Correlation of phenotype with the genotype of egg-contaminating Salmonella enterica serovar Enteritidis

Abstract

The genotype of Salmonella enterica serovar Enteritidis was correlated with the phenotype using DNA-DNA microarray hybridization, ribotyping, and Phenotype MicroArray analysis to compare three strains that differed in colony morphology and phage type. No DNA hybridization differences were found between two phage type 13A (PT13A) strains that varied in biofilm formation; however, the ribotype patterns were different. Both PT13A strains had DNA sequences similar to that of bacteriophage Fels2, whereas the PT4 genome to which they were compared, as well as a PT4 field isolate, had a DNA sequence with some similarity to the bacteriophage ST64b sequence. Phenotype MicroArray analysis indicated that the two PT13A strains and the PT4 field isolate had similar respiratory activity profiles at 37 degrees C. However, the wild-type S. enterica serovar Enteritidis PT13A strain grew significantly better in 20% more of the 1,920 conditions tested when it was assayed at 25 degrees C than the biofilm-forming PT13A strain grew. Statistical analysis of the respiratory activity suggested that S. enterica serovar Enteritidis PT4 had a temperature-influenced dimorphic metabolism which at 25 degrees C somewhat resembled the profile of the biofilm-forming PT13A strain and that at 37 degrees C the metabolism was nearly identical to that of the wild-type PT13A strain. Although it is possible that lysogenic bacteriophage alter the balance of phage types on a farm either by lytic competition or by altering the metabolic processes of the host cell in subtle ways, the different physiologies of the S. enterica serovar Enteritidis strains correlated most closely with minor, rather than major, genomic changes. These results strongly suggest that the pandemic of egg-associated human salmonellosis that came into prominence in the 1980s is primarily an example of bacterial adaptive radiation that affects the safety of the food supply.

FIG. 1.

Ribotype patterns of wild-type and biofilm-forming S. enterica serovar Enteritidis PT13A strains. Lane 1 contained digoxigenin-labeled markers with relative molecular weights of 23, 9.4, 6.5, 4.3, 2.3 and 2.0 kb (from top to bottom). Lanes 2 to 4 and lanes 6 and 7, wild-type S. enterica serovar Enteritidis PT13A strain (ESQRU accession numbers 21046, 21045, 21044, 21042, and 21041, respectively); lane 5, S. enterica serovar Typhimurium (ESQRU accession number 21043); lanes 8 to 11, biofilm-forming S. enterica serovar Enteritidis PT13A strain (ESQRU accession numbers 21030, 21029, 21028, and 21027, respectively).

FIG. 2.

Use of curvilinear analysis for detection of RA profile differences. Curvilinear RA profiles were obtained by application of the 4th polynomial (Slide Write Plus graphing function) to data in Table 4. (A) Results of Phenotype MicroArray analysis obtained from growth of S. enterica serovar Enteritidis at 25°C; (B) results obtained from growth at 37°C. ▪, S. enterica serovar Enteritidis PT4 field isolate; ▴, wild-type S. enterica serovar Enteritidis PT13A strain; •, biofilm-forming S. enterica serovar Enteritidis PT13A strain. The x axis indicates RA categories as follows: 1, inhibited; 2, low; 3, significant; 4, stimulated. The parameters used for setting RA categories are described in Table 2.