Measuring nasal bacterial load and its association with otitis media

Abstract

Background: Nasal colonisation with otitis media (OM) pathogens, particularly Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis, is a precursor to the onset of OM. Many children experience asymptomatic nasal carriage of these pathogens whereas others will progress to otitis media with effusion (OME) or suppurative OM. We observed a disparity in the prevalence of suppurative OM between Aboriginal children living in remote communities and non-Aboriginal children attending child-care centres; up to 60% and <1%, respectively. This could not be explained by the less dramatic difference in rates of carriage of respiratory bacterial pathogens (80% vs 50%, respectively). In this study, we measured nasal bacterial load to help explain the different propensity for suppurative OM in these two populations.

Methods: Quantitative measures (colony counts and real-time quantitative PCR) of the respiratory pathogens S. pneumoniae, H. influenzae and M. catarrhalis, and total bacterial load were analysed in nasal swabs from Aboriginal children from remote communities, and non-Aboriginal children attending urban child-care centres.

Results: In both populations nearly all swabs were positive for at least one of these respiratory pathogens. Using either quantification method, positive correlations between bacterial load and ear state (no OM, OME, or suppurative OM) were observed. This relationship held for single and combined bacterial respiratory pathogens, total bacterial load, and the proportion of respiratory pathogens to total bacterial load. Comparison of Aboriginal and non-Aboriginal children, all with a diagnosis of OME, demonstrated significantly higher loads of S. pneumoniae and M. catarrhalis in the Aboriginal group. The increased bacterial load despite similar clinical condition may predict persistence of middle ear effusions and progression to suppurative OM in the Aboriginal population. Our data also demonstrated the presence of PCR-detectable non-cultivable respiratory pathogens in 36% of nasal swabs. This may have implications for the pathogenesis of OM including persistence of infection despite aggressive therapies.

Conclusion: Nasal bacterial load was significantly higher among Aboriginal children and may explain their increased risk of suppurative OM. It was also positively correlated with ear state. We believe that a reduction in bacterial load in high-risk populations may be required before dramatic reductions in OM can be achieved.

Figure 1

Respiratory bacterial carriage in Aboriginal and non-Aboriginal children determined by RTQ-PCR.

Figure 2

Level of S. pneumoniae, H. influenzae, M. catarrhalis and total bacterial load by ear state in Aboriginal and non-Aboriginal children as determined by RTQ-PCR.

Figure 3

S. pneumoniae, H. influenzae and M. catarrhalis as a percentage of total load by ear state in Aboriginal and non-Aboriginal children as determined by RTQ-PCR.

Figure 4

Relationship between the level of S. pneumoniae and ear state.

Figure 5

Predicted probability of suppurative OM with bacterial load based on the logistic regression model of data acquired by RTQ-PCR.

Figure 6

Sensitivity (A) and specificity (B) for predicting suppurative OM with bacterial load based on data acquired by RTQ-PCR.