SummaryQuorum sensing (QS) governs the production of virulence factors and the architecture and sodium dodecyl sulphate (SDS) resistance of bio®lm-grown Pseudomonas aeruginosa. P. aeruginosa QS requires two transcriptional activator proteins known as LasR and RhlR and their cognate autoinducers PAI-1 (N-(3-oxododecanoyl)-L-homoserine lactone) and PAI-2 (Nbutyryl-L-homoserine lactone) respectively. This study provides evidence of QS control of genes essential for relieving oxidative stress. Mutants devoid of one or both autoinducers were more sensitive to hydrogen peroxide and phenazine methosulphate, and some PAI mutant strains also demonstrated decreased expression of two superoxide dismutases (SODs), Mn-SOD and Fe-SOD, and the major catalase, KatA. The expression of sodA (encoding Mn-SOD) was particularly dependent on PAI-1, whereas the in¯uence of autoinducers on Fe-SOD and KatA levels was also apparent but not to the degree observed with Mn-SOD. b-Galactosidase reporter fusion results were in agreement with these ®ndings. Also, the addition of both PAIs to suspensions of the PAI-1/2-de®cient double mutant partially restored KatA activity, while the addition of PAI-1 only was suf®cient for full restoration of Mn-SOD activity. In bio®lm studies, catalase activity in wild-type bacteria was signi®cantly reduced relative to planktonic bacteria; catalase activity in the PAI mutants was reduced even further and consistent with relative differences observed between each strain grown planktonically. While wild-type and mutant bio®lms contained less catalase activity, they were more resistant to hydrogen peroxide treatment than their respective planktonic counterparts. Also, while catalase was implicated as an important factor in bio®lm resistance to hydrogen peroxide insult, other unknown factors seemed potentially important, as PAI mutant bio®lm sensitivity appeared not to be incrementally correlated to catalase levels.

The ability of public health agencies to identify, through enhanced epidemiologic and surveillance techniques, raw fruits, vegetables, and unpasteurized juices as probable sources of infectious microorganisms, has undoubtedly resulted in increased numbers of documented outbreaks. Changes in agronomic, harvesting, distribution, processing, and consumption patterns and practices have also likely contributed to this increase. The risk of illness associated with raw produce and unpasteurized produce products can be reduced by controlling or preventing contamination, or by removing or killing pathogenic microorganisms by washing or treating them with sanitizers. However, the hydrophobic cutin, diverse surface morphologies, and abrasions in the epidermis of fruits and vegetables limit the efficacy of these treatments.

®rst documented outbreak of salmonellosis associated with the consumption of peanut butter was reported. This study was undertaken to determine survival characteristics of high (5Á68 log 10 cfu g À1 ) and low (1Á51 log 10 cfu g À1 ) inocula of a ®ve-serotype mixture of Salmonella in ®ve commercial peanut butters and two commercial peanut butter spreads. Populations in samples inoculated with 5Á68 log 10 cfu g À1 and stored for 24 weeks at 21 or 5 C decreased 4Á14±4Á50 log 10 cfu g À1 and 2Á86±4Á28 log 10 cfu g À1 , respectively, depending on the formulation. The order of retention of viability was: peanut butter spreads > traditional (regular) and reduced sugar, low-sodium peanut butters > natural peanut butter. Differences in rates of inactivation are attributed to variation in product composition as well as size and stability of water droplets in the colloidal matrix, which may in¯uence nutrient availability. With the exception of natural peanut butter, products initially inoculated with 1Á51 log 10 cfu of Salmonella g À1 (32 cfu g À1 ) were positive for the pathogen after storage for 24 weeks at 5 C. At 21 C, however, with the exception of one peanut butter spread, all products were negative for Salmonella after storage for 24 weeks. Post-process contamination of peanut butters and spreads with Salmonella may to result in survival in these products for the duration of their shelf life at 5 C and possibly 21 C, depending on the formulation.

Confocal scanning laser microscopy (CSLM) was used to demonstrate the attachment of Escherichia coli O157:H7 transformed with a plasmid encoding for green fluorescent protein (GFP) to the surface and within the internal structures of nonwaxed Red Delicious cv. apples. Apples at 2 or 25°C were inoculated with an E. coli O157:H7 cell suspension at 2 or 25°C. The effect of a negative temperature differential (cold inoculum, warm apple), a positive differential (warm inoculum, cold apple), and no differential (warm inoculum, warm apple), in combination with a pressure differential (atmospheric versus 10,130 Pa), on the attachment and infiltration of cells was determined. CSLM stereo images of external surfaces of apples subjected to all combinations of test parameters showed preferential cellular attachment to discontinuities in the waxy cuticle on the surface and to damaged tissue surrounding puncture wounds, where the pathogen was observed at depths up to 70 m below the skin surface. Attachment to lenticels was sporadic but was occasionally observed at depths of up to 40 m. Infiltration through the floral tube and attachment to seeds, cartilaginous pericarp, and internal trichomes were observed in all apples examined, regardless of temperature differential during inoculation. The pressure differential had no effect on infiltration or attachment of E. coli O157:H7. Image analysis to count cells at various depths within tissues was used to quantitatively compare the extent of infiltration into various apple structures as well as the effects of the temperature differential. Puncture wounds harbored greater numbers of the pathogen at greater depths than did other sites examined. Attachment or infiltration of cells was greater on the intact skin and in lenticels, russet areas, and the floral tube of apples inoculated under a negative temperature differential compared to those inoculated under no temperature differential. The results suggest that E. coli O157:H7 attached to internal core structures or within tissues of apples may evade decontamination treatments. Interventions designed to deliver disinfectants to these locations or to remove viable cells of E. coli O157:H7 and other pathogens from apples by other means need to be developed and validated.