Antigen expression in biofilm cells of Aeromonas hydrophila employed in oral vaccination of fish

Subinhibitory concentrations of florfenicol increase the biofilm formation of Piscirickettsia salmonis

Public health is facing a new challenge due to the increased bacterial resistance to most of the conventional antibacterial agents. Inadequate use of antibiotics in the Chilean aquaculture industry leads to the generation of multidrug resistance bacteria. Many fish pathogenic bacteria produce biofilm upon various sources of stress such as antibiotics, which provides several survival advantages for the bacterial life in community and can constitute a reservoir of pathogens in the marine environment. Being florfenicol a broad-spectrum antibiotic commonly used to treat infections in aquaculture, the aim of this study was to assess whether this antibiotic modulates in vitro the biofilm formation in several isolates of Piscirickettsia salmonis. Standard antibiotic-micro broth 96-flat well plates were used to determinate the minimal inhibitory concentration of florfenicol in eight different P. salmonis isolates. In vitro findings, with P. salmonis growing in the presence and absence of the antibiotic, exhibited a statistically significantly increase (p < .05) in biofilm formation in all the bacterial isolates cultivated with sub-MIC (defined as the half of the minimal inhibitory concentration in the presence of antibiotic) of florfenicol compared with controls (antibiotic-free broth). In conclusion, sub-MIC of florfenicol induced an increased biofilm formation in all P. salmonis isolates tested.

Protection of Teleost Fish against Infectious Diseases through Oral Administration of Vaccines: Update 2021

Immersion and intraperitoneal injection are the two most common methods used for the vaccination of fish. Because both methods require that fish are handled and thereby stressed, oral administration of vaccines as feed supplements is desirable. In addition, in terms of revaccination (boosting) of adult fish held in net pens, oral administration of vaccines is probably the only feasible method to obtain proper protection against diseases over long periods of time. Oral vaccination is considered a suitable method for mass immunization of large and stress-sensitive fish populations. Moreover, oral vaccines may preferably induce mucosal immunity, which is especially important to fish. Experimental oral vaccine formulations include both non-encapsulated and encapsulated antigens, viruses and bacteria. To develop an effective oral vaccine, the desired antigens must be protected against the harsh environments in the stomach and gut so they can remain intact when they reach the lower gut/intestine where they normally are absorbed and transported to immune cells. The most commonly used encapsulation method is the use of alginate microspheres that can effectively deliver vaccines to the intestine without degradation. Other encapsulation methods include chitosan encapsulation, poly D,L-lactide-co-glycolic acid and liposome encapsulation. Only a few commercial oral vaccines are available on the market, including those against infectious pancreatic necrosis virus (IPNV), Spring viremia carp virus (SVCV), infectious salmon anaemia virus (ISAV) and Piscirickettsia salmonis. This review highlights recent developments of oral vaccination in teleost fish.

Identification of novel vaccine candidates in the whole-cell Aeromonas hydrophila biofilm vaccine through reverse vaccinology approach

Biofilm vaccine has been recognised as one of the successful strategy to reduce the Aeromonas hydrophila infection in fish. But, the vaccine contains the protective and non-protective proteins, which may lead to show altered heterologous adaptive immunity response. Moreover, cross protection and effectiveness of previously developed biofilm vaccine was not tested against different geographical A. hydrophila isolates. Therefore, in the present study, whole-cell A. hydrophila biofilm vaccine was evaluated in rohu, vaccinated group showed increased antibody titer and protection against the different geographical A. hydrophila isolates namely KAH₁ and AAH₂ with 78.9% and 84.2% relative percentage survival, respectively. In addition, by using the immune sera of biofilm vaccinated group, a total of six protective proteins were detected using western blot assay. Further, the same proteins were identified by nano LC-MS/MS method, a total of fourteen candidate proteins showing the immunogenic property including highly expressed OMP's tolC, bamA, lamb, AH4AK4_2542, AHGSH82_029580 were identified as potential vaccine candidates. The STRING analysis revealed that, top candidate proteins identified may potentially interact with other intracellular proteins; involved in ribosomal and (tricarboxylic acid) TCA pathway. Importantly, all the selected vaccine candidate proteins contain the B-cell epitope region. Finally, the present study concludes that, whole-cell A. hydrophila biofilm vaccine able to protect the fish against the different geographical A. hydrophila isolates. Further, through reverse vaccinology approach, a total of fourteen proteins were identified as potential vaccine candidates against A. hydrophila pathogen.

