Biofilm formation by Aeromonas hydrophila on green-leafy vegetables: cabbage and lettuce

Prevalence and virulence potential of Aeromonas spp. isolated from human diarrheal samples in North East Italy

IMPORTANCE

In this work, we demonstrate the epidemiologic relevance of the Aeromonas genus as the cause of infective diarrhea in North East Italy, both in children and adult subjects, with the significative presence of highly pathogenic strains. Aeromonas strains possess a heterogeneous armamentarium of pathogenicity factors that allows the microbe to affect a wide range of human intestinal epithelial cell processes that justify the ability to induce diarrhea through different mechanisms and cause diseases of variable severity, as observed for other gastrointestinal pathogens. However, it remains to be determined whether specific genotype(s) are associated with clinical pictures of different severity to implement the diagnostic and therapeutic approaches for this relevant enteric pathogen.

Antibiotic Resistance, Virulence Gene Detection, and Biofilm Formation in Aeromonas spp. Isolated from Fish and Humans in Egypt

The genus Aeromonas is widely distributed in aquatic environments and is recognized as a potential human pathogen. Some Aeromonas species are able to cause a wide spectrum of diseases, mainly gastroenteritis, skin and soft-tissue infections, bacteremia, and sepsis. The aim of the current study was to determine the prevalence of Aeromonas spp. in raw fish markets and humans in Zagazig, Egypt; identify the factors that contribute to virulence; determine the isolates’ profile of antibiotic resistance; and to elucidate the ability of Aeromonas spp. to form biofilms. The examined samples included fish tissues and organs from tilapia (Oreochromis niloticus, n = 160) and mugil (Mugil cephalus, n = 105), and human skin swabs (n = 51) and fecal samples (n = 27). Based on biochemical and PCR assays, 11 isolates (3.2%) were confirmed as Aeromonas spp. and four isolates (1.2%) were confirmed as A. hydrophila. The virulence genes including haemolysin (hyl A) and aerolysin (aer) were detected using PCR in A. hydrophila in percentages of 25% and 50%, respectively. The antimicrobial resistance of Aeromonas spp. was assessed against 14 antibiotics comprising six classes. The resistance to cefixime (81.8%) and tobramycin (45.4%) was observed. The multiple antibiotic resistance (MAR) index ranged between 0.142–0.642 with 64.2% of the isolates having MAR values equal to 0.642. Biofilm formation capacity was assessed using a microtiter plate assay, and two isolates (18.1%) were classified as biofilm producers. This study establishes a baseline for monitoring and controlling the multidrug-resistant Aeromonas spp. and especially A. hydrophila in marine foods consumed in our country to protect humans and animals.

Biofilms as a microbial hazard in the food industry: A scoping review

Biofilms pose a serious public health hazard with a significant economic impact on the food industry. The present scoping review is designed to analyze the literature published during 2001-2020 on biofilm formation of microbes, their detection methods, and association with antimicrobial resistance (if any). The peer-reviewed articles retrieved from 04 electronic databases were assessed using PRISMA-ScR guidelines. From the 978 preliminary search results, a total of 88 publications were included in the study. On analysis, the commonly isolated pathogens were Listeria monocytogenes, Staphylococcus aureus, Salmonella spp., Escherichia coli, Bacillus spp., Vibrio spp., Campylobacter jejuni and Clostridium perfringens. The biofilm-forming ability of microbes was found to be influenced by various factors such as attachment surfaces, temperature, presence of other species, nutrient availability etc. A total of 18 studies characterized the biofilm-forming genes, particularly for S. aureus, Salmonella spp., and E. coli. In most studies, polystyrene plate and/or stainless-steel coupons were used for biofilm formation, and the detection was carried out by crystal violet assays and/or by plate counting method. The strain-specific significant differences in biofilm formation were observed in many studies, and few studies carried out analysis of multi-species biofilms. The association between biofilm formation and antimicrobial resistance wasn't clearly defined. Further, viable but non-culturable (VBNC) form of the foodborne pathogens is posing an unseen (by conventional cultivation techniques) but potent threat food safety. The present review recommends the need for carrying out systematic surveys and risk analysis of biofilms in food chain to highlight the evidence-based public health concerns, especially in regions where microbiological food hazards are quite prevalent.

