigorova and Norris, 1990) that is accurate and reliable but has long response times (in the range 24 to 72 h) and requires manual operations by trained personnel in a laboratory It is not suitable for in situ measurements, nor as the base for automatic instruments. The industrial standard for in-the-field MWF bacterial monitoring is based on dip-slide test kits that are timeconsuming (36 to 48 h for most bacterial species) and lack the accuracy of the PCT Another technique used to measure bacterial concentration is impedance microbiology (IM) (Firstenberg-Eden and Eden, 1984), exploiting the fact that bacterial metabolism transforms uncharged or weakly charged compounds into highly charged ones, producing a measurable change in the sample electrical parameters, allowing us to estimate the unknown bacterial concentration. The proposed instrument, exploiting information and communication technology (ICT) solutions to measure, process and transmit data, can be used, by anybody, for in situ microbial screening inside metalworking plants
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