Suppression of Francisella tularensis growth in the rat by co-infection with F. novicida.

We have previously shown that when cultured in vitro, peritoneal rat macrophages infected with Francisella novicida spontaneously release nitric oxide in sufficient quantities to inhibit bacterial growth. However, it is not known whether F. novicida can have a similar antimicrobial effect in vivo. Here we show that a co-infection of F. novicida with Francisella tularensis can suppress the number of F. tularensis cells in rat spleens by as much as 100-fold.

Phase variation in Francisella tularensis affecting intracellular growth, lipopolysaccharide antigenicity and nitric oxide production.

Many microbial pathogens, such as Mycobacterium spp. and Salmonella spp., use macrophage intracellular growth or antigenic variation as mechanisms for avoiding the host immune system. In this work we present evidence to show that the intracellular pathogen Francisella tularensis uses phase variation to alter antigenicity and the host macrophage nitric oxide response simultaneously, thereby modulating its intracellular growth. The lipopolysaccharide (LPS) and lipid A of F. tularensis fails to stimulate production of significant levels of nitric oxide (NO) by rat macrophages. However, spontaneous variants of F. tularensis expressing an antigenically distinct LPS induce rat macrophages to produce increased levels of NO, thereby suppressing microbial intramacrophage growth. Similarly, lipid A isolated from these variants stimulates increased levels of NO production. A reverse phase shift can occur, which returns the LPS to the original antigenic form, reduces NO production, and restores intramacrophage growth. These findings represent the first demonstration of a phase-variation phenomenon which modulates intracellular growth and an innate immune response. Furthermore, these results suggest that a microbial pathogen can exploit macrophage NO production for its own benefit, perhaps by prolonging the host-pathogen association during the acute phase of disease or during the process of establishing a carrier state.

Serum-sensitive mutation of Francisella novicida: association with an ABC transporter gene.

Francisella novicida is a facultative intracellular pathogen that can survive and grow in macrophages by preventing phagolysosomal fusion. In this study in vitro cassette mutagenesis was used to generate a library of insertion mutants of F.novicida. Two related mutants, KM14 and KM14S, initially identified as defective for growth in macrophages, were found to be sensitive to serum. These mutants were also found to grow approximately 1000-fold less well in the livers and spleens of infected mice. We cloned a genetic locus that was presumably mutagenized in these mutants and found that it included genes that had high similarity in their deduced amino acid sequence to those of msbA and orfE of Escherichia coli. The former is a member of the superfamily of ABC transporter proteins. We named the corresponding genes in F. novicida, valAB. Integration of a cloned valAB locus into the chromosome of KM14S partially restored the serum resistance phenotype found in wild-type F. novicida.

Isolation of ribosomal subunits containing intact rRNA from human placenta: estimation of functional activity of 80S ribosomes.

We have elaborated a method for the isolation of ribosomal subunits from fresh unfrozen human placenta containing intact rRNA and a complete set of ribosomal proteins. Activity of 80S ribosomes obtained by reassociation of 40S and 60S subunits in nonenzymatic poly(U)-dependent binding of Phe-tRNA(Phe) was equal to 80% (above 1.5 mol [14C]Phe-tRNA(Phe) is coupled to 1 mol of ribosomes). The activity of 80S ribosomes in poly(U)-directed synthesis of polyphenylalanine was tested in a polysome-free protein-synthesizing system from rabbit reticulocytes. About 100 mol of phenylalanine residue was polymerized by a mole of ribosomes at a rate of 0.83 residues per minute in this system (2 h, 37 degrees C).