Nature-Inspired Antimicrobial Surfaces and Their Potential Applications in Food Industries

Antimicrobial resistance (AMR) is a growing global concern and has called for the integration of different areas of expertise for designing robust solutions. One such approach is the development of antimicrobial surfaces to combat the emerging resistance in microbes against drugs and disinfectants. This review is a compressive summary of the work done in the field of material science, chemistry, and microbiology in the development of antimicrobial materials and surfaces that are inspired by examples in nature. The focus includes examples of natural antimicrobial surfaces, such as cicada wings or nanopillars, dragonfly wings, shrimp shells, taro leaves, lotus leaves, sharkskin, gecko skin, and butterfly wings, along with their mechanism of action. Techniques, compositions, and combinations that have been developed to synthetically mimic these surfaces against bacterial/viral and fungal growth in food-processing areas have also been discussed. The applications of synthetic mimics of natural antimicrobial surfaces in food-processing environments is still a naïve area of research. However, this review highlights the potential applications of natural antimicrobial surfaces in the food-processing environment as well as outlines the challenges that need mitigations.

Aeromonas spp. diversity in U.S. mid-Atlantic surface and reclaimed water, seasonal dynamics, virulence gene patterns and attachment to lettuce

Aeromonas, a ubiquitous taxon in water environments, is emerging as a foodborne pathogen of concern that remains understudied and under-reported. We evaluated the distribution of 331 Aeromonas spp. isolates collected from irrigation water over one year and characterised their virulence profile, attachment and ability to persist on lettuce. Water sources included non-tidal and tidal river, farm pond and reclaimed water. Twenty Aeromonas species were identified; A. veronii, A. hydrophila and A. jandaei predominated in all water types and seasons, comprising ~63% of isolates. Species distribution was most affected by water type. The highest and lowest diversity were detected in river and pond water, respectively. A. hydrophila and A. veronii ranked highest in frequency in fresh river and reclaimed water, while A. jandaei ranked first in pond water. Only two isolates carried all five virulence genes tested, while 46% of A. hydrophila (n = 50), 54% of A. veronii (n = 61) and 50% of A. jandaei (n = 32) isolates harboured multiple enterotoxin genes. Detection of alt and ast genes was more likely in summer collections, while ast detection was less likely in tidal brackish river and pond water isolates. Season was a factor in attachment to polystyrene, being strongest in spring isolates. The gene flaA was associated with strong attachment and was more likely to be detected in non-tidal fresh river isolates. A. hydrophila and A. jandaei isolates persisted on lettuce leaves for 24 h, but populations dwindled over 120 h, while loosely and strongly attached cells of A. veronii isolates persisted for 120 h. This study provides comprehensive data on Aeromonas species distribution and environmental traits. The associations revealed among diversity, water type, season, virulence factors and phyllosphere attachment capacity can inform agricultural water standards in novel ways. Moreover, understanding Aeromonas-plant interactions is an important step in advancing food safety of fruit and vegetables.

Outer Membrane Vesicles Released From Aeromonas Strains Are Involved in the Biofilm Formation

Aeromonas spp. are Gram-negative rod-shaped bacteria ubiquitously distributed in diverse water sources. Several Aeromonas spp. are known as human and fish pathogens. Recently, attention has been focused on the relationship between bacterial biofilm formation and pathogenicity or drug resistance. However, there have been few reports on biofilm formation by Aeromonas. This study is the first to examine the in vitro formation and components of the biofilm of several Aeromonas clinical and environmental strains. A biofilm formation assay using 1% crystal violet on a polystyrene plate revealed that most Aeromonas strains used in this study formed biofilms but one strain did not. Analysis of the basic components contained in the biofilms formed by Aeromonas strains confirmed that they contained polysaccharides containing GlcNAc, extracellular nucleic acids, and proteins, as previously reported for the biofilms of other bacterial species. Among these components, we focused on several proteins fractionated by SDS-PAGE and determined their amino acid sequences. The results showed that some proteins existing in the Aeromonas biofilms have amino acid sequences homologous to functional proteins present in the outer membrane of Gram-negative bacteria. This result suggests that outer membrane components may affect the biofilm formation of Aeromonas strains. It is known that Gram-negative bacteria often release extracellular membrane vesicles from the outer membrane, so we think that the outer membrane-derived proteins found in the Aeromonas biofilms may be derived from such membrane vesicles. To examine this idea, we next investigated the ability of Aeromonas strains to form outer membrane vesicles (OMVs). Electron microscopic analysis revealed that most Aeromonas strains released OMVs outside the cells. Finally, we purified OMVs from several Aeromonas strains and examined their effect on the biofilm formation. We found that the addition of OMVs dose-dependently promoted biofilm formation, except for one strain that did not form biofilms. These results suggest that the OMVs released from the bacterial cells are closely related to the biofilm formation of Aeromonas strains.

