Destruction of Listeria monocytogenes by Sodium Hypochlorite and Quaternary Ammonium Sanitizers

Combination of Blue Light and Chemical Sanitizers for Inactivation of Listeria monocytogenes Dried Cells on Inert Surfaces

Environmental contamination of ready-to-eat (RTE) foods with Listeria monocytogenes is a major food safety concern. Treatment of surfaces with antimicrobial blue light (aBL) has recently emerged as a technology that can supplement current sanitation practices. Chemical sanitizers are used extensively by the food industry, but their combination with aBL at 405 nm has not been evaluated. This project was undertaken to determine the combined effect of sanitizers with aBL to inactivate L. monocytogenes dried cells on inert surfaces. Peracetic acid (PAA), benzalkonium chloride (BAC), and sodium hypochlorite (NaClO) at time/concentrations that delivered less than 2 Log CFU viability reductions were combined with limited aBL treatments applied simultaneously or sequential on cells dried on stainless steel (SS) and polytetrafluoroethylene (PTFE) coupons. When five-strain mixtures were dried on SS and treated with aBL alone (575 J/cm2), average viability reductions of less than 2 Log CFU/coupon were observed. Single treatments with NaClO (20 ppm, 60 min) and PAA (30 ppm, 30 min) caused less than 1.5 Log CFU/coupon inactivation on SS. During independent experiments that combined the same concentration/time of NaClO and PAA applied simultaneously with the above aBL dose, viability reductions of 5.4 and 4.7 Log CFU/coupon, respectively, were measured. The results of sequential treatments of dried cells with sanitizers and aBL were variable depending on the sanitizer, concentration, surface, and the sequence order. Measured reductions of sequential treatments varied from 1.5 Log CFU/coupon with BAC (40 ppm/30 min) to 5.5 Log CFU/coupon with NaClO (75 ppm/60 min) applied before aBL on PTFE. The comparison between the results obtained at low sanitizer concentrations simultaneously used with aBL to the sum of the single results (3.5 vs. 5.4 Log CFU/coupon or 3.5 vs. 4.7 Log CFU/coupon) resulted in statistically significant differences (p < 0.05). These findings suggested that there was a synergistic effect between sanitizers and aBL.

Listeria monocytogenes and Listeriosis: The Global Enigma

Listeria monocytogenes is an intracellular, Gram-positive, non-spore-forming, non-encapsulated, facultative anaerobic, rod-shaped, and psychrotrophic food-borne pathogen that causes the infection, listeriosis, thus it attracts great attention following listeriosis outbreaks, which are often associated with high mortality rates. The prevalence of listeriosis is quite low globally; however, the most recent and deadliest outbreak occurred in South Africa, during which 216 persons lost their lives. L. monocytogenes is endowed with the potential to multiply through a wide range of harsh environmental conditions, forming biofilms on varying surfaces in the food industry, as well as having persistent and antibiotic-resistant cells, which pose a major threat and burden to the ready-to-eat food industry. A more frustrating characteristic of this bacterium is its strain divergence, alongside an increased level of antibiotic resistance registered among the strains of L. monocytogenes recovered from food, humans, and environmental sources, especially to those antibiotics involved in the treatment of human listeriosis. Antibiotic resistance exerted by and among pathogenic food-borne microbes is an ongoing public health menace that continues to be an issue. Against this background, a thorough search into different databases using various search engines was performed, which led to the gathering of salient information that was organised, chronologically, based on Listeria monocytogenes and listeriosis. Altogether, the findings elaborated in this study present up-to date knowledge on different aspects of this pathogen which will improve our understanding of the mystery associated with it and the ways to prevent and control its dissemination through ready-to-eat foods. In addition, constant monitoring of the antibiotic resistance profiles of strains of L. monocytogenes from varying sources detected changes, giving an update on the trend in antibiotic resistance. Overall, monitoring of bacterial contamination serves as the key aspect in the control of the food safety output in the food industry.

Process Technologies for Disinfection of Food-Contact Surfaces in the Dry Food Industry: A Review

The survival characteristics of bacterial pathogens, including Salmonella spp., Listeria monocytogenes, Staphylococcus aureus, and Escherichia coli, in foods with a low water activity (aw) have been extensively examined and reported. Microbial attachment on the food-contact surfaces can result in cross-contamination and compromise the safety of low-aw foods. The bactericidal potential of various conventional and novel disinfection technologies has been explored in the dry food industry. However, the attachment behavior of bacterial pathogens to food-contact surfaces in low-aw conditions and their subsequent response to the cleaning and disinfection practices requires further elucidation. The review summarizes the elements that influence disinfection, such as the presence of organic residues, persistent strains, and the possibility of microbial biotransfer. This review explores in detail the selected dry disinfection technologies, including superheated steam, fumigation, alcohol-based disinfectants, UV radiation, and cold plasma, that can be used in the dry food industry. The review also highlights the use of several wet disinfection technologies employing chemical antimicrobial agents against surface-dried microorganisms on food-contact surfaces. In addition, the disinfection efficacy of conventional and novel technologies against surface-dried microorganisms on food-contact surfaces, as well as their advantages and disadvantages and underlying mechanisms, are discussed. Dry food processing facilities should implement stringent disinfection procedures to ensure food safety. Environmental monitoring procedures and management techniques are essential to prevent adhesion and allow the subsequent inactivation of microorganisms.

