Microbial Biocatalysis, 2nd Edition

1. Introduction

Biocatalysis, leveraging the catalytic power of enzymes or whole microbial cells, has firmly established itself as a pivotal technology for sustainable chemical synthesis, environmental remediation, and the production of value-added compounds [1,2]. Whole-cell biocatalysts, in particular, offer distinct advantages over isolated enzymes. These include the inherent presence of multi-enzyme systems capable of mediating complex cascade reactions, simplified cofactor regeneration mechanisms within a single cellular host, and often the enhanced operational stability of enzymes in their native cellular environment [3,4]. Consequently, whole-cell biocatalysis is extensively applied in biosynthesis for producing pharmaceuticals and fine chemicals, in biotransformation for modifying complex molecules, and in biodegradation for the complete mineralization of persistent organic pollutants [5].

The advancement of biological catalytic processing using whole-cell biocatalysts is a multidisciplinary endeavor, encompassing biocatalyst engineering, bio-reaction engineering, and downstream processing [6]. Beyond traditional approaches like microbial strain screening and immobilization, modern biological tools have revolutionized the field. Genetic engineering, metabolic engineering, and synthetic biology now enable the rational design and construction of tailored whole-cell biocatalysts with desired activities, specificities, and stabilities [7,8]. Furthermore, innovative integrated processes are being developed to enhance catalytic efficiency. For instance, harnessing biocompatible chemistry to interface with microbial metabolism allows for novel chemical transformations within living systems [9].

A significant challenge in microbial biocatalysis, especially for biodegradation, involves the limited bioavailability of hydrophobic substrates. Recent research has shed light on how microorganisms can overcome these limitations, for example, by adsorbing to oil–water interfaces and forming Pickering emulsions, thereby directly accessing non-aqueous phase liquids (NAPLs) [10,11]. The interplay between cells, surfactants, and NAPLs is complex, where factors like cell surface hydrophobicity, often modifiable by polymers like carboxymethyl cellulose, can dramatically influence interfacial access and degradation efficiency [12]. Understanding these interfacial phenomena is crucial for optimizing bioremediation strategies [10,11].

This Special Issue, “Microbial Biocatalysis, 2nd Edition”, aimed to collect original research papers and reviews that highlight the recent progress in the application of living whole-cell biocatalysts across the spectrum of biosynthesis, biotransformation, and biodegradation. The contributions gathered herein showcase the breadth and depth of the current research, from fundamental investigations into microbial physiology and genetics to applied studies demonstrating practical solutions.

2. Overview of Published Articles

This Special Issue brings together five distinct contributions—four original research articles and one review—that illuminate the diverse applications of and ongoing advancements in microbial biocatalysis.

Addressing the critical challenge of environmental pollution, the work by Li et al. [contribution 1] focuses on the biodegradation of aniline, a hazardous organic pollutant. By ingeniously employing ¹³C-labeled aniline in conjunction with DNA-stable isotope probing technology, they successfully pinpointed key bacterial players such as Acinetobacter and Zoogloea, alongside less frequently reported genera involved in aerobic degradation processes. Their research further predicted crucial functional genes (atd, tdn, and dan), thereby offering valuable biomarkers and a mechanistic understanding essential for devising effective bioremediation strategies for aniline-contaminated environments.

The quest for novel and robust enzymes for industrial applications is a central theme in biocatalysis. In this vein, Lin et al. [contribution 2] tapped into the rich biodiversity of the microbial consortia associated with Tremella fuciformis. Through metagenomic sequencing, they identified, cloned, and characterized three xylanases. Notably, the enzyme AsXyn1 from Annulohypoxylon stygium demonstrated high thermostability and a broad pH tolerance, underscoring its potential for industries like bioenergy and pulp processing. The finding that post-translational modifications significantly influence enzyme properties also provides critical insights for future enzyme engineering.

The imperative to develop greener and more sustainable industrial processes is particularly acute in sectors like paper manufacturing. Patel et al. [contribution 3] address this by investigating the synergistic use of thermostable laccase and xylanase in optimizing the pre-bleaching of kraft pulp. They isolated a laccase-producing Bacillus licheniformis BK-1 and demonstrated that a synergistic mixture of its laccase and xylanase can significantly reduce chlorine usage in pulp pre-bleaching by 50%, while simultaneously enhancing pulp quality. This work elegantly showcases how microbial enzymes can pave the way for more environmentally friendly industrial practices.

Microbial biotransformation also plays a vital role in pharmaceutical research, particularly in understanding drug metabolism and discovering novel bioactive derivatives. Song et al. [contribution 4] illustrate this through their investigation of the microbial transformation of pimavanserin, a drug for Parkinson’s disease psychosis, using Cunninghamella blakesleeana AS 3.970. Their study led to the identification of ten previously unreported metabolites. The characterization of the major metabolite, M1, and its potential bioactivity, as suggested by molecular docking, highlights the power of microbial systems as models for metabolic studies and as sources of potential new therapeutic agents.

Finally, to effectively harness microbial degradation for hydrophobic organic compounds (HOCs), a profound understanding of interfacial phenomena and the role of additives is necessary. The review by Zhu et al. [contribution 5] provides a timely and comprehensive overview of the influence of surfactants on the interfacial microbial degradation of these compounds. It critically evaluates how surfactants can either enhance or inhibit HOC biodegradation by altering bioavailability, microbial adhesion, and cell viability. The review underscores the necessity for carefully tailored surfactant formulations to optimize bioremediation, offering crucial guidance for future research in remediating environments contaminated with HOCs.

3. Conclusions

The collection of articles within this Special Issue, “Microbial Biocatalysis, 2nd Edition”, collectively paints a vibrant picture of a field that is continuously evolving and expanding its impact. The research presented herein spans a remarkable range, from tackling persistent environmental pollutants like aniline and hydrophobic organic compounds, to the discovery and characterization of novel industrial enzymes such as xylanases, and the development of greener manufacturing processes for the pulp industry. Furthermore, the exploration of microbial biotransformation for drug metabolism studies underscores the versatility of microbial catalysts.

These contributions not only highlight innovative solutions, but also delve into the fundamental mechanisms underpinning microbial biocatalytic processes, employing sophisticated techniques like stable isotope probing, metagenomics, and detailed investigations of interfacial dynamics. The insights gained are crucial for the rational design of more efficient and robust biocatalytic systems.

As Guest Editors, we extend our sincere gratitude to all the authors for their high-quality contributions and their commitment to advancing the field. We are also deeply appreciative of the dedicated reviewers whose insightful comments were invaluable in shaping these articles. Finally, our thanks go to the editorial team at Catalysts for their unwavering support and professionalism in bringing this Second Edition to fruition.

It is our hope that the research and reviews compiled in this Special Issue will serve as a valuable resource and an inspiration for further innovation. The future of microbial biocatalysis is bright, promising even more sophisticated and sustainable solutions to global challenges in chemical production, environmental management, and human health.