In vitro antimicrobial activity of topical otological antimicrobials and Tris‐EDTA against resistant Staphylococcus pseudintermedius and Pseudomonas aeruginosa isolates from dogs

Abstract

Background -The value of susceptibility tests for the selection of topical otological antimicrobial agents is unclear. Laboratories test antibiotic concentrations substantially lower than concentrations supplied in topical formulations. Additionally, microbiological consensus statements are not available for topical antimicrobials.Hypothesis/Objectives -The primary aim of this study was to measure the minimum inhibitory (MIC) and bactericidal (MBC) concentrations of enrofloxacin, gentamicin, marbofloxacin, neomycin, orbifloxacin, polymyxin B and silver sulfadiazine (SSD) from concentrations available in otological formulations (COF) to 1:59,000 dilution. The secondary aim was to evaluate the effect of Tris-EDTA in conjunction with these antimicrobial agents.Methods and materials -Twenty resistant clinical isolates [Staphylococcus pseudintermedius (n = 10) and Pseudomonas aeruginosa (n = 10)] were tested by broth microdilution using a concentrated inoculum (3.75 9 10 7 cfu/mL).Results -Concentrations available in otological formulations were at least 269 greater than the MICs for S. pseudintermedius and P. aeruginosa. COFs of polymyxin B and SSD were 279 greater than the MBCs for P. aeruginosa, whereas all other antimicrobial COFs were equal to or less than the MBCs for both organisms. Tris-EDTA significantly reduced the MICs of all antimicrobials, except with SSD for P. aeruginosa, and it significantly increased the MIC of SSD for S. pseudintermedius.Conclusions and clinical importance -Further studies are warranted to validate the present results in vivo. COFs are inhibitory and less likely bactericidal, with few exceptions, against resistant strains of these organisms. Tris-EDTA may be advantageous for P. aeruginosa whereas no additional benefit is afforded against S. pseudintermedius. Susceptibility tests may not be useful for the selection of topical otological antimicrobial agents.

Antimicrobial resistance patterns of Pseudomonas aeruginosa isolated from canine clinical cases at a veterinary academic hospital in South Africa

“…Despite the widely accepted view that P. aeruginosa exhibits intrinsic resistance to aminoglycosides, available evidence suggests aminoglycosides such as amikacin or gentamicin are useful in the treatment of respiratory infections (Poole 2005 ). Furthermore, commercial topical preparations for treatment of ear infections also contain aminoglycosides that are known to be effective (Boyd, Santoro & Gram 2019 ).…”

“…In addition, the susceptibility testing method used in this study was the Kirby Bauer Disk Diffusion (KBDD) method, which is reported to be limited when compared with the minimum inhibitory concentration (MIC) clinical application. Moreover, topical preparations appear to still be effective against P. aeruginosa, despite the KBDD method indicating a higher prevalence of resistance (Boyd et al 2019 ). This notwithstanding, the KBDD method still provides accurate susceptibility testing to guide therapeutic options.…”

Preliminary evaluation of two commercial ear solutions in the treatment of canine otitis externa

“…However, to our knowledge, only one study has shown the in vivo efficacy of Otodine® in the treatment of OE in dogs (Bouassiba et al 2012), in which Otodine® was followed by an ear medication containing marbofloxacin, dexamethasone and clotrimazole. Tris-EDTA has been described as a bacteriostatic product capable of potentiating other antimicrobial agents, including antibiotics, and its synergistic effect against resistant bacteria associated with otitis has specifically been described (Wooley & Jones 1983, Farca et al 1997, Boyd et al 2019. In our study we have shown the efficacy of Otodine® alone, without the administration of any other antimicrobial or anti-inflammatory agents in the management of canine OE.…”

“…Otodine® ear solution contains chlorhexidine and Tris-EDTA. Previous studies have shown their in vitro effectiveness against microorganisms commonly associated with canine OE (Cole et al 2006, Cole et al 2007, Swinney et al 2008, Guardabassi et al 2010, Steen & Paterson 2012, Banovic et al 2013, Mason et al 2013, Boyd et al 2019, Chan et al 2019. In three of these studies in vitro evaluation of the same product used in this study (Otodine®) was carried out and the results obtained showed moderate activity against M. pachydermatis (Mason et al 2013 Each parameter was graded on a scale of 0 to 3; the OTIS3 index is the result of the sum of the values of erythema, oedema, erosion and exudate.…”

Chemical and microbiological stability of diluted ceftazidime in three different solutions under three storage temperatures over a 28 day period

“…Given the variability in the RCC as well as the MIC for PA of the TE+CAZ solutions, TE would appear to be more appropriate to use as a pre‐treatment ear flush rather than a vehicle for delivery of CAZ. Multiple studies have shown that the presence of TE can enhance the antimicrobial activity of antimicrobials against PA specifically 22–24 . TE also has antibiofilm activity, making it particularly useful for cases of chronic OE due to PA, where biofilm is present in 40–90% of cases 25–27 …”