Acute Otitis Media Microbiology

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Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis are the principal etiologic agents in bacterial AOM accounting for about 80% of the bacterial isolates (2,3). S. pneumoniae has constantly been found more commonly, irrespective of age group, but its predominance has tended to decrease following the introduction of the pneumococcal conjugate vaccine in 2000 (4), where the frequency of isolation of H. influenzae increased. Of special concern is the increased rate of isolation of penicillin-resistant strains of S. pneumoniae (5) and amoxicillin-resistant H. influenzae (5,6) from infected ears. The incidence of such strains may reach 50% in some areas.

Other organisms that less frequently cause AOM include group A beta-hemolytic streptococci (GABHS), Staphylococcus aureus, Turicella otitidis, Alloiococcus otitis Chlamydia spp., and Staphylococcus epidermidis, and various aerobic and faculatative gram-negative bacilli (7) including Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Proteus spp. Gram-negative bacilli and staphylococci are implicated as dominant etiologic agents in otitis media of the neonate. However, even among very young infants, S. pneumoniae and H. influenzae constitute the most common etiologic agents. Viruses were recovered in the middle-ear fluid of 14.3% of children (8).

The role of anaerobic bacteria was evaluated in four studies (9-13). In a study of 186 children (9,10), aerobic bacteria alone, predominantly pneumococci and H. influenzae, were isolated from 118 (63.4%) patients (Table 1). Anaerobes alone, most often Peptostreptococcus spp., were isolated from 24 (12.9%) patients. Mixed flora including aerobes and anaerobes were present in 26 (14%) patients. No bacterial growth was noted in 18 (9.7%) patients. Thus, the addition of anaerobic methodology to the processing of specimens enabled the isolation of bacteria from 90% of the patients studied. This rate is higher than that obtained in studies in which anaerobic techniques were not used (2). Even though the ear canal was not sterilized prior to the procedure, it is unlikely that the Peptostreptococcus spp. isolates were of ear canal origin as that site is mainly colonized by Propionibacterium acnes (14).

In the second study (11), where the tympanic membrane was disinfected, three anaerobes were recovered from 28 infants: two Clostridium spp. and one Peptostreptococcus magnus.

TABLE 1 Bacteria Isolated from 186 Cases of Acute Otitis Media

Percentage of patients

Isolates Number of isolates with positive cultures

Aerobic bacteria

TABLE 1 Bacteria Isolated from 186 Cases of Acute Otitis Media

Aerobic bacteria

Streptococcus pneumoniae

62

37

Haemophilus influenzae

52

30

Staphylococcus aureus

15

9

Group A beta-hemolytic streptococci

9

5

Pseudomonas aeruginosa

3

2

Group D Enterococcus

3

2

Others

12

7

Total number of aerobic bacteria

156

Anaerobic bacteria

Peptostreptococcus spp.

39

21

Propionibacterium spp. Others3

12

7

5

3

Total number of anaerobic bacteria

56

Total number of aerobic and anaerobic bacteria

212

a One each of Veillonella spp., Bifidobacterium spp., Eubacterium spp., Clostridium ramosum, and microaerophilic streptococci. Source: From Ref. 10.

In the third study, two anaerobes (Bacteroides fragilis and Porphyromonas gingivalis) were recovered from 2 of 80 children (13).

The fourth study was of middle-ear aspirates and external auditory canals of 50 children with spontaneous perforation (12). Bacterial growth was present in 51 of 61 ear aspirates obtained from 46 (92%) patients. The organisms isolated mainly from the external ear canal were S. epidermidis isolates, P. acnes, and alpha-hemolytic streptococci. Aerobic bacteria alone were found in the ear aspirates of 47 patients (92%), anaerobes alone in 1 (2%), and both aerobes and anaerobes in three (6%). The predominant middle-ear isolates were S. pneumoniae, H. influenzae, GABHS, and M. catarrhalis. The anaerobes recovered in the middle ear were Peptostreptococcus spp. (2) and P. acnes (2).

The study demonstrate that specimens of otorrhea collected from the external auditory canals can be misleading as only 44 of the 61 (72%) isolates recovered from the middle ear were also present in the ear canal. Peptostreptococci were recovered from 17% of inner ear aspirates but were recovered from only 3% of external ear canal specimens obtained in this study (12). This difference further supports their possible role in AOM. On the other hand, the rate of isolation of P. acnes from the external ear canal was 18%, which is higher than its recovery rate from inner ear aspirates (7%) (14).

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