Irradiation sensitivity of planktonic and biofilm-associated Escherichia coli O157:H7 isolates is influenced by culture conditions

Response of Paenibacillus polymyxa SC2 to the stress of polymyxin B and a key ABC transporter YwjA involved

Polymyxins are cationic peptide antibiotics and regarded as the "final line of defense" against multidrug-resistant bacterial infections. Meanwhile, some polymyxin-resistant strains and the corresponding resistance mechanisms have also been reported. However, the response of the polymyxin-producing strain Paenibacillus polymyxa to polymyxin stress remains unclear. The purpose of this study was to investigate the stress response of gram-positive P. polymyxa SC2 to polymyxin B and to identify functional genes involved in the stress response process. Polymyxin B treatment upregulated the expression of genes related to basal metabolism, transcriptional regulation, transport, and flagella formation and increased intracellular ROS levels, flagellar motility, and biofilm formation in P. polymyxa SC2. Adding magnesium, calcium, and iron alleviated the stress of polymyxin B on P. polymyxa SC2, furthermore, magnesium and calcium could improve the resistance of P. polymyxa SC2 to polymyxin B by promoting biofilm formation. Meanwhile, functional identification of differentially expressed genes indicated that an ABC superfamily transporter YwjA was involved in the stress response to polymyxin B of P. polymyxa SC2. This study provides an important reference for improving the resistance of P. polymyxa to polymyxins and increasing the yield of polymyxins. KEY POINTS: • Phenotypic responses of P. polymyxa to polymyxin B was performed and indicated by RNA-seq • Forming biofilm was a key strategy of P. polymyxa to alleviate polymyxin stress • ABC transporter YwjA was involved in the stress resistance of P. polymyxa to polymyxin B.

Riboflavin- and chlorophyllin-based antimicrobial photoinactivation of Brevundimonas sp. ESA1 biofilms

Some Brevundimonas spp. are globally emerging opportunistic pathogens that can be dangerous to individuals with underlying medical conditions and for those who are immunocompromised. Gram-negative Brevundimonas spp. can form resilient sessile biofilms and are found not only in different confined terrestrial settings (e.g., hospitals) but are also frequently detected in spacecraft which is inhabited by astronauts that can have altered immunity. Therefore, Brevundimonas spp. pose a serious health hazard in different environments, especially in its biofilm form. Conventional antimicrobials applied to disrupt, inactivate, or prevent biofilm formation have limited efficiency and applicability in different closed-loop systems. Therefore, new, effective, and safe biofilm control technologies are in high demand. The present work aimed to investigate antimicrobial photoinactivation (API) of Brevundimonas sp. ESA1 monocultural biofilms mediated by non-toxic, natural photosensitizers such as riboflavin (RF) and chlorophyllin (Chl) with an emphasis of this technology as an example to be safely used in closed-loop systems such as spacecraft. The present study showed that Chl-based API had a bactericidal effect on Brevundimonas sp. ESA1 biofilms at twice the lower irradiation doses than was needed when applying RF-based API. Long-term API based on RF and Chl using 450 nm low irradiance plate has also been studied in this work as a more practically applicable API method. The ability of Brevundimonas sp. ESA1 biofilms to reduce alamarBlue™ and regrowth analysis have revealed that after the applied photoinactivation, bacteria can enter a viable but non-culturable state with no ability to resuscitate in some cases.

A review of bacterial biofilm control by physical strategies

Biofilms are multicellular communities of microorganisms held together by a self-produced extracellular matrix, which contribute to hygiene problems in the food and medical fields. Both spoilage and pathogenic bacteria that grow in the complex structure of biofilm are more resistant to harsh environmental conditions and conventional antimicrobial agents. Therefore, it is important to develop eco-friendly preventive methodologies to eliminate biofilms from foods and food contact equipment. The present paper gives an overview of the current physical methods for biofilm control and removal. Current physical strategies adopted for the anti-biofilm treatment mainly focused on use of ultrasound power, electric or magnetic field, plasma, and irradiation. Furthermore, the mechanisms of anti-biofilm action and application of different physical methods are discussed. Physical strategies make it possible to combat biofilm without the use of biocidal agents. The remarkable microbiocidal properties of physical strategies are promising tools for antimicrobial applications.

The Antimicrobial Effect of Radiant Catalytic Ionization on the Bacterial Attachment and Biofilm Formation by Selected Foodborne Pathogens under Refrigeration Conditions

The decontamination of food contact surfaces is a major problem for the food industry. The radiant catalytic ionization (RCI) method, based on the ionization process, may be an alternative for conventional decontamination procedures. The advantage of this technique is the possibility of its application to household refrigerating appliances and industrial cold rooms. This study aimed to assess the effect of RCI on the reduction of Campylobacter jejuni, Listeria monocytogenes, and Salmonella Enteritidis from the biofilms formed on a glass surface under refrigeration conditions. Bacterial biofilms were exposed to RCI for 24 h and after 12 (variant I) and 72 h (variant II) of the glass surface contamination. In the last variant (III), the contaminated meat was placed on the glass surface in the refrigerator and subjected to RCI treatment for 72 h. The significantly highest values of absolute reduction efficiency coefficient E were found for the bacterial attachment stage of biofilm formation (variant I). The research proves the efficiency of the RCI method in the reduction of bacteria number from a glass surface.

