Potential Ototopical Antiseptics for the Treatment of Active
Chronic Otitis Media: An In Vitro Evaluation

Objectives: Primary: to compare, in vitro, the antimicrobial activity of different antiseptic agents versus quinolone drops, against the common organisms of chronic otitis media.

Secondary: to examine the possible role of pH on the antimicrobial activity of the antiseptic solutions.

Methods: Three antiseptic powders (boric acid; iodine; and a 1:1 combination of these two) and four solutions (2% boric acid; 2% acetic acid; 3.25% aluminum acetate; and 5% povidone iodine) were tested against five bacteria and two fungi common in chronic otitis media, using both agar plates and the modified broth dilution method. These results were compared with the antimicrobial activity of quinolone drops.

The potential role of pH of solutions was tested by reducing the acidity of the agents and repeating the broth dilution.

Results: Of the powders, iodine, and iodine/boric acid combined, are very effective against all organisms. Boric acid powder showed moderate effectiveness against all organisms. All solutions performed poorly on the agar plates.
5% povidone iodine has good effectivity when tested with the broth dilution method. 3.25% aluminum acetate had the best activity against Pseudomonas aeruginosa.

Conclusion: Boric acid powder and 5% povidone iodine solution show promise for clinical use. Boric acid powder has proven clinical effectiveness. 5% povidone iodine requires further clinical research. Although very effective in vitro, iodine powder is toxic to tissues and cannot be recommended for clinical use. The pH of solutions does not seem to play a significant role in their antimicrobial activity in vitro.

Key Words: Antiseptic—Alumunium acetate—Boric acid— Chronic otitis media—Iodine powder—Ofloxacin—Povidone iodine—Quinolone—Topical antiseptics—Supurative otitis media.

Otol Neurotol 41:e1060–e1065, 2020.

Chronic Otitis Media

Active chronic otitis media (COM) remains a prevalent otological disease and a major cause of acquired hearing loss in children (1). The quoted prevalence rates vary between different regions and ethnic groups, ranging from 1.5% in the United Kingdom, 0.4 to 4.2% in sub-Saharan Africa, 10.5 to 30.3% in indigenous Australian children, to 30 to 46% in the Inuit people of Alaska (1–5).

Treatment of Active COM

After excluding specific causes for COM (cholesteatoma and TB) the management starts by rendering active COM inactive (6). Topical therapies, being able to deliver a much higher dose locally, work better than systemic therapies (7). Topical quinolone drops with their wide antimicrobial activity and safety profile are
regarded as the gold standard (6,8). They are however expensive, rely on patient compliance, and, with widespread use, run the risk of developing bacterial resistance (6). With COM being more prevalent in developing nations (5), a cheaper yet effective alternative is required. Nonantibiotic, topical, inexpensive antiseptics are therefore an attractive possibility.

Address correspondence and reprint requests to Andries Francois van Straten, F.C.O.R.L. (SA), Faculty of Medicine and Health Sciences, Room 5054, Clinical Building, Building 91, Francie van Zijl Drive, Tygerberg, 7505 Cape Town, South Africa; E-mail: vanstraten.andre @gmail.com

Address for reprints: Same as for correspondence.
The authors disclose no conflicts of interest.
DOI: 10.1097/MAO.0000000000002728

Antiseptics in COM

Ototopical antiseptics have been used for the management of COM for decades, albeit not with consistent success. Acetic acid drops have been proven ineffective
(6). Burrow’s solution (13% aluminum acetate) and a dilution of the original 13% solution have been shown to have some effect (9,10). Boric acid in solution form is ineffective (11,12), but we have previously reported in a randomized controlled trial that boric acid powder is as effective as quinolone topical drops (6). Iodine powder mixed with boric acid powder seems ineffective (13), while a small trial reported good efficacy of 5% povidone iodine solution (14).
The healthy external ear canal is naturally slightly acidic and it is postulated that this acidity contributes to bacteriostasis. It has been suggested that lowering the pH of the external ear canal to a more bacteriostatic acidic range may contribute to treatment and prevention of COM. However, it is still uncertain what role, if any, the pH of treating substances plays in the treatment of COM (15,16).
Other antiseptics including gentian violet, castelani solution, and ethyl alcohol have all been shown to be ototoxic and are thus not used for COM (17,18).
An ideal agent would theoretically possess the following qualities:

