Aggregation via the Red, Dry, and Rough Morphotype Is Not a Virulence Adaptation in
Salmonella enterica
Serovar Typhimurium
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abstract
ABSTRACT
The
Salmonella
rdar (
r
ed,
d
ry,
a
nd
r
ough) morphotype is an aggregative and resistant physiology that has been linked to survival in nutrient-limited environments. Growth of
Salmonella enterica
serovar Typhimurium was analyzed in a variety of nutrient-limiting conditions to determine whether aggregation would occur at low cell densities and whether the rdar morphotype was involved in this process. The resulting cultures consisted of two populations of cells, aggregated and nonaggregated, with the aggregated cells preferentially displaying rdar morphotype gene expression. The two groups of cells could be separated based on the principle that aggregated cells were producing greater amounts of thin aggregative fimbriae (Tafi or curli). In addition, the aggregated cells retained some physiological characteristics of the rdar morphotype, such as increased resistance to sodium hypochlorite. Competitive infection experiments in mice showed that nonaggregative Δ
agfA
cells outcompeted rdar-positive wild-type cells in all tissues analyzed, indicating that aggregation via the rdar morphotype was not a virulence adaptation in
Salmonella enterica
serovar Typhimurium. Furthermore, in vivo imaging experiments showed that Tafi genes were not expressed during infection but were expressed once
Salmonella
was passed out of the mice into the feces. We hypothesize that the primary role of the rdar morphotype is to enhance
Salmonella
survival outside the host, thereby aiding in transmission.