Listeria monocytogenes Biofilms in Food-Associated Environments: A Persistent Enigma

Listeria monocytogenes (LM) is a bacterial pathogen responsible for listeriosis, a foodborne illness associated with high rates of mortality (20-30%) and hospitalisation. It is particularly dangerous among vulnerable groups, such as newborns, pregnant women and the elderly. The persistence of this organism in food-associated environments for months to years has been linked to several devastating listeriosis outbreaks. It may also result in significant costs to food businesses and economies. Currently, the mechanisms that facilitate LM persistence are poorly understood. Unravelling the enigma of what drives listerial persistence will be critical for developing more targeted control and prevention strategies. One prevailing hypothesis is that persistent strains exhibit stronger biofilm production on abiotic surfaces in food-associated environments. This review aims to (i) provide a comprehensive overview of the research on the relationship between listerial persistence and biofilm formation from phenotypic and whole-genome sequencing (WGS) studies; (ii) to highlight the ongoing challenges in determining the role biofilm development plays in persistence, if any; and (iii) to propose future research directions for overcoming these challenges.

Effectiveness of Quaternary Ammonium in Reducing Microbial Load on Eggs

Table eggs are an affordable yet nutritious protein source for humans. Unfortunately, eggs are a vector for bacteria that could cause foodborne illness. This study aimed to investigate the effectiveness of a quaternary ammonium compound (quat) sanitizer against aerobic mesophilic bacteria, yeast, and mold load on the eggshell surface of free-range and commercial farms and the post-treatment effect on microbial load during storage. Total aerobic mesophilic bacteria, yeast, and molds were enumerated using plate count techniques. The efficacy of the quaternary ammonium sanitizer (quat) was tested using two levels: full factorial with two replicates for corner points, factor A (maximum: 200 ppm, minimum: 100 ppm) and factor B (maximum: 15 min, minimum: 5 min). Quat sanitizer significantly (p < 0.05) reduced approximately 4 log10 CFU/cm² of the aerobic mesophilic bacteria, 1.5 to 2.5 log10 CFU/cm² of the mold population, and 1.5 to 2 log10 CFU/cm² of the yeast population. However, there was no significant (p ≥ 0.05) response observed between individual factor levels (maximum and minimum), and two-way interaction terms were also not statistically significant (p ≥ 0.05). A low (<1 log10 CFU/cm²) aerobic mesophilic bacteria trend was observed when shell eggs were stored in a cold environment up to the production expiry date. No internal microbial load was observed; thus, it was postulated that washing with quat sanitizer discreetly (without physically damaging the eggshell) does not facilitate microbial penetration during storage at either room temperature or cold storage. Current study findings demonstrated that the quat sanitizer effectively reduced the microbial population on eggshells without promoting internal microbial growth.

Comparative study on the efficacy of sodium hypochlorite, aqueous ozone, and peracetic acid in the elimination of Salmonella from cattle manure contaminated various surfaces supported by Bayesian analysis

Providing the dairy industry with an effective and safe disinfectant is considered a key step in improving the farm hygiene and biosecurity. Salmonella infection via foodborne transmission remains a major public health threat. The main objective of this study was therefore to characterize and compare the decontamination power of NaOCl, aqueous-O₃, and PAA against cattle manure based-Salmonella heavily contaminated various surfaces (plastic, nylon, rubber, and wood) using Bayesian analysis. In a crossover design, 14 strips of each material were randomly assigned between 3 groups, treatment (n = 6), positive-control (contaminated with feces-Salmonella mixture, but not exposed to disinfectants, n = 6), and negative control (laboratory blank, inoculated only with sterile water, n = 2). The strips were soaked in cattle manure inoculated with 10⁷–10⁸ of Salmonella Typhimurium-Choleraesuis (aSTC) and exposed to 50 mL of 200 ppm NaOCl, 9 ppm aqueous-O₃, or 400 ppm PAA for 4 minutes. Bayesian methods were used for analysis. On plastic and nylon surfaces, NaOCl, aqueous-O_3, or PAA reduce aSTC population to a safe level (>5.0-log₁₀) within 4 minutes. On rubber surface, PAA and aqueous-O₃ can produce a reduction in aSTC population 50% and 30% higher than NaOCl with posterior probabilities of 97% and 90%, respectively. However, PAA can produce reduction factor on wood surface 40% higher than aqueous-O₃ and NaOCl with posterior probabilities of 97% and 73%, respectively. We conclude that smooth surfaces were most effectively decontaminated. Peracetic acid of 400 ppm can provide an effective means for controlling Salmonella population heavily contaminated various surfaces in dairy operations. However, the safe residues and strong reactivity makes aqueous-O₃ and PAA attractive alternative disinfectants for improving farm hygiene and biosecurity.