Bacterial biofilm-derived antigens: a new strategy for vaccine development against infectious diseases

INTRODUCTION

Microorganisms can develop into a social organization known as biofilms and these communities can be found in virtually all types of environment on earth. In biofilms, cells grow as multicellular communities held together by a self-produced extracellular matrix. Living within a biofilm allows for the emergence of specific properties for these cells that their planktonic counterparts do not have. Furthermore, biofilms are the cause of several infectious diseases and are frequently inhabited by multi-species. These interactions between microbial species are often critical for the biofilm process. Despite the importance of biofilms in disease, vaccine antigens are typically prepared from bacteria grown as planktonic cells under laboratory conditions. Vaccines based on planktonic bacteria may not provide optimal protection against biofilm-driven infections.

AREAS COVERED

In this review, we will present an overview of biofilm formation, what controls this mode of growth, and recent vaccine development targeting biofilms.

EXPERT OPINION

Previous and ongoing research provides evidence that vaccine formulation with antigens derived from biofilms is a promising approach to prevent infectious diseases and can enhance the protective efficacy of existing vaccines. Therefore, research focusing on the identification of biofilm-derived antigens merits further investigations.

Current prospects and challenges in fish vaccine development in India with special reference to Aeromonas hydrophila vaccine

Infectious diseases are adversely affecting aquaculture practices throughout world and Asian countries are no exception. Indian aquaculture practices are facing serious setback due to a variety of infectious agent's which are responsible for severe mortality and morbidity of all the cultured freshwater fish species leading to severe economic losses. The emergence of antibiotic resistant pattern, residual effect and environmental degradation due to indiscriminate use of antibiotics has necessitates the development of suitable alternate prophylaxis measures for better protection. In this regard, vaccine(s) has proved to be an effective strategy against pathogens to improve the fish production. Over the years numerous studies have been conducted to develop vaccine(s) against different pathogens. While most of the efforts are made to develop vaccine against bacterial pathogens especially against Aeromoniasis and Edwardsiellosis, few attempts have also been made against certain other bacterial, parasitic and fungal pathogens as well. Despite various successful experimental attempts, till date no vaccines against any of the pathogens are commercially available for Indian aquaculture. This review principally focuses on the current state of art in the development of vaccine against different microbial pathogens in general and Aeromonas hydrophila in particular since the bacterium is a major pathogen which is involved in a number of disease conditions in all the cultured fish species in India. Herein in this review, details of various experimental approaches made to find out a potential vaccine candidate which in turn can induce protective immune responses in host alongwith the constraints associated with it in developing a suitable vaccine against this bacterium and its market potential have been illustrated from an Indian perspective.

An iTRAQ-Based Comparative Proteomics Analysis of the Biofilm and Planktonic States of Aeromonas veronii TH0426

Aeromonas veronii is a virulent fish pathogen that causes extensive economic losses in the aquaculture industry worldwide. In this study, a virulent strain of A. veronii TH0426 was used to establish an in vitro biofilm model. The results show that the biofilm-forming abilities of A. veronii TH0426 were similar in different media, peaking under conditions of 20 °C and pH 6. Further, isobaric tags for relative and absolute quantitation (iTRAQ)-based quantitative proteomics methods were used to compare the differential expression of A. veronii between the biofilm and planktonic cells. The results show alterations in 277 proteins, with 130 being upregulated and 147 downregulated. Pathway analysis and GO (Gene Ontology) annotations indicated that these proteins are mainly involved in metabolic pathways and the biosynthesis of secondary metabolites and antibiotics. These proteins are the main factors affecting the adaptability of A. veronii to its external environment. MRM (multiple reaction 27 monitoring) and qPCR (qPCR) were used to verify the differential proteins of the selected A. veronii. This is the first report on the biofilm and planktonic cells of A. veronii, thus contributing to studying the infection and pathogenesis of A. veronii.

