Diversity assessment of Listeria monocytogenes biofilm formation: Impact of growth condition, serotype and strain origin

Influence of different factors on biofilm formation of Listeria monocytogenes and the regulation of cheY gene

In a food-processing environment, bacterial cells often adhere to surfaces and form biofilms to protect themselves from external adverse influences. Our study aimed to identify the influence of environmental factors and cell properties on Listeria monocytogenes biofilm formation. Biofilm formation was quantified through measuring the optical density at 590 nm (OD_{590 nm}) after crystal violet staining. Neutral pH and 37oC were beneficial for biofilm formation whereas the influence of glucose (0.0-1.0%) and sodium chloride (0.0-1.0%) were strain-dependent. In general, the addition of sodium chloride and glucose increased biofilm formation in most strains compared to that in controls with no sodium chloride or glucose added. Bacteria with strong biofilm-forming capacity always produced large amounts of biofilm in most instances. Biofilm formation positively correlated with the cell surface hydrophobicity and motility but was independent from planktonic cell growth. The expression of flagella-related flaA, motB, and the two-component chemotactic system cheA/Y genes in biofilm cells increased compared to that in planktonic cells. Meanwhile, a cheY knockout mutant was constructed, and decreased biofilm-formation ability along with reduced cell-surface hydrophobicity were found in the non-motile mutant. Furthermore, the cheY knockout mutant showed no change in growth, and pH susceptibility compared to that in the wild-type strain.

Interference of sanitizers, NaCl and curing salts on Listeria monocytogenes adhesion and subsequent biofilm formation

Listeria monocytogenes, a well-known foodborne pathogen and the causative agent of listeriosis, has the ability to persist in food processing environments due to its high adhesion ability in different surfaces, playing an important role in the food industry. The aim of this study was to assess how the main stressing conditions, usually observed in meat processing facilities (sanitizers, NaCl, curing salts), interfere in L. monocytogenes adhesion and biofilm formation. The isolates, representatives of different L. monocytogenes lineages (n = 6) were subjected to four different sanitizers (S1: quaternary ammonium; S2: peracetic acid, hydrogen peroxide and glacial acetic acid, S3: biguanide polyhexamethylene hydrochloride, S4: hydrogen peroxide) to verify adhesion ability and susceptibility based on minimum inhibitory concentration (MIC). In addition, the isolates adhesion and biofilm were assessed up to 72 h under different conditions: sanitizers (MIC values), curing salts and NaCl (both at 5, 7·5, 10%), at different temperatures (4, 12 and 37°C). Despite the effectiveness of sanitizers, isolates presented higher biofilm development when compared to controls in the presence of quaternary ammonium (S1, 1: 1,024) at 4°C, over the tested time (P < 0·05). Furthermore, different responses were observed for the different L. monocytogenes strains tested, providing a better understanding of the persistence of this pathogen in the food processing facilities.

Improved understanding of biofilm development by Piscirickettsia salmonis reveals potential risks for the persistence and dissemination of piscirickettsiosis

Piscirickettsia salmonis is the causative agent of piscirickettsiosis, a disease with high socio-economic impacts for Chilean salmonid aquaculture. The identification of major environmental reservoirs for P. salmonis has long been ignored. Most microbial life occurs in biofilms, with possible implications in disease outbreaks as pathogen seed banks. Herein, we report on an in vitro analysis of biofilm formation by P. salmonis Psal-103 (LF-89-like genotype) and Psal-104 (EM-90-like genotype), the aim of which was to gain new insights into the ecological role of biofilms using multiple approaches. The cytotoxic response of the salmon head kidney cell line to P. salmonis showed interisolate differences, depending on the source of the bacterial inoculum (biofilm or planktonic). Biofilm formation showed a variable-length lag-phase, which was associated with wider fluctuations in biofilm viability. Interisolate differences in the lag phase emerged regardless of the nutritional content of the medium, but both isolates formed mature biofilms from 288 h onwards. Psal-103 biofilms were sensitive to Atlantic salmon skin mucus during early formation, whereas Psal-104 biofilms were more tolerant. The ability of P. salmonis to form viable and mucus-tolerant biofilms on plastic surfaces in seawater represents a potentially important environmental risk for the persistence and dissemination of piscirickettsiosis.

Prevalence and Characteristics of Bacillus cereus Group Isolated from Raw and Pasteurised Milk

The elimination of spore-forming bacteria is not guaranteed by current pasteurisation processes and is a challenging problem for the dairy industry. Given that Bacillus cereus sensu lato (B. cereus group) is an important foodborne pathogen and spoiler in the dairy industry, this study aimed at evaluating the prevalence and characteristics of B. cereus group in raw and pasteurised milk samples collected in Victoria, Australia. Isolated B. cereus group were tested for antimicrobial susceptibility, biofilm formation and virulence properties. Genetic diversity was assessed using ERIC-PCR. Proteomic profiling using MALDI-TOF MS and chemical profiling using Fourier-transform infrared (FTIR) spectroscopy were also applied for clustering of the isolates. Results showed 42.3% of milk samples contained B. cereus group, with a higher contamination level for pasteurised milk. Virulence studies identified genes nheA, nheB, hblA and nheC in most isolates and cyk gene in 46% of all isolates. Antimicrobial susceptibility testing showed a high prevalence of resistance towards ampicillin, ceftriaxone and penicillin. The biofilm-forming capacity of our isolates showed that most (53.7%) had the ability to form a biofilm. Genetic profiling using ERIC-PCR placed most B. cereus group isolates from pasteurised milk in the same cluster, indicating that they probably originated from a similar source. Raw milk isolates showed greater diversity indicating various sources. FTIR spectroscopy showed high agreement with genetic profiling. In contrast, low agreement between proteomic (MALDI-TOF MS) and genetic typing was observed. The present study showed that the FTIR spectroscopy could be adopted as a rapid tool for the typing of B. cereus group. Overall, the virulence and antimicrobial resistance characteristics, together with the ability of isolates to produce biofilm, indicate the importance of B. cereus group in the Australian dairy industry.

The use of a CDC biofilm reactor to grow multi-strain Listeria monocytogenes biofilm

Listeria monocytogenes is one of the most concerning pathogens for the food industry due to its ability to form biofilms, particularly in difficult-to-clean sites of processing facilities. There is a current industry-wide lack of data to refer to when selecting a strategy to control L. monocytogenes biofilms in the food premises. Many strategies have been developed to study biofilm formation of bacteria; however, few have targeted L. monocytogenes biofilms under dynamic conditions. This study addresses the biofilm formation ability of L. monocytogenes on stainless steel and polycarbonate under dynamic conditions using TSBYE or BHI as media culture at 30 °C or 37 °C. Higher cell counts were recovered at 30 °C in TSBYE on polycarbonate while lower counts were obtained at 37 °C in BHI on stainless steel (P < 0.05). Nonetheless, all factors (temperature, media and material) were statistically significant (P < 0.05) and an interaction between temperature and media was observed (P < 0.05). To our knowledge, this work represents an initial framework to develop L. monocytogenes biofilms under different dynamic conditions. The use of CDC Biofilm Reactor is not widely used yet in the food industry and represent a novel approach to help sanitary control strategies implementation.