Synergistic effects of ultrasound and sodium hypochlorite (NaOCl) on reducing Listeria monocytogenes ATCC19118 in broth, stainless steel, and iceberg lettuce

This study was performed in order to determine whether a combined treatment of ultrasound and sodium hypochlorite (NaOCl) is more effective than individual treatment on reducing Listeria monocytogenes ATCC19118 on stainless steel and iceberg lettuce. The bactericidal effect of ultrasound and NaOCl was investigated in tryptic soy broth (TSB), on stainless steel and iceberg lettuce. Various concentrations of NaOCl (50, 100, 150, and 200 ppm) were tested along with various ultrasound treatment times (5, 20, 40, 60, 80, and 100 min). The combined treatment of ultrasound and NaOCl resulted in greater bacterial reductions than either treatment alone, without causing any significant changes in lettuce texture. The synergistic values of combined ultrasound and NaOCl treatments in TSB, on stainless steel, and on iceberg lettuce were 0.01-0.99 log10 colony-forming units (CFU)/mL, 0.01-0.62 log 10 CFU/g, and 0.12-1.66 log10 CFU/g, respectively. These results suggest that the combination of ultrasound and NaOCl was more effective than each treatment against Listeria monocytogenes, and that this combination can effectively sanitize fresh products such as iceberg lettuce.

Evaluation of the removal and destruction effect of a chlorine and thiamine dilaurylsulfate combined treatment on L. monocytogenes biofilm

The present study investigated the efficacy of single and combined treatment of both chlorine and thiamine dilaurylsulfate (TDS) on the reduction of Listeria monocytogenes biofilms in microtiter plate. The disinfectants used in this study were 50, 100, and 200 mg/L chlorine and 100, 500, and 1000 mg/L of TDS. Biofilm-forming index (BFI) and culturable cell count were used to evaluate the disinfectant assay. The highest BFI reduction was 0.80, achieved by the combination of 200 mg/L chlorine and 1000 mg/L TDS. In contrast, the highest culturable cell count reduction was 4.80 log colony-forming units/well by the combination of 200 mg/L chlorine and 100 mg/L TDS. The BFI was reduced in a concentration-dependent manner while culturable cell count was significantly reduced only when all chlorine concentration was combined with 100 mg/L TDS. However, when chlorine was combined with a higher concentration of TDS, the reduction decreased significantly. The result in this study showed that the combination of the 200 mg/L chlorine and 1000 mg/L TDS could be a practical application in removing L. monocytogenes biofilms from surfaces in food industry, and for the 200 mg/L chlorine and 100 mg/L, it can be used for killing the pathogen biofilms. However, more studies are still needed in order to show its efficacy on foods surfaces as well as to develop an even more effective treatment in both killing and removing biofilms.

Development and Control of Bacterial Biofilms on Dairy Processing Membranes

Membrane fouling is a major operational problem that leads to reduced membrane performance and premature replacement of membranes. Bacterial biofilms developed on reverse osmosis membranes can cause severe flux declines during whey processing. Various types of biological, physical, and chemical factors regulate the formation of biofilms. Extracellular polymeric substances produced by constitutive microflora provide an effective barrier for the embedded cells. Cultural and microscopic techniques also revealed the presence of biofilms with attached bacterial cells on membrane surfaces. Presence of biofilms, despite regular cleaning processes, reflects ineffectiveness of cleaning agents. Cleaning efficiency depends upon factors such as pH of the cleaning agent, temperature, pressure, cleaning agent dose, optimum cleaning time, and cross-flow velocity during cleaning. Among different cleaning agents, surfactants help to prevent bacterial attachment to surfaces by reducing the surface tension of water and interfacial tension between the layers. Enzymes mixed with surfactants and chelating agents can be used to penetrate the biofilm matrix formed by microbes. Recent studies have shown the role of quorum-sensing-based cell-to-cell signaling, which provides communication within bacterial cells to form a mature biofilm, and also the role of applying quorum inhibitors to prevent biofilm formation. Major cleaning applications are also summarized in Table .

Molecular characterization of biofilm formation and attachment of Salmonella enterica serovar typhimurium DT104 on food contact surfaces

The molecular mechanism of biofilm formation by Salmonella Typhimuriun DT104 was characterized for a better understanding of its attachment and colonization in food processing environments. A library of random mutagenized clones was screened for phenotypic analyses of their ability to form biofilm, pellicle, curli, and cellulose. The genes identified were involved in lipopolysaccharide synthesis, assembly of flagella, regulation of rRNA biosynthesis, and outer membrane transportation and signaling. The insertion of transposon in flgK, rfbA, nusB, and pnp genes resulted in decreased biofilm formation. Alterations of flagellar and lipopolysaccharide production were confirmed in the flgK mutant and rfbA mutant, respectively. Biofilm formation by these four mutants in meat and poultry broths and their attachment on surfaces of stainless steel and glass were significantly reduced compared with those of the wild-type strain (P < 0.05). On the contrary, the mutation of STM4263 and yjcC genes in Salmonella Typhimuriun DT104 resulted in increased biofilm formation and attachment of the species in tested broths and on contact surfaces. Our findings suggest that many factors, such as production of exopolymeric substances and their efficient transportation through outer membrane, expression of flagella, and regulation of exoribonucleases and RNA-binding protein, could be involved in biofilm formation and attachment of Salmonella Typhimurium DT104 on contact surfaces.

