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.Escherichia coli O157:H7 is a pathogen causing serious disease that survives in a variety of hosts and environments, many of which are part of our food production system (24). Multilocus variable-number tandem-repeat analysis (MLVA) is an effective, high-throughput typing method that measures hypervariable tandem repeats (TRs) at various locations in the genome (8,13,16,22). However, any change in the rate of hypervariability in these TRs is problematic when trying to predict the phylogenetic relationships among isolates. Alterations in mutation rates due to stress in several studied systems have been previously reported (2,6,9,10,15,20,25,28) and recently reviewed (18). Mutation rate changes likely occur through the induction of a general-stress-response sigma factor (rpoS), leading to genome-wide instability through the activity of a strand-slippage mechanism (SSM) during replication (30) and/or to reduced fidelity of repair after DNA damage (SOS response and mismatch repair) (7, 21). Mutation rates have been shown to differ between MLVA loci (13,16,22) and between strains (31). Therefore, we hypothesized that environmental stress changes the mutation rate at MLVA TRs.Recovery of bacteria stressed in natural environments. Twenty-eight isolates of E. coli O157:H7 strain RM1484 (apple juice outbreak strain) were recovered from Arabidopsis thaliana seed about 60 days after 28 independently contaminated plants were planted as previously described (4). Hence, the bacteria survived in the phyllosphere during growth and maturation of the plant and seed. Ninety-six RM1484 isolates were also recovered from creek water (CW) after incubation for 7 days at 15°C. Each strain was analyzed by MLVA for 11 loci as previously published (Table 1) (3). No differences were found at any of the 11 loci in the recovered isolates compared to RM1484 (Table 2). These results are consistent with previous results that showed Erwinia amylovora repeat structures to be stable after log-term storage in sterile water at 25°C and passage in plants (26). Additionally, strain RM1484 was recovered from soil after growth of contaminated lettuce for 4 weeks at either 15°C or 23°C; the soil was then allowed to remain fallow and dry for an additional 4 weeks. One isolate from the 23°C soil and two isolates from the 15°C soil contained TR insertions (Table 2). Low nutrient levels in the phyllosphere or CW correlated with decreased DNA replication compared to the rhizosphere results (1). Detection of TR changes exclusively in rhizosphere isolates and not in phyllosphere and CW isolates substantiates the conclusion that replicat...

Multiple-Locus Variable-Number Tandem-Repeat Analysis in Genotyping Yersinia enterocolitica Strains from Human and Porcine Origins

“…The generated variations in one or more VNTRs might cause the MLVA results for identical clones to appear different (19). In addition, environmental stress can affect the stability of tandem repeats (31). Most of the mutations in VNTR units were shown to be single-repeat differ- ences (32), but multiple-repeat mutations also occur.…”

Effect of ultrasound on bioconversion of isoflavones and probiotic properties of parent organisms and subsequent passages of Lactobacillus

“…Our previous study has demonstrated that ultrasound, especially at 60 W for 3 min, significantly enhanced membrane permeability, which subsequently promoted the growth of lactobacilli and bifidobacteria and bioconversion of isoflavones in mannitolesoymilk (Yeo & Liong, 2011). Past study has demonstrated that bacterial cells expose to stressful condition may possibly exhibit heritable changes and thus the altered characteristics would also be prevalent in subsequent passages of cells (Cooley, Carychao, Nguyen, Whitehand, & Mandrell, 2010). However, the inheritance of beneficial effect by subsequent passages of ultrasound-treated cells remains unclear.…”

Evaluating the impacts of stressors of Pseudomonas syringae pathovar tomato on the effectiveness of multi-locus variable number tandem repeat analysis and multi-locus sequence typing in microbial forensic investigations

“…However, because MLVA relies on genetic loci having intrinsically high mutation rates [9], VNTR loci can be affected by treatments such as environmental stress and serial passaging, leading to alteration in the MLVA fingerprints of organisms of interest [9,13,14]. For this reason, a better understanding of the stability and mutational rates of VNTR loci is needed to permit the interpretation of MLVA results in forensic investigations [15,7].…”