PITTSBURGH—Culture-negative, antibiotic-resistant, chronic otitis media (OM) has been explained as a sterile, inflammatory process, but the presence of bacterial nucleic acids and proteins in effusions supports an alternative hypothesis: formation of bacterial biofilms on mucosae. Biofilms comprise complex communities of bacteria, some of whose reduced metabolic rates render them nearly impervious to antibiotics. Animal research now suggests that such biofilms could account for non-culturable chronic OM.[1]

Evidence for the persistence of viable bacteria prompted Garth D. Ehrlich, PhD, and colleagues to experimentally recreate chronic OM by injecting Haemophilus influenzae into the middle ears of 42 chinchillas. Three days later, ampicillin (150 mg/kg bid) was administered to produce sterile effusions and to minimize systemic infection. Middle-ear mucosal specimens taken at various times after injection were processed for microscopy. Results showed that mature biofilms had formed by five days postinoculation.

HOW BIOFILMS RESIST DEFENSES

Within a bacterial biofilm, “you have a large matrix, of which about 10% are the bacteria,” explained Dr. Ehrlich, Executive Director of the Center for Genomic Sciences at Allegheny Singer Research Institute in Pittsburgh. “In the matrix, [the bacteria] can’t be engulfed by phagocytes because they’re both enclosed and protected” by surrounding extracellular polymeric substances, Dr. Ehrlich said. The matrix also adsorbs complement and immunoglobulins, further safeguarding bacteria.

Besides defying immune defenses, biofilm bacteria can resist man-made antimicrobials. “Remember that a biofilm is not a single phenotype,” said Dr. Ehrlich. Within each stratum of the matrix, bacteria metabolically adapt to the specific microenvironment of pH, oxygen, and nutrients: Deeper bacteria divide infrequently, making them resistant to most antibiotics. Thus, argued Dr. Ehrlich, even if “you kill the ones in the periphery, you still have a nidus left,” which permits regrowth and planktonic spread of infection.

Mucosal biofilms at other sites may have a similar role. “If you have a child who has recurrent middle-ear infections, and if you remove the adenoids, it will dramatically reduce the number of ear infections,” he noted.

“In addition to the phenotypic plurality, there is a lot of genetic variability—different strains, passing back and forth genetic information,” said Dr. Ehrlich; this could also boost drug resistance. Moreover, “we know that some of these ‘professional pathogenic organisms’ turn on this [gene exchange] mechanism when they are under stress,” as when exposed to antimicrobial agents.

WHY CHRONIC OM YIELDS NEGATIVE CULTURES

While effusions from acute OM generally yield positive cultures, those from chronic OM typically do not. “Bacteria that grow as a biofilm are often recalcitrant to culture,” Dr. Ehrlich explained, adding, “Even if you do manage to culture bacteria, you get gross underestimates,” because multiple bacteria in a detached biofilm fragment are scored as a single colony. Furthermore, previous antibiotic administration typically reduces effusion bacterial counts while sparing biofilms in chronic OM patients, Dr. Ehrlich pointed out: “Antibiotic treatment tends to promote the biofilm phenotype.”

Alternatively, “DNA tests are based on polymerase chain reaction,” continued Dr. Ehrlich. “They’re well validated.” But, he remarked, “They’re not done as often as they should be.”

—Mimi Zucker, PhD

Reference
1. Ehrlich GD, Veeh R, Wang X, et al. Mucosal biofilm formation on middle-ear mucosa in the chinchilla model of otitis media. JAMA. 2002;287:1710-1715.

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