Honey bee larval peritrophic matrix degradation during infection with Paenibacillus larvae, the aetiological agent of American foulbrood of honey bees, is a key step in pathogenesis.

Paenibacillus larvae, the aetiological agent of American foulbrood (AFB) of honey bees, causes a fatal intestinal infection in larvae and invades the haemocoel by breaching the midgut. The peritrophic matrix lining the midgut epithelium in insects constitutes an effective barrier against abrasive food particles, xenobiotics, toxins and pathogens. Pathogens like P. larvae entering the host through the gut first need to overcome this barrier. To better understand AFB pathogenesis, we analysed the fate of the peritrophic matrix in honey bee larvae during P. larvae infection. Using histochemical techniques, we first established that chitin is a major component of the honey bee larval peritrophic matrix. Rearing larvae on a diet containing a fluorochrome blocking formation of the peritrophic matrix or a bacterial endochitinase revealed that a fully formed peritrophic matrix is essential for larval survival. Larvae infected by P. larvae showed total degradation of the peritrophic matrix enabling the bacteria to directly attack the epithelial cells. Carbon source utilization tests confirmed that P. larvae is able to metabolize colloidal chitin. We propose that P. larvae degrades the peritrophic matrix to allow direct access of the bacteria or of bacterial toxins to the epithelium to prepare the breakthrough of the epithelial layer.