Disruption of a kasB homolog gene (kasB) causes attenuation of cell invasion and virulence of Nocardia seriolae

Nocardia seriolae is the primary aetiological agent of nocardiosis in fish, which causes mass mortality in freshwater and marine fish. beta-ketoacyl-ACP synthase (KAS) is one of the essential enzymes in the synthesis of mycolic acids (MASs) in Mycobacterium spp. and has been chosen as the target for therapeutic intervention in mycobacterial diseases. In the present study, a kasB homologue gene (kasB) was identified in the genome of N. seriolae, and the gene-deficient mutant (Delta kasB) was generated based on a clinical isolate, XSYC-Ns. Compared to the wild-type (WT) strain, the Delta kasB showed a measurably growth defect in vitro but retained the acid-fastness in acid-fast staining. Observation of the cell ultrastructure showed some alterations in the cell wall of the Delta kasB strain. Compared to its original strain, the cell wall lipid layer seemed sparser, and a wider electron-transparent zone was observed in the cell wall of Delta kasB strain. Moreover, the Delta kasB strain showed impaired ability of cell invasion as well as intracellular survival in the cell line originating from the head-kidney of the large yellow croaker (LYC-hK), compared to its original strain. In addition, the deficiency of Delta kasB significantly attenuated the virulence of N. seriolae in largemouth bass. The present study suggested that the Delta kasB gene might be involved in the synthesis of extracellular cell-wall lipids in N. seriolae and play a crucial role in its pathogenicity.