Control of Salmonella in food related environments by chemical disinfection

Salmonella may be transferred to food through cross-contamination during processing and preparation. To minimise the risk of cross-contamination, proper cleaning and disinfection is essential for the food industry. Recently, disinfection of areas for preparation and storage of food has also gained increased popularity in households. There is a range of disinfectants available with different properties and usage areas, and care must be taken to choose the proper disinfectant for the specific application.There are many methods for testing the antimicrobial effect of disinfectants. To evaluate whether a disinfectant will be effective in practical settings, the test method should model real-life situations. Most disinfectants are effective against Salmonella at recommended user concentration in suspension tests. However, a number of factors may reduce the biocidal effect of disinfectants under practical conditions. This include properties of the surface to be disinfected, presence of soiling on the surface, the physiological state of the bacteria exposed to disinfection, including bacteria embedded in biofilms, and the effects of other stresses (e.g. desiccation, starvation and temperature).Here we review the effects of disinfectants used in food related areas in industries and in households against Salmonella. A general overview is given for disinfectants in use and methods used to evaluate effects. Effects of disinfectants against Salmonella in suspension and on surfaces, including biofilms, are presented and compared. Novel control strategies such as use of electrolysed water, antimicrobial surfaces, and anti-biofilm compounds are also covered. Finally, we review the ability of Salmonella to gain reduced susceptibility to disinfectants through adaptation and other physiological responses like biofilm formation.     

In situ characterization and analysis of Salmonella biofilm formation under meat processing environments using a combined microscopic and spectroscopic approach

Salmonella biofilm on food-contact surfaces present on food processing facilities may serve as a source of cross-contamination. In our work, biofilm formation by multi-strains of meat-borne Salmonella incubated at 20 °C, as well as the composition and distribution of extracellular polymeric substances (EPS), were investigated in situ by combining confocal laser scanning microscopy (CLSM), scanning electron microscope (SEM), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and Raman spectroscopy. A standard laboratory culture medium (tryptic soy broth, TSB) was used and compared with an actual meat substrate (meat thawing-loss broth, MTLB). The results indicated that Salmonella grown in both media were able to form biofilms on stainless steel surfaces via building a three-dimensional structure with multilayers of cells. Although the number of biofilm cells grown in MTLB was less than that in TSB, the cell numbers in MTLB was adequate to form a steady and mature biofilm. Salmonella grown in MTLB showed “cloud-shaped” morphology in the mature biofilm, whereas when grown in TSB appeared “reticular-shaped”. The ATR-FTIR and Raman analysis revealed a completely different chemical composition between biofilms and the corresponding planktonic cells, and some important differences in biofilms grown in MTLB and in TSB. Importantly, our findings suggested that the progress towards a mature Salmonella biofilm on stainless steel surfaces may be associated with the production of the EPS matrix, mainly consisting of polysaccharides and proteins, which may serve as useful markers of biofilm formation. Our work indicated that a combination of these non-destructive techniques provided new insights into the formation of Salmonella biofilm matrix.     

Biofilm formation of Salmonella Typhimurium on stainless steel and acrylic surfaces as affected by temperature and pH level

The effect of temperature (28, 37 and 42 °C) and pH (6 and 7) on the biofilm formation capability of Salmonella Typhimurium on stainless steel and acrylic was investigated. The rate of biofilm formation increased with increasing temperature and pH, while the number of attached cells after 240 h decreased with increasing temperature and was not different between pH 6 and 7. The surface hydrophobicity of bacterial cells was not significantly (p > 0.05) different among tested conditions. Electron-donating/accepting properties changed with pH and temperature, although these changes did not correlate with the ability to form biofilms under respective conditions. Attachment of S. Typhimurium showed a preference for stainless steel compared to acrylic surfaces under all conditions tested. The results suggest that salmonellae were less adherent to acrylic than to stainless steel surfaces; thus, acrylic-type surfaces should be considered for use in the food industry over stainless steel where applicable. The rate of biofilm formation increased at higher temperatures and pH levels within the tested ranges. Hurdle technology using lower temperatures reduced pH may help delay biofilm formation on food contact surfaces contaminated with S. Typhimurium.     