Strain variability in biofilm formation: A food safety and quality perspective

The inherent differences in microbial behavior among identically treated strains of the same microbial species, referred to as "strain variability", are regarded as an important source of variability in microbiological studies. Biofilms are defined as the structured multicellular communities with complex architecture that enable microorganisms to grow adhered to abiotic or living surfaces and constitute a fundamental aspect of microbial ecology. The research studies assessing the strain variability in biofilm formation are relatively few compared to the ones evaluating other aspects of microbial behavior such as virulence, growth and stress resistance. Among the available research data on intra-species variability in biofilm formation, compiled and discussed in the present review, most of them refer to foodborne pathogens as compared to spoilage microorganisms. Molecular and physiological aspects of biofilm formation potentially related to strain-specific responses, as well as information on the characterization and quantitative description of this type of biological variability are presented and discussed. Despite the considerable amount of available information on the strain variability in biofilm formation, there are certain data gaps and still-existing challenges that future research should cover and address. Current and future advances in systems biology and omics technologies are expected to aid significantly in the explanation of phenotypic strain variability, including biofilm formation variability, allowing for its integration in microbiological risk assessment.

Vibrio harveyi virulence gene expression in vitro and in vivo during infection in black tiger shrimp Penaeus monodon

Luminescent Vibrio harveyi is common in sea and estuarine waters. It produces several virulence factors and negatively affects larval penaeid shrimp in hatcheries, resulting in severe economic losses to shrimp aquaculture. Although V. harveyi is an important pathogen of shrimp, its pathogenicity mechanisms have yet to be completely elucidated. In the present study, isolates of V. harveyi were isolated and characterized from diseased Penaeus monodon postlarvae from hatcheries in Kerala, India, from September to December 2016. All 23 tested isolates were positive for lipase, phospholipase, caseinase, gelatinase and chitinase activity, and 3 of the isolates (MFB32, MFB71 and MFB68) showed potential for significant biofilm formation. Based on the presence of virulence genes, the isolates of V. harveyi were grouped into 6 genotypes, predominated by vhpA+ flaB+ ser+ vhh1- luxR+ vopD- vcrD+ vscN-. One isolate from each genotype was randomly selected for in vivo virulence experiments, and the LD50 ranged from 1.7 ± 0.5 × 103 to 4.1 ± 0.1 × 105 CFU ml-1. The expression of genes during the infection in postlarvae was high in 2 of the isolates (MFB12 and MFB32), consistent with the result of the challenge test. However, in MFB19, even though all genes tested were present, their expression level was very low and likely contributed to its lack of virulence. Because of the significant variation in gene expression, the presence of virulence genes alone cannot be used as a marker for pathogenicity of V. harveyi.

Comparison of the intensity of biofilm formation by Listeria monocytogenes using classical culture-based method and digital droplet PCR

Listeria monocytogenes is a Gram-positive bacterium, commonly found in food, water or sewage. This microorganism is capable of forming biofilm on different surfaces such as steel, glass, polypropylene etc. Recently an increase in cases of listeriosis has been noted, making L. monocytogenes the important health threat. Therefore, there is a need for rapid and sensitive detection of this pathogen. This study aimed to compare the number of L. monocytogenes cells recovered from the biofilm (prepared on steel and polypropylene) using the detection and amplification of the hlyA gene (droplet digital PCR, ddPCR) and the classical culture method. The research material consisted of 96 L. monocytogenes strains. A total of 58 isolates were obtained from clinical samples and 38 isolates derived from the municipal sewage treatment plant. Additionally, the reference strain ATCC®19111™ (WDCM00020) was used. The Pearson correlation coefficient for the results obtained by the classical culture-based method and ddPCR was 0.864 and 0.725, for biofilms produced on AISI 304 stainless steel surface and the polypropylene surface, respectively. Correlations were statistically significant (p ≤ 0.001), indicating that the ddPCR technique is an effective tool for the assessment of bacteria number in the biofilm.

Ascorbic Acid Changes Growth of Food-Borne Pathogens in the Early Stage of Biofilm Formation

Since bacterial biofilm may contribute to the secondary contamination of food during the manufacturing/processing stage there is a need for new methods allowing its effective eradication. Application of food additives such as vitamin C already used in food industry as antioxidant food industry antioxidants may be a promising solution. The aim of this research was evaluation of the impact of vitamin C (ascorbic acid), in a range of concentrations 2.50 µg mL-1-25.0 mg mL-1, on biofilms of Staphylococcus aureus, Escherichia coli, and Listeria monocytogenes strains isolated from food. The efficacy of ascorbic acid was assessed based on the reduction of optical density (λ = 595 nm). The greatest elimination of the biofilm was achieved at the concentration of vitamin C of 25.0 mg mL-1. The effect of the vitamin C on biofilm, however, was strain dependent. The concentration of 25.0 mg mL-1 reduced 93.4%, 74.9%, and 40.5% of E. coli, L. monocytogenes, and S. aureus number, respectively. For E. coli and S. aureus lower concentrations were ineffective. In turn, for L. monocytogenes the biofilm inhibition was observed even at the concentration of 0.25 mg mL-1. The addition of vitamin C may be helpful in the elimination of bacterial biofilms. Nonetheless, some concentrations can induce growth of the pathogens, posing risk for the consumers' health.

Growth and Survival of Listeria monocytogenes on Intact Fruit and Vegetable Surfaces during Postharvest Handling: A Systematic Literature Review

Listeria monocytogenes may be present in produce-associated environments (e.g., fields, packing houses); thus, understanding its growth and survival on intact, whole produce is of critical importance. The goal of this study was to identify and characterize published data on the growth and/or survival of L. monocytogenes on intact fruit and vegetable surfaces. Relevant studies were identified by searching seven electronic databases: AGRICOLA, CAB Abstracts, Center for Produce Safety funded research project final reports, FST Abstracts, Google Scholar, PubMed, and Web of Science. Searches were conducted using the following terms: Listeria monocytogenes, produce, growth, and survival. Search terms were also modified and "exploded" to find all related subheadings. Included studies had to be prospective, describe methodology (e.g., inoculation method), outline experimental parameters, and provide quantitative growth and/or survival data. Studies were not included if methods were unclear or inappropriate, or if produce was cut, processed, or otherwise treated. Of 3,459 identified citations, 88 were reviewed in full and 29 studies met the inclusion criteria. Included studies represented 21 commodities, with the majority of studies focusing on melons, leafy greens, berries, or sprouts. Synthesis of the reviewed studies suggests L. monocytogenes growth and survival on intact produce surfaces differ substantially by commodity. Parameters such as temperature and produce surface characteristics had a considerable effect on L. monocytogenes growth and survival dynamics. This review provides an inventory of the current data on L. monocytogenes growth and/or survival on intact produce surfaces. Identification of which intact produce commodities support L. monocytogenes growth and/or survival at various conditions observed along the supply chain will assist the industry in managing L. monocytogenes contamination risk.

