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Turn Up the Heat — Food and Clinical Escherichia coli Isolates Feature Two Transferrable Loci of Heat Resistance

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ABSTRACT

Heat treatment is a widely used process to reduce bacterial loads in the food industry or to decontaminate surfaces, e.g., in hospital settings. However, there are situations where lower temperatures must be employed, for instance in case of food production such as raw milk cheese or for decontamination of medical devices such as thermo-labile flexible endoscopes. A recently identified locus of heat resistance (LHR) has been shown to be present in and confer heat resistance to a variety of Enterobacteriaceae, including Escherichia coli isolates from food production settings and clinical ESBL-producing E. coli isolates. Here, we describe the presence of two distinct LHR variants within a particularly heat resistant E. coli raw milk cheese isolate. We demonstrate for the first time in this species the presence of one of these LHRs on a plasmid, designated pFAM21805, also encoding type 3 fimbriae and three bacteriocins and corresponding self-immunity proteins. The plasmid was highly transferable to other E. coli strains, including Shiga-toxin-producing strains, and conferred LHR-dependent heat resistance as well as type 3 fimbriae-dependent biofilm formation capabilities. Selection for and acquisition of this “survival” plasmid by pathogenic organisms, e.g., in food production environments, may pose great concern and emphasizes the need to screen for the presence of LHR genes in isolates.

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Heat resistance assays of E. coli wild-type strains, FAM21805 mutants, and K-12 MG1655 transconjugants. Relative survival compared to time point 0 of strains incubated at 55°C for 15, 30, and 45 min. (A) FAM21805 wild-type, -ΔLHR1, -ΔLHR2, and ΔLHR1ΔLHR2. (B) FAM21805 wild-type, K-12 MG1655 wild-type, and K-12 MG1655 transconjugants harboring LHR2 or both LHRs of FAM21805. (C) C604-10, K-12 MG1655 wild-type, and K-12 MG1655 transconjugants featuring LHR1 or LHR2 of C604-10. Error bars indicate standard deviations. Significances are indicated for the 45 min. time points based on one-tailed t-tests in (A,B), and Mann-Whitney rank sum tests in (C); *p < 0.05, **p < 0.01.
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Figure 4: Heat resistance assays of E. coli wild-type strains, FAM21805 mutants, and K-12 MG1655 transconjugants. Relative survival compared to time point 0 of strains incubated at 55°C for 15, 30, and 45 min. (A) FAM21805 wild-type, -ΔLHR1, -ΔLHR2, and ΔLHR1ΔLHR2. (B) FAM21805 wild-type, K-12 MG1655 wild-type, and K-12 MG1655 transconjugants harboring LHR2 or both LHRs of FAM21805. (C) C604-10, K-12 MG1655 wild-type, and K-12 MG1655 transconjugants featuring LHR1 or LHR2 of C604-10. Error bars indicate standard deviations. Significances are indicated for the 45 min. time points based on one-tailed t-tests in (A,B), and Mann-Whitney rank sum tests in (C); *p < 0.05, **p < 0.01.

Mentions: We have previously demonstrated that both LHRs in ESBL-producing E. coli isolate C604-10 confer heat resistance to this strain (Boll et al., 2016). To determine whether this was also the case with raw milk cheese E. coli isolate FAM21805, we generated a panel of LHR mutant strains with allelic replacements of designated regions with the kanr gene (Table 1). For FAM21805 ΔLHR1, we deleted the region spanning from orf2 (sHsp20c) to orf16. With regard to LHR2FAM21805, we deleted the region spanning from orf2 to the disrupted orf14, thereby excluding potential influence of removal of the inserted Microcin S system. As shown in Figure 4A, removal of either one of the LHRs did not affect survival of FAM21805 upon heat exposure. In contrast, survival of the FAM21805 double LHR mutant was severely reduced, thereby demonstrating that both LHRs of FAM21805 are functionally active and confer heat resistance.