Longitudinal Assessment of the Dynamics of Escherichia coli, Total Coliforms, Enterococcus spp., and Aeromonas spp. in Alternative Irrigation Water Sources: a CONSERVE Study

As climate change continues to stress freshwater resources, we have a pressing need to identify alternative (nontraditional) sources of microbially safe water for irrigation of fresh produce. This study is part of the center CONSERVE, which aims to facilitate the adoption of adequate agricultural water sources. A 26-month longitudinal study was conducted at 11 sites to assess the prevalence of bacteria indicating water quality, fecal contamination, and crop contamination risk (Escherichia coli, total coliforms [TC], Enterococcus, and Aeromonas). Sites included nontidal freshwater rivers/creeks (NF), a tidal brackish river (TB), irrigation ponds (PW), and reclaimed water sites (RW). Water samples were filtered for bacterial quantification. E. coli, TC, enterococci (∼86%, 98%, and 90% positive, respectively; n = 333), and Aeromonas (∼98% positive; n = 133) were widespread in water samples tested. Highest E. coli counts were in rivers, TC counts in TB, and enterococci in rivers and ponds (P < 0.001 in all cases) compared to other water types. Aeromonas counts were consistent across sites. Seasonal dynamics were detected in NF and PW samples only. E. coli counts were higher in the vegetable crop-growing (May-October) than nongrowing (November-April) season in all water types (P < 0.05). Only one RW and both PW sites met the U.S. Food Safety Modernization Act water standards. However, implementation of recommended mitigation measures of allowing time for microbial die-off between irrigation and harvest would bring all other sites into compliance within 2 days. This study provides comprehensive microbial data on alternative irrigation water and serves as an important resource for food safety planning and policy setting.IMPORTANCE Increasing demands for fresh fruit and vegetables, a variable climate affecting agricultural water availability, and microbial food safety goals are pressing the need to identify new, safe, alternative sources of irrigation water. Our study generated microbial data collected over a 2-year period from potential sources of irrigation (rivers, ponds, and reclaimed water sites). Pond water was found to comply with Food Safety Modernization Act (FSMA) microbial standards for irrigation of fruit and vegetables. Bacterial counts in reclaimed water, a resource that is not universally allowed on fresh produce in the United States, generally met microbial standards or needed minimal mitigation. We detected the most seasonality and the highest microbial loads in river water, which emerged as the water type that would require the most mitigation to be compliant with established FSMA standards. This data set represents one of the most comprehensive, longitudinal analyses of alternative irrigation water sources in the United States.

Metagenomic Characterization of Bacterial Communities on Ready-to-Eat Vegetables and Effects of Household Washing on their Diversity and Composition

Ready-to-eat (RTE) leafy salad vegetables are considered foods that can be consumed immediately at the point of sale without further treatment. The aim of the study was to investigate the bacterial community composition of RTE salads at the point of consumption and the changes in bacterial diversity and composition associated with different household washing treatments. The bacterial microbiomes of rocket and spinach leaves were examined by means of 16S rRNA gene high-throughput sequencing. Overall, 886 Operational Taxonomic Units (OTUs) were detected in the salads’ leaves. Proteobacteria was the most diverse high-level taxonomic group followed by Bacteroidetes and Firmicutes. Although they were processed at the same production facilities, rocket showed different bacterial community composition than spinach salads, mainly attributed to the different contributions of Proteobacteria and Bacteroidetes to the total OTU number. The tested household decontamination treatments proved inefficient in changing the bacterial community composition in both RTE salads. Furthermore, storage duration of the salads at refrigeration temperatures affected the microbiome, by decreasing the bacterial richness and promoting the dominance of psychrotropic bacteria. Finally, both salads were found to be a reservoir of opportunistic human pathogens, while washing methods usually applied at home proved to be inefficient in their removal.