Antimicrobial activity of gaseous Citrus limon var pompia leaf essential oil against Listeria monocytogenes on ricotta salata cheese

Contamination by Listeria monocytogenes is a particularly challenging problem in the food industry due to the ability of the bacterium to develop under conditions normally used for food preservation. Here, we show that the gaseous phase of Citrus limon var pompia leaf essential oil (hereafter PLEO) exerts specific anti-Listeria activity on ricotta salata cheese stored at 5 °C. The synergic effect of gaseous PLEO treatment and refrigeration was first confirmed in vitro on L. monocytogenes strains treated for 3 h with gaseous PLEO and then stored at 5 °C. Ricotta cheese was then inoculated with L. monocytogenes strains and subjected to hurdle technology with different concentrations of gaseous PLEO. Cell counts revealed gaseous PLEO to exert a bactericidal effect on L. monocytogenes 20600 DSMZ and a bacteriostatic effect on a mix of L. monocytogenes strains. Scanning and transmission electron microscopy analyses of L. monocytogenes cells suggested that gaseous PLEO targets the bacterial cell wall and plasma membrane. Chemical analyses of the liquid and vapor phases of PLEO indicated linalyl acetate to be the predominant compound, followed by limonene and the two isomers of citral, whereas EO composition analysis, although generally in line with previous findings, showed the presence of linalyl acetate for the first time. Solid-phase microextraction coupled with gas chromatography confirmed the presence of all crude oil components in the headspace of the box.

Biofilm Formation of Listeria monocytogenes Strains Under Food Processing Environments and Pan-Genome-Wide Association Study

Concerns about food contamination by Listeria monocytogenes are on the rise with increasing consumption of ready-to-eat foods. Biofilm production of L. monocytogenes is presumed to be one of the ways that confer its increased resistance and persistence in the food chain. In this study, a collection of isolates from foods and food processing environments (FPEs) representing persistent, prevalent, and rarely detected genotypes was evaluated for biofilm forming capacities including adhesion and sessile biomass production under diverse environmental conditions. The quantity of sessile biomass varied according to growth conditions, lineage, serotype as well as genotype but association of clonal complex (CC) 26 genotype with biofilm production was evidenced under cold temperature. In general, relative biofilm productivity of each strain varied inconsistently across growth conditions. Under our experimental conditions, there were no clear associations between biofilm formation efficiency and persistent or prevalent genotypes. Distinct extrinsic factors affected specific steps of biofilm formation. Sudden nutrient deprivation enhanced cellular adhesion while a prolonged nutrient deficiency impeded biofilm maturation. Salt addition increased biofilm production, moreover, nutrient limitation supplemented by salt significantly stimulated biofilm formation. Pan-genome-wide association study (Pan-GWAS) assessed genetic composition with regard to biofilm phenotypes for the first time. The number of reported genes differed depending on the growth conditions and the number of common genes was low. However, a broad overview of the ontology contents revealed similar patterns regardless of the conditions. Functional analysis showed that functions related to transformation/competence and surface proteins including Internalins were highly enriched.

Persistent Listeria monocytogenes Isolates from a Poultry-Processing Facility Form more Biofilm but Do Not Have a Greater Resistance to Disinfectants Than Sporadic Strains

Some strains of Listeria monocytogenes can persist in food-processing environments, increasing the likelihood of the contamination of foodstuffs. To identify traits that contribute to bacterial persistence, a selection of persistent and sporadic L. monocytogenes isolates from a poultry-processing facility was investigated for biofilm-forming ability (crystal violet assay). The susceptibility of sessile cells to treatments (five minutes) with sodium hypochlorite having 10% active chlorine (SHY: 10,000 ppm, 25,000 ppm, and 50,000 ppm) and benzalkonium chloride (BZK: 2500 ppm, 10,000 ppm, and 25,000 ppm) was also studied. All isolates exhibited biofilm formation on polystyrene. Persistent strains showed larger (p < 0.001) biofilm formation (OD580 = 0.301 ± 0.097) than sporadic strains (OD580 = 0.188 ± 0.082). A greater susceptibility to disinfectants was observed for biofilms of persistent strains than for those of sporadic strains. The application of SHY reduced biofilms only for persistent strains. BZK increased OD580 in persistent strains (2500 ppm) and in sporadic strains (all concentrations). These results indicate that the use of BZK at the concentrations tested could represent a public health risk. Findings in this work suggest a link between persistence and biofilm formation, but do not support a relationship between persistence and the resistance of sessile cells to disinfectants.

Dual-Species Model Biofilm Consisting of Listeria monocytogenes and Salmonella Typhimurium: Development and Inactivation With Cold Atmospheric Plasma (CAP)

Most environmental biofilms contain a variety of species. These species can establish cooperative and competitive interactions, possibly resulting in an increase or a decrease in antimicrobial resistance. Therefore, results obtained following inactivation of single-species biofilms by means of different technologies (e.g., Cold Atmospheric Plasma, CAP) should be validated for multi-species biofilms. First, a strongly adherent and mature Listeria monocytogenes and S. Typhimurium dual-species biofilm was developed by altering different incubation conditions, i.e., growth medium, incubation temperature, inoculum ratio of L. monocytogenes and S. Typhimurium cells, and incubation time. Adherence and maturity were quantified by means of optical density measurements and viable plate counts, respectively. Secondly, both the (1 day old) reference biofilm and a more mature 7 days old biofilm were treated for different CAP treatment times (0-30 min). Viable plate counts were again used to determine the (remaining) cell density. For both the biofilm development and inactivation, predictive models were applied to describe the growth/inactivation kinetics. Finally, the kinetics of the [1 and 7 day(s) old] dual-species biofilms were compared with those obtained for the corresponding single-species biofilms. Results implied that a strongly adherent and mature reference dual-species biofilm was obtained following 24 h of incubation at 25°C using 20-fold diluted TSB and an inoculum ratio of 1:1. Main observations regarding CAP inactivation were: (i) the dual-species biofilm age had no influence on the CAP efficacy, although a longer treatment time was required for the oldest biofilm, (ii) for the 1 day old biofilms, CAP treatment became less efficient for S. Typhimurium inactivation when this species was part of the dual-species biofilm, while L. monocytogenes inactivation was not influenced by the biofilm type, and (iii) for the 7 days old biofilms, CAP inactivation of both species became more efficient when they were part of the dual-species biofilms. It can be concluded that the efficacy of the CAP treatment is altered when cells become part of a dual-species biofilm, which is quite important with respect to a possible application of CAP for biofilm inactivation within the food industry.

