Today, most commercialized technology to directly detect bacteria requires complicated chemistry and/or advanced optics. SBT Instruments offers a new affordable approach, where bacteria are measured with no sample pretreatment, no incubation time, and no advanced user interaction. This is accomplished by using an electrically based flow cytometry technology referred to as impedance flow cytometry. The electrical detection principle revolves around measuring the impedance in a small microfluidic flow cell, which enables us to detect bacteria immediately.

Working Principle

The concept of impedance flow cytometry in industry usage is novel. The concept is best explained by dividing it up in its three main components; cytometry, flow, and impedance. Cytometry is the science of measuring characteristics of cells including bacteria cells. Flow cytometry is the science of measuring characteristics of cells in a flow. Impedance flow cytometry involves measuring the characteristics of cells in a flow using impedance measurements. Most are familiar with the concept of electrical resistance, and impedance can be described as a more complex form of electrical resistance.

The working principle in SBT Instruments' product range involves circulating a liquid through a microfluidic flow cell with integrated electrodes. Bacteria, and other particles, flow across the electrodes, which results in a change in impedance between the electrodes. The impedance change for bacteria is uniquely different compared to other non-organic particles, and it is therefore possible to provide an almost immediate estimate of the bacteria in the sample. The working principle detects all bacteria species in the sample provided they have an intact cell membrane. Bacteria with destroyed cell membranes are not counted as bacteria. The unit of measurement is therefore membrane intact bacteria per milliliter (MIC/ml).

Total bacteria count vs. CFU vs. ATP

SBT Instruments' technology directly counts bacteria on a single cell basis based on their electrical properties. We do not operate in secondary units such as relative light units, enzymatic reaction times, or even colony forming units, which may differ from measurement to measurement and from system to system. Instead we operate in actual counts that can not be misinterpreted.

Colony forming units (CFU) measurements have been the standard method for measuring bacteria for hundreds of years. Different bacteria species grow better at different temperatures, at different ambient conditions, and on different nutritional media. The preferred growth conditions are usually well-known for pure cultures of bacteria, and the correlation between plate counting and SBT's technology is linear and approximately 1:1 when measuring pure cultures. However, the correlation is a different in a typical environmental sample, such as a water sample, as only a fraction of the bacteria in the sample will actually give rise to colonies and therefore be counted with plate counting. SBT's technology measures all bacteria in a sample, independent of their preferred growth conditions, so the bacteria count will typically be orders of magnitude higher than a corresponding CFU count and therefore also provide a much more accurate view of the bacterial content of the environmental sample.

Environmental ATP tests for surface and water samples emerged as a response to the notoriously slow measurement time of CFU measurements. ATP tests measures all biological material but the method is unable to determine if any of the measured material is bacteria. Furthermore, since the ATP measurement is based on a chemical reactions it is notoriously prone to react with surface disinfectants and provide false read-outs. The technology employed by SBT Instruments directly measures bacteria. Additionally, we do not use any chemical reactions and therefore surface disinfectants have no influence on our measurements.