Compared
with the chemically defined synthetic wastewater (SynWW),
real wastewater has been reported to exhibit distinct effects on microbial
community development. Whether and how soluble microbial products
in real wastewater contribute to different effects of synthetic and
real wastewater on the fate of exogenous bacteria remains elusive.
In this study, using a model wastewater bacterium Comamonas
testosteroni, we first examined the influences of microfiltration
filter-sterilized real wastewater (MF-WW) and SynWW on the retention
of C. testosteroni in established wastewater flocs
during bioaugmentation. In bioreactors fed with MF-WW, augmentation
of C. testosteroni to wastewater flocs resulted in
a substantially higher abundance of the augmented bacterial cells
than those fed with SynWW. To identify the soluble microbial products
in MF-WW contributing to the observed differences between bioaugmentation
reactors fed with MF-WW and SynWW, we examined the effect of MF-WW
and SynWW on the growth, floc formation, and biofilm development of C. testosteroni. When C. testosteroni grew
in MF-WW, visible flocs formed within 2 h, which is in contrast to
cell growth in SynWW where floc formation was not observed. We further
demonstrated that the observed differences were mainly attributed
to the high molecular weight fraction of the soluble extracellular
polymeric substances (EPS) in MF-WW, in particular, proteins and extracellular
DNA. The DLVO analysis suggested that, in the presence of soluble
EPS, the bacterial cell surface exhibits an increased hydrophobicity
and a diminished energy barrier, leading to irreversible attachment
of planktonic cells and floc formation. The RNA-seq based transcriptional
analysis revealed that, in the presence of soluble EPS, genes involved
in nonessential metabolisms were downregulated while genes coding
for Cco (cbb3-type) and Cox (aa3-type)
oxidases with different oxygen affinities were upregulated, facilitating
bacterial survival in flocs. Taken together, this study reveals the
mechanisms underlying the contribution of soluble EPS in real wastewater
to the recruitment of exogenous bacteria by microbial aggregates and
provides implications to bioaugmentation.