• active against the organisms associated with COM
• not subject to microbial resistance
• not cause any ototoxicity or adverse reactions
• inexpensive
• not require patient compliance
• have an easy mode of administration

The micro-organisms that this agent should be effective against have been well documented in the literature.

Commonly isolated aerobic organisms include Pseudomonas aeruginosa (18—67%), Staphylococcus aureus (14—33%); Enterobacteriaceae including Proteus spp., Klebsiella spp., and Escherichia spp. (4—43%); Haemophilus influenzae (1—11%); while anaerobic micro-organisms include Bacteroides spp. (1—91%) and Fusobacterium spp. (4—15%) (19). Fungi have been isolated in up to 29% of ears (20). Common fungi include Aspergillus flavus, Aspergillus niger, Aspergillus fumigatus, Penicillium, Candida albicans, Candida parapsilosis (21,22).

Aim of This Study

Following the success of our clinical trial with boric acid powder (6), we decided to examine the in vitro activity of a variety of different ototopical antiseptics of potential use in the treatment of active COM.

OBJECTIVES

We sought to determine the antimicrobial activity (specifically against the common organisms of COM) of three antiseptic powders and four antiseptic solutions.
These would be compared with the antimicrobial activity of quinolone drops.
Our secondary aim was to determine the role of the pH of the antiseptic solutions in their antimicrobial activity.

METHOD

Study Design

This was an observational laboratory-based in-vitro study of
the activity of different topical antiseptics against organisms
commonly associated with COM.

TABLE 1. Micro-organisms tested with their specific American Type Culture Collection (ATCC) control strain identifier

Micro-organisms Included

A range of American Type Culture Collection control organisms, representing the common isolates from ears with COM, were used—as shown in Table 1. In the case of Proteus mirabilis, a clinical isolate was used. The isolate was obtained
from the NHLS Microbiology laboratory at Tygerberg Hospital, and had been identified using the commercial Vitek 2 system (BioMe´rieux, France).
Owing to the complexities involved in performing in-vitro susceptibility tests on moulds we did not include Aspergillus species or other filamentous fungi in the study.

Antiseptics Included

Only antiseptics that had previously been used in ears with tympanic membrane perforations were included. Agents known to be ototoxic were excluded. The optimal strength/percentage of a solution previously tolerated by patients in trials was chosen. Agents tested were:

• Boric acid powder
• Iodine powder
• Boric acid and iodine powder in combination (1:1)
• 2% boric acid in distilled water solution
• 2% acetic acid in distilled water solution
• 3.25% aluminum acetate solution
• 5% povidone iodine solution

Efficacy was compared with that of a quinolone:

• 0.3% ofloxacin (Octin drops; Cipla)

The antibacterial activity of the various compounds was tested by both a modified agar diffusion technique as well as by broth dilution.

Agar Diffusion Method

A 0.5 McFarland inoculum (equivalent to 1–2  108CFU/ml) of each organism was prepared from fresh cultures using a nephelometer (DensiChek; BioMe´rieux), in accordance with the internationally accepted technique for performing susceptibility testing in diagnostic microbiology laboratories. This was then
inoculated onto the surface of 2% horse blood agar plates by evenly distributing it over the complete surface using a sterile transport swab (Transystem; Copan Italia). A circular well was punched out in the center of each agar plate using the back of a sterile Pasteur pipette, and 40 ul of the antiseptic agent was placed
into the central well (in the case of the powders, the well was completely filled with the powder). The agar plateswere incubated overnight in 5% CO2 at 378C, based on *CLSI and EUCAST recommendations for bacterial susceptibility testing. After 24 hours the zone of inhibition was measured in millimeters using a standard caliper. This processwas repeated in triplicate to ensure accuracy and repeatability.
(CLSI is the US based Clinical and Laboratory Standards Institute; EUCAST is the European Committee for Antimicrobial Susceptibility Testing) (23,24).

