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An Accessory Protein Is Required for Relaxosome Formation by Small Staphylococcal Plasmids. Matthew C. A. Smith, 2004.Mobilization of the staphylococcal plasmid pC221 requires at least one plasmid-encoded protein, MobA, in order to form a relaxosome . pC221 and closely related plasmids also possess an overlapping reading frame encoding a protein of 15 kDa, termed MobC . By completing the nucleotide sequence of plasmid pC223, we have found a further example of this small protein, and gene knockouts have shown that MobC is essential for relaxosome formation and plasmid mobilization in both pC221 and pC223 . Primer extension analysis has been used to identify the nic site in both of these plasmids, located upstream of the mobC gene in the sense strand . Although the sequence surrounding the nic site is highly conserved between pC221 and pC223, exchange of the oriT sequence between plasmids significantly reduces the extent of relaxation complex formation, suggesting that the Mob proteins are selective for their cognate plasmids in vivo . Transmission and Infectious Dose of Escherichia coli O157:H7 in Swine. N. A. Cornick, 2004.Escherichia coli O157:H7 is only occasionally isolated from healthy swine, but some experimentally infected animals will shed the organism in their feces for at least 2 months . Potential explanations for the paucity of naturally occurring infections in swine, as compared to cattle, include a lack of animal-to-animal transmission so that the organism cannot be maintained within a herd, a high infectious dose, or herd management practices that prevent the maintenance of the organism in the gastrointestinal tract . We hypothesized that donor pigs infected with E . coli O157:H7 would transmit the organism to naïve pigs . We also determined the infectious dose and whether housing pigs individually on grated floors would decrease the magnitude or duration of fecal shedding . Infected donor pigs shedding <104 CFU of E . coli O157:H7 per g transmitted the organism to 6 of 12 naïve pigs exposed to them . The infectious dose of E . coli O157:H7 for 3-month-old pigs was approximately 6 x 103 CFU . There was no difference in the magnitude and duration of fecal shedding by pigs housed individually on grates compared to those housed two per pen on cement floors . These results suggest that swine do not have an innate resistance to colonization by E . coli O157:H7 and that they could serve as a reservoir host under suitable conditions .
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