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

Cronobacter is a ubiquitous Gram-negative pathogen bacterium capable of surviving in low water activity environments, in particular powdered infant formula (PIF). Seven Cronobacter strains representing four different species (C. sakazakii, n = 4; C. malonaticus, n = 1; C. muytjensii, n = 1; C. turicensis, n = 1) were subjected to dry stress and stored in PIF at room temperature. The resulting survivor curves showed that Cronobacter sp. can survive for extended periods of at least 3 months with a significant, but moderate, variability regarding the level of resistance between species; however, no correlation was evident regarding the origin of strains. These results are evaluated with regard to other key characteristics, including genomic profiles and biofilm formation capacities of the strains. SIGNIFICANCE AND IMPACT OF THE STUDY: Cronobacter can survive extended periods of at least 3 months in PIF, with moderately significant interspecific variability in desiccation resistance. Results are evaluated with regard to genomic profiles and biofilm formation capacities of the strains, and contribute to an improved understanding of the environmental persistence of Cronobacter in contaminated PIF, and subsequent risk to infant exposure.

Impact of persistent and nonpersistent generic Escherichia coli and Salmonella sp. recovered from a beef packing plant on biofilm formation by E. coli O157.

AIMS: To examine the influence of meat plant Escherichia coli and Salmonella sp. isolates on E. coli O157 biofilm formation. METHODS AND RESULTS: Biofilm formation was quantified by crystal violet staining (A570 nm ) and viable cell numbers for up to 6 days at 15°C. All five persistent E. coli genotypes formed strong biofilms when cultured alone or co-cultured with E. coli O157, with A570 nm values reaching ≥4·8 at day 4, while only two of five nonpersistent genotypes formed such biofilms. For E. coli O157:H7 co-culture biofilms with E. coli genotypes 136 and 533, its numbers were ≥1·5 and ≥1 log CFU per peg lower than those observed for its mono-culture biofilm at days 2 and 4, respectively. The number of E. coli O157:NM in similar co-culture biofilms was 1 log CFU per peg lower than in its mono-culture biofilm at day 4 and 6, respectively. Salmonella sp. lowered the number of E. coli O157:NM by 0·5 log unit, once, at day 6. CONCLUSION: Generic E. coli may outcompete E. coli O157 strains while establishing biofilms. SIGNIFICANCE AND IMPACT OF THE STUDY: Findings advance knowledge regarding inter-strain competition for a similar ecological niche and may aid development of biocontrol strategies for E. coli O157 in food processing environments.

Expression of the cystine-glutamate exchanger (xc-) in retinal ganglion cells and regulation by nitric oxide and oxidative stress.

The cystine-glutamate exchanger, system x(c)(-), mediates the Na(+)-independent exchange of cystine into cells, coupled to the efflux of intracellular glutamate. System x(c)(-) plays a critical role in glutathione homeostasis. Early studies of brain suggested that system x(c)(-) was present primarily in astrocytes but not neurons. More recent work indicates that certain brain neurons have an active system x(c)(-). In the retina, system x(c)(-) has been demonstrated in Müller and retinal pigment epithelial cells. We have recently suggested that two protein components of system x(c)(-), xCT and 4F2hc, are present in ganglion cells of the intact retina. Here, we have used (1) molecular and immunohistochemical assays to determine whether system x(c)(-) is present in primary ganglion cells isolated from neonatal mouse retinas and (2) functional assays to determine whether its activity is regulated by oxidative stress in a retinal ganglion cell line (RGC-5). Primary mouse ganglion cells and RGC-5 cells express xCT and 4F2hc. RGC-5 cells take up [(3)H]glutamate in the absence of Na(+), and this uptake is blocked by known substrates of system x(c)(-) (glutamate, cysteine, cystine, quisqualic acid). Treatment of RGC-5 cells with NO and reactive oxygen species donors leads to increased activity of system x(c)(-) associated with an increase in the maximal velocity of the transporter with no significant change in the substrate affinity. This is the first report of system x(c)(-) in primary retinal ganglion cells and RGC-5 cells. Oxidative stress upregulates this transport system in RGC-5 cells, and the process is associated with an increase in xCT mRNA and protein but no change in 4F2hc mRNA or protein.

Evidence and characterization of a gene cluster required for the production of viscosin, a lipopeptide biosurfactant, by a strain of Pseudomonas fluorescens.

The genetic control of viscosin production was examined in a strain of Pseudomonas fluorescens (PfA7B) that causes broccoli head rot. Viscosin is a potent lipopeptide biosurfactant that enables the bacteria to come into intimate contact with the difficult-to-wet waxy heads of broccoli. Tn5 mutagenesis completely disrupted viscosin production as shown by HPLC analysis of the mutagenized cell lysates. The Vis- mutants retained their pectolytic capability and were able to decay potato slices. On broccoli, however, the Vis- mutants caused decay of wounded florets, but the decay failed to spread to adjacent nonwounded florets as had occurred with the wild-type PfA7B. Triparental matings of the Vis- mutants with their corresponding wild-type clones and the helper Escherichia coli HB101 carrying the mobilization plasmid pPK2013 resulted in three stable viscosin-producing transconjugants that caused typical decay of broccoli tissue. Linkage maps of clones and protein profiles showed that a 25-kb chromosomal DNA region of PfA7B affected the production of three high molecular mass proteins required for viscosin synthesis. These proteins, approximately 218, 215, and 137 kDa in size, likely compose a synthetase complex that assembles the nine amino acid peptide of viscosin and subsequently attaches this to the hydrophobic fatty acid component of the molecule. A probe made from this DNA region hybridized with DNA fragments of other phytopathogenic pseudomonads to varying degrees.