Measuring bacterial growth by refractive index tapered fiber optic biosensor

Abstract

A single-mode tapered fiber optic biosensor was utilized for real-time monitoring of the Escherichia coli (E. coli K-12) growth in an aqueous medium. The applied fiber tapers were fabricated using heat-pulling method with waist diameter and length of 6–7 μm and 3 mm, respectively. The bacteria were immobilized on the tapered surface using Poly-l-Lysine. By providing the proper condition, bacterial population growth on the tapered surface increases the average surface density of the cells and consequently the refractive index (RI) of the tapered region would increase. The adsorption of the cells on the tapered fiber leads to changes in the optical characteristics of the taper. This affects the evanescent field leading to changes in optical throughput. The bacterial growth rate was monitored at room temperature by transmission of a 1558.17 nm distributed feedback (DFB) laser through the tapered fiber. At the same condition, after determining the growth rate of E. coli by means of colony counting method, we compared the results with that obtained from the fiber sensor measurements. This novel sensing method, promises new application such as rapid analysis of the presence of bacteria.

Introduction

Escherichia coli (E. coli) naturally exist in the intestinal tract of humans and warm-blooded animals. The bacterial genes are comparatively simple; therefore, these genes can be easily-manipulated or duplicated through a process of reproduction. A series of genetic manipulating systems have also been developed which allows the production of recombinant proteins using E. coli. One of the useful applications of recombinant DNA technology is in the production of human insulin [1], and moreover modified E. coli have been used in vaccine development, bioremediation and production of immobilized enzymes [2]. On the other hand, some of E. coli strains such as serotype O157:H7 can cause serious food poisoning in human, and are occasionally responsible for costly product recalls [3]. In view of the fact that, E. coli are not always confined to the intestine, their ability to survive for brief periods outside the body makes them an ideal microorganism to test environmental samples for fecal contamination. The positive and negative features of E. coli places them among the best-studied model organisms and one of the most important species in biotechnology and microbiology.

Measuring the growth rate of these bacteria has become an important issue in laboratories, food industry, and pharmaceutical companies. The effect of temperature variation and media composition on the growth rate of the bacteria has been studied extensively [4]. In the conventional methods, the contaminated sample is grown in an enriched medium for 6–18 h. Some other convenient and safe counting methods have led to the development of a number of sensors for real-time bioprocess monitoring of bacterial growth. These methods involve in situ monitoring systems which were developed for measuring dissolved oxygen [5], [6], carbon-dioxide [7], optical density (OD) [8], [9], [10], [11], heat exchange [12], [13], and pH [5], [6]. Several real-time bioprocess monitoring systems have been introduced including piezoelectric sensors [14], [15], impedance spectroscopy measurement [16], [17], [18], [19], and micromechanical oscillator [20] which are illustrated in Table 1.

Refractive index (RI) measurement in small volumes plays a vital role in many areas of biophysics, biochemistry and biomedicine. For instance, chemical or composition changes inside the cells can be measured by RI method [21], while the traditional bulk refractometers are not appropriate for such an application. Currently, the RI measurement method is a new and powerful way for determining the growth rate of living cells.

Fiber based RI sensors (FBRIS) are one of the RI measurement structures which represent several advantageous features such as small size, immunity from electromagnetic interference, corrosion resistance, and remote sensing capability. In addition, FBRISs could be easily multiplexed on a single fiber network. These characteristics give them preference over other techniques for biological applications.

FBRISs are classified into long period grating (LPG) [22], [23], [24], [25], fiber Bragg grating (FBG) [26], [27], [28], hybrid LPG–FBG structures [29], [30], metal coated fiber using surface Plasmon resonance [31], [32], localized surface Plasmon [33], multi-mode tapered fiber (MMTF) [34], and single-mode tapered fiber (SMTF) [35]. Although these configurations were proposed as fiber sensors, there are still some limitations for the RI measurement. Some of the limitations and advantages of the mentioned approaches are summarized in Table 2.

Previous work in the bacterial growth rate was monitored by 480 nm light transmission through the tapered fiber [36] which showed that changes in transmission was a measure of change in absorption of the evanescent field (EVF). In this study the bacterial growth rate was monitored by transmission of a 1558.17 nm distributed feedback (DFB) laser through the tapered fiber which studies the effect of bacterial growth on RI and laser radiation effect on bacterial growth rate were investigated. RI variation was calculated by measuring bacterial growth rate and RI model for cell which used in Ref. [37]. Our sensor is believed to demonstrate modestly enhanced sensitivity and the simplicity of design compared to other previous sensor [37].

The aim of this study is to fabricate a RI tapered fiber optic biosensor (TFOBS) in order to determine the growth rate of E. coli K-12 at room temperature which is useful in measuring the bacterial contamination in food industries.