Otitis media (OM) encompasses a spectrum of inflammatory conditions of the middle ear and is a leading cause of healthcare visits and antibiotic prescriptions worldwide, particularly in the pediatric population [1]. Among its various forms, recurrent acute otitis media (RAOM) and chronic suppurative otitis media (CSOM) pose significant clinical challenges and contribute substantially to the burden of disease [2]. RAOM is defined by repeated, distinct episodes of acute infection of the middle ear, with resolution of symptoms and signs between episodes, typically affecting young children [3]. In contrast, CSOM is characterized by persistent or intermittent purulent discharge (otorrhea) through a non-intact tympanic membrane for a duration of more than six weeks [4].

The pathophysiology underlying these two conditions is fundamentally different. RAOM is primarily a consequence of Eustachian tube dysfunction in the context of a susceptible host, allowing pathogens from the nasopharynx to colonize the middle ear cleft [1]. CSOM, however, involves a chronic breach in the anatomical barrier of the tympanic membrane, which not only permits persistent infection but also allows for the ingress of flora from the external auditory canal [5]. This chronic, open state is thought to foster a different microbial environment, often involving biofilm formation, which contributes to the recalcitrant nature of the disease [6].

Effective management of both RAOM and CSOM relies heavily on appropriate antimicrobial therapy. However, the choice of empirical antibiotics should be guided by knowledge of the likely causative pathogens and their local susceptibility patterns. While the microbiology of acute otitis media (AOM) has been well-studied, demonstrating the predominance of Streptococcus pneumoniae, non-typeable Haemophilus influenzae, and Moraxella catarrhalis, the widespread use of pneumococcal conjugate vaccines (PCV) has led to shifts in pathogen prevalence and serotype distribution [7]. The microbiology of CSOM is often reported to be dominated by opportunistic pathogens such as Pseudomonas aeruginosa and Staphylococcus aureus, which are less common in AOM [8].

Despite this general understanding, there is a paucity of studies that directly and contemporaneously compare the complete microbiological landscape and resistance profiles of RAOM and CSOM within the same patient population. Such a comparison is critical, as it can confirm whether these clinically distinct entities are also microbiologically distinct, thereby validating the need for fundamentally different therapeutic approaches. Understanding these differences is essential for developing evidence-based guidelines, optimizing treatment outcomes, and promoting antimicrobial stewardship. Therefore, this study aimed to comprehensively compare the bacteriological profiles and in vitro antibiotic susceptibility patterns of isolates from patients with RAOM and those with active mucosal CSOM.

Study Design and Setting: A cross-sectional, comparative study was conducted at the Department of Otolaryngology of a tertiary university hospital.

Study Population: Two distinct patient groups were enrolled consecutively.

1. RAOM Group: This group comprised children aged 6 months to 12 years diagnosed with RAOM, defined as experiencing three or more distinct episodes of AOM in the preceding 6 months, or four or more episodes in the preceding 12 months. All patients in this group were undergoing elective myringotomy with tympanostomy tube insertion for management of their condition and had evidence of middle ear effusion at the time of surgery.
2. CSOM Group: This group included patients of any age with a clinical diagnosis of active mucosal CSOM, defined as persistent purulent otorrhea for over six weeks through a chronic tympanic membrane perforation, without evidence of cholesteatoma.

Exclusion criteria for both groups included the use of systemic or topical antibiotics within the 14 days prior to sample collection, known immunodeficiency disorders, craniofacial anomalies, and a history of cholesteatoma.

Sample Collection: All samples were collected by trained otolaryngologists using aseptic techniques.

• RAOM Group: After cleaning the external auditory canal with 70% ethanol, a myringotomy incision was made in the anteroinferior quadrant of the tympanic membrane. Middle ear fluid was then aspirated using a sterile Alden-Senturia suction trap.
• CSOM Group: The external auditory canal was meticulously cleaned of debris and discharge. Under microscopic guidance, a sterile calcium alginate micro-swab was passed through the tympanic membrane perforation to collect a sample directly from the middle ear mucosa. Care was taken to avoid contamination from the external canal.

All collected samples were immediately placed in Stuart's transport medium and transported to the microbiology laboratory for processing within two hours.

