R-plasmid RP1 increases sensitivity of Proteus mirabilis to normal body defences.

The bactericidal action of non-immune whole blood on Proteus mirabilis was increased when the bacteria contained the R-plasmid RP1. This effect was due mainly to increased phagocytosis. Iron-depleted stationary-phase cells were more sensitive than carbon-depleted cells. The contribution of serum was usually negligible but was increased during a minor non-specific infection. Most plasmid-containing phenotypes were more sensitive than were those without plasmids but there were considerable differences between stationary and exponentially-growing cells. The R-plasmid-mediated increase in sensitivity to phagocytosis may be due in part to the presence of additional glycosylated proteins in the outer membrane.