Thermosensitive Nature of IncHI1 Plasmid Transfer

The report by Phan and colleagues (3) continues to highlight the importance of IncHI1 plasmids in the spread of antibiotic resistance in Salmonella enterica serovar Typhi by analyzing variability in a collection of plasmids isolated between 1970 and 2004. The authors state correctly that IncHI1 plasmids transfer at higher frequencies (3 to 4 logs) at ambient temperatures (27°C and below) than at an in vivo temperature (37°C). It was disappointing to read that they attributed this to a personal communication only. In fact, thermosensitive transfer of H plasmids, including those now known as IncHI1 plasmids, was first reported by Smith in 1974 (4). Subsequently, my laboratory showed that conjugation depended on the incubation temperature of the mating mixture, but the growth temperature of the donor was also important, whereas donor cells grown at 26°C could not facilitate transfer at 37°C (5). Transmission electron microscopy of H pili after labeling with an H-pilus-specific bacteriophage (1) was used to demonstrate that H pili, encoded by the IncHI1 plasmid R27, were extended by extrusion from the cell surface and not by addition of pilin subunits to the pilus tip. The relatively slow synthesis of H pili, which require at least 15 min for maturation at 27°C, enabled us to document this critical finding (1). We proposed earlier that the thermosensitive nature of IncHI1 plasmid transfer makes these plasmids potential vectors in the dissemination of antibiotic resistance among pathogenic and indigenous bacteria in soil and water environments (2, 6). This notion is supported by the recent studies of Phan et al. (3).