Prevalence, Molecular Typing, and Determination of the Biofilm-Forming Ability of Listeria monocytogenes Serotypes from Poultry Meat and Poultry Preparations in Spain

A study was undertaken of the presence of Listeria monocytogenes in 260 samples of poultry meat obtained from retail outlets in northwestern Spain. L. monocytogenes was detected in 20 samples (7.7%). Twenty strains (one strain per positive sample) were characterized. The strains belonged to 10 serotypes: 1/2a (2 strains), 1/2b (2), 1/2c (2), 3a (1), 3b (2), 3c (2), 4a (2), 4b (4), 4c (1), and 4d (2). Cluster analysis (ribotyping; EcoRI) showed a strong genetic relationship between strains isolated from samples coming from different outlets. Ribotyping permitted some isolates of the same serotype to be differentiated, which points to the possible usefulness of this technique in the epidemiological surveillance of L. monocytogenes. All strains formed biofilm on polystyrene, as shown by confocal laser scanning microscopy. The biovolume (between 621.7 ± 36.0 µm3 and 62,984.0 ± 14,888.2 µm3 in the observational field of 14,161 μm2), percentage of surface coverage (from 2.17 ± 0.84% to 94.43 ± 3.97%), roughness (between 0.399 ± 0.052 and 0.830 ± 0.022), and maximum thickness (between 9.00 ± 0.00 µm and 24.00 ± 14.93 µm) of biofilms varied between strains (p < 0.05). These results expand knowledge of the characteristics of L. monocytogenes isolates from poultry.

Characterization of Listeria Monocytogenes Originating from the Spanish Meat-Processing Chain

Using agglutination techniques, 118 Listeria monocytogenes isolates from red meat and poultry were serotyped. Strains were ascribed to the serotypes 4b/4e (44.1% of the strains), 1/2 (a, b or c; 28.0%), 4c (6.8%), 4d/4e (5.9%) and 3 (a, b or c; 2.5%). Among these are the serotypes most frequently involved in cases of human listeriosis. The susceptibility of 72 strains to 26 antibiotics of clinical importance was determined by disc diffusion (Clinical and Laboratory Standards Institute; CLSI). High levels of resistance were observed to cefoxitin (77.8% of the strains showed resistance), cefotaxime (62.5%), cefepime (73.6%), and nalidixic acid (97.2%), nitrofurantoin (51.4%) and oxacillin (93.1%). Less than 3% of the strains showed resistance to the antibiotic classes used in human listeriosis therapy (i.e., ampicillin, gentamicin, rifampicin, chloramphenicol, enrofloxacin, vancomycin, trimethoprim-sulfamethoxazole, erythromycin, and tetracycline). The influence of species and serotype on the growth kinetics (modified Gompertz equation) and on the adhesion ability (crystal violet staining) of nine isolates of L. monocytogenes (serotypes 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, and 4d), and one strain of Listeria ivanovii were investigated. The maximum growth rate (ΔOD420-580/h) varied between 0.073 ± 0.018 (L. monocytogenes 1/2a) and 0.396 ± 0.026 (L. monocytogenes 4b). The isolates of L. monocytogenes belonging to serotypes 3a and 4a, as well as L. ivanovii, showed a greater (p < 0.05) biofilm-forming ability than did the remaining strains, including those that belong to the serotypes commonly implied in human listeriosis (1/2a, 1/2b, 1/2c and 4b). The need for training in good hygiene practices during the handling of meat and poultry is highlighted to reduce the risk of human listeriosis.

CRISPR-Cas9 knockout of qseB induced asynchrony between motility and biofilm formation in Escherichia coli

Generally, cell motility and biofilm formation are tightly regulated. The QseBC two-component system (TCS) serves as a bridge for bacterial signal transmission, in which the protein QseB acts as a response regulator bacterial motility, biofilm formation, and virulence. The mechanisms that govern the interaction between QseBC and their functions have been studied in general, but the regulatory role of QseB on bacterial motility and biofilm formation is unknown. In this study, the CRISPR-Cas9 system was used to construct the Escherichia coli MG1655ΔqseB strain (strain ΔqseB), and the effects of the qseB gene on changes in motility and biofilm formation in the wild type (WT) were determined. The motility assay results showed that the ΔqseB strain had higher (p < 0.05) motility than the WT strain. However, there was no difference in the formation of biofilm between the ΔqseB and WT strains. Real-time quantitative PCR illustrated that deletion of qseB in the WT strain downregulated expression of the type I pili gene fimA. Therefore, we might conclude that the ΔqseB induced the downregulation of fimA, which led to asynchrony between motility and biofilm formation in E. coli, providing new insight into the functional importance of QseB in regulating cell motility and biofilm formation.

Facultative Anaerobes Shape Multispecies Biofilms Composed of Meat Processing Surface Bacteria and Escherichia coli O157:H7 or Salmonella enterica Serovar Typhimurium

This study investigated the microbial dynamics in multispecies biofilms of Escherichia coli O157:H7 strain 1934 (O157) or Salmonella enterica serovar Typhimurium ATCC 14028 (ST) and 40 strains of meat processing surface bacteria (MPB). Biofilms of O157 or ST with/without MPB were developed on stainless steel coupons at 15°C for up to 6 days. Bacteria in suspensions (inoculum, days 2 and 6) and biofilms (days 2 and 6) were enumerated by plating. The composition of multispecies cultures was determined by 16S rRNA gene sequencing. In suspensions, levels of O157 and ST were ∼2 log higher in single-species than in multispecies cultures on both sampling days. ST was 3 log higher in single-species than in multispecies biofilms. A similar trend, though to a lesser extent, was observed for O157 in biofilms on day 2 but not on day 6. No difference (P > 0.05) in bacterial counts was noted for the two MPB-pathogen cocultures at any time during incubation. Bacterial diversity in multispecies cultures decreased with incubation time, irrespective of the pathogen or culture type. The changes in the relative abundance of MPB were similar for the two MPB-pathogen cocultures, though different interbacterial interactions were noted. Respective fractions of ST and O157 were 2.1% and 0.97% initially and then 0.10% and 0.07% on day 2, and 0.60% and 0.04% on day 6. The relative proportions of facultative anaerobes in both multispecies cultures were greater in both suspensions and biofilms than in the inoculum. Citrobacter, Hafnia, Aeromonas, and Carnobacterium predominated in biofilms but not always in the planktonic cultures.IMPORTANCE Results of this study demonstrate that Salmonella enterica serovar Typhimurium and E. coli O157:H7 can integrate into biofilms when cocultured with bacteria from meat plant processing surfaces. However, the degree of biofilm formation for both pathogens was substantially reduced in the presence of the competing microbiota, with S. Typhimurium more greatly affected than E. coli O157:H7. The expression of extracellular determinants such as curli and cellulose appears to be less important for biofilm formation of the pathogens in multispecies cultures than in monoculture. In contrast to previous reports regarding food processing surface bacteria, data collected here also demonstrate that facultative anaerobes may have a competitive edge over strict aerobes in establishing multispecies biofilms. It would be important to take into account the presence of background bacteria when evaluating the potential persistence of a pathogen in food processing facilities.

