Biofilms on seeds and sprouts
Bacterial adherence to seeds is a process that strongly
influences rhizosphere colonization. Suppliers often
deliberately coat their seed stocks with microbial biofilms
to inoculate the developing rhizosphere. Additionally,
biofilms on seeds and sprouts used for human consumption
are common sources of infection. P. putida adheres
effectively to seeds and will subsequently colonize the
rhizosphere [37]. Several P. putida mutants, including one
in the lapA homologue of P. fluorescens, are deficient in seed
adherence and biofilmformation on inert surfaces, emphasizing
the overlap between these activities. Recently,
Coombs and Franco [45] identified endophytic populations
of nonpathogenic actinobacteria in wheat tissues and
determined that these were derived from interior colonization
of surface-sterilized seeds. Endophytic seed populations
help ensure future rhizosphere colonization.
Other studies of seed colonization have observed rodshaped
and coccal bacteria embedded within EPS in
scanning electronmicrographs of alfalfa seeds and sprouts
[46,47]. Biofilms are notoriously resistant to washing and
other common antibacterial treatments. Fett et al. found
that both Escherichia coli O157:H7 and Salmonella populations
on alfalfa sprouts required treatments much harsher
than simple water washing to reduce the numbers of
adherent microbes, and full removal was never achieved
[48,49]. It seems likely that the surviving bacteria resided
within biofilms, although this was not addressed.