###### Abstract  
Nonmodel microbes with unique and diverse metabolisms have become rising stars in synthetic biology; however, the lack of efficient gene engineering techniques still hinders their development. Recently, the use of base editors has emerged as a versatile method for gene engineering in a wide range of organisms including nonmodel microbes. This method is a fusion of impaired CRISPR/Cas9 nuclease and base deaminase, enabling the precise point mutation at the target without inducing homologous recombination. This review updates the latest advancement of base editors in microbes, including the conclusion of all microbes that have been researched by base editors, the introduction of newly developed base editors, and their applications. We provide a list that comprehensively concludes specific applications of BEs in nonmodel microbes, which play important roles in industrial, agricultural, and clinical fields. We also present some microbes in which BEs have not been fully established, in the hope that they are explored further and so that other microbial species can achieve arbitrary base conversions. The current obstacles facing BEs and solutions are put forward. Lastly, the highly efficient BEs and other developed versions for genome-wide reprogramming of cells are discussed, showing great potential for future engineering of nonmodel microbes. b 2022  
the production of the target metabolites, synthetic biology and metabolic engineering are developed to optimize genetic and regulatory processes within cells. One of the most useful tools to learn and modify biosynthetic pathways is gene engineering techniques, which can ultimately alter microbial genotypes and phenotypes by insertions, deletions, and mutations of nucleotides [7]. Model microbes such as _Escherichia coli_ (_E. coli_) and _Saccharomyces cerevisiae_ (_S. cerevisiae_) have been researched and used for industrial biochemical production in the long term [8]. Despite their well-characterized metabolisms and availability of a large genetic toolbox for rapid gene modification [9], these model strains are unable to produce all desirable products to satisfy the increasing industrial needs. In contrast, nonmodel microbes that are derived from a more complex environment and have evolved to utilize cheaper and more environmentally friendly sources of carbon than model microbes, often possess versatile physiology and metabolic capabilities that are important for the future production of biofuel, as well as chemical and novel antibiotics that may ease the problem of multi-drug resistance [8; 10]. They are also able to tolerate extreme industrial processing environments, such as low pH, high salt, and high temperatures; therefore, nonmodel microbes have become one of the hot spots in metabolic engineering. However, the lack of facile gene editing tools hinders the genetic characterizations and modifications of nonmodel microbes [11]. Moreover, the understanding and mitigating drug-resistance mechanisms require the gene engineering approaches to aid in targeting and editing pathogenic microbial genomes [12]. The advent of the CRISPR/Cas9 (CRISPR, clustered regularly interspaced short palindromic repeats/Cas9, the dual RNA-guided DNA endonuclease) opens the door to manipulate the genome of microbes that are traditionally difficult to be edited (such as nonmodel bacteria [13; 14; 15; 16], fungi [17], protozoan parasites [18; 19], etc.). As shown in Figure 1A, the guide RNA (gRNA) directs Cas9 to the targeted DNA containing the protospacer adjacent motif (PAM, 5'-NGG-3') to introduce DNA double-stranded breaks (DSBs) [20]. This induces homologous recombination (HR) to replace the DNA sequence with homologous templates. However, the generation of DSBs is reported to be lethal to some bacteria, such as _Clostridium cellulolyticum_[21] and _E. coli_[22], and it induces nonhomologous end joining (NHE), which results in random insertions or deletions (indels) [23; 24]. The lack of a strong HR system in some microbes, such as _Mycobacterium tuberculosis_ (_M.