Resistance to benzalkonium chloride, peracetic acid and nisin during formation of mature biofilms by Listeria monocytogenes

Increase of resistance to the application of benzalkonium chloride (BAC), peracetic acid (PA) and nisin during biofilm formation at 25 °C by three strains of Listeria monocytogenes (CECT 911, CECT 4032, CECT 5873 and BAC-adapted CECT 5873) in different scenarios was compared. For this purpose, resistance after 4 and 11-days of biofilm formation was quantified in terms of lethal dose 90% values (LD₉₀), determined according with a dose-response logistic mathematical model. Microscopic analyses after 4 and 11-days of L. monocytogenes biofilm formation were also carried out. Results demonstrated a relation between the microscopic structure and the resistance to the assayed biocides in matured biofilms. The worst cases being biofilms formed by the strain 4032 (in both stainless steel and polypropylene), which showed a complex “cloud-type” structure that correlates with the highest resistance of this strain against the three biocides during biofilm maturation. However, that increase in resistance and complexity appeared not to be dependent on initial bacterial adherence, thus indicating mature biofilms rather than planctonic cells or early-stage biofilms must be considered when disinfection protocols have to be optimized. PA seemed to be the most effective of the three disinfectants used for biofilms. We hypothesized both its high oxidizing capacity and low molecular size could suppose an advantage for its penetration inside the biofilm. We also demonstrated that organic material counteract with the biocides, thus indicating the importance of improving cleaning protocols. Finally, by comparing strains 5873 and 5873 adapted to BAC, several adaptative cross-responses between BAC and nisin or peracetic acid were identified.     

Combating biofilms of foodborne pathogens with bacteriocins by lactic acid bacteria in the food industry

Most foodborne pathogens have biofilm-forming capacity and prefer to grow in the form of biofilms. Presence of biofilms on food contact surfaces can lead to persistence of pathogens and the recurrent cross-contamination of food products, resulting in serious problems associated with food safety and economic losses. Resistance of biofilm cells to conventional sanitizers urges the development of natural alternatives to effectively inhibit biofilm formation and eradicate preformed biofilms. Lactic acid bacteria (LAB) produce bacteriocins which are ribosomally synthesized antimicrobial peptides, providing a great source of nature antimicrobials with the advantages of green and safe properties. Studies on biofilm control by newly identified bacteriocins are increasing, targeting primarily onListeria monocytogenes, Staphylococcus aureus, Salmonella, and Escherichia coli. This review systematically complies and assesses the antibiofilm property of LAB bacteriocins in controlling foodborne bacterial-biofilms on food contact surfaces. The bacteriocin-producing LAB genera/species, test method (inhibition and eradication), activity spectrum and surfaces are discussed, and the antibiofilm mechanisms are also argued. The findings indicate that bacteriocins can effectively inhibit biofilm formation in a dose-dependent manner, but are difficult to disrupt preformed biofilms. Synergistic combination with other antimicrobials, incorporation in nanoconjugates and implementation of bioengineering can help to strengthen their antibiofilm activity. This review provides an overview of the potential and application of LAB bacteriocins in combating bacterial biofilms in food processing environments, assisting in the development and widespread use of bacteriocin as a promising antibiofilm-agent in food industries.     

Salmonella enterica Enteritidis biofilm formation and viability on regular and triclosan-impregnated bench cover materials

Contamination of food contact surfaces by microbes such as Salmonella is directly associated with substantial industry costs and severe foodborne disease outbreaks. Several approaches have been developed to control microbial attachment; one approach is the development of food contact materials incorporating antimicrobial compounds. In the present study, Salmonella enterica Enteritidis adhesion and biofilm formation on regular and triclosan-impregnated kitchen bench stones (silestones) were assessed, as was cellular viability within biofilms. Enumeration of adhered cells on granite, marble, stainless steel, and silestones revealed that all materials were prone to bacterial colonization (4 to 5 log CFU/cm(2)), and no significant effect of triclosan was found. Conversely, results concerning biofilm formation highlighted a possible bacteriostatic activity of triclosan; smaller amounts of Salmonella Enteritidis biofilms were formed on impregnated silestones, and significantly lower numbers of viable cells (1 × 10(5) to 1 × 10(6) CFU/cm(2)) were found in these biofilms than in those on the other materials (1 × 10(7) CFU/cm(2)). All surfaces tested failed to promote food safety, and careful utilization with appropriate sanitation of these surfaces is critical in food processing environments. Nevertheless, because of its bacteriostatic activity, triclosan incorporated into silestones confers some advantage for controlling microbial contamination.     

