Microbiologic dilemmas in AOMT: is a recovered organism indicative of pathogenicity?

Specimen collection and processing greatly impact the type and number of organisms recovered, and this can subsequently change microbiologic outcomes in studies of acute otitis media through tympanostomy tubes (AOMT). For example, in a recent AOMT study that we conducted, a Juhn Tym-Tap device was placed at the lumen of the tympanostomy tube to collect the otorrhea. This was done after the ear canal had been cleaned and suctioned. This device was used to minimize contamination of the otorrhea specimen by organisms from the outer ear canal. The otorrhea specimen was expelled onto two swabs, one for recovery of bacteria and the other for extraction of total DNA, followed by polymerase-chain-reaction-based detection and identification of all bacteria present.

A comparison of the findings from the two swabs led to two important observations: (1) the pretherapy culture-positive rate increased from 74% (recovered bacteria) to 80% (recovered and detected bacteria) and (2) numerous anaerobes were present in the otorrhea specimen that were not being recovered (unpublished data). In a subsequent study, we were able to increase the pretherapy culture-positive rate to approximately 80% by changing the transport media that were used when the specimens were sent to the clinical microbiology laboratory and by using media and incubation conditions that were specifically designed for anaerobes (unpublished data).

Multiple species

Even with elaborate techniques employed to eliminate the possibility of contamination of otorrhea specimens, multiple bacterial species are recovered from many AOMT specimens. Of 339 specimens we collected with the aid of a Juhn Tyro-Tap, more contained two or more bacterial species than contained just one (table). This situation poses a dilemma when we are trying to determine which species is the pathogen or, in cases where only one bacterial species is recovered, in determining whether it is the actual pathogen.

One approach preferred by some to address this question is to examine Gram's-stained otorrhea smeared onto a glass slide at the time of collection. A microscopic examination can reveal the presence of white cells (an indicator of the host immune response), as well as the type of bacteria (i.e., gram-positive or gram-negative) and the shape of the bacteria (i.e., cocci, rods, bacilli, coryneforms, etc.). This approach can provide supportive data to address the question of pathogenicity, although bacteria in low numbers can easily be missed during the microscopic examination. Some would argue that the presence of white cells in a smear sample is sufficient evidence to guide therapy in the absence of culture data.

The "What is a pathogen?" dilemma

A common example of the "What is a pathogen?" dilemma in AOMT is Staphylococcus epidermidis, which is usually considered to be a commensal organism in the external auditory canal. In a recent clinical study conducted by Alcon Laboratories (unpublished data), S epidermidis was recovered in 133 of 822 (16.2%) AOMT patients. S epidermidis was the sole organism recovered in 72 of the 133 (54.1%) cases, and it was recovered in 39 of 133 (29.3%) cases that involved one or more of the five major pathogens (Haemophilus influenzae, Streptococcus pneumoniae, Moraxella catarrhalis, Staphylococcus aureus, and Pseudomonas aeruginosa). In the remaining 22 cases (16.5%), S epidermidis was recovered with other bacterial species.

These findings present us with more questions than answers. For example, say S epidermidis is the sole bacterium recovered pretherapy and it is eradicated post-therapy and the patient's AOMT resolves clinically. Does this finding support the notion that S epidermidis was indeed the pathogen?

In another example of the "What is a pathogen ?" dilemma, we have characterized and described a novel species of Pseudomonas called Pseudomonas otitidis. (1) This species was recovered from 31 patients with acute otitis externa (AOE). In 11 of these patients, P otitidis was the only organism recovered; in the other 20 patients, the most common organism isolated with P otitidis was either P aeruginosa or Staphylococcus caprae. The fact that P otitidis was the only bacterium recovered in several instances supports the notion that it is indeed a pathogen.

We recently received a supportive communication from a clinical microbiologist in Seoul, Korea, who reported that he had recovered two pseudomonads from an AOE patient; one was P aeruginosa and the other matched the description of P otitidis. The Korean isolate of P otitidis was imipenem-resistant, as were three of the AOE P otitidis isolates that we found. Does this finding support the concept that P otitidis is a pathogen? These and other pertinent questions certainly deserve further consideration.

David Stroman, PhD

Reference

(1.) Clark LL, Dajcs JJ, McLean CH, et al. Pseudomonas otitidis sp. nov., isolated from patients with otic infections. Int J Syst Evol Microbiol 2006;56:709-14.

Table. Recovery of bacterial species from 339 AOMT
specimens collected with a Juhn Tym-Tap

                                               No. (%)
No. of bacterial species                     of specimens

Specimens with 0 bacterial species             70 (20.6)
Specimens with 1 bacterial species            117 (34.5)
Specimens with 2 bacterial species             85 (25.1)
Specimens with [greater than or equal to]      67 (19.8)
 3 bacterial species