infantis resulted from a general increase in different biotypes or a successful spread of 1 clone .
infantis identified in israel during 20072009 ( 21 human sources , 28 poultry sources , and 22 food sources ) and 16 historical strains isolated during 19702005 ( 12 human sources , 2 poultry sources , and 2 food sources ) were subjected to pulsed - field gel electrophoresis ( pfge ) .
macrorestriction with the xbai enzyme discriminated the isolates into 23 distinct profiles ( pulsotypes ) , designated i1i23 .
although the historical isolates showed high diversity in their pfge patterns , most ( 58/71 , 82% ) recent ( 20072009 ) isolates were homogeneous and showed an indistinguishable pfge profile ( pulsotype i1 ) , which was not found among the historical isolates ( figure 2 ; table a1 ) .
these results indicate that most of the emerging isolates belong to 1 genetic clone that probably started to spread in israel sometime during 20052007 .
furthermore , comparison of the i1 pulsotype with other pfge profiles through pulsenet ( www.cdc.gov/pulsenet/ ) and pulsenet europe ( www.pulsenetinternational.org/networks/europe.asp ) indicated a pattern not reported elsewhere , suggesting the emerging clone is endemic to israel .
pulsed - field gel electrophoresis ( pfge ) patterns of salmonella enterica serovar infantis isolates from clinical , food , and poultry sources isolated in israel , 19702009 , showing a high degree of clonality .
macrodigestion performed using xbai restriction enzyme and genetic similarity ( in % ) was based on dice coefficients .
pfge was conducted according to the standardized salmonella protocol centers for disease prevention and control pulsenet as described ( 4 ) by using s. enterica ser .
a complete list is provided in table a1 . to further characterize the isolates , we performed susceptibility tests to 16 antimicrobial compounds .
overall , resistance to 11 antimicrobial agents was detected ( table ; table a1 ) .
first , although 6/16 ( 38% ) of the historical strains were sensitive to all tested antimicrobial agents and 5/16 ( 31% ) were resistant to only 1 ( nitrofurantoin ) , none of the 20072009 isolates were sensitive to all of the tested antimicrobial agents .
most ( 68/71 , 96% ) of the recent isolates were resistant to > 3 antimicrobial agents , which suggests a process of resistance acquisition over time .
second , whereas isolates from 19702005 did not share any obvious resistance pattern , most ( 66/71 , 93% ) of the 20072009 strains showed a combined resistance pattern to nalidixic acid , nitrofurantoin , and tetracycline with or without additional resistance to trimethoprim / sulfamethoxazole ( table ) .
the convergence of the recent serovar infantis clones to a dominant resistance pattern is consistent with their common pfge profile and shows that they share high similarity on phenotypic and genotypic levels . *
pfge , pulsed - field gel electrophoresis ; i1 , emerging pfge pattern ; d , different from the emerging pattern .
next , we characterized the molecular mechanisms responsible for the common antimicrobial drug  resistance phenotype . in bacteria ,
an efficient means of acquisition and dissemination of resistance genes is through mobile genetic elements such as plasmids , transposons , or integrons ( 5 ) .
plasmid analysis for 15 emerging ( 20072009 ) and 7 historical ( 19702005 ) randomly selected isolates demonstrated that all possessed 1 large plasmid of 100 kb .
to identify antimicrobial drug resistance genes that are possibly encoded on this plasmid , mating experiments were conducted with a plasmid - free , rifampin - resistant escherichia coli j53 strain and recent s. enterica ser .
conjugation experiments showed the obtained e. coli transconjugants received the large ( 100-kb ) plasmid and acquired the tetracycline resistance phenotype but remained susceptible to nalidixic acid and nitrofurantoin .
molecular analysis by pcr showed the teta gene encoded within the tn1721 transposon in 6 of 6 randomly selected emerging isolates but in only 1 of 5 older historical strains .
we examined class 1 integrons using pcr primers designed to amplify the variable region of class 1 integrons .
sulfamethoxazole followed by the orfc gene of unknown function . in contrast , 3/5 historical isolates did not possess any integron , and 2/5 contained a disparate integron with a variable region of 1.3 kb .
sequencing analysis indicated a different cassette encoded by the aminoglycoside adenyltransferase aada1 gene conferring resistance to spectinomycin and streptomycin .
resistance to quinolones is often associated with point mutations in the quinolone - resistance determining region of the gyra gene ( 6 ) . to examine this possibility , we determined the gyra sequence from 6 recent naladixic acid  resistant and 4 naladixic acid  sensitive isolates .
all resistant clones showed the same nucleotide substitution from guanine to thymine at position 259 ( g259 t ) in the gyra gene , resulting in the exchange of asparagine in position 87 to tyrosine ( asp87tyr ) in the quinolone resistance  determining region domain .
no mutations were found in the gyra sequence of the naladixic acid  sensitive isolates , suggesting that the asp87tyr point mutation is responsible for the observed naladixic acid  resistance phenotype .
it is likely that environmental selective pressure caused by use of antimicrobial drugs has led to the distribution of appropriate resistant genes .
nitrofurans and sulfonamides , for example , have been widely used to treat infections and promote growth of livestock ( 7 ) . because the emerging clone was dominant in all levels of the food chain , including broiler chickens , it is possible that the emerging clone was originally introduced from a poultry source .
recent studies from other countries identified healthy poultry as a potential reservoir of s. enterica ser .
infantis isolates from different sources and regions in israel showed high homogeneity of emerging isolates that differ genetically and phenotypically from previously isolated strains .
we showed that the emerging clone is multidrug resistant and is characterized by a large conjugative plasmid harboring the tn1721 transposone and teta gene , which provides reduced susceptibility to tetracyclines .
additional characteristics include a class 1 integron containing the dfra1 cassette , a gyra mutation that mediates nalidixic acid resistance and furthers resistance to nitrofurantoin .
our results suggest the recent emergence of serovar infantis is an outcome of a clonal expansion and establishment of a specific biotype that took place during a relatively short period .
 to determine whether rapid emergence of salmonella enterica serovar infantis in israel resulted from an increase in different biotypes or spread of 1 clone , we characterized 87 serovar infantis isolates on the genotypic and phenotypic levels . 
 the emerging strain comprised 1 genetic clone with a distinct pulsed - field gel electrophoresis profile and a common antimicrobial drug resistance pattern . 
The Study
Conclusions