Turn Up the Heat — Food and Clinical Escherichia coli Isolates Feature Two Transferrable Loci of Heat Resistance
Heat resistance assays of E. coli wild-type strains, FAM21805 mutants, and K-12 MG1655 transconjugants. Relative survival compared to time point 0 of strains incubated at 55°C for 15, 30, and 45 min. (A) FAM21805 wild-type, -ΔLHR1, -ΔLHR2, and ΔLHR1ΔLHR2. (B) FAM21805 wild-type, K-12 MG1655 wild-type, and K-12 MG1655 transconjugants harboring LHR2 or both LHRs of FAM21805. (C) C604-10, K-12 MG1655 wild-type, and K-12 MG1655 transconjugants featuring LHR1 or LHR2 of C604-10. Error bars indicate standard deviations. Significances are indicated for the 45 min. time points based on one-tailed t-tests in (A,B), and Mann-Whitney rank sum tests in (C); *p < 0.05, **p < 0.01.
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Figure 4: Heat resistance assays of E. coli wild-type strains, FAM21805 mutants, and K-12 MG1655 transconjugants. Relative survival compared to time point 0 of strains incubated at 55°C for 15, 30, and 45 min. (A) FAM21805 wild-type, -ΔLHR1, -ΔLHR2, and ΔLHR1ΔLHR2. (B) FAM21805 wild-type, K-12 MG1655 wild-type, and K-12 MG1655 transconjugants harboring LHR2 or both LHRs of FAM21805. (C) C604-10, K-12 MG1655 wild-type, and K-12 MG1655 transconjugants featuring LHR1 or LHR2 of C604-10. Error bars indicate standard deviations. Significances are indicated for the 45 min. time points based on one-tailed t-tests in (A,B), and Mann-Whitney rank sum tests in (C); *p < 0.05, **p < 0.01.
Mentions: We have previously demonstrated that both LHRs in ESBL-producing E. coli isolate C604-10 confer heat resistance to this strain (Boll et al., 2016). To determine whether this was also the case with raw milk cheese E. coli isolate FAM21805, we generated a panel of LHR mutant strains with allelic replacements of designated regions with the kanr gene (Table 1). For FAM21805 ΔLHR1, we deleted the region spanning from orf2 (sHsp20c) to orf16. With regard to LHR2FAM21805, we deleted the region spanning from orf2 to the disrupted orf14, thereby excluding potential influence of removal of the inserted Microcin S system. As shown in Figure 4A, removal of either one of the LHRs did not affect survival of FAM21805 upon heat exposure. In contrast, survival of the FAM21805 double LHR mutant was severely reduced, thereby demonstrating that both LHRs of FAM21805 are functionally active and confer heat resistance.

View Article: PubMed Central - PubMed

ABSTRACT

Heat treatment is a widely used process to reduce bacterial loads in the food industry or to decontaminate surfaces, e.g., in hospital settings. However, there are situations where lower temperatures must be employed, for instance in case of food production such as raw milk cheese or for decontamination of medical devices such as thermo-labile flexible endoscopes. A recently identified locus of heat resistance (LHR) has been shown to be present in and confer heat resistance to a variety of Enterobacteriaceae, including Escherichia coli isolates from food production settings and clinical ESBL-producing E. coli isolates. Here, we describe the presence of two distinct LHR variants within a particularly heat resistant E. coli raw milk cheese isolate. We demonstrate for the first time in this species the presence of one of these LHRs on a plasmid, designated pFAM21805, also encoding type 3 fimbriae and three bacteriocins and corresponding self-immunity proteins. The plasmid was highly transferable to other E. coli strains, including Shiga-toxin-producing strains, and conferred LHR-dependent heat resistance as well as type 3 fimbriae-dependent biofilm formation capabilities. Selection for and acquisition of this &ldquo;survival&rdquo; plasmid by pathogenic organisms, e.g., in food production environments, may pose great concern and emphasizes the need to screen for the presence of LHR genes in isolates.

No MeSH data available.


Related in: MedlinePlus