Effects of environmental stress on stability of tandem repeats in Escherichia coli O157:H7

Abstract

Multilocus variable-number tandem-repeat analysis (MLVA) is used for source tracking Escherichia coli O157:H7 in agricultural environments. Tandem repeats were stable after limited replication but changed after exposure to irradiation, elevated temperatures, and starvation conditions. The pO157 plasmid was frequently lost under these stress conditions. Environmental stresses may increase phylogenetic diversity as measured by MLVA.

FIG. 1.

Mutation rate of strain RM1484 after exposure to different stress conditions. Ninety-six cell lines were propagated serially at different temperatures, or after exposure to the indicated stress condition, for a combined total of 20,000 generations. The numbers of mutations are indicated as single, double, triple, and quadruple step, depending on the size of the mutation. An adjusted total is indicated that takes into consideration the larger mutations. The sum of the mutation rates (Combined) determined for each separate stress component (UV-C [orange bar], 43°C [pink bar], and starvation [blue bar]) is compared with the mutation rate determined in the triple-stress experiment. Statistical analysis used a generalized linear model fitted to adjusted totals with the GENMOD procedure according to the instructions provided by the SAS Institute, Inc. (11, 17). A Poisson distribution, data appropriate for counting, and a probability level of 0.05 were employed in testing differences for significance determinations (indicated by letters above the bars). Bars with the same letter represent results that were not significantly different.