Production of biofilm and quorum sensing by Escherichia coli O157:H7 and its transfer from contact surfaces to meat, poultry, ready-to-eat deli, and produce products

Multistate outbreaks of Escherichia coli O157:H7 infections through consumption of contaminated foods including produce products have brought a great safety concern. The objectives of this study were to determine the effect of biofilm and quorum sensing production on the attachment of E. coli O157:H7 on food contact surfaces and to evaluate the transfer of the pathogen from the food contact to various food products. E. coli O157:H7 produced maximum levels of AI-2 signals in 12 h of incubation in tested meat, poultry, and produce broths and subsequently formed strong biofilm in 24 h of incubation. In general, E. coli O157:H7 formed stronger biofilm on stainless steel than glass. Furthermore, E. coli O157:H7 that had attached on the surface of stainless steel was able to transfer to meat, poultry, ready-to-eat deli, and produce products. Strong attachment of the transferred pathogen on produce products (cantaloupe, lettuce, carrot, and spinach) was detected (>10(3) CFU/cm2) even after washing these products with water. Our findings suggest that biofilm formation by E. coli O157:H7 on food contact surfaces can be a concern for efficient control of the pathogen particularly in produce products that require no heating or cooking prior to consumption.

A putative ABC transporter is involved in negative regulation of biofilm formation by Listeria monocytogenes

Listeria monocytogenes may persist for long periods in food processing environments. In some instances, this may be due to aggregation or biofilm formation. To investigate the mechanism controlling biofilm formation in the food-borne pathogen L. monocytogenes, we characterized LM-49, a mutant with enhanced ability of biofilm formation generated via transposon Tn917 mutagenesis of L. monocytogenes 4b G. In this mutant, a Tn917 insertion has disrupted the coding region of the gene encoding a putative ATP-binding cassette (ABC) transporter permease identical to Lmof2365_1771 (a putative ABC transporter permease) presented in the sequenced strain L. monocytogenes strain 4b F2365. This disrupted gene, denoted lm.G_1771, encoded a protein with 10 transmembrane helices. The revertant, LM-49RE, was obtained by replacing lm.G_1771::Tn917 with lm.G_1771 via homologous recombination. We found that LM-49RE formed the same amount of biofilm biomass as the wild-type strain. Furthermore, transcription of the downstream lm.G_1770 gene was not influenced by the upstream Tn917 insertion, and the presence of Tn917 has no effect on biofilm formation. These results suggest that lm.G_1771 was solely responsible for the negative regulation of biofilm formation by L. monocytogenes 4b G. The immediate gene upstream of lm.G_1771 encoded an ATP-binding protein. Bioinformatics analysis suggested that these two genes were organized into an operon and that their proteins formed an export ABC transporter. Here, we report the characterization of the mutant and identification of a novel ABC transporter that functions in negative regulation of biofilm formation in L. monocytogenes.

Biofilm formation by multidrug-resistant Salmonella enterica serotype typhimurium phage type DT104 and other pathogens

The biofilm-forming capability of Salmonella enterica serotypes Typhimurium and Heidelberg, Pseudomonas aeruginosa, Listeria monocytogenes, Escherichia coli O157:H7, Klebsiella pneumoniae, and Acinetobacter baumannii isolated from humans, animal farms, and retail meat products was evaluated by using a microplate assay. The tested bacterial species showed interstrain variation in their capabilities to form biofilms. Strong biofilm-forming strains of S. enterica serotypes, E. coli O157: H7, P. aeruginosa, K. pneumoniae, and A. baumannii were resistant to at least four of the tested antibiotics. To understand their potential in forming biofilms in food-processing environments, the strong biofilm formers grown in beef, turkey, and lettuce broths were further investigated on stainless steel and glass surfaces. Among the tested strains, Salmonella Typhimurium phage type DT104 (Salmonella Typhimurium DT104) isolated from retail beef formed the strongest biofilm on stainless steel and glass in beef and turkey broths. K. pneumoniae, L. monocytogenes, and P. aeruginosa were also able to form strong biofilms on the tested surface materials. Salmonella Typhimurium DT104 developed a biofilm on stainless steel in beef and turkey broths through (i) initial attachment to the surface, (ii) formation of microcolonies, and (iii) biofilm maturation. These findings indicated that Salmonella Typhimurium DT104 alongwith other bacterial pathogens could be a source of cross-contamination during handling and processing of food.

Chlorine resistance of Listeria monocytogenes biofilms and relationship to subtype, cell density, and planktonic cell chlorine resistance

Strains of Listeria monocytogenes vary in their ability to produce biofilms. This research determined if cell density, planktonic chlorine resistance, or subtype are associated with the resistance of L. monocytogenes biofilms to chlorine. Thirteen strains of L. monocytogenes were selected for this research based on biofilm accumulation on stainless steel and rep-PCR subtyping. These strains were challenged with chlorine to determine the resistance of individual strains of L. monocytogenes. Planktonic cells were exposed to 20 to 80 ppm sodium hypochlorite in 20 ppm increments for 5 min in triplicate per replication, and the experiment was replicated three times. The number of tubes with surviving L. monocytogenes was recorded for each isolate at each level of chlorine. Biofilms of each strain were grown on stainless steel coupons. The biofilms were exposed 60 ppm of sodium hypochlorite. When in planktonic culture, four strains were able to survive exposure to 40 ppm of chlorine, whereas four strains were able to survive 80 ppm of chlorine in at least one of three tubes. The remaining five strains survived exposure to 60 ppm of chlorine. Biofilms of 11 strains survived exposure to 60 ppm of chlorine. No association of biofilm chlorine resistance and planktonic chlorine resistance was observed; however, biofilm chorine resistance was similar for strains of the same subtype. Biofilm cell density was not associated with chlorine resistance. In addition, biofilms that survived chlorine treatment exhibited different biofilm morphologies. These data suggest that chlorine resistance mechanisms of planktonic cells and biofilms differ, with planktonic chlorine resistance being more affected by inducible traits, and biofilm chlorine resistance being more affected by traits not determined in this study.

