Wednesday, October 7, 2009

Antimicrobial Resistance-Conferring Plasmids with Similarity to Virulence Plasmids from Avian Pathogenic Escherichia coli Strains in Salmonella enterica Serovar Kentucky Isolates from Poultry.

W. Florian Fricke, Patrick F. McDermott, Mark K. Mammel, Shaohua Zhao, Timothy J. Johnson, David A. Rasko, Paula J. Fedorka-Cray, Adriana Pedroso, Jean M. Whichard,
J. Eugene LeClerc, David G. White, Thomas A. Cebula, and Jacques Ravel

Salmonella enterica
is a common cause of food –borne gastroenteritis. This combined with the rise of multidrug resistant S. enterica isolates is a grave medical concern. The S enterica subsp. enterica serovar Kentucky is the most common serotype found in chickens [1,2]. Moreover this serotype is often found to be resistant to antibiotics such as tetracycline and streptomycin [2]. The goal of the study was to find clues to the development of multi-drug resistance in S. Kentucky. The authors analyzed the complete sequences of the 3 large plasmids (pCVM29188_146, pCVM29188_101, pCVM29188_46) that they isolated from S. Kentucky CVM29188. Only the two large plasmids (pCVM29188_146 and pCVM29188_101) were found to carry antibiotic resistance genes. Thus, genes coding for resistance to aminoglycosides (strAB) and tetracyclins (tetRA) were found on pCVM29188_146 and those coding for resistance to cephalosporins (bla CMY-2) were found on pCVM29188_101. Both resistance plasmids (pCVM29188_101 and pCVM29188_146) in this study were found to have intact transfer regions, while the smaller plasmid pCVM29188_46 (46kb) did not have any transfer genes. Sequence similarity of the replication and transfer genes to other plasmids suggest that plasmid pCVM29188_101 may belong to the IncI1 group while plasmid pCVM29188_46 may belong to the IncFII group. The backbone of plasmid pCVM29188_146 is very similar to two plasmids isolated from avian pathogenic E. coli strains and also have the same virulence factors. The plasmid pCVM29188_46 has little similarity to other plasmids and has a lot of hypothetical proteins. They next conducted mating experiments with plasmids pCVM29188_146 and pCVM29188_101 and showed their transfer to two strains of Salmonella and a strain of E. coli. Next they wanted to test the abundance of the virulence genes on other isolates of S. Kentucky from meat, clinical and agricultural sources. So they screened 287 S. Kentucky isolates for the presence of virulence genes by PCR with primers specific for the 5 loci of pCVM29188_146 that were responsible for encoding virulence factors. They found that 64% of all S. Kentucky strains tested positive for the presence of at least one locus associated with virulence. The association was even stronger among the S. Kentucky strains that were isolated from chicken. This however was not the case for the 6 other Salmonella serovars that were isolated from chicken Moreover, all S. Kentucky strains that had at least one virulence locus of Pcvm29188_146 also had resistance to tetracycline and a subgroup of these had resistance to streptomycin. This suggests that a strong association may exist between S. Kentucky and plasmids like pCVM29188_146 that encode both virulence factors and resistance to antibiotics such as tetracycline and streptomycin. One explanation that the authors offer for this association is that pathogenic E. coli strains bearing virulence plasmids may have encountered resistance plasmids, leading to integration of the resistance genes into the virulence plasmid. This new plasmid could have transferred into a Salmonella strain such as the S. Kentucky commonly found in chickens. They do acknowledge that this does not explain why other Salmonella strains isolated from chicken do not have this plasmid type. The second explanation is that virulence plasmids may help S. Kentucky in coping with stress or other enterobacteria and hence, the association. This again does not explain why the association is only seen in S. Kentucky isolated from chicken.
This is an interesting study of plasmids from a strain of bacterium that has medical relevance to us. It is indeed surprising that there is such a clear association of the virulence and antibiotic resistance encoding plasmid with the S. Kentucky strain isolated from chicken. Their explanations for the association seem a little weak.


1. FDA. 2008. National Antimicrobial Resistance Monitoring System for Enteric Bacteria (NARMS). Retail meat annual report, 2006. FDA, Bethesda MD.
2. USDA. 2008. National Antimicrobial Resistance Monitoring System for Enteric
Bacteria (NARMS). Veterinary isolates final report, slaughter isolates,
2006. USDA, Washington, DC.

Diya Sen
University of Idaho

Monday, October 5, 2009

Mobile Microenvironments

Horizontal Transfer of the Tetracycline Resistance Gene tetM Mediated by pCF10 Among Enteroccus faecalis in the House Fly Alimentary Canal
Mastura Akhtar, Helmut Hirt, Ludek Zurek
Environmental Microbiology

Vectors have largely aided the spread of microorganisms. Vectors move bacteria from one place to another as they themselves go about their life cycle. The movements of a house fly would be a prime example of this type of vector. However, a role of the vector not considered as often is its role as a habitat for a bacterium itself. During transport, or as a permanent environment, bacteria encounter a unique combination of other bacteria and nutrients that only the vector could assemble. This provides for a microenvironment that can play a key role in the evolution of and dissemination of traits beneficial to bacterial species. While inside the fly these traits can be transferred horizontally through conjugation, transduction and transformation. This study considers how plasmid mediated horizontal gene transfer in the gut of a house fly can mediate tetracycline resistance to transfer between bacterial species.
Two strains of enterocci, bacteria normally residing in the gastrointestinal tract, were selected for donor and recipient. The donor contained the tetM gene to identify transformants using selective media. Flies were separated into two groups, one that received the donor first, via infected food supply, and the other received the recipient strain first. After twelve hours flies were given food source containing the opposite strain for one hour. Each group was then subdivided so that while flies were checked for the presence of donor, recipient and transformants over the next five days and half would have their eating appendage sterilized and half would not.
Results showed that regardless of whether the donor or recipient was introduced first, both groups established concentrations of donor and recipient cells that were similar. The was also no statistical difference in rate of gene transfer between the two groups. Concentration of donor and recipient cells in the digestive tract was similar to that of the surface sterilized, suggesting that observed donor, recipient and transformants were localized to the gut.
Transformants began to be detected 24 hours after both stains were combined. Their presence was screened for using selective media. Groups of flies were sterilized at the surface to eliminate the possibility of surface contamination and transformation outside the vector. Portions of the food supplied to the flies were periodically screened for transformants with very little occurrence, suggesting this did not play a key role. However, it is possible that transfer is taking place on the eating appendage. The conditions in which intestinal gene transfer are best suited are not well understood. The final possible explanation could be that transformants could be the product of high plasmid transfer rate and subsequent rapid clonal expansion of transformants.
Horizontal transfer in a vector could lead to the spread of various genes and provide a unique microenvironment for evolution. The house fly’s unique combination of contact with decaying organic matter and food provides ample opportunity for transfer of traits between bacteria evolved to live in harsh environments to those that are common in food. This potential introduces a need to better understand horizontal gene transfer and evolution in microenvironments that can directly affect humans.

Brian Lohman
University of Idaho