Biofilm formation and multidrug-resistant Aeromonas spp. from wild animals

OBJECTIVES

The 'One Health' concept recognises that the health of humans, animals and the environment are interconnected. Therefore, knowledge on the behaviour of micro-organisms from the most diverse environmental niches is important to prevent the emergence and dissemination of antimicrobial resistance. Wild animals are known to carry antimicrobial-resistant micro-organisms with potential public health impact. However, no data are available on the behaviour of sessile bacteria from wild animals, although antimicrobial resistance is amplified in biofilms. This study characterised the ciprofloxacin susceptibility and the adhesion and biofilm formation abilities of 14 distinct Aeromonas spp. (8 Aeromonas salmonicida, 3 Aeromonas eucrenophila, 2 Aeromonas bestiarum and 1 Aeromonas veronii) isolated from wild animals and already characterised as resistant to β-lactam antibiotics.

METHODS

The ciprofloxacin MIC was determined according to CLSI guidelines. A biofilm formation assay was performed by a modified microtitre plate method. Bacterial surface hydrophobicity was assessed by sessile drop contact angle measurement.

RESULTS

All Aeromonas spp. strains were resistant to ciprofloxacin (MICs of 6-60μg/mL) and had hydrophilic surfaces (range 2-37mJ/m²). These strains were able to adhere and form biofilms with distinct magnitudes. Biofilm exposure to 10×MIC of ciprofloxacin only caused low to moderate biofilm removal.

CONCLUSIONS

This study shows that the strains tested are of potential public health concern and emphasises that wild animals are potential reservoirs of multidrug-resistant strains. In fact, Aeromonas spp. are consistently considered opportunistic pathogens. Moreover, bacterial ability to form biofilms increases antimicrobial resistance and the propensity to cause persistent infections.

Characterization of virulence, cell surface characteristics and biofilm-forming ability of Aeromonas spp. isolates from fish and sea water

Members of the genus Aeromonas are emerging human pathogens, causing a variety of extra-intestinal, systemic and gastrointestinal infections in both immunocompetent and immunocompromised persons. Aeromonas virulence is multifaceted and involves surface-associated molecules, motility, biologically active extracellular products and biofilm formation. Aeromonads, isolated from diverse freshwater fish species as well as sea water, were screened for biofilm formation, with varying physicochemical parameters including temperature, agitation and nutrient availability. Motility, cell surface characteristics (auto-aggregation, hydrophobicity and S layer), and extracellular virulence factor production (haemolysis, proteolysis, DNase production) were also assessed to identify potential associations with the biofilm phenotype. Biofilm formation was influenced by environmental conditions, with isolates preferentially forming biofilms in nutrient-rich media at 30 °C, although strong biofilm formation also occurred at 37 °C. Strong biofilm formation was observed for Aeromonas culicicola isolates following exposure to nutrient-rich conditions, while Aeromonas allosaccharophila isolates preferred nutrient-poor conditions for biofilm formation. Source-/species-specific correlations, ranging from weak to strong, were observed between biofilm formation and motility, cell surface characteristics and/or extracellular virulence factor production. Understanding the specific mechanisms by which Aeromonas species adhere to abiotic surfaces may aid in preventing and/or treating disease outbreaks in aquaculture systems and could lead to effective eradication of these fish pathogens.