Comparison of the chlorine inactivation of Yersinia enterocolitica in chlorine demand and demand-free systems

Inactivation of Yersinia enterocolitica by chlorine (0.6 to 20 ppm) was investigated in distilled water and in tryptic soy broth (TSB, 0.015%) at different temperatures (4, 20, and 40 degrees C). In distilled water, chlorine inactivation of Y. enterocolitica was enhanced by increasing the temperature from 4 to 20 degrees C, and survival curves were described by a model that assumed first-order kinetics followed by tailing in which the microbial concentration remained constant. The presence of TSB increased chlorine resistance of Y. enterocolitica, and survival curves were concave downward. These survival curves were described by a model based on the Weibull distribution. Chlorine decay in distilled water was independent of temperature and of the initial concentration of available chlorine and was modeled by first-order reaction kinetics. Chlorine decay in TSB was independent of the initial chlorine concentration but depended on the treatment temperature and was modeled by the addition of two first-order decay equations. The increased resistance of Y. enterocolitica to chlorine in TSB was not due only to the chlorine demand by the TSB components. These components protected Y. enterocolitica cells from the antimicrobial effect of chlorine.

Elimination of Listeria monocytogenes biofilms by ozone, chlorine, and hydrogen peroxide

This study evaluated the efficacy of ozone, chlorine, and hydrogen peroxide to destroy Listeria monocytogenes planktonic cells and biofilms of two test strains, Scott A and 10403S. L. monocytogenes was sensitive to ozone (O3), chlorine, and hydrogen peroxide (H2O2). Planktonic cells of strain Scott A were completely destroyed by exposure to 0.25 ppm O3 (8.29-log reduction, CFU per milliliter). Ozone's destruction of Scott A increased when the concentration was increased, with complete elimination at 4.00 ppm O3 (8.07-log reduction, CFU per chip). A 16-fold increase in sanitizer concentration was required to destroy biofilm cells of L. monocytogenes versus planktonic cells of strain Scott A. Strain 10403S required an ozone concentration of 1.00 ppm to eliminate planktonic cells (8.16-log reduction, CFU per milliliter). Attached cells of the same strain were eliminated at a concentration of 4.00 ppm O3 (7.47-log reduction, CFU per chip). At 100 ppm chlorine at 20 degrees C, the number of planktonic cells of L. monocytogenes 10403S was reduced by 5.77 log CFU/ml after 5 min of exposure and by 6.49 log CFU/ml after 10 min of exposure. Biofilm cells were reduced by 5.79 log CFU per chip following exposure to 100 ppm chlorine at 20 degrees C for 5 min, with complete elimination (6.27 log CFU per chip) after exposure to 150 ppm at 20 degrees C for 1 min. A 3% H2O2 solution reduced the initial concentration of L. monocytogenes Scott A planktonic cells by 6.0 log CFU/ml after 10 min of exposure at 20 degrees C, and a 3.5% H2O2 solution reduced the planktonic population by 5.4 and 8.7 log CFU/ml (complete elimination) after 5 and 10 min of exposure at 20 degrees C, respectively. Exposure of cells grown as biofilms to 5% H2O2 resulted in a 4.14-log CFU per chip reduction after 10 min of exposure at 20 degrees C and in a 5.58-log CFU per chip reduction (complete elimination) after 15 min of exposure.

Formation of biofilms by Listeria monocytogenes under various growth conditions

Eight strains of Listeria monocytogenes (7644, 19112, 15313, Scott A, LCDC, 10403S, SLCC, and 1370) produce biofilms when grown on polyvinyl chloride microtiter well plates. The growth medium (tryptic soy broth [TSB] or modified Welshimer's broth [MWB] at 32 degrees C) influenced the amount of biofilm formed; maximum biofilms were formed in MWB by six strains and in TSB by the remaining two strains. This result suggests that the growth medium is critical in development of L. monocytogenes biofilm. This organism also produced biofilms on stainless steel chips. Biofilm formation on these chips was observed following growth in TSB at 4, 20, and 37 degrees C. After 20 h of incubation at 20 or 37 degrees C, the cell density was approximately 10(6) CFU per chip, and after 4 days incubation at 4 degrees C, the cell density was 10(5) CFU per chip. L. monocytogenes strain Scott A biofilm formation on stainless steel chips was visualized using scanning electron microscopy, which revealed dense aggregates of cells held together by meshlike webbing.

Relationship between inactivation kinetics of a Listeria monocytogenes suspension by chlorine and its chlorine demand

AIMS

Chlorine demand by Listeria monocytogenes cells and inactivation of L. monocytogenes by chlorine (0.6-1.0 mg l(-1)) at different temperatures (4, 20 and 30 degrees C) have been investigated in a batch reactor.

