11.1 Introduction

The concept of bioelectronics, proposed in 1968, means the combination between the

biological system and the field of electronics. This field focuses on the different me­

chanisms of electron transfer in a biological system and their potential applications.

However, bioelectronics, according to pioneer Göpel, aimed at the direct combination

between biomolecular and electronic structures [1]. This concept is defined by the

International Union of Pure and Applied Chemistry (IUPAC) as the application of bio­

molecular principles to microelectronics [2]. Recently, with the discovery of electrically

conductive bacterial structures, termed microbial nanowires, new fields had emerged in

bioelectronics. The bacterial nanowires structure is an extracellular appendage that has

been serving as a strategy for electron transport in diverse microbes. Generally, two

mechanisms can be implicated in microbes electrochemically direct and mediated elec­

tron transfer. In direct electron transfer (DET), the electron transfers between micro­

organisms and solid acceptors implicate on bacteria cell membrane’s contact. The DET

may also involve insoluble Fe (III) or an anode of microbial bioelectrochemical systems

(BES). This type was first discovered and described in Geobacter species which are ef­

fective in the bioremediation of subsurface contaminants [3]. Recently, several studies

demonstrated the electron transport along bacterial nanowires. This discovery has been

observed in some Geobacter, Shewanella, and Cyanobacterium species [4]. The electronically

conducting structure termed nanowires permits these bacteria to reach the solid electron

acceptors without cell contact. The second mechanism is called mediated electron transfer

(MET). The MET is facilitated by using artificial mediator compounds or by a biocatalyst

and classified according to the origin and redox species. For growth, the bacteria can

generate energy by using diverse strategies. The nanowires represent nano-objects pro­

duced by microbes that enable the transfer of electrons to extracellular electron acceptors.

The electron transfered through the nanowires permit possibilities for cell-cell and cell-

surface interaction. Through that, their potential applications in bioremediation, bioengi­

neering, and bioelectronics have been demonstrated. In this chapter, we describe different

non-flagella proteinaceous appendages such as Chaperone-Usher pili (CU pili), curli pili,

and type 3 and 4 secretion system pili (T3SS and T4SSs). The diverse types of extracellular

electron transfer exhibited by microbial cells were presented. A brief taxonomy, principal

characteristics, and even the pili type 4 assemblage of the two types of species implicated in

electron transfer via nanowires Geobacter and Shewanella were detailed. We also discuss

important aspects of microbial nanowires (MNWs) including their types, roles, and me­

chanisms of electron transfer in Geobacter and Shewanella species. Along with that, the po­

tential applications of both bacteria in the field of bioremediation, bioelectricity, and energy

production are also reviewed.

11.2 Microbial Pilus: From Fimbriae to Nanowires

The term “pili” (Latin for hair), first identified by Duguid et al. [5], were non-flagellar

proteinaceous appendages that occur among Gram-negative bacteria. The pili have been

mostly referred to as “filaments,” “bristles,” “fimbriae,” or “pili” by Ottow [6]. The pili,

introduced in 1959 [7], were distinguished mainly based on morphology, which can be

172

Bioelectronics