Aeromonas biofilm on stainless steel: efficiency of commonly used disinfectants

Aeromonas spp. are ubiquitous bacteria widely distributed among aquatic environments that have the ability to form biofilms. This aptitude allows them to persist in water distribution systems, contaminating drinking water, food processing surfaces and ultimately food. For this study, the biofilm‐forming ability of aeromonads was evaluated after 48‐h incubation on stainless steel discs at both 4 and 20 °C. Subsequently, disinfectants based on amphoteric surfactants and chlorine compounds were evaluated regarding the capacity to eradicate preformed biofilm and inhibit biofilm formation. Results obtained demonstrated that all strains under analysis were able to form biofilm at both room and refrigeration temperatures. The chlorine‐based disinfectant demonstrated to be efficient in removing preformed biofilm, but both were unsuccessful in preventing biofilm formation, highlighting the importance of adequate cleaning and disinfection procedures, with emphasis on food processing surfaces.     

Microbial biofilm detection on food contact surfaces by macro-scale fluorescence imaging

Hyperspectral fluorescence imaging methods were utilized to evaluate the potential detection of pathogenic bacterial biofilm formations on five types of food contact surface materials: stainless steel, high-density polyethylene (HDPE), plastic laminate (Formica), and two variations of polished granite. The main objective of this study was to determine a minimal number of spectral fluorescence bands suitable for detecting microbial biofilms on surfaces commonly used to process and handle food. Spots of biofilm growth were produced on sample surfaces by spot-inoculations of pathogenic Escherichia coli O157:H7 and Salmonella followed by room temperature storage for 3 days. Subsequently, hyperspectral fluorescence images were acquired from 421 to 700 nm using ultraviolet-A excitation. Both E. coli O157:H7 and Salmonella biofilms emitted fluorescence predominantly in the blue to green wavelengths with emission maxima at approximately 480 nm. A single-band image at 559 nm was able to detect the biofilm spots on stainless steel. On HDPE and granite, algorithms using different two-band ratios provided better separation of the biofilm spots from background areas than any single-band images did. The biofilm spots on stainless steel, HDPE, and granite could be detected with overall detection rate of 95%. On Formica, too many false positives were present to accurately determine an effective biofilm detection rate. This may have been due to the lower cell population density that was observed for the biofilm spots grown on Formica (approximately 4.3–6.4 log cfu cm⁻²) as compared to the other surfaces. These findings can be incorporated into developing portable hand-held imaging devices for sanitation inspection of food processing surfaces.     

Influence of serotype on the growth kinetics and the ability to form biofilms of Salmonella isolates from poultry

The influence of the serotype on the growth behaviour and the ability to form biofilms of Salmonella enterica strains was investigated. The relationships between biofilm formation and growth kinetic parameters were also determined. A total of 69 strains (61 isolates from poultry and 8 reference strains from culture collections) belonging to 10 serotypes (S. enterica serotype Typhimurium, S. Newport, S. Paratyphi B, S. Poona, S. Derby, S. Infantis, S. Enteritidis, S. Virchow, S. Agona and S. Typhi) were tested. All Salmonella strains produced biofilms on polystyrene micro-well plates (crystal violet assay). Isolates were classified as weak (35 strains), moderate (22), or strong (12) biofilm producers. S. Agona and S. Typhi produced the most substantial (P < 0.001) biofilms. Growth curves were performed at 37 °C in tryptone soy broth by means of optical density (OD_420–580) measurements from 0 to 48 h. Growth kinetic parameters (Gompertz model) varied between serotypes. The maximum growth rate (ΔOD_420–580/h) ranged from 0.030 ± 0.002 (S. Typhi) to 0.114 ± 0.011 (S. Agona). The ability of Salmonella strains to form biofilms was not related to their growth kinetic parameters. The formation of biofilms by Salmonella on polystyrene constitutes an issue of concern because plastic materials are frequently used in food facilities. The findings suggest that special efforts must be made for the effective control of Salmonella in food-processing environments when S. Agona or S. Typhi strains are present.     

