Our data indicate that Salmonella Cerro strains circulating in the US represent a clonal subtype. The vast majority of cattle isolates, most isolates from other domestic animals, and all human isolates in our study shared a single XbaI PFGE pattern (CU-213), while nearly all other PFGE patterns differed by only one band from this predominant PFGE pattern. Moreover, all isolates were susceptible to all antimicrobial drugs tested, again supporting the hypothesis of one clonal subtype. While further studies are clearly needed, the absence of antimicrobial resistance genes among the 237 tested isolates might indicate that limited antimicrobial selection pressures have so far acted on this serotype, which might be congruent with a predominantly low virulence of this serotype. Our data might further suggest that host or environmental factors, rather than genetic changes, contributed to the recent increase in Salmonella Cerro prevalence. However, genetic changes not detectable by XbaI PFGE or a combination of host and pathogen factors might have also caused the increase in prevalence, and further studies are clearly needed.
Cerro being extremely common among dairy cattle in the US, human cases are rarely associated with this serotype. We found though that all three available human Salmonella
Cerro isolates had the same XbaI
PFGE pattern that was predominant among dairy cattle isolates. It is thus tempting to speculate that at least some Salmonella
Cerro strains have some capacity to cause human disease. However, specific virulence characteristics remain to be elucidated. While it is tempting to speculate that the rare occurrence of human Salmonella
Cerro cases may be due to reduced virulence of this subtype, scarcity of exposure cannot be excluded as one determining cause. All tested isolates lacked spvA
, a generally plasmid-mediated virulence factor found among some strains of certain, predominantly host adapted, serotypes such as Abortusovis, Dublin, or Typhimurium (Chu and Chiu, 2006
; Gassama et al., 2006
; Guney et al., 1994
). Still, absence of this virulence gene is unlikely to explain the rare association of serotype Cerro with human cases.
Importantly, our data also suggest that genetic diversity in contemporary US Salmonella
Cerro isolates is low or cannot be adequately captured by standard XbaI
PFGE. The development and validation of improved subtyping methods, an integral component of veterinary epidemiology, is clearly needed to allow further studies of this potentially emerging serotype (Belkum et al., 2007