The Immittance Response of Escherichia Coli Bacteria

Research Article

J Bacteriol Mycol. 2014;1(2): 6.

The Immittance Response of Escherichia Coli Bacteria

Mohammad A Alim1*, Sudip Bhattacharjee2 and Josh Herring3

1Department of Electrical Engineering and Computer Science, Alabama A & M University, P.O. Box 297 Huntsville, Alabama 35762, USA

2Department of Mechanical and Civil Engineering, Alabama A & M University, P.O. Box 367 Huntsville, Alabama 35762, USA

3Department of Food and Animal Science, Alabama A & M University, P.O. Box 1628 Huntsville, Alabama 35762, USA

*Corresponding author: Mohammad A Alim, Department of Electrical Engineering and Computer Science, Alabama A & M University, P.O. Box 297 Huntsville, Alabama 35762, USA

Zhang H, Z-BioMed, Inc., 15725 Crabbs Branch Way, Rockville, MD 20855, USA

Received: November 08, 2014; Accepted: November 20, 2014; Published: November 21, 2014

Abstract

In this study the ac small-signal electrical data are obtained for the sterile BHI and E. Coli bacteria suspended BHI material systems to extract an equivalent circuit model via Bode plot in conjunction with the complex plane plot. It is observed that the non-blocking conductive nature of the underlying operative electrical paths between the two opposite electrodes across the sample (material system) exist satisfying dc condition of the equivalent circuit model. Thus, the proposed equivalent circuit model verifies the existence of the shunt resistance that provides a meaningful representation of the material system. The non-Debye behavior is not addressed as this work is emphasized to establish correct equivalent circuit model.

Keywords: E. Coli; Impedance; Immittance; Bode plot; Equivalent circuit model

Introduction

The brain heart infusion (BHI) broth based material system used as a medium for the Escherichia coli (E. Coli) bacteria growth has been investigated [1, 2] via impedance or admittance (immittance) measurements to determine underlying operative mechanisms. The immittance measurement provides a powerful tool to understand the detection process of E Coli as well as the total behavior of the same in the suspended medium. In order to detect bacteria, relative or absolute changes in the constituting immittance parameters are used [3]. The electrical response of the bacterial behavior is ascertained by specific pattern of the data display and subsequent proposition of the equivalent circuit model [5-14].

The ac small-signal data when displayed in the complex plane formalisms [15-20] the underlying conduction processes between the two electrodes across the material system is delineated for the operative phenomena. This display supports traditional Bode plot and thus, does not violate the general requirements of the elements constituting the equivalent circuit model. The advantage of the complex plane plot is that an equivalent circuit model can easily be extracted unless dual representation of the complex plane plot of the same data is observed. Semicircular relaxation achieved in more than one complex plane provides at least one reasonable operative equivalent circuit. This has been demonstrated for a number of solid state material systems [21-30]. The sterile and bacterial media give rise to equivalent circuit model comprising of the simplistic series R-C (resistive-capacitive) combination [1-14]. This type of equivalent circuit model is blocking in nature that prohibits the flow of direct current (dc). The immittance data generation for a variety of peakto- peak voltages indicates that there is no existence of the blocking element prohibiting the flow of dc through the material system. Invariably the dc condition can be extrapolated from the convenient Bode plot [15-17].

The ac electrical measurements and complex plane plotting techniques are used to establish operative mechanisms via representative equivalent circuit model for the material system regardless of the state. Such an approach has proved to be a useful tool/technique in characterizing the electrical nature of a number of heterogeneous systems [18-30]. This technique unravels the underlying competing phenomena via lumped parameter/complex plane analysis (LP/CPA). The total ac response of the material system can be modeled in terms of equivalent circuit elements which identifies conduction mechanisms in an operating electrical path between the electrode terminals. Based on the slope of the straight line of the Bode plots and the semicircular relaxation of the complex plane plots Debye and non-Debye responses can be delineated. Nevertheless, the elements of an equivalent circuit model can be related to the charge carrying species.