Effect of carvacrol and thymol on Salmonella spp. biofilms on polypropylene

The present study evaluated the effects of carvacrol and thymol against Salmonella spp. biofilm on polypropylene. The efficacy of the compounds was assessed by quantifying Salmonella spp. cells during and after biofilm formation on polypropylene and performing scanning electron microscopy. During biofilm formation, carvacrol and thymol, at subinhibitory concentrations, reduced bacterial counts about 1–2 log, while established Salmonella spp. biofilms were reduced about 1–5 log by carvacrol and thymol, at MIC or 2× MIC. The greatest reduction in carvacrol‐treated biofilms, about 5 log, was observed with 156 and 312 μg mL^?1 (MIC and 2× MIC) in established Salmonella Typhimurium ATCC 14028 biofilms. Thymol showed the greatest reduction, about 4 log, at 624 μg mL^?1 (2× MIC) against mature Salmonella Enteritidis biofilm. Carvacrol and thymol reduced the number of Salmonella spp. cells on polypropylene, suggesting their potential for the control of Salmonella spp. biofilms.     

Biofilm-producing ability of Staphylococcus aureus isolates from Brazilian dairy farms

This study aimed to investigate the in silico biofilm production ability of Staphylococcus aureus strains isolated from milking parlor environments on dairy farms from São Paulo, Brazil. The Staph. aureus isolates were obtained from 849 samples collected on dairy farms, as follows: milk from individual cows with subclinical mastitis or history of the disease (n = 220); milk from bulk tank (n = 120); surfaces of milking machines and utensils (n = 389); and milk handlers (n = 120). Thirty-one Staph. aureus isolates were obtained and categorized as pulsotypes by pulsed-field gel electrophoresis and submitted to assays for biofilm formation on polystyrene, stainless steel, rubber, and silicone surfaces. Fourteen (45.2%) pulsotypes were considered producers of biofilm on the polystyrene microplate assay, whereas 13 (41.9%) and 12 (38.7%) pulsotypes were biofilm producers on stainless steel and rubber, respectively. None of the pulsotypes evaluated produced biofilms on silicone. Approximately 45% of Staph. aureus pulsotypes isolated from different sources on dairy farms showed the ability to produce biofilms in at least one assay, indicating possible persistence of this pathogen in the milking environment. The potential involvement of Staph. aureus in subclinical mastitis cases and its occurrence in milk for human consumption emphasize the need to improve hygiene practices to prevent biofilm formation on the farms studied.     

The effects of stainless steel finish on Salmonella Typhimurium attachment,biofilm formation and sensitivity to chlorine

Bacterial colonization and biofilm formation on stainless steel (SS) surfaces can be sources for cross contamination in food processing facilities, possessing a great threat to public health and food quality. Here the aim was to demonstrate the influence of surface finish of AISI 316 SS on colonization, biofilm formation and susceptibility of Salmonella Typhimurium to disinfection.     

Mixed species biofilms of Listeria monocytogenes and Lactobacillus plantarum show enhanced resistance to benzalkonium chloride and peracetic acid

We investigated the formation of single and mixed species biofilms of Listeria monocytogenes strains EGD-e and LR-991, with Lactobacillus plantarum WCFS1 as secondary species, and their resistance to the disinfectants benzalkonium chloride and peracetic acid. Modulation of growth, biofilm formation, and biofilm composition was achieved by addition of manganese sulfate and/or glucose to the BHI medium. Composition analyses of the mixed species biofilms using plate counts and fluorescence microscopy with dual fluorophores showed that mixed species biofilms were formed in BHI (total count, 8-9 log₁₀ cfu/well) and that they contained 1-2 log₁₀ cfu/well more L. monocytogenes than L. plantarum cells. Addition of manganese sulfate resulted in equal numbers of both species (total count, 8 log₁₀ cfu/well) in the mixed species biofilm, while manganese sulfate in combination with glucose, resulted in 1-2 log₁₀ more L. plantarum than L. monocytogenes cells (total count, 9 log₁₀ cfu/well). Corresponding single species biofilms of L. monocytogenes and L. plantarum contained up to 9 log₁₀ cfu/well. Subsequent disinfection treatments showed mixed species biofilms to be more resistant to treatments with the selected disinfectants. In BHI with additional manganese sulfate, both L. monocytogenes strains and L. plantarum grown in the mixed species biofilm showed less than 2 log₁₀ cfu/well inactivation after exposure for 15 min to 100 μg/ml benzalkonium chloride, while single species biofilms of both L. monocytogenes strains showed 4.5 log₁₀ cfu/well inactivation and single species biofilms of L. plantarum showed 3.3 log₁₀ cfu/well inactivation. Our results indicate that L. monocytogenes and L. plantarum mixed species biofilms can be more resistant to disinfection treatments than single species biofilms.     