METHODS AND RESULTS

Chlorine demand depended on the microbial concentration and was independent on the initial chlorine concentration and temperature. Chlorine decay was modelled by the addition of two first-order decay equations. Inactivation of L. monocytogenes by chlorine depended on the initial microbial concentration, initial chlorine concentration and temperature. A mathematical model based on a biphasic inactivation properly described survival curves of L. monocytogenes and a tertiary model was developed that satisfactorily predicted the inactivation of L. monocytogenes by different concentrations of initial chlorine at different temperatures.

CONCLUSIONS

Both available chlorine decay and inactivation of L. monocytogenes by chlorine were biphasic and can be modelled by a two-term exponential model.

SIGNIFICANCE AND IMPACT OF THE STUDY

The biphasic nature of survival curves of L. monocytogenes did not reflect the effect of a change of available chlorine concentration during the treatment. The microbial inactivation was caused by successive reactions that occur after the consumption of the chlorine by the bacterial cell components.

Effectiveness of chlorine washing disinfection and effects on the appearance of artichoke and borage

AIM

Optimal conditions for chlorine application to obtain a reasonable decrease in the microbial counts without damaging the appearance of artichoke and borage have been established.

METHODS AND RESULTS

The influence of chlorine concentration (0-200 mg l(-1)), pH, addition of organic acids, contact time and presence of protective structures on the microflora and vegetal appearance were studied. When pH was not controlled the effect of chlorine depended on its concentration until the pH increase caused by addition of chlorine reached 8.8. Any further increase in chlorine concentration was nullified by the pH increase. When pH was adjusted to 4.5 with acetic acid, the effectiveness increased with concentration. However, the use of citric acid to control pH caused a sharp decrease in effectiveness at concentration about 250 mg l(-1). The higher effectiveness of chlorine on homogenized plant extracts compared with the whole plant showed the impact of the vegetal structures on the resistance of the microorganisms. For artichoke, a relationship between the effectiveness of chlorine disinfection and its structures was also found. Extended washing times did not affect the total counts. However, in both vegetables, the appearance was affected by the extended contact times.

CONCLUSIONS

The solutions rendering the highest microbial reduction with minimum damages were: 50 mg l(-1) free chlorine without pH control for artichoke and 100 mg l(-1) free chlorine at pH 7.0 for borage.

SIGNIFICANCE AND IMPACT OF THE STUDY

Specific conditions for chlorine disinfection of artichoke and borage were determined to reduce the microorganisms in minimally processed artichoke and borage without damaging their appearance.

Postadaptational resistance to benzalkonium chloride and subsequent physicochemical modifications of Listeria monocytogenes

Many studies have demonstrated that bacteria, including Listeria monocytogenes, are capable of adapting to disinfectants used in industrial settings after prolonged exposure to sublethal concentrations. However, the consequent alterations of the cell surface due to sanitizer adaptation of this pathogen are not fully understood. Two resistant and four sensitive L. monocytogenes strains from different sources were progressively subcultured with increasing sublethal concentrations of a surfactant, benzalkonium chloride (BC). To evaluate the effects of acquired tolerance to BC, parent and adapted strains were compared by using several morphological and physiological tests. Sensitive strains showed at least a fivefold increase in the MIC, while the MIC doubled for resistant strains after the adaptation period. The hydrophobicities of cells of parent and adapted strains were similar. Serological testing indicated that antigen types 1 and 4 were both present on the cell surface of adapted cells. The data suggest that efflux pumps are the major mechanism of adaptation in sensitive strains and are less important in originally resistant isolates. A different, unknown mechanism was responsible for the original tolerance of resistant isolates. In an originally resistant strain, there was a slight shift in the fatty acid profile after adaptation, whereas sensitive strains had similar profiles. Electron micrographs revealed morphological differences after adaptation. The changes in cell surface antigens, efflux pump utilization, and fatty acid profiles suggest that different mechanisms are used by resistant and sensitive strains for adaptation to BC.

Inactivation and removal of Bacillus cereus by sanitizer and detergent

This study was conducted to compare the susceptibility of planktonic cells, attached single cells and biofilm cells of Bacillus cereus to sodium hypochlorite and Spartec, a quaternary ammonium compound (QAC). Removal of B. cereus biofilm by simulating clean-in-place (CIP) procedures employing 1.5% Spec-Tak 1000, an alkaline detergent; 0.65% Dilac, an acid detergent and 1.5% Diverform Plus, a sodium hypochlorite containing alkaline detergent was also examined. The results show that B. cereus cells in a biofilm were most resistant to chemical sanitizers followed by the attached single cell and cells in a planktonic state. When B. cereus cells were in a planktonic state, 25 ppm hypochlorite or 100 ppm QAC induced a more than 5.0 log CFU/ml reduction of cell numbers within 15 s. However, the same sanitizer used in this study showed little effect on cells in a biofilm even when the exposure time was extended to 5 min. The sanitizers tested were less effective against cells in biofilm formed on milk pre-soiled than on unsoiled stainless steel chips. It was also noted that Spec-Tak 1000 treatment at 70 degrees C for 10 min and water rinse in the first step of the long-hot CIP procedure can effectively remove the biofilm B. cereus cells from the milk pre-soiled stainless chip. With this long-hot CIP procedure, the biofilm cells of B. cereus were removed by ca. 6.10 log CFU/chip compared to ca. 1.47 log CFU/chip noted in the control procedure.

