The Principles of Microbial Biosensors

Introduction

The underlying basis of a microbial biosensor is the close proximity between an immobilized microorganism that serves as a specific recognition element and an electrochemical or optical sensing transducer that is used to convert the biochemical signal into an electronic signal that can be processed.

The manufacture of a microbial biosensor requires the immobilization of the microorganisms on a transducer by chemical or physical methods. Because the response, operational stability, and long-term use of a microbial biosensor is a function of the immobilization strategy employed, immobilization technology plays a key role in the successful design of microbial biosensors, and therefore, the choice of immobilization method is vital.

Microbe Immobilization Methods

We have two techniques that are used to immobilise microbes:

• Chemical methods
• Physical methods

Chemical Methods

The chemical techniques of microbe immobilization include covalent binding and cross-linking. Covalent binding techniques rely on the formation of a stable covalent bond between functional groups of the cell wall components of the microorganism and the transducer. To successfully achieve this objective, whole cells are exposed to harsh chemical reactions that can damage the microbial cell membrane and decrease the biological viability of the cells. Determining how to overcome this problem remains a practical challenge. Contrariwise, cross-linking involves bridging between functional groups on the outer cell membrane by multifunctional reagents like glutaraldehyde to form a network. Because of the speed and simplicity, this technique has found wide acceptance for the immobilization of microorganisms. The cells may be cross-linked directly onto the transducers surface or on a removable support membrane that can then be placed on the transducer. Although cross-linking has merits over covalent bonding, the cell viability can be affected by the cross-linking agents. Thus, cross-linking is suitable in fabricating microbial biosensors where cell viability is not important and only the intracellular enzymes are involved in the detection.

Related: What is a Biosensor?

Physical Methods

The physical techniques of microbe immobilization include adsorption and entrapment. Since these methods do not involve covalent bond formation with microbes and provide relatively small perturbation of microorganism native structure and function, these techniques are preferred when viable cells are needed. Physical adsorption is the simplest method for microbe immobilization. Typically, a microbial suspension is incubated with the electrode or an immobilization matrix, such as glass bead. The microbes are immobilized due to adsorptive interactions (i.e. ionic or polar bonding) and hydrophobic interaction. Nonetheless, immobilization using adsorption alone generally leads to poor long-term stability because of desorption of microbes. The immobilization of microorganisms by entrapment can be achieved, for instance, by the retention of the cells in close proximity of the transducer surface using a dialysis membrane. However, a major drawback of entrapment immobilization is the additional diffusion resistance offered by the entrapment material, which will result in a lower sensitivity and detection unit.

Microbial biosensors typically involve the assimilation of organic compounds by the microorganisms, followed by a change in respiration activity (metabolism) or the production of specific electrochemically active metabolites, such as H₂, CO₂ or NH₃, that are secreted by the microorganism.

Examples of Microbial Biosensors

Examples of microbial biosensors include ammonia (NH₃) and nitrogen dioxide (NO₂) sensors that utilize nitrifying bacteria as the biological sensing component. An ammonia biosensor can be produced based on nitrifying, such as Nitrosomonas sp. that use ammonia as a source of energy and oxidize ammonia as follows:

This oxidation proceeds at a high rate, and the amount of oxygen consumed by the immobilized bacteria can be measured directly by a polarographic oxygen electrode behind the bacteria.

Nitric oxide (NO) and NO₂ are the two principal pollution gases of nitrogen in the atmosphere. The principle of NO₂ biosensor is demonstrated below:

NO₂ gas diffuses through the gas-permeable membrane it is oxidized by the Nitrobacter sp. bacteria as follows:

The consumption of O₂ around the membrane is determined by an electrochemical oxygen electrode.

Employing microbial cells in electrochemical sensors has several merits over enzyme-based electrodes for example they offer increased electrode lifetime to several weeks, contrariwise, microbial sensors may be less favorable compared with enzyme electrodes with respect to specificity and response time.

Also read: Blood Glucose Sensors

Application of Microbial Biosensors

Microbial biosensors are used in control of biochemical processes in a number of environmental, food and pharmaceutical applications.