Mycofloral profile and the radiation sensitivity (D10 values) of solar dried and gamma irradiated Pleurotus ostreatus (Jacq.Ex. Fr.) Kummer fruitbodies stored in two different packaging materials

The presence of fungi in our foods poses serious health risks as some genera of fungi may produce certain mycotoxins which have carcinogenic, mutagenic, teratogenic, and immunosuppressive effect on humans and animals alike. Fruitbodies of Pleurotus ostreatus were solar dried at a moisture content of 12.5 ± 0.2% and stored in polythene and polypropylene packs, gamma irradiated at doses of 0 (control), 1, 2, 3, 4, and 5 kGy at a dose rate of 1.7 kGy/hr from a Cobalt 60 source (SLL, 515, Hungary) and stored at room temperature 28-30°C for a period of 12 months. Mycological analyses were done at intervals of 0, 3, 6, and 12 months. A total of eleven (11) fungi belonging to eight fungal genera were isolated on both Cooke's and DRBC media; Aspergillus (A. niger, A. flavus, A. fumigatus, A. tamarii), Rhizopus (R. oligosporus), Mucor (M. racemosus), Fusarium (F. oxysporum), Penicillium (Penicillium sp.), Trichoderma (T. viride), and Rhodotorula sp. were recorded. There was a significant (p < .05) reduction in initial mycofloral population by an average of 2.2 log cycles as well as in species numbers with increasing doses of radiation. Radiation sensitivity (D₁₀ values) also ranged between 1.68-2.78 kGy. Gamma irradiation treatment is one way which can enhance food safety through the reduction in potential pathogens and has been recommended as part of a comprehensive program to enhance food safety.

Growth of Shiga toxin-producing Escherichia coli (STEC) and impacts of chilling and post-inoculation storage on STEC attachment to beef surfaces

Concern has been expressed surrounding the utility of studies describing the efficacy of antimicrobial interventions targeting the Shiga toxin-producing Escherichia coli (STEC) that inoculate chilled versus non-chilled beef carcasses. The objectives of this study were to evaluate the effects of chilling (non-chilled, chilled to surface temperature of ≤5 °C) on STEC attachment to brisket surfaces, and the effects of post-inoculation storage on STEC recovery. Paired briskets from split carcasses were separated; one brisket from each pair was kept non-chilled, while the other was chilled to a surface temperature of ≤5 °C prior to inoculation. Briskets were inoculated with a cocktail of eight STEC and then stored at 5 or 25 °C. At 0, 30, 60, 90 and 120 min post-inoculation, 30 cm(2) of tissue was aseptically excised, followed by selective enumeration of strongly and loosely attached STEC. A significant, though small (0.4 log10 CFU/cm(2)), difference in the numbers of strongly attached cells was observed between non-chilled and chilled briskets (p < 0.05). Significant effects on cell attachment by the interaction of chilling and post-inoculation storage period, or chilling and post-inoculation storage temperature, were identified (p < 0.05). Results indicate beef chilling and post-inoculation storage conditions influenced STEC attachment to beef.

Escherichia coli O157:H7 biofilm formation on Romaine lettuce and spinach leaf surfaces reduces efficacy of irradiation and sodium hypochlorite washes

Escherichia coli O157:H7 contamination of leafy green vegetables is an ongoing concern for consumers. Biofilm-associated pathogens are relatively resistant to chemical treatments, but little is known about their response to irradiation. Leaves of Romaine lettuce and baby spinach were dip inoculated with E. coli O157:H7 and stored at 4 degrees C for various times (0, 24, 48, 72 h) to allow biofilms to form. After each time, leaves were treated with either a 3-min wash with a sodium hypochlorite solution (0, 300, or 600 ppm) or increasing doses of irradiation (0, 0.25, 0.5, 0.75, or 1 kGy). Viable bacteria were recovered and enumerated. Chlorine washes were generally only moderately effective, and resulted in maximal reductions of 1.3 log CFU/g for baby spinach and 1.8 log CFU/g for Romaine. Increasing time in storage prior to chemical treatment had no effect on spinach, and had an inconsistent effect on 600 ppm applied to Romaine. Allowing time for formation of biofilm-like aggregations reduced the efficacy of irradiation. D(10) values (the dose required for a 1 log reduction) significantly increased with increasing storage time, up to 48 h postinoculation. From 0 h of storage, D(10) increased from 0.19 kGy to a maximum of 0.40 to 0.43 kGy for Romaine and 0.52 to 0.54 kGy for spinach. SEM showed developing biofilms on both types of leaves during storage. Bacterial colonization of the stomata was extensive on spinach, but not on Romaine. These results indicate that the protection of bacteria on the leaf surface by biofilm formation and stomatal colonization can reduce the antimicrobial efficacy of irradiation on leafy green vegetables.

