Antibiotic susceptibility profile of Pseudomonas plecoglossicida, the pathogen of visceral white-spot disease in large yellow croaker (Larimichthys crocea)

The large yellow croaker (Larimichthys crocea)is one of the most important marine aquaculture fish species in China, with over 80% of production in Ningde City, Fujian Province. In recent years, visceral white-spot disease caused by Pseudomonas plecoglossicida has seriously affected the development of large yellow croaker aquaculture industry. In this study, antibiotic susceptibility and resistance genes of 19 strains of P. plecoglossicida isolated from 2013 to 2023 were analyzed. The 19 strains were tested for susceptibility to 31 antibiotic drugs using the Kirby-Bauer paper slide method. The results showed that P. plecoglossicida isolated from different years exhibited high susceptibility to to aminoglycosides, quinolones, tetracyclines, carbapenems, but low susceptibility to other classes of antibiotics. P. plecoglossicida strains isolated in 2023 appeared to be insensitive to antibiotics of enrofloxacin and doxycycline. The results of resistance gene detection showed that carriage rates of the resistance genes aph(3ʹ)-Ia, qnrS, floR, Sul1 were 100%, while that of tetA was 95%. The carriage rates of other resistance genes were below 90%, and the resistance phenotypes did not correspond exactly to the rate of detection of resistance genes, indicating that the formation of resistance may involve the complexity of multiple gene, suggesting that the formation of drug resistance may involve the complex interaction of multiple genes or the existence of other unrevealed drug resistance mechanisms. Furthermore, we found that the outer membrane protein OprM deletion mutant (ΔoprM), the transcriptional regulator DeoR deletion mutant (ΔdeoR), and T3SS effector molecule ExoU deletion mutant (ΔexoU), the transcriptional regulator protein ExsA deletion mutant (ΔexsA), the translocator proteins PopB-PopD deletion mutant (ΔpopBD), and the acinar structural protein PscC-PscB, were found in the same strain, and the resistance phenotype corresponded exactly to the detection rate of resistance genes, suggesting that resistance may involve complex interactions among multiple genes or other unknown mechanisms. (ΔpscC-F), a mutant with deletion of the structural proteins PscC-PscF (ΔpscC-F), showed enhanced sensitivity to antibiotics such as enrofloxacin and florfenicol, suggesting that the outer membrane proteins OprM, T3SS, and the regulator protein DeoR play important roles in influencing the susceptibility of P. plecoglossicida to antibiotics. These findings provide valuable insights for the prevention and control of visceral white-spot disease in large yellow croaker.