In this work, the sterile BHI (hereafter BHI) and E. Coli bacteria suspended BHI (hereafter E. Coli) are investigated using ac smallsignal electrical measurements. Time dependence of these material systems is also investigated. These data are analyzed via complex plane formalisms as well as conventional Bode plot. From these plots it is revealed that the R-C series combination is shunted to support the non-blocking nature of the dc operative path of these material systems. Thus, the existence of the shunt resistor is verified for the sterile BHI and E. Coli.

The development of the correct equivalent circuit representation of both BHI and E. Coli systems is the underlying objective in this investigation. Distinction between Debye and non-Debye responses obtained from the complex plane formalisms in conjunction with the Bode plots is not included in this work. This is because the correct representation of the equivalent circuit model is addressed which is derived from the measured data. The Bode plot alone does not provide obvious understanding upon visual inspection whether it represents Debye or non-Debye response unless the slope of the straight line therein is determined. In this way the equivalent circuit presented in this work is not the same as seen elsewhere [5-14]. Often R-C series circuit is extracted from the Bode plot as a final equivalent circuit model of this material system. That is why the correct representation of the equivalent circuit model is addressed which is derived from the measured data. In this way the equivalent circuit presented herein is not the same as seen elsewhere [5-14].

Experimental

(a) Escherichia Coli (E. Coli) cultures and media

The bacteria used were from a non-pathogenic E. Coli culture obtained from American Type Culture Collection (ATCC 8739) in Manassas, Virginia. The cultures were activated in a manner similar to Mason and Powelson [31] by first transferring the bacterial inoculum from a refrigerated slant to tryptic soy agar (TSA - Difco, Sparks, MD) plates and incubated at 37°C for 24h. A well-isolated single colony forming unit (CFU) was inoculated into 100mL of brain heart infusion (BHI) broth (Difco, Sparks, MD) and incubated aerobically at 37°C. The bacteria were grown in a manner similar to Al-Qadiri et al [32] in BHI at 37°C and transferred every 24h for at least 3days prior to use. A 24h culture of 3µL was inoculated into 30mL of BHI in a culture tube equipped with stainless steel probes for electrical impedance measurements. Commercially dehydrated or concentrated media were used and reconstituted according to the manufacturer’s directions.

(b) Electrical measurements

In this study sterile BHI medium and the E. Coli were used in the acquisition of the immittance data applying ac small-signal voltage (1 V peak-to-peak). Two identical sterile BHI samples in test tubes were used for this purpose. One used as the sterile BHI sample for which the measurement was noted as zero time. The other BHI contained suspended E. Coli for which the measurement was noted at certain intervals after the introduction of the bacteria. Thus, the measurements were recorded at 180min, 390min, 600min, 990min and 1440min.

Two electrodes inserted in the test tube were connected to the impedance analyzer (HP 4192A). The ac small-signal electrical data were obtained in the frequency range 5Hz ≤ f ≤ 10MHz. These data were acquired in the admittance form (Y* = GP + j ω CP) where GP is the conductance, CP is the capacitance (ω × CP is the susceptance), ω (= 2p ƒ) is the angular frequency, and j = √-1 . The measurements were performed at room temperature in the vicinity of about 25oC. These data were analyzed via complex plane formalisms and used in the Bode plane representation.

Results and Discussion

(a) Representation of the measured data in the complex plane formalism

Figure 1 depicts the terminal ac electrical data in the admittance (Y*) plane of the sterile BHI medium (in blue) as well as the growing bacteria suspended in the BHI medium. The observed relaxation is semicircular on both positive and negative domains of the Y*-plane. This is identical to the behavior observed earlier in the solid-state system [33]. The negative domain is attributed to the inductive response of the material system as noted in the negative capacitance domain. The positive domain semicircle exhibits center below the x-axis indicating depressed semicircle. Thus, each curve portrays non-Debye response of the material system as a whole. Overall, the conductance is increasing with time reflecting gradual multiplication of the E. Coli bacteria.