the bacterial isolates were collected throughout finland during 19982007 as described ( 10 ) . of bionumerics version 5.1 software ( applied maths , kortrijk , belgium ) was used for sequence assembly . allele numbers , sequence types ( sts ) , and clonal complexes ( ccs ) were assigned by using the pubmlst database ( 5 ) . a serum sensitivity assay was conducted with 73 c. jejuni isolates according to a described protocol ( 8) . the same pool of serum samples from 10 healthy blood donors was used in all experiments . c. jejuni all statistical analyses were performed by using graphpad prism version 4.03 ( graphpad software , san diego , ca , usa ) and pasw statistics version 18 ( spss inc . , chicago , il , usa ) . a total of 72 c. jejuni isolates from blood were successfully typed by mlst ; 1 isolate had a mixed mlst pattern . five isolates were in unassigned sts , and the rest were distributed among 11 ccs ( table ) . genetic relatedness of these isolates was further confirmed by using pulsed - field gel electrophoresis . * mlst , multilocus sequence typing ; st , sequence type ; nd , none detected . however , bacteremia episodes caused by st-677 cc isolates were exclusively diagnosed during the seasonal peak during may  august ( figure 1 ) . of c. jejuni blood culture isolates detected during may  august , most ( 64% ) were st-677 cc . furthermore , st-677 cc was the most prevalent complex in 4 geographic regions of finland . annual and seasonal distribution of 72 camplyobacter jejuni blood culture isolates belonging either to the st-677 clonal complex ( cc ) or to the other multilocus sequence typing ( mlst ) ccs . one isolate with a mixed multilocus sequence type was not included . c. jejuni bacteremia was diagnosed during may  august ( m  a ) or during any other month of the year ( o ) . susceptibility to human serum varied between c. jejuni isolates from different ccs ( figure 2 ) . st-677 cc isolates were significantly less susceptible to human serum than all other isolates ( p<0.0001 ) . st-45 cc isolates were significantly more susceptible to human serum than all other isolates ( p<0.0001 ) . percentage of surviving bacteria in human serum for 73 blood culture isolates of campylobacter jejuni ( cj ) , grouped according to major multilocus sequence typing clonal complexes ( ccs ) , and for controls c. jejuni cj11168 and c. fetus . we characterized a unique collection of 73 c. jejuni isolates from blood obtained during a nationwide study in finland over a 10-year period . despite the high population diversity of c. jejuni , nearly half of the isolates from blood showed clustering within the st-677 cc , a rare cc in other countries ( 12,13 ) . thus , invasiveness of blood culture isolates could not be solely explained by their serum resistance , although the predominant isolates of st-677 cc were more serum resistant than other isolates . c. jejuni has high st diversity . as of may 2 , 2013 , a total of 6,564 sts were registered ( 5 ) . in this study , we detected clustering of c. jejuni isolates from blood in an uncommon st-677 cc . further studies are needed to clarify whether bacterial characteristics might explain this finding . in our previous study , which included human fecal c. jejuni isolates obtained in finland from the mid-1990s through 2007 , which is nearly the same period as in the current nationwide study , 11.7% of the isolates belonged to st-677 cc ( 11 ) . the 2 most prevalent ccs in that study , st-45 cc ( 43.6% of fecal isolates ) and st-21 cc ( 19.4% of fecal isolates ) , were detected only among 12 ( 16% ) and 10 ( 14% ) of blood culture isolates , respectively , in the present study . st-45 cc and st-21 cc have been shown to be prevalent in several countries ( 4,13 ) . however , our results suggest that these 2 ccs are not common among c. jejuni isolates from blood in finland , which cluster more in the st-677 cc . on the basis of the present results , we speculate that st-677 cc might have a special invasive capability or has adapted to the environment in finland . in general , complement - mediated killing of serum - susceptible isolates plays a major role in restricting access of pathogens to the bloodstream . however , available information about possible serum sensitivity of c. jejuni isolates from blood is scarce ( 8,9 ) . in our study of nonselected c. jejuni isolates from blood , susceptibility to human serum varied according to mlst cc . in conclusion , in this nationwide study during a 10-year period in finland , we found by mlst analysis that half of the bacteremia isolates of c. jejuni clustered within an otherwise uncommon st-677 cc . whether this finding indicates special adaptation of st-677 cc to finland or to the human bloodstream our findings emphasize the role of using well - defined clinical materials in studies on bacterial pathogenicity and severity of human disease .
campylobacter jejuni bacteria are highly diverse enteropathogens . 
 seventy - three c. jejuni isolates from blood collected in finland were analyzed by multilocus sequence typing and serum resistance . 
 approximately half of the isolates belonged to the otherwise uncommon sequence type 677 clonal complex . 
 isolates of this clonal complex were more resistant than other isolates to human serum .