Variation in stress resistance patterns among stx genotypes and genetic lineages of shiga toxin-producing Escherichia coli O157

Abstract

To evaluate the relationship between bacterial genotypes and stress resistance patterns, we exposed 57 strains of Shiga toxin-producing Escherichia coli (STEC) O157 to acid, freeze-thaw, heat, osmotic, oxidative, and starvation stresses. Inactivation rates were calculated in each assay and subjected to univariate and multivariate analyses, including principal component analysis (PCA) and cluster analysis. The stx genotype was determined for each strain as was the lineage-specific polymorphism assay (LSPA6) genotype. In univariate analyses, strains of the stx(1) stx(2) genotype showed greater resistance to heat than strains of the stx(1) stx(2c) genotype; moreover, strains of the stx(1) stx(2) genotype showed greater resistance to starvation than strains of the stx(2) or stx(2c) genotypes. LSPA6 lineage I (LI) strains showed greater resistance to heat and starvation than LSPA6 lineage II (LII) strains. PCA revealed a general trend that a strain with greater resistance to one type of stress tended to have greater resistance to other types of stresses. In cluster analysis, STEC O157 strains were grouped into stress-resistant, stress-sensitive, and intermediate clusters. In stx genotypes, all strains of the stx(1) stx(2) genotype were grouped with the stress-resistant cluster, whereas 72.7% (8/11) of strains of the stx(1) stx(2c) genotype grouped with the stress-sensitive cluster. In LI strains, 77.8% (14/18) of the strains were grouped with the stress-resistant cluster, whereas 64.7% (11/17) of LII strains were grouped with the stress-sensitive cluster. These results indicate that the genotypes of STEC O157 that are frequently associated with human illness, i.e., LI or the stx(1) stx(2) genotype, have greater multiple stress resistance than do strains of other genotypes.

Fig 1

The results of cluster analysis in biplots by principal component analysis (PCA). Distances between plots represent the similarities between strains. Each stx genotype (A) and LSPA6 lineage (B) of STEC O157 corresponds to a color, as shown. Plots located inside the circle belong to each cluster: red, cluster 1; green, cluster 2; blue, cluster 3. The distance between plots shows the similarity of their stress resistance patterns. PCA shows that higher values on the x axis indicate greater multiple stress resistance to six stresses and higher values on the y axis indicate greater resistance to acid, heat, and starvation stresses and lower resistance to freeze-thaw, osmotic pressure, and oxidative stresses.

Fig 2

Associations between clusters and stx genotypes (A) and LSPA6 lineages (B) of STEC O157. Cluster 1 and cluster 3 were characterized as stress-resistant and stress-susceptible groups, respectively. Stress resistances of strains in cluster 2 varied depending on the stress.