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Background  Several different IgA-proteases exist in  Haemophilus influenzae . The variants have been suggested to play differential roles in pathogenesis, but there is limited information on their distribution in clinical isolates. The objective of this study was to investigate the distribution of IgA-protease genotypes in  H. influenzae  and assess the association between IgA-protease genotype and type of clinical infection.    Methods  We performed PCR-screening of the IgA-protease gene variants in two cohorts of clinical  H. influenzae.  The first cohort consisted of 177 isolates from individuals with respiratory tract infection in January 2010, 2011 and 2012. Information on age, gender and clinical infection was available in this cohort. The second cohort comprised 53 isolates, including NTHi from bloodstream, cerebrospinal fluid (CSF) and urogenital origin as well as encapsulated isolates respresenting all capsule types. We assessed associations between IgA protease genotype and clinical predictors using basic statistical tests of association as well as regression analysis.    Results  The  igaB  gene was found in 46% of isolates in the respiratory tract cohort, and no evident trend could be seen during the study years. However, the  igaB  gene was significantly less common among invasive isolates (19%),  p  = 0.003 (Fischer’s exact test), even when encapsulated isolates were excluded (21%),  p  = 0.012. A significantly negative association between bacteraemia and  igaB  genotype remained after adjusting for covariates. We did not identify a significant association between IgA-protease gene variants and type of respiratory tract infection, but isolates with an  igaA2  genotype were overrepresented in pre-school children.    Conclusions  The distribution of IgA-protease gene variants in Swedish  H. influenzae  highlighted the widespread abundance of the  igaB  in isolates from cases of respiratory tract infection, but the  igaB  gene variant was significantly less common in invasive (bloodstream and CSF) isolates of  H. influenzae  compared with respiratory tract isolates.    Electronic supplementary material  The online version of this article (10.1186/s12879-018-3464-3) contains supplementary material, which is available to authorized users.

Differential distribution of IgA-protease genotypes in mucosal and invasive isolates of Haemophilus influenzae in Sweden