Survival of Listeria monocytogenes in commercial food-processing equipment cleaning solutions and subsequent sensitivity to sanitizers and heat

AIMS

To determine the ability of Listeria monocytogenes to survive exposure to commercial food-processing equipment cleaning solutions and subsequent treatment with sanitizers or heat.

METHODS AND RESULTS

Cells of five strains of L. monocytogenes were suspended in 1% solutions of eight commercial cleaners (pH 7.1-12.5) or in water (control) and incubated at 4 degrees C for 30 min or 48 h before populations were determined by plating on tryptose phosphate agar. After exposure of cells to cleaning solutions for 30 min, populations of the most resistant strain of L. monocytogenes were reduced by < or = 1.63 log10 cfu ml(-1). In only three highly alkaline cleaning solutions (pH 11.6-12.4) were populations reduced significantly (P < or = 0.05) compared with reductions in water. After 48 h, populations were significantly higher in one cleaning solution (pH 10.4) than in water, while populations in six of the other seven cleaning solutions were reduced by > or = 4.72 log10 cfu ml(-1). Cells exposed to cleaning solutions for 30 min became sensitive to 4.0 or 6.0 mg l(-1) free chlorine and to 50 or 100 mg l(-1) benzalkonium chloride and cetylpyridinium chloride, common components of quaternary ammonium sanitizers. Cells exposed to four of the five test cleaners had D56 degrees C values less than or equal to those of the control cells.

CONCLUSIONS

Listeria monocytogenes tolerates exposure to a high concentration of alkaline cleaning solutions but consequently becomes sensitized to sanitizers.

SIGNIFICANCE AND IMPACT OF THE STUDY

The elimination of L. monocytogenes surviving exposure to alkaline cleaning solutions widely used for food-processing equipment is essential and the appropriate use of sanitizers for subsequent application to equipment is important in achieving this goal.

Effect of benzalkonium chloride on viability and energy metabolism in exponential- and stationary-growth-phase cells of Listeria monocytogenes

The difference in killing exponential- and stationary-phase cells of Listeria monocytogenes by benzalkonium chloride (BAC) was investigated by plate counting and linked to relevant bioenergetic parameters. At a low concentration of BAC (8 mg liter(-1)), a similar reduction in viable cell numbers was observed for stationary-phase cells and exponential-phase cells (an approximately 0.22-log unit reduction), although their membrane potential and pH gradient were dissipated. However, at higher concentrations of BAC, exponential-phase cells were more susceptible than stationary-phase cells. At 25 mg liter(-1), the difference in survival on plates was more than 3 log units. For both types of cells, killing, i.e., more than 1-log unit reduction in survival on plates, coincided with complete inhibition of acidification and respiration and total depletion of ATP pools. Killing efficiency was not influenced by the presence of glucose, brain heart infusion medium, or oxygen. Our results suggest that growth phase is one of the major factors that determine the susceptibility of L. monocytogenes to BAC.

Occurrence of and a possible mechanism for resistance to a quaternary ammonium compound in Listeria monocytogenes

In a study of 200 Listeria monocytogenes isolates, 10% were determined to be resistant to benzalkonium chloride (BC). Serial subcultivation of initially BC sensitive (BC(S)) and BC resistant (BC(R)) isolates in sublethal concentrations of BC resulted in enhanced and approximately equal resistance of all strains to the compound. Fifty per cent of the BC(R) isolates showed resistance to ethidium bromide (EB) as well. A proton motive force (pmf)-dependent efflux of EB was demonstrated in BC(R) isolates, and in originally sensitive strains adapted to grow in BC. This efflux was not found in BC(S) strains. The result indicate that BC can induce a broad resistance mechanism based on a pmf-driven efflux pump. There was no indication that this type of resistance was related to resistance to antibiotics.

Low sensitivity of Listeria monocytogenes to quaternary ammonium compounds

Ninety-seven epidemiologically unrelated strains of Listeria monocytogenes were investigated for their sensitivities to quaternary ammonium compounds (benzalkonium chloride and cetrimide). The MICs for seven serogroup 1/2 strains were high. Three came from the environment and four came from food; none were isolated from human or animal samples. All 97 strains carried the mdrL gene, which encodes a multidrug efflux pump, and the orfA gene, a putative transcriptional repressor of mdrL. The absence of plasmids in four of the seven resistant strains and the conservation of resistance after plasmid curing suggested that the resistance genes are not plasmid borne. Moreover, PCR amplification and Southern blot hybridization experiments failed to find genes phylogenetically related to the qacA and smr genes, encoding multidrug efflux systems previously described for the genus Staphylococcus. The high association between nontypeability by phages and the loss of sensitivity to quaternary ammonium compounds are suggestive of an intrinsic resistance due to modifications in the cell wall.

