The epidemic character of community-associated methicillin-resistant Staphylococcus aureus, especially the geographically widespread clone USA300, is poorly understood. USA300 isolates carry a type IV staphylococcal chromosomal cassette mec (SCCmec) element conferring beta-lactam antibiotic class resistance and a putative pathogenicity island, arginine catabolic mobile element (ACME). Physical linkage between SCCmec and ACME suggests that selection for antibiotic resistance and for pathogenicity may be interconnected. We constructed isogenic mutants containing deletions of SCCmec and ACME in a USA300 clinical isolate to determine the role played by these elements in a rabbit model of bacteremia. We found that deletion of type IV SCCmec did not affect competitive fitness, whereas deletion of ACME significantly attenuated the pathogenicity or fitness of USA300. These data are consistent with a model in which ACME enhances growth and survival of USA300, allowing for genetic "hitchhiking" of SCCmec. SCCmec in turn protects against exposure to beta-lactams.

Much knowledge about microbial gene regulation and virulence is derived from genetic and biochemical studies done outside of hosts. The aim of this review is to correlate observations made in vitro and in vivo with two di¡erent bacterial pathogens in which the nature of regulated gene expression leading to virulence is quite di¡erent. The ¢rst is Vibrio cholerae, in which the concerted action of a complicated regulatory cascade involving several transcription activators leads ultimately to expression of cholera toxin and the toxin-coregulated pilus. The regulatory cascade is active in vivo and is also required for maintenance of V. cholerae in the intestinal tract during experimental infection. Nevertheless, speci¢c signals predicted to be generated in vivo, such as bile and a temperature of 37 8C, have a severe downmodulating e¡ect on activation of toxin and pilus expression. Another unusual aspect of gene regulation in this system is the role played by inner membrane proteins that activate transcription. Although the topology of these proteins suggests an appealing model for signal transduction leading to virulence gene expression, experimental evidence suggests that such a model may be simplistic. In Streptococcus pyogenes, capsule production is critical for virulence in an animal model of necrotizing skin infection. Yet capsule is apparently produced to high levels only from mutation in a two-component regulatory system, CsrR and CsrS. Thus it seems that in V. cholerae a complex regulatory pathway has evolved to control virulence by induction of gene expression in vivo, whereas in S. pyogenes at least one mode of pathogenicity is potentiated by the absence of regulation.

ABSTRACT. Maize White Spot (MWS), a foliar disease caused by Pantoea ananatis, could cause up to 60% yield loss. Some strains of P. ananatis harboring the ice nucleation gene inaA catalyze the formation of ice nuclei, causing tissue damage at temperatures slightly below freezing. Little is known about the relationship between the presence of the ina gene in this maize pathogen and its expression during the phenomenon of ice nucleus formation. Here, we attempted to verify the presence of the inaA gene and the expression of phenotype in vitro. The identity of the isolates and the presence of the inaA gene were determined by P. ananatis speciesspecific primers. The expression of the inaA gene was assessed in vitro by the visualization of ice-crystal formation in water at subzero temperatures. A total of ninety P. ananatis isolates from MWS lesions were characterized. The presence of the inaA gene was confirmed by gel electrophoresis of the 350-400-bp PCR products. The inaA primers did not lead to DNA fragment amplification in three isolates. The ice nucleation phenotype was expressed in 83.34% of the isolates carrying the inaA gene. Our study showed that the ice nucleation in P. ananatis isolated from MWS lesions was dependent on the presence of a functional ina gene in the genome. We also found evidence indicating that some P. ananatis strains have a mutated form of the inaA gene, producing a non-functional ice nucleation protein. This is the first report on inaA gene characterization in P. ananatis isolates from Maize White Spot.

ABSTRACT. Measures to control maize white spot (MWS) caused by Pantoea ananatis are preferentially based on resistant cultivars. A lack of knowledge on the genetic variability of pathogens could interfere with the development and utilization of controlling strategies in this pathosystem. The main goals of this study were to investigate the genetic variability of 90 P. ananatis isolates from three different eco-geographical regions of Brazil by amplified fragment length polymorphism (AFLP), and to determine the presence of a universal P. ananatis plasmid in isolates from tropical Brazil. Analysis of genetic similarity by AFLP allowed us to categorize the 90 isolates into two groups. However, no correlation between the collecting sites and genetic groupings was observed. The polymorphism percentage found in P. ananatis ranged between 24.64 and 92.46%, and genetic diversity was calculated to be 0.07-0.09. The analysis of molecular variance showed that 99.18% of genetic variability was within the populations, providing evidence that evolutionary forces were acting on these populations. All P. ananatis isolates showed the P. ananatis universal plasmid (280 or 352 kb). This is the first report on the presence of a universal P. ananatis plasmid from MWS lesions in the tropical area.