Roles of outer membrane protein W (OmpW) on survival,morphology, and biofilm formation under NaCl stresses in Cronobacter sakazakii

Cronobacter sakazakii is an important foodborne pathogen associated with rare but severe infections through consumption of powdered infant formula. Tolerance to osmotic stress in Cronobacter has been described. However, the detailed factors involved in tolerance to osmotic stress in C. sakazakii are poorly understood. In this study, roles of outer membrane protein W (OmpW) on survival rates, morphologic changes of cells, and biofilm formation in C. sakazakii under different NaCl concentrations between wild type (WT) and OmpW mutant (ΔOmpW) were determined. The survival rates of ΔOmpW in Luria-Bertani medium with 3.5% or 5.5% NaCl were reduced significantly, and morphological injury of ΔOmpW was significantly increased compared with survival and morphology of WT. Compared with biofilm formation of the WT strain, biofilms in ΔOmpW were significantly increased in Luria-Bertani with 3.5% or 5.5% NaCl using crystal violet staining assay after 48 and 72 h of incubation. Detection of biofilms using confocal laser scanning microscopy and scanning electron microscopy further confirmed the changes of biofilm formation under different NaCl stresses. This study demonstrates that OmpW contributes to survival of cells in planktonic mode under NaCl stresses, and biofilm formation is increased in ΔOmpW in response to NaCl stress.     

Interaction and inactivation of Listeria and Lactobacillus cells in single and mixed species biofilms exposed to different disinfectants

Listeria spp. are ubiquitously found in both the natural and the food processing environment, of which Listeria monocytogenes is of an important health risk. Here, we report on the formation of single and mixed species biofilms of L. monocytogenes/Listeria innocua and Lactobacillus plantarum strains in 24‐well polystyrene microtiter plates and on the inactivation of 24‐hr and 72‐hr biofilms using quaternary ammonium compound‐, tertiary alkyl amine‐, and chlorine‐based disinfectants. Fluorescent in situ hybridization (FISH) and LIVE/DEAD BacLight staining were applied for 72‐hr L. innocua–L. plantarum mixed biofilms in the LabTek system for the species identification and the reaction of biofilm cells to disinfectants, respectively. L. monocytogenes/L. innocua were more resistant to disinfectants in 72‐hr than in 24‐hr biofilms, whereas L. plantarum strains did not show any significant differences between 72‐hr and 24‐hr biofilms. Furthermore, L. innocua when grown with L. plantarum was more resistant to all disinfection treatments, indicating a protective effect from lactobacilli in the mixed species biofilm. The biofilm formation and reaction to disinfectants, microscopically verified using fluorescence in situ hybridization and LIVE/DEAD staining, showed that L. innocua and L. plantarum form a dense mixed biofilm and also suggested the shielding effect of L. plantarum on L. innocua in the mixed species biofilm.     

Removal of Salmonella enterica serovar Typhimurium and Cronobacter sakazakii biofilms from food contact surfaces through enzymatic catalysis

Bacterial biofilms are highly difficult to control, hence significant economic resources have been allocated to develop strategies to eradicate them. This study evaluated the effect of an enzymatic treatment to be used as a cleaning product to control the presence of biofilms. Two different materials used in the food industry, polystyrene and stainless steel, were tested using Salmonella Typhimuirum and Cronobacter sakazakii. Biofilm formation was carried out by inoculating the surfaces with a standardized concentration of 4 log (CFU cm⁻²) and incubated for 48 hr with renewal of nutrients. The biofilm formation and subsequent enzymatic treatment were quantified using fluorescence microscopy and the conventional culture method. The enzymatic treatment showed significant reductions of 2–3 log (CFU cm⁻²) in biofilm cells, which was attributed to the degradation of the extracellular matrix and the further detachment of both microorganisms. The maximum biofilm detachment obtained with the preventive formula was 46.67%; however, this percentage could be increased by applying an aggressive treatment or by adding a subsequent disinfection step that would eliminate adhered microbial cells. Further, the enzymatic cleaning treatment could be exploited as a potent technology to control bacterial adherence and biofilm formation in the food industry.     

