Abstract
Microbial screening is a primary concern for many products. Traditional techniques based on standard plate count (SPC) are accurate, but time consuming. Furthermore, they require a laboratory environment and qualified personnel. The impedance technique (IT) looking for changes in the electrical characteristics of the sample under test (SUT) induced by bacterial metabolism represents an interesting alternative to SPC since it is faster (3–12 h vs. 24–72 h for SPC) and can be easily implemented in automatic form. With this approach, the essential parameter is the time for bacteria concentration to reach a critical threshold value (about 10 7 cfu mL −1) capable of inducing significant variations in the SUT impedance, measured by applying a 100 mV peak-to-peak 200 Hz sinusoidal test signal at time intervals of 5 min. The results of this work show good correlation between data obtained with the SPC approach and with impedance measurements lasting only 3 h, in the case of highly contaminated samples (10 6 cfu mL −1). Furthermore, this work introduces a portable system for impedance measurements composed of an incubation chamber containing the SUT, a thermoregulation board to control the target temperature and an impedance measurement board. The mix of cheap electronics and fast detection time provides a useful tool for microbial screening in industrial and commercial environments.
Highlights
The need for microbial screening is a primary concern for various market segments, such as, for example, the medical, environmental, food and military ones (Alocilja and Radke, 2003)
Microbial concentration is traditionally measured by means of the standard plate count (SPC) technique (Kaspar and Tartera, 1990), performed by inoculating the culture plate with a dilute solution of the sample under test (SUT) and counting the number of cells grown in the resultant culture
The results indicate that Mc Conkey medium is suitable for electrical detection of bacterial concentration
Summary
The need for microbial screening is a primary concern for various market segments, such as, for example, the medical, environmental, food and military ones (Alocilja and Radke, 2003). Microbial concentration is traditionally measured by means of the standard plate count (SPC) technique (Kaspar and Tartera, 1990), performed by inoculating the culture plate with a dilute solution of the sample under test (SUT) and counting the number of cells grown in the resultant culture This approach is characterized by high accuracy but it features long times (24 to 72 hours, depending on culture broth and bacteria type) and, in practice, requires a microbiology laboratory with qualified personnel. The Impedance Technique (IT) based on the monitoring of changes in the SUT electrical characteristics induced by bacterial metabolism (Fistemberg-Eden and Eden, 1984) provides a good solution for many cases of practical interest, since it: a) measures total bacterial concentration with response time ranging from 3 to 12 hours (depending on the sample microbial concentration); b) can be implemented in automated form with cheap electronics. The other sensor is a platinum resistance temperature detector (PT100) working on a broad range of temperatures (-200 to +850 °C) and whose resistance is linearly related to the temperature
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