Comparison of molecular serotyping methods for Actinobacillus pleuropneumoniae and analysis of atypical serotypes detected in routine diagnostics

Clinical outbreaks due to Actinobacillus pleuropneumoniae (APP) and subclinical infections have high impact on swine health status worldwide although several commercial vaccines are available. Autogenous vaccination programs are implemented when APP outbreaks occur in commercially vaccinated herds. The identification and characterization of the involved APP serotypes is therefore crucial for the implementation of preventive strategies and antimicrobial usage reduction on the farm. Interpretation of serotyping results obtained by different methods might be difficult in case of mismatching results or untypable APP isolates. In this study results of two routine serotyping methods- a capsular gene based and an apx toxin gene PCR- were compared in 151 APP field and 19 APP reference strains. APP species was identified after bacterial culture by MALDI-TOF-MS followed by serotyping. Toxin profiles were not in accordance with the serotype defined by capsule gene PCR in 37 % of APP field strains which were grouped in those with (1) atypical capsule (cps) gene patterns (22 %) and those with (2) atypical apxIV toxin gene length (78 %). Selected atypical APP strains were further analysed by whole genome sequencing. The toxin gene-based PCR robustly identified the apxI-III toxin genes in all strains and revealed highly variable apxIV toxin gene patterns. For thirteen isolates a cps-gene type 6 and apxIV toxin gene pattern of serotype 2/8/15 could be confirmed via WGS. For three serotype 9/11 isolates the failure of the cps gene typing was found to be due to a deletion at the 3' of the cpsF gene. A standardized, precise description of the apx-toxin gene pattern as well as the cps-gene-based serotype for APP strains can be recommended (e.g. APP cps type 2, apx gene profile apxIB, apxII, apxIII).