Characterization of Salmonella isolates from retail foods based on serotyping, pulse field gel electrophoresis, antibiotic resistance and other phenotypic properties

Sixteen Salmonella strains isolated from a variety of foods during 2000 and 2003 by the Florida State Department of Agriculture were characterized by various genotypic and phenotypic tests. Among 16 isolates, 15 different serotypes were identified. Pulse-field gel electrophoresis (PFGE) fingerprinting profiles obtained using restriction endonucleases XbaI and BlnI revealed that 16 Salmonella isolates were genetically diverse with 16 unique PFGE patterns. The PFGE pattern of eight isolates matched with the CDC/FDA data base of previous outbreaks and clinical isolates indicating their potential to cause disease. With the exception of isolates obtained from alligator meat (tetracycline resistant) and orange juice (chloramphenicol and sulfisoxazole resistant), the remainder of the isolates were susceptible to the panel of 15 antimicrobials tested. Molecular subtyping was further complimented by a variety of phenotypic tests such as acid-tolerance, Caco-2 cell invasion and biofilm formation which have often been used as a gauge of virulence and infection potential of Salmonella isolates. The induced acid tolerance level of the isolate obtained from orange juice was not significantly different from the laboratory reference strain S. enterica serovar Typhimurium SL1344. Six isolates exhibited very low levels of constitutive acid-tolerance, of which four isolates failed to infect differentiated Caco-2 cells. Although all isolates formed biofilms, there was no clear relation between the ability to form biofilms, infect differentiated Caco-2 cells and induce acid-tolerance. This study indicated that different serotypes of Salmonella were present in a variety of retail foods and exhibited diverse phenotypic characteristics.