Expression of virulence and stress response genes in Aeromonas hydrophila under various stress conditions

Aeromonas hydrophila has emerged as an important human pathogen as it causes gastroenteritis and extra-intestinal infections. Information regarding the influence of environmental stresses on gene expression profile of A. hydrophila is lacking. The impact of nutrient replenishment, nutrient deprivation, acid stress, and cold shock on housekeeping, general stress-response, and virulence genes was studied using quantitative real-time PCR in two A. hydrophila strains, CECT 839^T , and A331. These sub-lethal stresses invoked significant changes in the expression of these genes in a strain-dependent manner. Overall, nutrient replenishment and deprivation significantly induced the expression of housekeeping (rpoD), general stress regulators (uspA and rpoS), and virulence (aer) genes, indicating their importance in regulating the survival and virulence of A. hydrophila under these stress conditions. rpoS gene was significantly induced under cold shock; whereas, acid stress significantly induced the expression of uspA gene. This is the first study to investigate the effect of environmental parameters on the expression of stress-response and virulence genes in A. hydrophila strains.

Pan-genome analysis of Aeromonas hydrophila, Aeromonas veronii and Aeromonas caviae indicates phylogenomic diversity and greater pathogenic potential for Aeromonas hydrophila

Aeromonas species are important pathogens of fishes and aquatic animals capable of infecting humans and other animals via food. Due to the paucity of pan-genomic studies on aeromonads, the present study was undertaken to analyse the pan-genome of three clinically important Aeromonas species (A. hydrophila, A. veronii, A. caviae). Results of pan-genome analysis revealed an open pan-genome for all three species with pan-genome sizes of 9181, 7214 and 6884 genes for A. hydrophila, A. veronii and A. caviae, respectively. Core-genome: pan-genome ratio (RCP) indicated greater genomic diversity for A. hydrophila and interestingly RCP emerged as an effective indicator to gauge genomic diversity which could possibly be extended to other organisms too. Phylogenomic network analysis highlighted the influence of homologous recombination and lateral gene transfer in the evolution of Aeromonas spp. Prediction of virulence factors indicated no significant difference among the three species though analysis of pathogenic potential and acquired antimicrobial resistance genes revealed greater hazards from A. hydrophila. In conclusion, the present study highlighted the usefulness of whole genome analyses to infer evolutionary cues for Aeromonas species which indicated considerable phylogenomic diversity for A. hydrophila and hitherto unknown genomic evidence for pathogenic potential of A. hydrophila compared to A. veronii and A. caviae.

Inhibition of pathogenic and spoilage bacteria by a novel biofilm-forming Lactobacillus isolate: a potential host for the expression of heterologous proteins

BACKGROUND

Bacterial biofilms are a preferred mode of growth for many types of microorganisms in their natural environments. The ability of pathogens to integrate within a biofilm is pivotal to their survival. The possibility of biofilm formation in Lactobacillus communities is also important in various industrial and medical settings. Lactobacilli can eliminate the colonization of different pathogenic microorganisms. Alternatively, new opportunities are now arising with the rapidly expanding potential of lactic acid bacteria biofilms as bio-control agents against food-borne pathogens.

RESULTS

A new isolate Lactobacillus plantarum PA21 could form a strong biofilm in pure culture and in combination with several pathogenic and food-spoilage bacteria such as Salmonella enterica, Bacillus cereus, Pseudomonas fluorescens, and Aeromonas hydrophila. Exposure to Lb. plantarum PA21 significantly reduced the number of P. fluorescens, A. hydrophila and B. cereus cells in the biofilm over 2-, 4- and 6-day time periods. However, despite the reduction in S. enterica cells, this pathogen showed greater resistance in the presence of PA21 developed biofilm, either in the planktonic or biofilm phase. Lb. plantarum PA21 was also found to be able to constitutively express GFP when transformed with the expression vector pMG36e which harbors the gfp gene as a reporter demonstrating that the newly isolated strain can be used as host for genetic engineering.

CONCLUSION

In this study, we evaluate the ability of a new Lactobacillus isolate to form strong biofilm, which would provide the inhibitory effect against several spoilage and pathogenic bacteria. This new isolate has the potential to serve as a safe and effective cell factory for recombinant proteins.

