Methicillin-resistant Staphylococcus aureus (MRSA) infections contribute significantly to the mortality rate associated with antimicrobial resistance, while also increasing medical complications when compared to methicillin-sensitive isolates. Consequently, increasing the efficacy of β-lactams may alleviate the burden associated with this drug–pathogen combination. Dialysed extracts, derived from Pelvetia canaliculata, were further partitioned to generate phlorotannin enriched extracts (PEEs). Minimum inhibitory concentrations (MICs) of extracts were used to determine the antimicrobial and antibiotic-modulating activity against seven clinical MRSA isolates. Whole-transcriptomic sequencing was carried out to determine potential mechanisms of action on the strongest extract–antibiotic combination against an MRSA isolate, determined by the fractional inhibitory concentration index (FICI). The most pronounced β-lactam modulatory effects were seen in the >100 kDa PEE from P. canaliculata in conjunction with amoxicillin (MIC fold-reductions = 12.8–214), while >30 kDa PEE resulted in the strongest FIC index value when combined with penicillin (0.23), indicating a synergistic effect. Whole-transcriptome analysis of this treatment identified multiple differentially expressed genes (Log2FC +1/−1) of statistical significance (p adj < 0.05), namely related to accelerated autolysis, cell-envelope stability, protein synthesis, DNA replication, iron homeostasis and active transport. This study highlights preliminary transcriptomic evidence of biological processes associated with penicillin modulation by P. canaliculata > 30 kDa PEE, providing a foundation for further investigation of its antibiotic-modulating activity.

Research background. The rapid emergence of antimicrobial resistance (AMR) has led to numerous pathogens developing resistance to available antibiotics. Staphylococcus aureus is a leading cause of hospital-acquired infections, and the emergence of resistant strains like methicillin-resistant Staphylococcus aureus (MRSA) has led to increased clinical complications and higher mortality rates. As such, restoration of first-line β-lactam antibiotic activity is a viable strategy to combat AMR. The antimicrobial activity of edible mushroom extracts is well reported; however, their use as a source of antibiotic modulators is relatively unexplored, and current reports often do not adhere to drug combination guidelines. The aim of this study is to evaluate the potential of extracts derived from ten edible mushroom species to modulate β-lactam antibiotics against MRSA with reference to the recommended guidelines. Experimental approach. Antimicrobial activity (minimum inhibitory concentration (MIC)) and antibiotic-modulating activity (fractional inhibitory concentration index (FICI)) were assessed using the broth microdilution method. The cell viability assay was used to assess cell membrane integrity following treatment to offer insight into potential mechanisms of action. Results and conclusions. Both antimicrobial and antibiotic modulatory activity were found only in ethyl acetate extracts. Hericium erinaceus extract showed the strongest antimicrobial activity against S. aureus (MIC=0.8 mg/mL), while Agaricus bisporus extract exhibited the strongest antibiotic-modulating properties, exhibiting a 25-fold reduction in the MIC of ampicillin, amoxicillin and penicillin against an MRSA isolate (FICI=0.29), indicating synergism. The mechanism of action indicates reduced cell membrane integrity, which may increase intracellular antibiotic concentration. Novelty and scientific contribution. These results indicate the potential use of ethyl acetate extracts from edible mushrooms, specifically A. bisporus, as a source of antibiotic-modulating compounds. To our knowledge, this is the first study to evaluate the antibiotic-modulating effects of H. erinaceus, Hypsizygus tessulatus and Pholiota adiposa. This study also highlights the necessity to adhere to FICI guidelines when evaluating the antibiotic-modulating potential of edible mushroom extracts.