Modified Broth Dilution Method

Owing to the nature of the methodology only antiseptics in solution could be tested by broth dilution. To achieve a solution, iodine powder was dissolved to its maximal concentration in distilled water (0.29%). As described earlier, a 0.5 McFarland solution of each organism was prepared and then diluted 1:150 in nutrient broth. This inoculum was added to serial 2-fold dilutions of the antiseptic solution in a microwell plate, with final dilutions of the antiseptic agent ranging from 1:2 to 1:1024. The micro-well plate was then incubated overnight as described above. After 24 hours the micro-wells were read manually for any turbidity, indicating organism growth, and the dilution that first resulted in
organism growth was recorded. Assays were performed in triplicate to ensure accuracy and repeatability.

Determining the Effect of pH on Antimicrobial
Activity

The pH of each antiseptic solution was measured using a Beckman Coulter PHi-510 pH meter. Five drops of 50% NaOH were added to 15 ml antiseptic solution to reduce its acidity, whereafter the pH was measured again. The modified broth
dilution was then repeated using serial dilutions of the less acidic antiseptic. The dilution that first resulted in organism growth was compared with that of the original antiseptic and any difference noted.

Ethical Considerations

Ethics approval was obtained through the University of Stellenbosch, Faculty of Medicine and Health Sciences Ethics Committee.

FIG. 1. Zones of inhibition. Zone of inhibition is measured in millimeters around the central well with the antiseptic. Nil inhibition of growth is evident where the organism displayed growth up to the central well filled with the antiseptic. > 80 ml inhibition of growth is evident where there was no growth of the organism on the agar plate around the central well filled with the antiseptic.

DISCUSSION

We sought to identify antiseptic agents, powders, and solutions, which might potentially be effective in treating active COM. We recognize that the action of an antiseptic on organisms in vitro does not necessarily equate to its clinical effectiveness. It does however give us a good indication of its efficacy against the specific organisms tested, and allows one to identify agents that show potential for clinical trials.

Antiseptics Previously Employed for Managing COM

An evaluation of the literature for studies on the use of antiseptics in active COM revealed a dearth of good evidence for effectiveness.
Acetic acid 1 and 2% eardrops have long been used in active COM, but have had mixed results for clearing otorrhoea in clinical trials (6,15,22,25,26).
Daily application of cotton wicks soaked in Burrow’s solution placed against the tympanic membrane or middle ear has been used to treat active COM with results
comparable to topical gentamycin drops (9). Thorp et al. (10), in a clinical trial, demonstrated 1/4 strength Burrow’s solution to effectively clear 75% of active COM after 2 weeks of three times daily application. This concentration was better tolerated by patients than full-strength Burrow’s solution (10).

Iodine in powder and solution form has also been studied for the use in COM. Browning et al. (13) reported a mixture of iodine and boric acid powder inflated into ears to be ineffective, while a small underpowered randomized trial by Jaya et al. (14) found 5% povidone iodine to be as effective as quinolone drops as well as safe—not causing any deterioration in hearing. Boric acid as powder and solution has been used for active COM—the powder mostly proving effective, but
droplet form being ineffective (15,16). Loock (6) showed in a randomized control trial of 159 patients that once-off impaction of boric acid powder into the external ear canal is as effective as a course of topical quinolone drops.
A literature search revealed no animal studies on the efficacy of antiseptic substances on COM. Various animal studies all pertain to the possible ototoxic effects of antiseptics. Alper et al. (27) conducted a randomized controlled trial using monkeys to investigate the efficacy and safety of topical tobramycin (not an antiseptic) and to analyze the contribution of adding steroid to the topical
treatment. They found that tobramycin was effective in treating active COM and eradicating P aeruginosa in monkeys, and that the addition of dexamethasone contributes to this efficacy with a more rapid response to treatment and decreased risk of recurrence after treatment.

