Investigation into the potential of Bjerkandera adusta to biotransform the recalcitrant fluoroquinolone antibiotic norfloxacin

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Fluoroquinolones are among the most widely used broad-spectrum antibiotics worldwide. Due to inefficient removal by wastewater treatment plants, they accumulate in aquatic and terrestrial environments. Their persistence is concerning because it promotes the emergence of multidrug-resistant bacteria. This is alarming since the World Health Organization (WHO) classifies fluoroquinolones as critically important antibiotics, given the limited treatment options for severe bacterial infections in humans. Additionally, their accumulation can have toxic effects on various organisms. Previous studies reported the potential of ligninolytic fungi in transforming fluoroquinolones. In particular, a strain of Bjerkandera adusta (TM11), isolated in Tunisia, showed promising potential for biotransforming a broad spectrum of fluoroquinolones. Interestingly, B. adusta exhibited no laccase activity but peroxidase activity. Here we investigated the impact of culture medium composition on the strain’s peroxidase activity and ability to biotransform norfloxacin, used as a model fluoroquinolone. We performed proteomic analysis of the secretomes from B. adusta cultured in four media, with and without antibiotics, to identify enzymes potentially involved in biotransformation. This study demonstrated that medium composition significantly influenced both peroxidase activity and norfloxacin biotransformation. The highest biotransformation rates, together with elevated peroxidase activity, were observed in media containing corn steep solids and yeast extract as nitrogen source after 7 days. Proteomic analysis revealed that presence of the antibiotic in the media induced the secretion of specific heme-peroxidase enzymes. Among them, two manganese peroxidases (MnP2 and MnP4), known for their ligninolytic activity, emerged as particularly promising candidates for future studies on the enzymatic biotransformation of fluoroquinolones.