Contribution of the thermotolerance genomic island to increased thermal tolerance in Cronobacter strains

Abstract

Cronobacter spp. are opportunistic pathogens associated with serious infections in neonates. Increased stress tolerance, including the thermotolerance of some Cronobacter strains, can promote their survival in production facilities and thus raise the possibility of contamination of dried infant formula which has been identified as a potential source of infection. Some Cronobacter strains contain a genomic island, which might be responsible for increased thermotolerance. By analysis of Cronobacter sequenced genomes this determinant was found to be present in only 49/73 Cronobacter sakazakii strains and in 9/14 Cronobacter malonaticus strains. The island was also found in 16/17 clinical isolates originating from two hospitals. Two configurations of the locus were detected; the first one with the size of 18 kbp containing the thrB-Q genes and a shorter version (6 kbp) harbouring only the thrBCD and thrOP genes. Strains containing the thermotolerance island survived significantly better at 58°C comparing to a C. sakazakii isogenic mutant lacking the island and strains with the longer version of the island were 2-10 times more tolerant than those with the shortened sequence. The function of the genomic island was further confirmed by its cloning into a low-copy vector and transforming it into the isogenic mutant. Different levels of rpoS, encoding for stress-response sigma factor, expression were also associated with variability in strain thermotolerance.

Genetic determinants of heat resistance in Escherichia coli

“…The promoter was predicted to interact with the OmpR, a transcription factor coordinating gene expression in response to osmotic stress (Mizuno and Mizushima, 1990 ). The LHR is over-expressed in response to osmotic stress (Riedel and Lehner, 2007 ), which corresponds to the observation that E. coli AW1.7 is resistant to heat only when incubated in growth media containing 1–4% NaCl (Ruan et al, 2011 ; Pleitner et al, 2012 ), as well as the observation that deletion of the LHR reduces the tolerance of C. sakazakii to osmotic stress (Orieskova et al, 2013 ). The LHR may thus function in response to osmotic and heat stress and its function may be partially dependent on the extracellular concentration of compatible solutes.…”

“…Eight ORFs contain predicted transmembrane domains, including Orfs8-10 and the proteases Orf15 and Orf16. One putative gene, orf13 , is predicted to encode a sodium/hydrogen antiporter, which corresponds to the interplay of osmotic and heat stress in strains expressing the LHR (Pleitner et al, 2012 ; Orieskova et al, 2013 ). Orf16, a predicted membrane protease, possesses a similar domain structure to DegS, a protease involved in the activation of the σ E stress pathway in E. coli (Alba and Gross, 2004 ).…”

“…Heterologous expression of orf7-orf10 from C. sakazakii in E. coli also resulted in an increase in thermotolerance (Gajdosova et al, 2011 ), but the heat resistance of the resulting transgenic strains was substantially lower than the level of resistance that was observed in E. coli AW1.7 carrying the entire LHR (Figures 1 , 5 , 7 ). Deletion of the LHR substantially reduced the resistance of C. sakazakii to heat (Orieskova et al, 2013 ).…”

Microorganism preservation by convective air-drying—A review

“…The inactivation, however, occur within a range of water concentration rather than at exact concentration due to biological variation of microorganisms in the culture [15]. The variation could arise from as little as difference in stages of cell growth or as much as the presence of different strains or species such as in mixed cultures [57,68]. The two mechanisms may, nevertheless, occur simultaneously during thermal drying process [29].…”

Survival of Cronobacter in powdered infant formula and their variation in biofilm formation

“…The production of a typical heteropolysaccharide capsule by some Cronobacter strains enhances desiccation resistance and increases biofilm formation (Orieskova et al . ). Caubilla‐Barron and Forsythe () reported prolonged survival of C. sakazakii for over 2 years in infant formula which was especially pronounced for capsulated strains.…”