Antibiotic underdosing linked to stronger pathogen survival finds study.
Sub-inhibitory doses of the antibiotic florfenicol can trigger defensive responses in Piscirickettsia salmonis, the bacterium behind salmon farming’s most costly disease, according to new research from Chile.
The study, carried out by scientists from Universidad Austral de Chile, Universidad de Concepción, Universidad Andrés Bello, Universidad Mayor, Berking Biotechnology and Centro Incar, found that low concentrations of the drug can activate genes linked to antibiotic resistance and stimulate biofilm formation.
Biofilms allow bacteria to cluster together and protect themselves, making infections harder to eliminate and increasing the risk of persistence in farming environments.
Researchers observed that key resistance systems were switched on under these conditions, including mechanisms that help bacteria pump out antibiotics and adapt to stress.
The findings raise questions about how antibiotics are used in practice, particularly where dosing may fall short of fully suppressing infections.
The issue is especially relevant in Chile, where florfenicol dominates antimicrobial use in salmon farming. Data from Sernapesca show the drug accounted for 96.8% of total antibiotic use at farming sites in the first half of 2025, largely linked to the control of piscirickettsiosis (SRS).
Researchers noted that biofilms formed under antibiotic stress could act as reservoirs for the pathogen in the marine environment, increasing the likelihood of reinfection within production systems.
The results add to ongoing scrutiny of antibiotic use in aquaculture and may support efforts to refine dosing strategies and accelerate the development of alternative treatments.