Abstract
BackgroundNew therapeutic strategies are needed to face the rapid spread of multidrug-resistant staphylococci in veterinary medicine. The objective of this study was to identify synergies between antimicrobial and non-antimicrobial drugs commonly used in companion animals as a possible strategy to restore antimicrobial susceptibility in methicillin-resistant Staphylococcus pseudintermedius (MRSP).ResultsA total of 216 antimicrobial/non-antimicrobial drug combinations were screened by disk diffusion using a clinical MRSP sequence type (ST) 71 strain resistant to all six antimicrobials tested (ampicillin, ciprofloxacin, clindamycin, doxycycline, oxacillin and trimethoprim/sulfamethoxazole). The most promising drug combination (doxycycline-carprofen) was further assessed by checkerboard testing extended to four additional MRSP strains belonging to ST71 or ST68, and by growth inhibition experiments.Seven non-antimicrobial drugs (bromhexine, acepromazine, amitriptyline, clomipramine, carprofen, fluoxetine and ketoconazole) displayed minimum inhibitory concentrations (MICs) ranging between 32 and >4096 mg/L, and enhanced antimicrobial activity of one or more antimicrobials. Secondary screening by checkerboard assay revealed a synergistic antimicrobial effect between carprofen and doxycycline, with the sum of the fractional inhibitory concentration indexes (ΣFICI) ranging between 0.3 and 0.5 depending on drug concentration. Checkerboard testing of multiple MRSP strains revealed a clear association between synergy and carriage of tetK, which is a typical feature of MRSP ST71. An increased growth inhibition was observed when MRSP ST71 cells in exponential phase were exposed to 0.5/32 mg/L of doxycycline/carprofen compared to individual drug exposure.ConclusionsCarprofen restores in vitro susceptibility to doxycycline in S. pseudintermedius strains carrying tetK such as MRSP ST71. Further research is warranted to elucidate the molecular mechanism behind the identified synergy and its linkage to tetK.
Highlights
New therapeutic strategies are needed to face the rapid spread of multidrug-resistant staphylococci in veterinary medicine
In small animal veterinary medicine, infections caused by methicillin-resistant Staphylococcus pseudintermedius (MRSP) pose a major therapeutic challenge since some MRSP strains, such as the European epidemic clone sequence type (ST) 71, are virtually resistant to all systemic antimicrobial products licensed for use in dogs [2]
Seven of the 36 non-antimicrobial drugs tested in the primary screening were shown to enlarge the edge of the inhibition zone of at least one antimicrobial disk: acepromazine (CLI, OXA), amitriptyline (AMP, OXA), bromexine (OXA), clomipramine (OXA), carprofen (AMP, DOX), fluoxetine (CIP, SXT) and ketoconazole (OXA, SXT)
Summary
New therapeutic strategies are needed to face the rapid spread of multidrug-resistant staphylococci in veterinary medicine. The objective of this study was to identify synergies between antimicrobial and non-antimicrobial drugs commonly used in companion animals as a possible strategy to restore antimicrobial susceptibility in methicillin-resistant Staphylococcus pseudintermedius (MRSP). In small animal veterinary medicine, infections caused by methicillin-resistant Staphylococcus pseudintermedius (MRSP) pose a major therapeutic challenge since some MRSP strains, such as the European epidemic clone sequence type (ST) 71, are virtually resistant to all systemic antimicrobial products licensed for use in dogs [2]. Combination therapy is one of the possible strategies that can be used to manage severe MRSP infections that cannot be cured by topical antiseptic treatment. Some antimicrobial combinations such as amoxicillin clavulanate and potentiated sulphonamides are widely used in human and veterinary medicine. Promising results have been shown by combining antimicrobials with small non-antimicrobial helper molecules interfering with resistance [4]
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