Lactobacilli such as Lacticaseibacillus (Lcb) paracasei are generally regarded as safe and health-promoting microbes, and have been widely applied in food and pharmaceutical industries. However, the genetic bases of their beneficial properties were mostly uncertain because of the lack of effective genetic manipulation tools. The type II CRISPR-Cas9 system is the largest family present in lactobacilli, but none of them yet have been developed for genetic modifications. Here, we establish the first endogenous CRISPR-Cas9 genome-editing system in lactobacilli. With a validated protospacer adjacent motif (PAM) and customized single guide RNA (sgRNA) expression cassette, the native CRISPR-Cas9 system was reprogrammed to achieve gene deletion and chromosomal insertion at over 90% efficiency, as well as nucleotide substitution at ≥50% efficiency. We also effectively accomplished deletions of large genomic fragments (5-10 kb) and simultaneous deletion of multiple genes at distal loci, both of which are the first cases in lactobacilli when either endogenous or exogenous CRISPR-Cas systems were employed. In addition, we designed a controllable plasmid-targeting sgRNA expression module and integrated it into the editing plasmid. The all-in-one vector realized gene deletion and plasmid curing at high efficiency (>90%). Collectively, the present study develops a convenient and precise genetic tool in Lcb. paracasei and contributes to the genetics and engineering of lactobacilli.
Keywords: endogenous CRISPR-Cas9 system; genome editing; lactobacilli; plasmid curing.