Disinfectant Susceptibility of Biofilm Formed by Listeria monocytogenes under Selected Environmental Conditions

Listeria monocytogenes is a one of the most important food-borne pathogens. Its ability to form biofilm contributes to increased resistance to disinfectants and inefficient disinfection, posing a serious threat for the food industry, and in the end the consumer. The aim of this study was the comparison of the biofilm formation ability of L. monocytogenes strains on stainless steel, under different environmental conditions (temperature, pH, NaCl concentration, nutrients availability), and the assessment of biofilm susceptibility to disinfectants. The bactericidal activity of four disinfectants in two concentrations (100% and 50% of working solution) against biofilm was conducted on four clinical strains, four strains isolated from food and one reference strain ATCC 19111. It was found that biofilm susceptibility to disinfectants was influenced by environmental conditions. Biofilm susceptibility correlated with the decrease of temperature, pH, nutrients availability and salinity of the environment. The least sensitive to disinfectants was biofilm produced at pH = 4 (the bacterial number ranged from 0.25 log CFU × cm-2 to 1.72 log CFU × cm-2) whereas the most sensitive was biofilm produced at pH = 9 (5.16 log CFU × cm-2 to 7.84 log CFU × cm-2). Quatosept was the most effective disinfectant, regardless of the conditions. In conclusion, biofilm susceptibility to disinfectants is strain-dependent and is affected by environmental conditions.

Biofilms: The Microbial "Protective Clothing" in Extreme Environments

Microbial biofilms are communities of aggregated microbial cells embedded in a self-produced matrix of extracellular polymeric substances (EPS). Biofilms are recalcitrant to extreme environments, and can protect microorganisms from ultraviolet (UV) radiation, extreme temperature, extreme pH, high salinity, high pressure, poor nutrients, antibiotics, etc., by acting as "protective clothing". In recent years, research works on biofilms have been mainly focused on biofilm-associated infections and strategies for combating microbial biofilms. In this review, we focus instead on the contemporary perspectives of biofilm formation in extreme environments, and describe the fundamental roles of biofilm in protecting microbial exposure to extreme environmental stresses and the regulatory factors involved in biofilm formation. Understanding the mechanisms of biofilm formation in extreme environments is essential for the employment of beneficial microorganisms and prevention of harmful microorganisms.

Propidium iodide staining underestimates viability of adherent bacterial cells

Combining membrane impermeable DNA-binding stain propidium iodide (PI) with membrane-permeable DNA-binding counterstains is a widely used approach for bacterial viability staining. In this paper we show that PI staining of adherent cells in biofilms may significantly underestimate bacterial viability due to the presence of extracellular nucleic acids (eNA). We demonstrate that gram-positive Staphylococcus epidermidis and gram-negative Escherichia coli 24-hour initial biofilms on glass consist of 76 and 96% PI-positive red cells in situ, respectively, even though 68% the cells of either species in these aggregates are metabolically active. Furthermore, 82% of E. coli and 89% S. epidermidis are cultivable after harvesting. Confocal laser scanning microscopy (CLSM) revealed that this false dead layer of red cells is due to a subpopulation of double-stained cells that have green interiors under red coating layer which hints at eNA being stained outside intact membranes. Therefore, viability staining results of adherent cells should always be validated by an alternative method for estimating viability, preferably by cultivation.

Expression levels of the agr locus and prfA gene during biofilm formation by Listeria monocytogenes on stainless steel and polystyrene during 8 to 48 h of incubation 10 to 37 °C

The objective of this study was to compare the gene expression levels of the agr locus and prfA gene during adhesion and biofilm formation by four L. monocytogenes isolates (2 biofilm-forming and 2 non-forming) on stainless steel and polystyrene surfaces at different temperatures (10 °C, 20 °C and 37 °C), and times (8 h, 12 h, 24 h and 48 h). The agrA and prfA genes were expressed at higher levels than the agrBCD genes. The levels of agr locus expression were higher in the biofilm-forming strains, and the greatest difference between biofilm-forming and non-forming isolates was observed for the agrB, agrC and agrD genes. However, no difference in the expression of the prfA gene was seen among the isolates, independent of the biofilm-forming ability. Maximum expression of the agr locus and prfA gene was observed at 37 °C, whereas expression was lowest at 10 °C. The agr locus, and particularly the agrB, agrC and agrD genes, is important in the initial adhesion phase of biofilm production by L. monocytogenes, with this expression independent of prfA. In addition, the agr locus and prfA gene expression levels were strongly influenced by time and temperature.

Influence of some parameters on the ability of Listeria monocytogenes, Listeria innocua, and Escherichia coli to form biofilms

AIM

The present study was conducted to evaluate the capacity of Listeria monocytogenes (L.m), Listeria innocua (L.i), and Escherichia coli to form biofilms on polystyrene support under different parameters by performing crystal violet (CV) staining technique.

MATERIALS AND METHODS

Different suspensions were prepared with single strains and with multiple combinations of strains including two serogroups of L.m (IIa and IIb), L.i, and E. coli strains at different microbial load. Selected strains and combinations were grown in biofilms for 6 days attached to polystyrene microplates under aerobic and microaerophilic conditions. The evaluation of the power of adhesion and biofilm formation was determined by CV staining followed by the measurement of optical density at 24 h, 72 h, and 6 days incubation time with and without renewal of the culture medium.

RESULTS

All the strains tested, presented more or less adhesion power depending on the variation of the studied parameters as well as the ability to form multispecies biofilms. Their development is more important by renewing the culture medium and increasing the initial load of bacteria. The ability to adhere and form biofilms differs from one serogroup to another within the same species. In bacterial combination, strains and species of bacteria adopt different behaviors.

CONCLUSION

The ability to form biofilms is a key factor in the persistence of tested strains in the environment. Our study showed that L.m, L.i, and E. coli could adhere to polystyrene and form biofilms under different conditions. More researches are necessary to understand the mechanisms of biofilm formation and the influence of different parameters in their development.