The study did not address potential vestibulotoxicity and did not address efficacy of treatment for other micro-organisms. Further animal studies all pertain to the possible ototoxic effects of antiseptics. Singh and Blakley (28) performed a systematic review of the literature assessing ototoxicity of presurgical antiseptic preparations, which included six animal studies assessing the ototoxicity of
iodine-based solutions. They found that povidone–iodine scrub which contains detergent, and povidone–iodine in 70% alcohol showed evidence of ototoxicity at relatively low concentration levels and with short exposure times. For the povidone–iodine solution there was no consensus regarding ototoxicity. They conclude that: ‘‘Iodine based, nonalcoholic, nondetergent solutions may be the least ototoxic but all should be used with caution.’’ Perez et al. (29) studied ototoxicity of chlorhexidine gluconate, povidone iodine 10%, as well as 70%
alcohol in sand rats. They found that whereas chlorhexidine and alcohol had clear ototoxic effects, povidone iodine did not (29).

pH as a Mechanism

In their studies using boric acid powder, both Saunders and Hearn speculated on the benefit of an acidic powder that lowers the pH of an infected ear canal to the more bacteriostatic acidic range of a healthy ear canal (15,16).

We found no obvious correlation between the pH of an antiseptic and its relative efficacy. Neither did decreasing the acidity of an antiseptic bring about a consistent change in their potency, with two exceptions. Decreasing the acidity of 0.29% iodine changed its pH to 13.0, and at this extreme pH the iodine solution was more active. 2% acetic acid, the most acidic solution tested, proved marginally less effective after increasing its pH to 5.1, with growth appearing at a single dilution earlier for each organism than its more acidic counterpart. In doing
antibacterial MIC testing, a single dilution difference is generally considered to be of little to no significance, as it can reflect interobserver/interlaboratory variability.
Therefore, it is difficult to draw firm conclusions about the effect of pH adjustment based on this data. The effect of adding NaOH on the underlying chemical composition of the substance may also need further investigation to
draw better conclusions.

Potential Clinical Application

We have already had great clinical success using boric acid powder for patients with active mucosalCOM. Loock (6) demonstrated that over 220 patients can be treated by impacting boric acid powder into the ear canal for the same price as a single course of the cheapest quinolone drops, with equivalent results. Using this technique, treatment with boric acid powder is cheap, easy to administer and
requires minimal patient compliance. This in-vitro study has confirmed its antimicrobial effect against the common organisms associated with COM.
5% povidone iodine is an antiseptic solution with potential for clinical use in active COM, and should be studied further. In their randomized controlled trial
Jaya et al. (14) demonstrated both its clinical effectiveness and its bactericidal activity in vitro, but commented that their study was on a small sample size and recommended more studies with larger patient numbers to confirm their results. This might prove a cheap alternative to quinolone drops. With an antiseptic reducing reliance on topical fluoroquinolones, it may help delay resistance to this antibiotic.

Caution on Iodine Powder

In this in-vitro trial iodine powder demonstrated remarkable antimicrobial activity against all of the tested microorganisms suggesting a possible very effective ototopical antiseptic. With the Pubmed literature search, the authors found no report of adverse effects of iodine powder when used in the ear. Browning et al. (13) had insufflated a mixture of boric acid and iodine powder into ears in one
treatment arm in their randomized controlled trial and reported no adverse effects. However, a further Google-based search revealed that pure iodine powder has a corrosive effect when coming in contact with skin (30). We therefore DO NOT recommend iodine powder for the use in ears, and indeed caution strongly against its clinical use.