Vibrio harveyi biofilm as immunostimulant candidate for high-health pacific white shrimp, Penaeus vannamei farming

The study was to develop Vibrio harveyi biofilm-based novel microbial product and its oral delivery for high health Penaeus vannamei farming. Yield of bacterial biofilm was optimized on chitin substrate (size: <360, 360-850 and 850-1250 μm; concentration: 0.3, 0.6 and 0.9%) in tryptone soy broth (0.15%). The biofilm was characterized by crystal violet assay, SEM and LSCM imaging; protein profiling by SDS-PAGE and LC-ESI-MS/MS. The immune stimulatory effect of the biofilm in yard experiments was evaluated by relative quantification of immune genes using real-time PCR effect on overall improvement on health status under field trials. The highest biofilm yield (6.13 ± 0.2 × 10⁷ cfu/ml) was obtained at 0.6% of <360 μm chitin substrate. The biofilm formation was stabilized by 96 h of incubation at 30 °C. Protein profiling confirmed expression of six additional proteins (SDS-PAGE) and 11 proteins were differentially expressed (LC-ESI-MS/MS) in biofilm cells over free cells of V. harveyi. Oral administration of the biofilm for 48 h confirmed to enhance expression of antimicrobial peptides, penaeidin, crustin and lysozyme in P. vannamei. Further Oral administration of biofilm for two weeks to P. vannamei (1.8 ± 0.13 g) improved the growth (2.66 ± 0.06 g) and survival (84.44 ± 1.82%) compared to control (2.15 ± 0.03 g; 70.94 ± 0.66%) Nursery trials showed a significant reduction in occurrence of anatomical deformities like antenna cut (12.67 ± 0.66%), rostrum cut (4.66 ± 0.87%), and tail rot (3.33 ± 0.88%), compared to animals fed with normal diet which was 24.33 ± 2.72; 14 ± 1.52 and 10.66 ± 1.45% respectively. In vitro and in vivo studies suggest inactivated biofilm cells of V. harveyi on chitin substrate express additional antigenic proteins and when administered orally through feed at regular intervals stimulates immune response and improve growth, survival and health status of shrimp.

A Unique Sugar l-Perosamine (4-Amino-4,6-dideoxy-l-mannose) Is a Compound Building Two O-Chain Polysaccharides in the Lipopolysaccharide of Aeromonas hydrophila Strain JCM 3968, Serogroup O6

Lipopolysaccharide (LPS) is the major glycolipid and virulence factor of Gram-negative bacteria, including Aeromonas spp. The O-specific polysaccharide (O-PS, O-chain, O-antigen), i.e., the surface-exposed part of LPS, which is a hetero- or homopolysaccharide, determines the serospecificity of bacterial strains. Here, chemical analyses, mass spectrometry, and ¹H and ¹³C NMR spectroscopy techniques were employed to study the O-PS of Aeromonas hydrophila strain JCM 3968, serogroup O6. MALDI-TOF mass spectrometry revealed that the LPS of A. hydrophila JCM 3968 has a hexaacylated lipid A with conserved architecture of the backbone and a core oligosaccharide composed of Hep₆Hex₁HexN₁HexNAc₁Kdo₁P₁. To liberate the O-antigen, LPS was subjected to mild acid hydrolysis followed by gel-permeation-chromatography and revealed two O-polysaccharides that were found to contain a unique sugar 4-amino-4,6-dideoxy-l-mannose (N-acetyl-l-perosamine, l-Rhap4NAc), which may further determine the specificity of the serogroup. The first O-polysaccharide (O-PS1) was built up of trisaccharide repeating units composed of one α-d-GalpNAc and two α-l-Rhap4NAc residues, whereas the other one, O-PS2, is an α1→2 linked homopolymer of l-Rhap4NAc. The following structures of the O-polysaccharides were established: O-PS1

→3)-α-l-Rhap4NAc-(1→4)-α-d-GalpNAc-(1→3)-α-l-Rhap4NAc-(1→

O-PS2

→2)-α-l-Rhap4NAc-(1→

The present paper is the first work that reveals the occurrence of perosamine in the l-configuration as a component of bacterial O-chain polysaccharides.

Vaccines Directed Against Microorganisms or Their Products Present During Biofilm Lifestyle: Can We Make a Translation as a Broad Biological Model to Tuberculosis?