Genetic Relationship, Virulence Factors, Drug Resistance Profile and Biofilm Formation Ability of Vibrio parahaemolyticus Isolated From Mussel

The objective of this study was to investigate the virulence factors, genetic relationship, antibiotic resistance profile and the biofilm formation ability of Vibrio parahaemolyticus isolates on shrimp and mussel surfaces at 30°C. In this study, eight (n = 8) V. parahaemolyticus isolated from mussel were examined. We used the polymerase chain reaction (PCR) to examine the distribution of different genes, and Repetitive Extragenic Palindromic-PCR (REP-PCR) to compare the genetic relationship. Disk diffusion technique was used to assess antibiotic and multiple-antibiotic resistance. The biofilm formation assay, and field emission scanning electron microscopy (FE-SEM) were used to evaluate biofilm formation ability. Transmission Electron Microscope (TEM) was used to observe the morphological structure of bacterial cell. Our results indicated that the biofilm-associated genes, 16S rRNA, toxR, and tdh, were present in all the tested V. parahaemolyticus isolates (n = 8). Approximately, 62.5% (5 isolates among 8 isolates) isolates showed strong multiple-antibiotic resistance index with an average value of 0.56. All isolates (n = 8) showed strong genetic relationship and significant biofilm formation ability on shrimp and mussel surfaces. This study demonstrated that the presence of virulence factors, high multiple antibiotic resistance index (MARI) values, and effective biofilm formation ability of V. parahaemolyticus isolates could be a great threat to human health and economic values in future. It was also suggested that a high resistance rate to antibiotic could be ineffective for treating V. parahaemolyticus infections. The continuous monitoring of V. parahaemolyticus antibiotic, molecular and biofilm characteristics is needed to increase seafood safety.

Susceptibility of Listeria monocytogenes planktonic cultures and biofilms to sodium hypochlorite and benzalkonium chloride

The susceptibility of four L. monocytogenes isolates from pork to sodium hypochlorite (SHY) and benzalkonium chloride (BZK) was tested. Minimum inhibitory concentration (MIC) values of 3500 ppm (SHY), or between 3 ppm and 13 ppm (BZK), were found. Minimum bactericidal concentration (MBC) values ranged from 3500 ppm to 4500 ppm (SHY), and from 3 ppm to 14 ppm (BZK). The effect of SHY and BZK on the architecture and cellular viability of 24-h-old biofilms formed by such strains on polystyrene was determined through confocal laser scanning microscopy (CLSM) in conjunction with fluorescent dyes for live cells (SYTO 9) and dead cells (propidium iodide). Strains were able to form biofilm (biovolume values in the observation field of 14,161 μm² ranged between 103,928.3 ± 6730.2 μm³ and 276,030.9 ± 42,291.9 μm³). Treatment of biofilms for 10 min with SHY (1MIC or 1.5MIC) or BZK (0.5MIC, 1MIC or 1.5MIC) decreased the biovolume of live (potentially dangerous) cells. SHY reduced the cellular viability of biofilms by more than 90%. On the other hand, BZK was able to remove most biofilm mass (live and dead cells), but decreased cellular viability only to a lesser extent, this suggesting strong biofilm detachment and dissemination of live cells.

Drug-susceptibility, biofilm-forming ability and biofilm survival on stainless steel of Listeria spp. strains isolated from cheese

The aim of the study was to analyze the contamination of mold cheese (Brie, Camembert, Gorgonzola, Munster and Roquefort) with Listeria spp. and assessment of culturable cells number recovered from the biofilm formed on the surface of stainless steel by obtained strains. Identified isolates (MALDI TOF MS technique) were subjected to susceptibility testing (disk-diffusion method) and their genetic similarity (PFGE method), ability to form biofilm (quantitative method), biofilm dry weight, and biofilm survival on stainless steel were evaluated. Out of 250 samples of cheese 26 (10.4%) were Listeria spp. positive, including 15 isolates (6.0% of samples) of L. monocytogenes, 7 isolates of L. innocua (2.8% of samples) and 4 isolates of L. welshimeri species (1.6% of samples). Of the 26 isolates tested, 22 strains were genetically different. It was shown that L. innocua and L. welshimeri strains were sensitive to all antibiotics tested, while two (16.7%) L. monocytogenes strains were resistant to penicillin and one (8.3%) to erythromycin. L. monocytogenes formed biofilm most intensively on stainless steel, while L. welshimeri the least effectively. The median of bacteria number recovered from the biofilm for L. monocytogenes was 6.81 log CFU × cm^-2, for L. innocua - 5.63 log CFU × cm^-2, and for L. welshimeri - 4.93 log CFU × cm^-2. The survival in the biofilm of Listeria spp. strains decreased along with the increase in a storage temperature of steel coupons. The longest survival time was reported at 4 °C, i.e. 47.58-124.41 days, with an elimination rate of 0.06-0.13 log CFU × day^-1. Collectively, L. monocytogenes is the most prevalent species of Listeria genus in the mold cheese. The ability of L. monocytogenes strains to form biofilm on stainless steel and survive in the food processing environment increases chance of the secondary contamination of food posing risk to the consumer health.

Biofilms in Food Processing Environments: Challenges and Opportunities

This review examines the impact of microbial communities colonizing food processing environments in the form of biofilms on food safety and food quality. The focus is both on biofilms formed by pathogenic and spoilage microorganisms and on those formed by harmless or beneficial microbes, which are of particular relevance in the processing of fermented foods. Information is presented on intraspecies variability in biofilm formation, interspecies relationships of cooperativism or competition within biofilms, the factors influencing biofilm ecology and architecture, and how these factors may influence removal. The effect on the biofilm formation ability of particular food components and different environmental conditions that commonly prevail during food processing is discussed. Available tools for the in situ monitoring and characterization of wild microbial biofilms in food processing facilities are explored. Finally, research on novel agents or strategies for the control of biofilm formation or removal is summarized.