Effect of plasma processing and organosilane modifications of polyethylene on Aeromonas hydrophila biofilm formation

The aim of our research was to study how the modifications of polyethylene—a material commonly used in medicine and water industry—influence bacterial cell attachment and biofilm formation. The native surface was activated and modified using two-step process consisting in the activation of native surface with a H₂O vapor plasma followed by its treatment with various organosilanes, namely, [3(tertbutylamine-2hydroxy) propyloxypropyl] diethoxymethylsilane, 1H,1H,2H,2H-perfluorooctylmethyldimethoxysilane, dimethoxydimethylsilane, and isobutylmethyldimethoxysilane. The effect of polyethylene modification after chemical treatment was analyzed using surface tension measurement. The adhesive properties of Aeromonas hydrophila LOCK0968 were studied in water with a low concentration of organic compounds, using luminometric and microscopic methods, and the viability of the adhered bacterial cells was evaluated using the colony forming units method. After two-week incubation the chemically modified materials exhibited better antiadhesive and antibacterial characteristics in comparison to the native surface. Among the examined modifying agents, dimethoxydimethylsilane showed the best desired properties.

Influence of glucose concentrations on biofilm formation, motility, exoprotease production, and quorum sensing in Aeromonas hydrophila

Aeromonas hydrophila recently has received increased attention because it is opportunistic and a primary human pathogen. A. hydrophila biofilm formation and its control are a major concern for food safety because biofilms are related to virulence. Therefore, we investigated biofilm formation, motility inhibition, quorum sensing, and exoprotease production of this opportunistic pathogen in response to various glucose concentrations from 0.05 to 2.5% (wt/vol). More than 0.05% glucose significantly impaired (P < 0.05) quorum sensing, biofilm formation, protease production, and swarming and swimming motility, whereas bacteria treated with 0.05% glucose had activity similar to that of the control (0% glucose). A stage shift biofilm assay revealed that the addition of glucose (2.5%) inhibited initial biofilm formation but not later stages. However, addition of quorum sensing molecules N-3-butanoyl-DL-homoserine lactone and N-3-hexanoyl homoserine lactone partially restored protease production, indicating that quorum sensing is controlled by glucose concentrations. Thus, glucose present in food or added as a preservative could regulate acyl-homoserine lactone quorum sensing molecules, which mediate biofilm formation and virulence in A. hydrophila.

Attachment strength and biofilm forming ability of Bacillus cereus on green-leafy vegetables: cabbage and lettuce

The present study was designed to investigate the ability of six Bacillus cereus strains to attach and form biofilm on cabbage and lettuce surfaces. These six strains were; a reference strain DSMZ 345 and five biofilm-producing strains (aquatic strains; TUB8, TUB30, TUB31, TUB32 and TUB33) isolated from drinking-water distribution network. Hydrophobicity, biofilm formation ability, attachment strength (S(R)) of spores and vegetative cells of the six B. cereus strains were also determined. Due to their high hydrophobicity, spores of all strains had high ability to attach polystyrene and did not affect by dilution of tryptone soy broth (TSB, 1:20 v/v) in the in vitro experiment. Significant (p < 0.05) enhancement in vitro biofilm formation by vegetative cells of B. cereus was recorded in the diluted TSB. The highest biofilm formation on cabbage and lettuce surfaces was obtained by spores and vegetative cells of all tested strains on the 4(th) hour of the incubation period. These populations were significantly (p < 0.05) increased by elongating incubation time from 4 h to 24 h except DSMZ 345 and TUB8. Biofilm formation behavior obtained by B. cereus spores and vegetative cells on the polystyrene surface was different compared with that recorded on produce surface. The S(R) of both spores and vegetative cells of the studied strains to the lettuce surface was higher than that of the cabbage surface. The hydrophobicity, biofilm formation and S(R) of spores and vegetative cells of the biofilm-producing strains were higher than that of the reference strain DSMZ 345. Scanning electron microscopy (SEM) exposed random distribution of cells either on the surface or cut edge, without clear obvious affinity for the surface structures. Increasing in the presence of large clusters of cells on leaf surfaces was demonstrated after 4 and 24 h. In conclusion, use of aquatic environmental isolates is more useful for studying biofilm formation than the reference strain. Lettuce surface supported the attachment of B. cereus spores and vegetative cells compared with the cabbage surface. Further investigations are required to improve our knowledge of biofilm formation mechanisms by the human pathogenic microorganisms, especially by using the environmental and clinical isolates. To ensure safety level of green-leafy vegetables, biofilm formation after harvest should be considered as critical control point during handling of these vegetables.