In vitroantimicrobial activity of seven adjuvants against common pathogens associated with canine otitis externa

Abstract

Background -An antibiotic adjuvant is a chemical substance used to modify or augment the effectiveness of primary antimicrobial agents against drug-resistant micro-organisms. Its use provides an alternative approach to address the global issue of antimicrobial resistance and enhance antimicrobial stewardship.Hypothesis/Objectives -To determine the antimicrobial activity of a panel of potential antimicrobial adjuvants against common pathogens associated with canine otitis externa (OE).Animals/Isolates -A number of type strains and clinical isolates (n = 110) from canine OE were tested including Staphylococcus pseudintermedius, b-haemolytic Streptococcus spp., Pseudomonas aeruginosa, Proteus mirabilis and Malassezia pachydermatis.Methods and materials -Antimicrobial activities of monolaurin, monocaprin, N-acetylcysteine (NAC), polymyxin B nonapeptide, Tris-EDTA, Tris-HCL and disodium EDTA were tested using microdilution methodology according to CLSI guidelines.Results -N-acetylcysteine, Tris-EDTA and disodium EDTA had antimicrobial activity against both type strains and otic pathogens. The other adjuvants tested had limited to no efficacy. NAC had a minimal inhibitory concentration (MIC) range of 2,500-10,000 lg/mL for the various organisms. Pseudomonas aeruginosa isolates were eight times more susceptible to disodium EDTA in the presence of Tris-HCL in comparison to disodium EDTA alone. Malassezia pachydermatis isolates were most susceptible to Tris-EDTA (MIC 90 = 190/60 lg/mL) and disodium EDTA (MIC 90 = 120 lg/mL).Conclusions and clinical relevance -N-acetylcysteine, Tris-EDTA and disodium EDTA have intrinsic antimicrobial activity and represent promising adjuvants that could be used to enhance the efficacy of existing antibiotics against Gram-negative and multidrug-resistant bacterial infections. These agents could be combined with other antimicrobial agents in a multimodal approach for mixed ear infections in dogs.

Biofilm production by pathogens associated with canine otitis externa, and the antibiofilm activity of ionophores and antimicrobial adjuvants

“…Further studies of the antimicrobial adjuvants N‐acetylcysteine (NAC), Tris‐EDTA and disodium EDTA revealed activity against both Gram‐positive and Gram‐negative bacteria (Chan, Khazandi, Hickey, Page, Trott, et al, ). A synergistic effect was seen against P. aeruginosa when narasin was combined with either Tris‐EDTA or disodium EDTA (Chan et al, , submitted). As chronic OE in dogs can be complicated by the presence of biofilms, this study aimed to determine the ability of S. pseudintermedius and P. aeruginosa isolates from cases of canine OE to form biofilm, and to explore the antibiofilm activity of ionophores and adjuvants against these pathogens.…”

“…We have previously reported the in vitro efficacy of repurposed ionophores and antimicrobial adjuvants on pathogens associated with canine OE (Chan et al, 2018a;Chan et al, 2018b;Chan, Khazandi, Hickey, Page, Trott, et al, 2018). The ionophores narasin and monensin, both anticoccidial agents widely used in production animals, were shown to be effective against Gram-positive canine otic pathogens including methicillin-resistant S. pseudintermedius (MRSP), but were not effective against Gram-negative bacteria such as P. aeruginosa (Chan et al, 2018a(Chan et al, , 2018. Further studies of the antimicrobial adjuvants N-acetylcysteine (NAC), Tris-EDTA and disodium EDTA revealed activity against both Gram-positive and Gram-negative bacteria (Chan, Khazandi, Hickey, Page, Trott, et al, 2018).…”

In vitroantimicrobial activity of narasin and monensin in combination with adjuvants against pathogens associated with canine otitis externa

“…Due to the technical difficulties of performing multiple checkerboard assays, American Type Culture Collection (ATCC) strains of Staphylococcus aureus (ATCC SA 29213) and Pseudomonas aeruginosa (ATCC PA 27853 and P. aeruginosa biofilm producer ATCC PAO1) were used to assess minimum inhibitory concentrations (MICs). This was considered appropriate because previous studies demonstrated that susceptibility to the test compounds was consistent between the ATCC type strains and a range of clinical isolates from cases of canine OE (n = 110) . This was the case even with meticillin‐susceptible S. pseudintermedius, MRSP and MDRSP.…”

“…We showed that Gram‐positive organisms including MRSP and MDRSP from cases of canine OE were highly susceptible to narasin and monensin although these agents lacked activity against Gram‐negative bacteria and had weak anti‐yeast activity . In a study to identify potential adjuvants that might broaden the spectrum of activity of these agents, we reported that N ‐acetylcysteine (NAC), Tris‐EDTA and disodium EDTA had intrinsic antimicrobial activity against otic pathogens, including Gram‐negative organisms …”

“…18,19 In a study to identify potential adjuvants that might broaden the spectrum of activity of these agents, we reported that N-acetylcysteine (NAC), Tris-EDTA and disodium EDTA had intrinsic antimicrobial activity against otic pathogens, including Gram-negative organisms. 20 The aim of this study was to determine the in vitro antimicrobial activity of narasin and monensin in the presence of an adjuvant (NAC, Tris-EDTA or disodium EDTA) against bacterial strains representing pathogens associated with canine OE.…”