Behavior of Foodborne Pathogens Listeria monocytogenes and Staphylococcus aureus in Mixed-Species Biofilms Exposed to Biocides

In nature and man-made environments, microorganisms reside in mixed-species biofilms, in which the growth and metabolism of an organism are different from these behaviors in single-species biofilms. Pathogenic microorganisms may be protected against adverse treatments in mixed-species biofilms, leading to health risk for humans. Here, we developed two mixed five-species biofilms that included one or the other of the foodborne pathogens Listeria monocytogenes and Staphylococcus aureus The five species, including the pathogen, were isolated from a single food-processing environmental sample, thus mimicking the environmental community. In mature mixed five-species biofilms on stainless steel, the two pathogens remained at a constant level of ∼105 CFU/cm2 The mixed five-species biofilms as well as the pathogens in monospecies biofilms were exposed to biocides to determine any pathogen-protective effect of the mixed biofilm. Both pathogens and their associate microbial communities were reduced by peracetic acid treatments. S. aureus decreased by 4.6 log cycles in monospecies biofilms, but the pathogen was protected in the five-species biofilm and decreased by only 1.1 log cycles. Sessile cells of L. monocytogenes were affected to the same extent when in a monobiofilm or as a member of the mixed-species biofilm, decreasing by 3 log cycles when exposed to 0.0375% peracetic acid. When the pathogen was exchanged in each associated microbial community, S. aureus was eradicated, while there was no significant effect of the biocide on L. monocytogenes or the mixed community. This indicates that particular members or associations in the community offered the protective effect. Further studies are needed to clarify the mechanisms of biocide protection and to identify the species playing the protective role in microbial communities of biofilms.IMPORTANCE This study demonstrates that foodborne pathogens can be established in mixed-species biofilms and that this can protect them from biocide action. The protection is not due to specific characteristics of the pathogen, here S. aureus and L. monocytogenes, but likely caused by specific members or associations in the mixed-species biofilm. Biocide treatment and resistance are a challenge for many industries, and biocide efficacy should be tested on microorganisms growing in biofilms, preferably mixed systems, mimicking the application environment.

A Comparative Study of the Susceptibility of Listeria Species to Sanitizer Treatments When Grown under Planktonic and Biofilm Conditions

Listeria species are ubiquitous in nature and can adapt to survive in a variety of niches, including food processing environments. Listeria species that colonize these environments may also have the potential to persist. Food safety strategies designed to manage these niches include regular cleaning and disinfection with proven sanitizers containing biocide-active compounds. Typically, these sanitizers are effective against bacteria growing under planktonic conditions, but their efficacy may be compromised when bacteria are contained in biofilms. The susceptibility of persistent Listeria isolates, i.e., those capable of forming biofilms, to a selection of sanitizers was investigated. A quaternary ammonium compound-based sanitizer was the biocide most effective against planktonic bacteria, with a MIC of 0.0015 to 0.006%. In contrast, ethanol-based sanitizers were the least effective. Although, no triclosan tolerance was observed for planktonic Listeria isolates, triclosan was the only biocide that resulted in a significant biomass reduction. Differences between Listeria species were observed; L. monocytogenes and L. welshimeri biofilms were more tolerant to quaternary ammonium compound-based sanitizers than were L. innocua biofilms. These findings extend our understanding of the application of commonly used sanitizers in the food industry and the efficacy of these sanitizers against Listeria species and their associated biofilms.

Influence of incubation conditions on the formation of model biofilms by Listeria monocytogenes and Salmonella Typhimurium on abiotic surfaces

AIMS

This research aims to develop strongly adherent and mature model biofilms (on a 20 cm² polystyrene surface) for two pathogenic species, i.e. Listeria monocytogenes and Salmonella Typhimurium. These model biofilms can be used as standards to study biofilms or to study/compare the influence of different inactivation technologies.

METHODS AND RESULTS

Three influencing factors on the formation of biofilms are investigated, i.e. growth medium, incubation temperature and incubation time, which are three easily controllable environmental factors. Optical density measurement and plate counts were used to evaluate the adherence and the maturity of the biofilms, respectively. Confocal laser scanning microscopy was used to verify most important findings obtained with previously mentioned assays. Results indicated that mature and strongly adherent L. monocytogenes biofilms are obtained following 13 h of incubation at 30°C with BHI as growth medium. For S. Typhimurium, an incubation period of 19 h at 25°C was required with 20-fold diluted TSB as growth medium.

CONCLUSIONS

Based on previously mentioned assays, a protocol for the formation of reproducible model biofilms was obtained.

SIGNIFICANCE AND IMPACT OF THE STUDY

The developed model biofilms can be applied as a standard to study biofilms (in different research fields) and their subsequent inactivation by different methods. In addition, the results of this study could be used to control biofilm formation (e.g. by setting a maximum allowed surface temperature).

Helicobacter pylori Biofilm Formation Is Differentially Affected by Common Culture Conditions, and Proteins Play a Central Role in the Biofilm Matrix

The concept of Helicobacter pylori biofilm formation is relatively new. To help provide a foundation for future biofilm studies, we characterized the biofilm formation ability of a common H. pylori lab strain, G27. The goal of this study was to evaluate biofilm formation by G27 in response to common culture conditions and to explore the biofilm matrix. Our results indicate that while various types of growth media did not dramatically affect biofilm formation, surface selection had a significant effect on the final biofilm mass. Furthermore, enzymatic assays and confocal microscopy revealed that proteins appear to be the primary structural component of the H. pylori extracellular matrix; extracellular DNA (eDNA) and polysaccharides were also present but appear to play a secondary role. Finally, we found that two well-characterized antibiofilm cationic peptides differentially affected early and late-stage biofilms. Together these results provide interesting avenues for future investigations that will seek to understand H. pylori biofilm formation.IMPORTANCE The study of H. pylori biofilm formation is still in its infancy. As such, there is great variability in how biofilm assays are performed across labs. While several groups have begun to investigate factors that influence H. pylori biofilm formation, it is not yet understood how H. pylori biofilm formation may vary based on commonly used conditions. These inconsistencies lead to difficulties in interpretation and comparison between studies. Here, we set out to characterize biofilm formation by a commonly available lab strain, G27. Our findings provide novel insight into optimal biofilm conditions, the biofilm matrix, and possible mechanisms to block or disrupt biofilm formation.

Listeria monocytogenes Response to Propionate Is Differentially Modulated by Anaerobicity

Propionate is a common food preservative and one of the major fermentation acids in the intestines. Therefore, exposure to propionate is frequent for foodborne pathogens and likely takes place under suboxic conditions. However, it is not clear whether the absence of oxygen affects how pathogens respond to propionate. Here, we investigated how propionate exposure affects Listeria monocytogenes growth and virulence factor production under aerobic or anaerobic conditions and showed that oxygen indeed plays a key role in modulating L. monocytogenes response to propionate. Under aerobic conditions, propionate supplementations had no effect on planktonic growth but resulted in decreased adherent growth. Under anaerobic conditions, propionate supplementations resulted in a pH-dependent inhibition of planktonic growth and increased adherent growth. Cultures grown with propionate accumulated higher levels of acetoin under aerobic conditions but lower levels of ethanol under both aerobic and anaerobic conditions. Metabolic perturbations by propionate were also evident by the increase in straight chain fatty acids. Finally, propionate supplementations resulted in increased listeriolyin O (LLO) production under anaerobic conditions but decreased LLO production under aerobic conditions. These results demonstrate for the first time that the presence or absence of oxygen plays a critical role in shaping L. monocytogenes responses to propionate.