The growth and resistance to sodium hypochlorite of Listeria monocytogenes in a steady-state multispecies biofilm

A constant-depth film fermenter (CDFF) was used to culture a steady-state multispecies biofilm consisting of one strain each of Listeria monocytogenes, Pseudomonas fragi and Staphylococcus xylosus. These bacteria were initially grown together in a conventional chemostat to achieve a steady state before being inoculated into the CDFF over an 18-h period. A dilute tryptone soya broth (TSB) medium was supplied to the CDFF and the biofilm allowed to develop over a 28-d period. This mature biofilm was then subjected to increasing levels of sodium hypochlorite solution to measure any antimicrobial effect. The three organisms were seen to reach a steady state after 6 d in the chemostat before being transferred to the CDFF where the mature multispecies biofilm reached steady state at 17 d. Listeria monocytogenes in both planktonic and biofilm growth stabilized at 1. 8 and 1.5%, respectively, of the total plate counts, while Ps. fragi and Staph. xylosus were the predominant organisms in the biofilm at 59% and 39.5%, respectively, of the total microbial population. Steady-state biofilms in the CDFF were exposed to increasing strengths of sodium hypochlorite; 200, 500 and 1000 p.p.m. free chlorine, but a substantial two-log cycle drop in bacterial numbers was only achieved at 1000 p.p.m. free chlorine. In planktonic culture all three organisms were completely eliminated when exposed to 10 p.p.m. free chlorine for a 30-s period.

Survival of Listeria monocytogenes in commercial cheese brines

The survival of Listeria monocytogenes was determined in commercial cheese brines collected from cheese factories in Wisconsin and northern Illinois. Survival of L. monocytogenes inoculated into commercial cheese brines ranged from < 7 d to over 259 d. Survival did not correlate with pH, salt content, nitrogen content, mineral content, or inherent microbial populations but was negatively associated with addition of sodium hypochlorite at the dairy plant. The L. monocytogenes generally survived longer in brines held at 4 degrees C than at 12 degrees C. Sodium hypochlorite or hydrogen peroxide inactivated L. monocytogenes when added to commercial brines in the lab at 10 to 100 ppm or 0.001% to 0.02%, respectively. Addition of 1% potassium sorbate or 1% sodium benzoate also decreased survival of L. monocytogenes. Laboratory filtration of commercial brines had a negative effect on survival in one of three brines tested. The L. monocytogenes did not grow during incubation in any of the commercial brine samples tested.

Sources and routes of contamination of raw milk with Listeria monocytogenes and its control

To identify the source of contamination of raw bulk milk with Listeria, we attempted to isolate the bacteria from various samples such as cattle-related samples, bulk storage tanks and the environments on three farms. On farms A and B, Listeria monocytogenes was repeatedly isolated from raw milk, while on farm C, it was scarcely isolated from it. On the former farms, Listeria was detected in cattle-related samples and the environments. On the other hand, only one fecal sample on the latter farm was Listeria-positive. Especially, we demonstrated that the bulk tank on farm A was contaminated with L. monocytogenes. Then, L. monocytogenes was controlled by continuously washing the bulk tank on farm A with alkaline detergent.

Significance of microbial biofilms in food industry: a review

Biofilms have been of considerable interest in the context of food hygiene. Of special significance is the ability of microorganisms to attach and grow on food and food-contact surfaces under favourable conditions. Biofilm formation is a dynamic process and different mechanisms are involved in their attachment and growth. Extracellular polymeric substances play an important role in the attachment and colonization of microorganisms to food-contact surfaces. Various techniques have been adopted for the proper study and understanding of biofilm attachment and control. If the microorganisms from food-contact surfaces are not completely removed, they may lead to biofilm formation and also increase the biotransfer potential. Therefore, various preventive and control strategies like hygienic plant lay-out and design of equipment, choice of materials, correct use and selection of detergents and disinfectants coupled with physical methods can be suitably applied for controlling biofilm formation on food-contact surfaces. In addition, bacteriocins and enzymes are gaining importance and have an unique potential in the food industry for the effective biocontrol and removal of biofilms. These newer biocontrol strategies are considered important for the maintenance of biofilm-free systems, for quality and safety of foods.

Microbial biofilms in the food processing industry--should they be a concern?

Biofilm formation will occur on solid surfaces in contact with a liquid. Organic and inorganic material in the liquid sediment onto the solid material. Subsequently, biologically active microorganisms will be attracted to this conditioned surface and adhere to it. The microbial cells will initiate growth, form an attachment matrix and develop into a complex community forming a microbial biofilm. Such microbial biofilms are common on solid surfaces in contact with many different kinds of liquids, fresh water, sea water, oil, milk and so on. These biofilms may be of benefit or be detrimental to the environment where they form. The goal of this review has been to summarize the literature on the development of microbial biofilms in these different environments with particular emphasis on what occurs in the environment of a food processing plant. Methods to control adherent microorganisms and subsequent biofilms in the food processing plant are discussed. It is apparent from the data that has been reviewed that the potential for the development of microbial biofilms in the environment of the food processing plant exists. However, the cleaning and sanitizing practices carried out in the food industry have been shown to control biofilm formation on food contact surfaces. Microbial attachment has been shown to occur on non-food contact surfaces and these attached microbes, if left undisturbed, will form biofilms. The potential for contamination of food with undesirable spoilage and pathogenic bacteria from attached microbes and biofilms exists in these food processing systems. Biofilm formation on non-food contact surfaces needs to be studied further and methods developed to prevent and control these biofilms.