Tuberculosis (TB) remains as a global public health problem. In recent years, experimental evidence suggesting the relevance of in vitro pellicle (a type of biofilm formed at the air-liquid interface) production as a phenotype mimicking aspects found by Mycobacterium tuberculosis-complex bacteria during in vivo infection has started to accumulate. There are still opportunities for better diagnostic tools, therapeutic molecules as well as new vaccine candidates to assist in TB control programs worldwide and particularly in less developed nations. Regarding vaccines, despite the availability of a live, attenuated strain (Mycobacterium bovis BCG) since almost a century ago, its variable efficacy and lack of protection against pulmonary and latent disease has prompted basic and applied research leading to preclinical and clinical evaluation of up to 15 new candidates. In this work, I present examples of vaccines based on whole cells grown as biofilms, or specific proteins expressed under such condition, and the effect they have shown in relevant animal models or directly in the natural host. I also discuss why it might be worthwhile to explore these approaches, for constructing and developing new vaccine candidates for testing their efficacy against TB.

Vaccination strategies to protect goldfish Carassius auratus against Aeromonas hydrophila infection

Ornamental goldfish Carassius auratus were treated with whole cell (WC), extracellular product (ECP), outer membrane protein (OMP) and biofilm (BF) vaccines developed from the virulent Aeromonas hydrophila (AHV1; GenBank HQ331525.1) with and without the immunoadjuvant Asparagus racemosus. On various days post-vaccination (dpv), the treated fish were challenged with virulent A. hydrophila. These fish were monitored for survival, growth, specific bacterial reduction, and biochemical, haematological and immunological parameters. C. auratus attained 100% mortality within 7 d in non-vaccinated groups, whereas the vaccines helped to significantly (p ≤ 0.001) increase survival after 25 and 50 dpv. The vaccines with immunoadjuvant (ECP₂, OMP₂ and BF₂ treatments) helped to reduce the Aeromonas load after the challenge, and serum albumin, globulin and protein levels were significantly (p < 0.01) improved in the OMP₂- and BF₂-treated groups. Haemoglobin and red blood cell counts were also significantly improved (p < 0.05) in the vaccinated groups compared to the control group. Additionally, haemagglutination occurred at the 1:12 dilution level in the vaccine plus immunoadjuvant-treated groups. Supplementing the vaccines with immunoadjuvant helped to improve phagocytosis to 54.07%, serum bactericidal activity to 14.6% and the albumin:globulin ratio to 7.6% in BF₂ after 50 dpv. Its positive effect significantly (p < 0.05) increased in vaccinated groups compared to controls. Based on the results, especially with the OMP and BF vaccines, the immunoadjuvant A. racemosus helped to improve the efficiency of the vaccines. This approach will aid in the development of more efficient vaccines against bacterial infections affecting the aquaculture industry.

Structural and immunochemical studies of the lipopolysaccharide from the fish pathogen, Aeromonas bestiarum strain K296, serotype O18

Chemical analyses and mass spectrometry were used to study the structure of the lipopolysaccharide (LPS) isolated from Aeromonas bestiarum strain K296, serotype O18. ESI-MS revealed that the most abundant A. bestiarum LPS glycoforms have a hexa-acylated or tetra-acylated lipid A with conserved architecture of the backbone, consisting of a 1,4′-bisphosphorylated β-(1→6)-linked d-GlcN disaccharide with an AraN residue as a non-stoichiometric substituent and a core oligosaccharide composed of Kdo₁Hep₆Hex₁HexN₁P₁. 1D and 2D NMR spectroscopy revealed that the O-specific polysaccharide (OPS) of A. bestiarum K296 consists of a branched tetrasaccharide repeating unit containing two 6-deoxy-l-talose (6dTalp), one Manp and one GalpNAc residues; thus, it is similar to that of the OPS of A. hydrophila AH-3 (serotype O34) in both the sugar composition and the glycosylation pattern. Moreover, 3-substituted 6dTalp was 2-O-acetylated and additional O-acetyl groups were identified at O-2 and O-4 (or O-3) positions of the terminal 6dTalp. Western blots with polyclonal rabbit sera showed that serotypes O18 and O34 share some epitopes in the LPS. The very weak reaction of the anti-O34 serum with the O-deacylated LPS of A. bestiarum K296 might have been due to the different O-acetylation pattern of the terminal 6dTalp. The latter suggestion was further confirmed by NMR.