Modeling the effect of pH, water activity, and ethanol concentration on biofilm formation of Staphylococcus aureus

In this work, the effect of environmental factors on Staphylococcus aureus (ATCC 13150) biofilm formation in tryptic soy broth was investigated under different ranges of pH (3.0-9.5), ethanol concentration (EtOH 0.0-20.0%), and a_w (NaCl, 0.866-0.992). Biofilm formation was quantified using the crystal violet staining method and optical density (OD: 590 nm) measurements. Biofilm formation was significantly stronger at pH and a_w close to S. aureus optimal growth conditions, while it was high at EtOH around 2.5-3.5%. Data sets from the difference between the OD measurements of the test and control (ΔOD) were fitted to the cardinal parameter model (CPM) and cardinal parameter model with inflection (CPMI) to describe the effect of the environmental factors. The models showed good quality of fit for the experimental data in terms of calculated RMSE, with the latter ranging from 0.276 to 0.455. CPM gave a good quality of fit compared to CPMI for the environmental factors tested. Optimal pH was close to neutral (6.76-6.81) and biofilm formation was possible till pH = 3.81-3.78 for CPM and CPMI, respectively. Optimum EtOH and a_w conditions for biofilm formation were in the range of 1.99-2.75 and 0.98-0.97, respectively. Predicted OD values observed using strain 13150 were very closely correlated to the OD values predicted with strain 12600 with R² of 0.978, 0.991, and 0.947 for pH, EtOH, and a_w, respectively. The cultivable bacterial cells within the biofilm were enumerated using standard plate counting and a linear model was applied to correlate the attached biofilm cells to ΔOD of biofilm formation. It was found that the biofilm formation correlated with S. aureus population growth. At 2.5-3.5% of EtOH the maximum population density was lower than that observed at 0.0% of EtOH. As 2.5-3.5% of EtOH initiated a stronger biofilm formation, biofilm formation seems to be induced by ethanol stress. The development of cardinal parameter models to describe the effect environmental factors of importance to biofilm formation, offers a promising predictive microbiology approach to decrypting the S. aureus population growth and survival ability on food processing surfaces.

Listeria monocytogenes strains show large variations in competitive growth in mixed culture biofilms and suspensions with bacteria from food processing environments

Interactions and competition between resident bacteria in food processing environments could affect their ability to survive, grow and persist in microhabitats and niches in the food industry. In this study, the competitive ability of L. monocytogenes strains grown together in separate culture mixes with other L. monocytogenes (L. mono mix), L. innocua (Listeria mix), Gram-negative bacteria (Gram- mix) and with a multigenera mix (Listeria + Gram- mix) was investigated in biofilms on stainless steel and in suspensions at 12 °C. The mixed cultures included resident bacteria from processing surfaces in meat and salmon industry represented by L. monocytogenes (n = 6), L. innocua (n = 5) and Gram-negative bacteria (n = 6; Acinetobacter sp., Pseudomonas fragi, Pseudomonas fluorescens, Serratia liquefaciens, Stenotrophomonas maltophilia). Despite hampered in growth in mixed cultures, L. monocytogenes established in biofilms with counts at day nine between 7.3 and 9.0 log per coupon with the lowest counts in the Listeria + G- mix that was dominated by Pseudomonas. Specific L. innocua inhibited growth of L. monocytogenes strains differently; inhibition that was further enhanced by the background Gram-negative microbiota. In these multispecies and multibacteria cultures, the growth competitive effects lead to the dominance of a strong competitor L. monocytogenes strain that was only slightly inhibited by L. innocua and showed strong competitive abilities in mixed cultures with resident Gram-negative bacteria. The results indicates complex patterns of bacterial interactions and L. monocytogenes inhibition in the multibacteria cultures that only partially depend on cell contact and likely involve various antagonistic and bacterial tolerance mechanisms. The study indicates large variations among L. monocytogenes in their competitiveness under multibacterial culture conditions that should be considered in further studies towards understanding of L. monocytogenes persistence in food processing facilities.

Impact of nutritional stress on drug susceptibility and biofilm structures of Burkholderia pseudomallei and Burkholderia thailandensis grown in static and microfluidic systems

Burkholderia pseudomallei is the causative agent of melioidosis and regarded as a bioterrorism threat. It can adapt to the nutrient-limited environment as the bacteria can survive in triple distilled water for 16 years. Moreover, B. pseudomallei exhibits intrinsic resistance to diverse groups of antibiotics in particular while growing in biofilms. Recently, nutrient-limited condition influenced both biofilm formation and ceftazidime (CAZ) tolerance of B. pseudomallei were found. However, there is no information about how nutrient-limitation together with antibiotics used in melioidosis treatment affects the structure of the biofilm produced by B. pseudomallei. Moreover, no comparative study to investigate the biofilm architectures of B. pseudomallei and the related B. thailandensis under different nutrient concentrations has been reported. Therefore, this study aims to provide new information on the effects of four antibiotics used in melioidosis treatment, viz. ceftazidime (CAZ), imipenem (IMI), meropenem (MEM) and doxycycline (DOX) on biofilm architecture of B. pseudomallei and B. thailandensis with different nutrient concentrations under static and flow conditions using confocal laser scanning microscopy. Impact of nutritional stress on drug susceptibility of B. pseudomallei and B. thailandensis grown planktonically or as biofilm was also evaluated. The findings of this study indicate that nutrient-limited environment enhanced survival of B. pseudomallei in biofilm after exposure to the tested antibiotics. The shedding planktonic B. pseudomallei and B. thailandensis were also found to have increased CAZ tolerance in nutrient-limited environment. However, killing activities of MEM and IMI were stronger than CAZ and DOX on B. pseudomallei and B. thailandensis both in planktonic cells and in 2-day old biofilm. In addition, MEM and IMI were able to inhibit B. pseudomallei and B. thailandensis biofilm formation to a larger extend compared to CAZ and DOX. Differences in biofilm architecture were observed for biofilms grown under static and flow conditions. Under static conditions, biofilms grown in full strength modified Vogel and Bonner’s medium (MVBM) showed honeycomb-like architecture while a knitted-like structure was observed under limited nutrient condition (0.1×MVBM). Under flow conditions, biofilms grown in MVBM showed a multilayer structure while merely dispersed bacteria were found when grown in 0.1×MVBM. Altogether, this study provides more insight on the effect of four antibiotics against B. pseudomallei and B. thailandensis in biofilm under different nutrient and flow conditions. Since biofilm formation is believed to be involved in disease relapse, MEM and IMI may be better therapeutic options than CAZ for melioidosis treatment.