Attachment and biofilm formation of Pseudomonas fluorescens PSD4 isolated from a dairy processing line

The effects of different nutrient sources and temperatures on attachment and biofilm formation of Pseudomonas fluorescens PSD4, a dairy isolate, were studied. Initial adherence and attachment capabilities among different strains were studied using microtitre plate assays. Biofilm development was observed using confocal microscopy. Strongly adherent cells were seen in protein rich media. Citrate as a carbon source enhanced biofilm formation. Glucose did not favor biofilm development. Psychrotrophic P. fluorescens PSD4 formed strongly adherent biofilms having high metabolic activities at low temperatures. P. fluorescens PSD4 with spoilage potential was capable of forming strong biofilms in dairy processing environments. Biofilm formation was influenced by nutrient availability and growth conditions. These factors should be considered for design of effective anti-biofilm strategies.     

Biofilm Formation of Staphylococcus aureus on Various Surfaces and Their Resistance to Chlorine Sanitizer

This study investigated the effect of material types (polystyrene, polypropylene, glass, and stainless steel) and glucose addition on Staphylococcus aureus biofilm formation, and the relationship between biofilm formation measured by crystal violet (CV) staining and the number of biofilm cells determined by cell counts was studied. We also evaluated the efficacy of chlorine sanitizer on inhibiting various different types of S. aureus biofilms on the surface of stainless steel. Levels of biofilm formation of S. aureus were higher on hydrophilic surfaces (glass and stainless steel) than on hydrophobic surfaces (polypropylene and polystyrene). With the exception of biofilm formed on glass, the addition of glucose in broth significantly increased the biofilm formation of S. aureus on all surfaces and for all tested strains (P ≤ 0.05). The number of biofilm cells was not correlated with the biomass of the biofilms determined using the CV staining method. The efficacy of chlorine sanitizer against biofilm of S. aureus was not significantly different depending on types of biofilm (P > 0.05). Therefore, further studies are needed in order to determine an accurate method quantifying levels of bacterial biofilm and to evaluate the resistance of bacterial biofilm on the material surface.     

The biofilm forming potential of bacterial species in the genus Campylobacter

The biofilm forming abilities of 16 strains representative of 14 of the 16 species comprising the genus Campylobacter were determined on glass, stainless steel, and polystyrene plastic. The formation of biofilms has been suggested as a means by which Campylobacter is able to persist within an inhospitable environment. Of the eight microaerophilic Campylobacter species, including two strains each of Campylobacter jejuni and Campylobacter fetus, only C. jejuni strain 81–176 reliably produced a visible biofilm on multiple surfaces. Alternately, all six strains of the anaerobic Campylobacter species reliably produced visible biofilms on multiple surfaces. Electron micrographs of the individual biofilms showed relatively homogeneous biofilms produced by the anaerobic strains, while the microaerophilic C. jejuni strain 81–176 produced a biofilm containing similar quantities of both the spiral and coccoid forms. This survey suggests a difference in the biofilm forming potentials and the morphologies of the bacteria comprising the biofilms between anaerobic and microaerophilic species of Campylobacter. Additionally, differences observed in the biofilm forming ability of two strains of C. jejuni suggest the need for a further investigation of the biofilm forming potential of this species using a larger number of strains.     

Differences in biofilm formation of produce and poultry Salmonella enterica isolates and their persistence on spinach plants

Spinach plants were irrigated biweekly with water containing 2.1 log CFU Salmonella/100 ml water (the maximum Escherichia coli MPN recommended by the Leafy Greens Marketing Agreement; LGMA), or 4.1 CFU Salmonella/100 ml water to determine Salmonella persistence on spinach leaves. Green Fluorescent protein expressing Salmonella were undetectable by most-probable number (MPN) at 24 h and 7 days following each irrigation event. This study indicates that Salmonella are unlikely to persist on spinach leaves when irrigation water is contaminated at a level below the LGMA standards. In a parallel study, persistence of Salmonella isolated from poultry or produce was compared following biweekly irrigation of spinach plants with water containing 6 log CFU Salmonella/100 ml. Produce Salmonella isolates formed greater biofilms on polystyrene, polycarbonate and stainless steel surfaces and persisted at significantly higher numbers on spinach leaves than those Salmonella from poultry origin during 35 days study. Poultry Salmonella isolates were undetectable (<1 log CFU/g) on spinach plants 7 days following each irrigation event when assayed by direct plating. This study indicates that Salmonella persistence on spinach leaves is affected by the source of contamination and the biofilm forming ability of the strain.