Microbiological Processing: Samples were inoculated onto 5% Sheep Blood Agar, Chocolate Agar, MacConkey Agar, and Sabouraud Dextrose Agar plates. Blood and Chocolate Agar plates were incubated at 37°C in a 5% CO₂ atmosphere for 48 hours. MacConkey Agar plates were incubated aerobically at 37°C for 24 hours. Bacterial isolates were identified based on colony morphology, Gram staining, and a panel of standard biochemical tests. Final identification and antibiotic susceptibility testing were performed using the VITEK 2 automated system (bioMérieux, France).

Antibiotic Susceptibility Testing: Susceptibility testing was performed and interpreted according to the prevailing Clinical and Laboratory Standards Institute (CLSI) guidelines. The antibiotic panel was selected based on clinical relevance and included penicillin, amoxicillin-clavulanate, cefuroxime, ceftriaxone, erythromycin, and trimethoprim-sulfamethoxazole for common respiratory pathogens. For S. aureus, testing included oxacillin (using a cefoxitin disk to detect methicillin resistance) and vancomycin. For Gram-negative isolates like P. aeruginosa, the panel included ciprofloxacin, ceftazidime, piperacillin-tazobactam, meropenem, and amikacin.

Statistical Analysis: Data were analyzed using SPSS Statistics for Windows, Version 25.0 (IBM Corp., Armonk, NY). Descriptive statistics were used to summarize patient demographics and microbiological data. The Chi-square test or Fisher's exact test was used to compare categorical variables, such as the prevalence of specific bacteria and resistance rates, between the two groups. A p-value of <0.05 was considered statistically significant.

Patient Demographics and Culture Results: A total of 177 patients meeting the inclusion criteria were enrolled, consisting of 85 patients in the RAOM group and 92 in the CSOM group. The mean age of patients in the RAOM group was 3.2 ± 1.5 years (range: 0.8–9 years), whereas the mean age in the CSOM group was significantly higher at 25.6 ± 12.1 years (range: 6–65 years; p < 0.001).

Of the 85 middle ear fluid samples from the RAOM group, 75 (88.2%) yielded a positive bacterial culture. In the CSOM group, 88 of 92 (95.7%) middle ear swabs were culture-positive. Polymicrobial growth (≥2 organisms) was significantly more common in the CSOM group (34/88, 38.6%) compared to the RAOM group (13/75, 17.3%; p = 0.003).

Comparative Microbiological Profile: A striking difference was observed in the distribution of bacterial pathogens between the two groups (Table 1). The RAOM group was dominated by the classic otopathogens: Streptococcus pneumoniae (36.0%), non-typeable Haemophilus influenzae (29.3%), and Moraxella catarrhalis (14.7%). Conversely, the CSOM group was predominantly colonized by Pseudomonas aeruginosa (46.6%), Staphylococcus aureus (26.1%), and other Gram-negative bacilli like Proteus mirabilis (11.4%). The prevalence of each of these key pathogens differed significantly between the two groups (p < 0.001). Fungal isolates (Aspergillus spp.) were found only in the CSOM group (4.5%).

Table 1. Frequency of Microbial Isolates in RAOM and CSOM Groups

Antibiotic Susceptibility Patterns of Gram-Positive Isolates: The susceptibility patterns of the most common Gram-positive isolates are shown in Table 2. S. pneumoniae isolates from the RAOM group showed high susceptibility to amoxicillin-clavulanate (96.3%) and ceftriaxone (100%), but 18.5% were resistant to penicillin. All S. aureus isolates were susceptible to vancomycin. However, the rate of methicillin resistance (MRSA) was significantly higher among isolates from the CSOM group (30.4%) compared to the RAOM group (12.5%; p = 0.041).

Table 2. Antibiotic Susceptibility of Key Gram-Positive Isolates

Antibiotic Susceptibility Patterns of Gram-Negative Isolates: Among Gram-negative isolates, H. influenzae from the RAOM group was highly susceptible to amoxicillin-clavulanate (95.5%) and third-generation cephalosporins, with a beta-lactamase production rate of 27.3%. P. aeruginosa, the dominant pathogen in CSOM, exhibited significant resistance (Table 3). While susceptibility to amikacin and meropenem was high, 22.0% of isolates were resistant to ciprofloxacin, a commonly used topical agent.

