Think you know biofilms? You don't know biofilms.

In mBio this week: new findings on what is and is not needed for Streptococcus pneumoniae to make a biofilm on the nasopharynx. Between 10 and 15% of adults are colonized with S. pneumoniae at any given time, a status that's thought to precede invasive pneumococcal disease, but many - if not most - studies of pathogen biofilm formation are done with in vitro models.

Fig. 2 Pneumococcal biofilms are present on mucosal epithelial cells in the septa of experimentally infected mice.

Blanchette-Cain et al. used scanning electron microscopy to study the development of biofilm aggregates of increasing complexity in vivo on murine nasal septa after intranasal inoculation. Among other observations, they note that the morphology of S. pneumoniae biofilms in vivo is strikingly distinct from that of biofilms formed on the surface of polystyrene microtiter plates. Importantly, biofilms within the nasopharynx "were neither confluent nor contiguous and incorporated host cells". Biofilms on mucosal epithelial cells transitioned from spotty aggregates on top of ciliated cells to larger clumps and structures attached directly to the basement membrane. These are phenomena that you simply can't study in vitro.

There are plenty of reasons for using in vitro models for studying biofilms, but what are we missing when we do?