Influence of plasmid pO157 on Escherichia coli O157:H7 Sakai biofilm formation

The role of plasmid pO157 in biofilm formation was investigated using wild-type and pO157-cured Escherichia coli O157:H7 Sakai. Compared to the wild type, the biofilm formed by the pO157-cured mutant produced fewer extracellular carbohydrates, had lower viscosity, and did not give rise to colony morphology variants that hyperadhered to solid surfaces.

Escherichia coli O157:H7 requires colonizing partner to adhere and persist in a capillary flow cell

The ability of a strain of waterborne Escherichia coli O157:H7 to colonize a glass flow cell and develop microcolonies when grown alone and with Pseudomonas aeruginosa PAO1 was examined. When introduced alone, planktonic E. coil were unable to attach to the glass surface. When introduced simultaneously with P. aeruginosa (co-inoculation), the two species coadhered to the surface. When E. coliwere introduced into a flow cell precolonized with a P. aeruginosa biofilm (precolonized), 10-fold more cells were retained than in the co-inoculated case. Both species were monitored nondestructively by time-lapse confocal microscopy, direct microscopy of the filtered effluent, and effluent plate counts. While more E. coli initially adhered in the precolonized system, E. coli microcolony formation occurred only in the co-inoculated system, where E. coil comprised 1% of the total surface-associated biovolume but greater than 50% of the biovolume near the edges of the flow cell. The hydrodynamics in the flow cell were evaluated using the finite volume analysis program CFX, revealing that shear stress was likely important in both initial attachment and steady-state colonization patterns. This research elucidates key factors which promote retention and subsequent biofilm development of E. coli 0157:H7.

INTRODUCTION

Bacteria exist in nature primarily in communities known as biofilms. These biofilms are usually characterized by differentiated structures, exhibit a different phenotype than their planktonic counterparts, and in nature most often consist of multispecies consortia (1, 2). An important process in shaping the formation and structure of some multispecies biofilms is the ability of certain species to coaggregate. In this process, planktonic cells adhere to genetically distinct cells in a biofilm or to other planktonic cells (3), thereby increasing biofilm formation. This process is growth-phase-dependent and is turned on and off by cells, suggestive that it may also play a role in dispersal and dissemination (4). Due to these and other complexities of the biofilm mode of growth, multiple species can coexist despite one organism having a much higher growth rate than another (5-7). In many cases, bacteria have been shown to gain a fitness advantage when residing in a mixed-species versus single-

Relative efficacy of sodium hypochlorite wash versus irradiation to inactivate Escherichia coli O157:H7 internalized in leaves of Romaine lettuce and baby spinach

Pathogenic bacteria that become internalized in leaf tissues are protected from the antimicrobial effects of surface treatments. Ionizing radiation is known to penetrate food tissues, but the efficacy of the process against internalized bacteria is unknown. Leaves of Romaine lettuce and baby spinach were cut into pieces, submerged in a cocktail mixture of three isolates of Escherichia coli O157:H7, and subjected to a vacuum perfusion process to force the bacterial cells into the intercellular spaces in the leaves. Scanning electron microscopy was used to evaluate the efficacy of the perfusion process. The inoculated leaves were then treated with a 3-min water wash, a 3-min wash with a sodium hypochlorite sanitizing solution (300 or 600 ppm), or various doses of ionizing radiation (0.25 to 1.5 kGy). Leaves were stomached to recover the internalized pathogen cells, which were enumerated. The vacuum perfusion effectively forced bacteria into the leaf vasculature and apoplast, as confirmed by scanning electron microscopy. For spinach leaf pieces, neither the water nor the sodium hypochlorite washes resulted in significant reductions of E. coli O157:H7 cells relative to the untreated control. For Romaine lettuce leaf pieces, 300 and 600 ppm sodium hypochlorite each resulted in less than 1-log reduction; water wash was ineffective. Ionizing radiation, in contrast, significantly reduced the pathogen population, with 4-log (Romaine lettuce) or 3-log (spinach) reductions at the highest dose tested. In Romaine leaves, the reduction was dose dependent across the range of doses tested, with a D10-value (the amount of irradiation necessary to reduce the population by 1 log unit) of 0.39 kGy. In spinach leaves, the pathogen had a biphasic response, with a D10-value of 0.27 kGy in the range of 0 to 0.75 kGy but only slight additional reductions from 0.75 to 1.5 kGy. In this study, ionizing radiation but not chemical sanitizers effectively reduced viable E. coli O157:H7 cells internalized in leafy green vegetables, but the response of the pathogen to irradiation was more complex in spinach leaves than in Romaine lettuce leaves.