we report on the prevalence of different antimicrobial - drug resistance profiles among danish salmonella ser . furthermore , we show results indicating that a dt104 outbreak strain lost resistance genes by shifting to the sgi1-b variant during the outbreak . consequently , the outbreak included isolates with both the acssut and the asu phenotype . typhimurium dt104 and 16 dt104b were obtained from persons with clinical disease in denmark . several outbreaks and clusters of dt104 the 323 isolates were characterized by mlva ( 97% of isolates ) , pfge ( 78% ) , and susceptibility testing ( 100% ) . susceptibility to a standard panel of antimicrobial agents ( www.danmap.org ) was determined by microbroth dilution and interpreted according to clinical and laboratory standards institute ( clsi ) guidelines ( 9 ) , except for ciprofloxacin ( > 0.125 g / ml was used as breakpoint ) . ( 10 ) and pfge , according to the pulsenet protocol ( 11 ) , were performed and analyzed as previously described ( 5 ) . to obtain a cluster - independent data set , we reduced the 323 isolates to 146 unique strains on the basis of the mlva and resistance profile ( i.e. , with different mlva and/or resistance profile ) ( table 1 ) . * one isolate per multilocus variable number of tandem repeats analysis ( mlva ) and antimicrobial resistance profile is represented in the data set of 146 strains with unique types . a , ampicillin ; c , chloramphenicol ; s , streptomycin ; su , sulfonamides ; t , tetracycline ; tm , trimethoprim . genotype determined by the sgi1-related pcr assays described in the text ( 1 isolate per  unique type  ) ; see table 2 for selected details . additional antimicrobial - drug resistances : nalidixic acid and ciprofloxacin ( 22 strains ) , amoxicillin / clavulanic acid ( 3 strains ) , gentamicin ( 1 strain ) . only a small subset of 10 acssut strains was tested by pcr for the presence of sgi1 genes . additional antimicrobial - drug resistances : nalidixic acid , ciprofloxacin , and gentamicin ( 1 strain ) . strains with phenotypes lacking resistance to > 1 of the antimicrobial agents ( acssut ) typical for sgi1 were further investigated for the presence of sgi1 variants . pcr was performed for detection of the sgi1 left and right junctions by using primers u7-l12 , lj - r1 , 104-rj , and c9-l2 ( 2 ) ; detection of the integron conserved segments ( 5-cs and 3-cs ) was done by using primers l1 and r1 ( 3 ) ; and detection of the antimicrobial resistance genes aada2 , qac / sul1 , flor , tet(g ) , and blap1 was performed by using the primers described by boyd et al . most dt104 and dt104b isolates displayed the phenotype acssut ( table 1 ) , which is compatible with the possession of sgi1 ; 122 ( 84% ) of the 146 strains were resistant to acssut with or without additional antimicrobial drug resistance that is regarded as independent of the sgi1 ( e.g. , flouroquinolones ) . three percent of the strains were additionally resistant to trimethoprim ( tm ) and therefore likely to possess the sgi1-a variant , and 6% of the strains were fully sensitive ( all of these were dt104 ) . the remaining strains had a variety of resistance profiles , including profiles that comply with the descriptions for variants sgi1-b and sgi1-c ( table 1 ) . this outbreak was shown to be caused by raw beef , served as carpaccio at a restaurant in a limited period of 4 weeks ( 6 ) . the isolates displayed variation with all applied typing methods : phage typing , mlva , pfge , and resistance profile ; 33 isolates were typical mdr dt104 ( acssut ) with 1 pfge and mlva profile . one isolate differed in phage type ( nontypeable ) and had a variant of the pfge profile that differed by 1 band . three isolates had different mlva types ( variation in 1 of the 5 loci ) ( 6 ) . detection of sgi1-related genes and regions by pcr showed that the acssut isolates were positive in all pcrs as expected for sgi1 ( table 2 ) , whereas the 3 asu isolates were positive for the left / right junction , but only 1 integron and pse-1 and qac / sul1 , in accordance with the phenotype and the sgi1-b variant , were positive ( 3,12 ) . the most likely mechanism for the creation of variants of sgi1 is considered to be homologue recombination between identical dna segments ( 12 ) . recombination could have taken place in the intestine of the infected animal , during processing or storage of the beef , or during infection of the patients . it is less likely that the outbreak was caused by multiple unrelated strains of dt104 because both the pfge and mlva types , including the variants , were distinctive and unique to this outbreak . * mlva , multilocus variable number of tandem repeats analysis ; pfge , pulsed - field gel electrophoresis . in addition to the 3 isolates related to the carpaccio outbreak , 21 human salmonella dt104 isolates in 20032006 had the phenotype asu . all of these had the same pfge profile ( worldwide , the typical dt104 pfge profile ) ( 13,14 ) but 3 different mlva profiles ( represented by 18 isolates , 2 isolates , and 1 isolate each ) . the cluster of 18 isolates had the phenotype asu and the mlva profile 133 ( table 2 ) , and most of these isolates were likely part of an outbreak related to pork from denmark ( december 2003march 2004 ) . another cluster of 3 isolates was received in the laboratory within a 3-day period in june 2003 from patients living in the same region . the phenotype of 2 of these isolates was asu , and 1 was acssut ( table 2 ) . no outbreak investigation was carried out at that time , and we can only speculate that this is a similar occurrence of shift in antimicrobial - drug resistance during an outbreak . a cluster of 9 isolates with mlva 443 occurred late in 2005 and in january 2006 ; most patients lived in a specific geographic area . eight of these isolates had the phenotype ssu ( equivalent to sgi1-c ) , and 1 isolate , which appeared late in the cluster , was assuttm . pcr confirmed the presence in all 9 mlva 443 isolates of the left and right junction , the typical sgi1 genes conferring resistance to ssu , and 1 cassette of class 1 integrons ( lacking the pse1-associated cassette ) ( table 2 ) . , it seems likely that the variation in phenotype was caused not by variation in sgi1 but rather by acquisition of antimicrobial - drug resistance genes independent of sgi1 ( e.g. , on plasmids ) . most of the salmonella dt104 isolates from patients in denmark had the typical acssut phenotype ; however , a small fraction ( 6% ) of dt104 isolates were fully sensitive and correspondingly negative for all sgi1-related genes and regions tested for by pcr . approximately 10% of the isolates had a different phenotype , and most of these had an antimicrobial - drug resistance profile corresponding to sgi1-a , -b , or c , which could be verified by pcr assays mapping parts of sgi1 . a number of clusters and outbreaks occurred in the study period , and shifts in antimicrobial - drug resistance phenotype and genotype seem to have taken place in several outbreaks . these shifts were most clearly seen in the carpaccio outbreak ( 6 ) , in which all evidence pointed at a single source outbreak , though the applied typing methods showed some variation among isolates , including a shift from sgi1 to sgi1-b in 3 of 40 isolates . we therefore conclude that variability of both genotypic and phenotypic characteristics can be expected during a foodborne outbreak ; this variability should be taken into consideration when defining outbreak - related cases .