Ultrastructural preservation of biofilms formed by non-typeable Hemophilus influenzae

Abstract

There is growing evidence to suggest that non-typeable Hemophilus influenzae (NTHi), an important cause of otitis media in children, is able to grow as a biofilm in the middle ear. This observation may help to explain bacterial persistence in chronic infections. We evaluate the usefulness of rapid freezing and freeze substitution as a means of preparing biofilms for ultrastructural examination by comparing the morphology of cryofixed specimens with the morphology produced using more conventional chemical fixation and dehydration methods. Chemical fixation and dehydration methods produced substantial ultrastructural damage to individual NTHi in the biofilm and loss of extracellular matrix, even in the presence of ruthenium red. In comparison, cryofixed and freeze-substituted NTHi biofilms showed significantly improved preservation of bacterial ultrastructure and biofilm organization. The intracellular contents of NTHi prepared using the cryomethods showed little evidence of aggregation, and bacteria within the biofilm were closely packed and surrounded by an abundant extracellular matrix. Although high-pressure freezing of NTHi biofilms followed by freeze substitution was highly effective for preserving ultrastructure when examined by transmission electron microscopy, immersion in liquid propane offered an alternative, “less technical”, freezing method. Immersion in liquid propane followed by freeze substitution and critical point drying was most effective for preserving ultrastructural details in specimens examined by scanning electron microscopy.