This study examined S. Senftenberg isolates from humans and animals to assess the genetic relatedness of S. Senftenberg from various hosts. In total, 98 strains of S. Senftenberg from various locations in the United States associated with humans and animal hosts were assessed using PFGE, MLST and antimicrobial susceptibility analysis (NARMS).
Pulsed field gel (PFGE) analysis of the isolates found that most S. Senftenberg isolates examined had profiles that appeared to be unique to the individual strains; among the 98 strains tested 93 unique profiles were identified. Cluster analysis identified four primary clusters at approximately 58% similarity; with most clusters composed of ST 14 and a single cluster consisting of ST 185. It was evident that PFGE provided greater differentiation than MLST alone which would have created two clusters only. This observation was supported by the diversity indices which found that PFGE resulted in the greatest rate of diversity over MLST and antimicrobial susceptibility testing. Similar studies by our lab investigating S. Typhimurium found that PFGE provided greater differentiation for the strains than MLST alone . It has been suggested that housekeeping genes can be too conservative and greater differentiation may be possible by expansion of the panel to include virulence genes where inherent variation may be greater . In a recent study, Liu et al  used two virulence genes (sseL and fimH) and a clustered regularly interspaced short palindromic repeat loci (CRISPR) as an alternative MLST analysis for subtyping the major serovars of Salmonella enterica sub species enterica. The MLST scheme using only the two virulence genes corresponded well with the serotypes but failed to discriminate between outbreak strains. Incorporation of the CRISPR sequences enhanced the discriminatory power to differentiate at the strain outbreak level, suggesting that modification of the MLST can enhance differentiation ability.
Antimicrobial susceptibility analysis of all isolates found that the human strains were susceptible to all of the antimicrobials of the NARMS panel; in contrast, the animal isolates showed a range of resistances with most isolates being resistant to two or more antimicrobials. The rate of resistance to antimicrobials was somewhat similar across the host species (13 porcine, 11 bovine and 12 poultry) with 11 isolates displaying resistance to 6 or more agents. Further studies to determine the nature of the resistance observed is ongoing but it is possible that mobile genetic elements such as integrons may be responsible for some of the high resistance levels observed in porcine isolates .
Sequence analysis of the isolates found that the most common sequence type (ST) observed among all isolates were ST 14 and ST 185, one isolate identified as ST 145 was recovered from a pig. ST 14 isolates were the most common being found in S. Senftenberg of porcine, equine, bovine, turkey, feline, canine, and human origin. Comparison of our data with the MLST database indicates that ST 14 is relatively common in a range of hosts including poultry, soya, fishmeal, lizard, and humans (http://www.mlst.net). Of interest, this ST has been found worldwide and is included in the SARB collection . In contrast, the ST 185 isolates of this study were relatively unique and found only in a small collection of turkey, bovine, and human hosts. When compared with the MLST database, this strain type was not as common being found only in isolates associated with animal feed and humans and primarily among strains recovered in Europe.
While relatively little is known about S. Senftenberg, the organism does appear to be associated with human disease and has been found to persist in feed, and feed materials in feed factories as well as poultry, poultry farms and the processing environment [8, 40–44]. Among CDC data, S. Senftenberg appears to be primarily associated with non-human clinical disease however, the organism has been associated with human illness and with a range of foods including fennel seed tea, nuts, herbs, baby cereal, poultry, and cattle and most recently spices [8, 45, 48–50, 52] and appears to be emerging in plant and plant products [46, 47, 51]. One of the limitations of this study is that traits and characteristics of S. Senftenberg have only been assessed in animal and human isolates and it is unknown if these observations hold true for isolates of plants (herbs, spices etc.). It is also interesting to speculate as to the nature of S. Senftenberg as it appears to be an emerging strain in human illness and animals as both a commensal but possibly also as an opportunistic pathogen. Ongoing analyses in our lab may clarify further the nature and pathogenesis of this serotype.