Improvement rate of acute otitis media caused by Haemophilus influenzae at 1 week is significantly associated with time to recovery.

Acute otitis media (AOM) is the most common upper respiratory tract infection in childhood. Children with AOM were enrolled at Tohoku Rosai Hospital between July 2006 and June 2011 if their middle ear fluid cultures after tympanocentesis yielded only Haemophilus influenzae. The susceptibilities of the isolates to ampicillin were determined, and microtiter biofilm assays and invasion assays using BEAS-2B cells were performed. The association between these bacterial characteristics and clinical relapses of AOM and treatment failures was evaluated. Seventy-four children (39 boys and 35 girls) with a median age of 1 year (interquartile range [IQR], 0.25 to 2 years) were enrolled. Among 74 H. influenzae isolates, 37 showed intermediate resistance or resistance to ampicillin (MIC, ≥ 2 μg/ml). In the microtiter biofilm assay, the median optical density at 600 nm (OD600) was 0.68 (IQR, 0.24 to 1.02), and 70 isolates formed biofilms. The median invasion rate was 15% (IQR, 0 to 10%), and 46 isolates invaded BEAS-2B cells. Relapses and treatment failures occurred in 19 and 6 children, respectively. There was no significant difference in the invasion rates between patients with and those without relapses or treatment failures. Also, there was no significant association between biofilm formation and relapse or treatment failure. The improvements in the severity scores after 1 week were significantly associated with the recovery time (P < 0.0001). We did not identify any significant association between relapse or treatment failure and bacterial factors. AOM has a multifactorial etiology, and this may explain why we could not find a significant association. An improvement in the severity score after 1 week of treatment may be a useful predictor of the outcome of AOM.