Comparison of Genetic and Physiological Properties of Salmonella enterica Isolates from Chickens Reveals One Major Difference Between Serovar Kentucky and Other Serovars: Response to Acid Rolf D. Joerger, Casey A. Sartori, and Kalmia E. Kniel Abstract For unknown reasons, Salmonella enterica Kentucky has become the serovar most frequently isolated from chickens and chicken carcasses in the United States. In an attempt to identify traits that may underlie this phenomenon, genetic and physiological features of 30 serovar Kentucky chicken isolates were compared with those of chicken isolates belonging to a range of other S. enterica serovars. Most of the well-known Salmonella virulence genes were detected in the serovar Kentucky isolates by PCR, but the cdtB, spvB, spvC, and pefA genes were not found. The serovar Kentucky isolates were as invasive as the non-Kentucky isolates in in vitro assays involving chicken embryo hepatocytes, but were less invasive than the Enteritidis, Mbandaki, and Typhimurium isolates when incubated with human HCT-8 cells. Statistical comparison of growth, biofilm formation, and stress survival data from the serovar Kentucky and those from the serovar Enteritidis, Hadar, Mbandaka, Senftenberg, Typhimurium, and Worthington isolates demonstrated either no differences or differences with only a few of the serovars; however, three data sets were different. These data sets included the OD 600 values of cultures grown in tryptic soy broth (TSB) adjusted to pH 5.5 with acetic acid and survival counts of cells grown in either TSB pH 7 or TSB adjusted to pH 5.5 with acetic acid and then transferred into TSB adjusted to pH 2.5 with HCl. Most notable was the log 10 reduction for acetic acid pre-exposed Kentucky isolates of 3.1 versus <1 log 10 for the other isolates upon transfer to pH 2.5. The connection, if any, between this acid response phenotype and the prevalence of the serovar Kentucky in poultry remains to be elucidated, but it is possible that slightly better growth in the presence of acetic acid in conjunction with not mounting a strong, energy-consuming acetic acid–induced adaptive acid response provides a small competitive advantage to this serovar in low acid environments such as the cecum where the pH is around 5.5. Introduction I n recent years surveys have demonstrated an increase in Salmonella enterica serovar Kentucky isolation frequency from broilers and ground chicken, and this serovar is now the most common isolate from these animals and animal products (McCrea et al., 2006; Li et al., 2007; Parveen et al., 2007; USDA Food Safety and Inspection Service, 2007). The high-frequency occurrence in chickens is in stark contrast to the very low fre- quency of isolation of this serovar from humans. In fact, ser- ovar Kentucky is not among the top 30 serovars from human sources reported to the CDC and was found only 81 times among 36,184 human Salmonella isolates in 2005 (CDC, 2007). The reasons why S. enterica Kentucky has become such a prolific colonizer of chickens, and especially the cecum (Pedroso et al., 2007), but not a threat to human health like, for example, S. enterica Enteritidis or Typhimurium are not known. Obviously, serovar Kentucky must be deficient in genes important to causing human disease. At the same time, one or more traits must be present in this serovar that underlie its success as colonizer of chickens. One can hypothesize that these traits could include those facilitating specific host– bacterial interactions, but also those that enhance survival in modern poultry farm environments. Therefore, we deter- mined and compared some genetic properties of Kentucky and non-Kentucky Salmonella isolates from chickens, as well Department of Animal and Food Sciences, University of Delaware, Newark, Delaware. FOODBORNE PATHOGENS AND DISEASE Volume 6, Number 4, 2009 ª Mary Ann Liebert, Inc. DOI: 10.1089=fpd.2008.0144 503