Table 3. Antibiotic Susceptibility of Key Gram-Negative Isolates

This study provides a direct, comparative microbiological analysis of RAOM and CSOM, revealing two distinct infectious disease profiles. Our principal finding is the clear dichotomy between the pathogens colonizing the middle ear in these two conditions. RAOM is confirmed to be a disease primarily caused by typical encapsulated respiratory pathogens that ascend from the nasopharynx, whereas CSOM is dominated by opportunistic and often highly resistant bacteria, reflecting its chronic nature and the compromised anatomy of the middle ear.

The microbiological profile of the RAOM group, with S. pneumoniae, H. influenzae, and M. catarrhalis as the leading isolates, aligns with the established etiology of AOM [1, 7]. This reinforces the understanding of RAOM as a disease of repeated acute infections rather than a chronic indolent one. The antibiotic susceptibility patterns observed in this group largely support current treatment guidelines, which recommend high-dose amoxicillin or amoxicillin-clavulanate as first-line therapy [3]. The penicillin resistance rate of 18.5% for S. pneumoniae underscores the importance of appropriate dosing and continued surveillance.

In stark contrast, the bacteriology of CSOM was characterized by a significant shift towards environmental and skin-commensal organisms. The predominance of P. aeruginosa (46.6%) and S. aureus (26.1%) is consistent with numerous studies worldwide [8, 9]. This microbial shift is likely driven by several factors inherent to CSOM. The chronic perforation allows direct inoculation of bacteria from the external auditory canal, where P. aeruginosa and S. aureus are common inhabitants [5]. Furthermore, the chronically inflamed, hypoxic, and moist middle ear environment creates an ideal niche for these opportunistic pathogens to thrive [10]. The significantly higher rate of polymicrobial infections in CSOM also suggests a complex synergistic ecosystem, possibly involving biofilm formation, which is a known virulence factor for P. aeruginosa and contributes to treatment failure [6, 11].

The antibiotic resistance patterns observed in the CSOM group are of major clinical concern. The 30.4% MRSA rate among S. aureus isolates and the 22.0% ciprofloxacin resistance rate in P. aeruginosa pose significant therapeutic challenges. The high prevalence of resistance is likely a result of selective pressure from previous, often multiple, courses of antibiotics, which are common in the history of CSOM patients [12]. This finding strongly advocates for a shift away from empirical treatment towards culture-directed therapy in the management of CSOM. While topical quinolones remain a mainstay of treatment, the observed resistance highlights that treatment failure can occur and susceptibility testing is invaluable in refractory cases [13, 14].

Our study's strength lies in its direct comparative design, using standardized collection and processing methods for both groups within a single center, which minimizes confounding from geographical or methodological variations. However, some limitations should be noted. As a cross-sectional study, it captures a single snapshot in time and cannot comment on the evolution of the microbiome. We did not employ molecular techniques, which might have detected additional, non-culturable organisms or provided information on viral co-infections. Finally, the findings are from a single tertiary care center, and local resistance patterns may vary. Despite these limitations, the clear and significant differences observed provide robust evidence of the microbiological divergence of RAOM and CSOM.

The clinical implications are clear. RAOM and CSOM should not be viewed as points on a single continuum but as separate entities requiring distinct management strategies. For RAOM, empirical therapy targeting common respiratory pathogens remains appropriate. For CSOM, clinicians must anticipate opportunistic and resistant organisms, and obtaining a sample for culture and sensitivity testing at the outset of treatment is highly recommended to guide effective therapy, prevent further resistance, and improve patient outcomes [15].

Recurrent acute otitis media and chronic suppurative otitis media are driven by fundamentally different microbiological profiles. RAOM is an infection caused by common respiratory pathogens, whereas CSOM is a complex, often polymicrobial infection dominated by opportunistic bacteria like P. aeruginosa and S. aureus with significantly higher rates of antibiotic resistance. This evidence strongly supports the need for distinct clinical guidelines for empirical antibiotic selection and reinforces the crucial role of microbiological culture and susceptibility testing in the management of chronic, discharging ears.

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