Teleost intestinal immunology

Teleosts clearly have a more diffuse gut associated lymphoid system, which is morphological and functional clearly different from the mammalian GALT. All immune cells necessary for a local immune response are abundantly present in the gut mucosa of the species studied and local immune responses can be monitored after intestinal immunization. Fish do not produce IgA, but a special mucosal IgM isotype seems to be secreted and may (partly) be the recently described IgZ/IgT. Fish produce a pIgR in their mucosal tissues but it is smaller (2 ILD) than the 4-5 ILD pIgR of higher vertebrates. Whether teleost pIgR is transcytosed and cleaved off in the same way needs further investigation, especially because a secretory component (SC) is only reported in one species. Teleosts also have high numbers of IEL, most of them are CD3-ɛ+/CD8-α+ and have cytotoxic and/or regulatory function. Possibly many of these cells are TCRγδ cells and they may be involved in the oral tolerance induction observed in fish. Innate immune cells can be observed in the teleost gut from first feeding onwards, but B cells appear much later in mucosal compartments compared to systemic sites. Conspicuous is the very early presence of putative T cells or their precursors in the fish gut, which together with the rag-1 expression of intestinal lymphoid cells may be an indication for an extra-thymic development of certain T cells. Teleosts can develop enteritis in their antigen transporting second gut segment and epithelial cells, IEL and eosinophils/basophils seem to play a crucial role in this intestinal inflammation model. Teleost intestine can be exploited for oral vaccination strategies and probiotic immune stimulation. A variety of encapsulation methods, to protect vaccines against degradation in the foregut, are reported with promising results but in most cases they appear not to be cost effective yet. Microbiota in fish are clearly different from terrestrial animals. In the past decade a fast increasing number of papers is dedicated to the oral administration of a variety of probiotics that can have a strong health beneficial effect, but much more attention has to be paid to the immune mechanisms behind these effects. The recent development of gnotobiotic fish models may be very helpful to study the immune effects of microbiota and probiotics in teleosts.

Structural characterization of the O-specific polysaccharide from the lipopolysaccharide of the fish pathogen Aeromonas bestiarum strain P1S

The O-specific polysaccharide obtained by mild-acid degradation of lipopolysaccharide of Aeromonas bestiarum P1S was studied by sugar and methylation analyses along with (1)H and (13)C NMR spectroscopy. The sequence of the sugar residues was determined using (1)H,(1)H NOESY and (1)H,(13)C HMBC experiments. The O-specific polysaccharide was found to be a high-molecular-mass polysaccharide composed of tetrasaccharide repeating units of the structure [formula in text]. Since small amounts of a terminal Quip3N residue were identified in methylation analysis, it was assumed that the elucidated structure also represented the biological repeating unit of the O-specific polysaccharide.

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

Aeromonas hydrophila is the most well known of the six species of Aeromonas, which has been linked to two groups of human diseases: septicemia and gastroenteritis. Reference strain ATCC 7966 and biofilm strains TUB19, TUB20, and TUB21 were investigated for their ability to form biofilm in vitro (after 48 h on polystyrene surface) and on the surface of two green-leafy vegetables, cabbage and lettuce (after 1, 2, 4, and 24 h). Attachment strength (S(R)) of these strains to the vegetable surface was also measured in the same time intervals. The ATCC 7966 and TUB19 had high ability to form biofilm in vitro compared with TUB20 and TUB21 in full strength tryptone soy broth or under starvation conditions in diluted tryptone soy broth (1:20, v/v). Cell surface hydrophobicity of the biofilm strains was lower than that of the reference strain. The biofilm of all tested strains on polystyrene surfaces differed from that on the vegetable surfaces. All strains studied rapidly attached to both green leafy vegetables (after 1 h). S(R) and cell populations (loosely and strongly attached cells) significantly (p < 0.05) increased with contact time; however, no significant (p > 0.05) differences in cell populations were recorded after 4 and 24 h. The highest S(R) and cell population (log CFU cm⁻²) were recorded by TUB19. In conclusion, the use of A. hydrophila strains isolated from environmental biofilm samples may be more useful for understanding biofilm formation on green-leafy vegetables than the reference or laboratory strains. The attachment of A. hydrophila was significantly affected by the surfaces of green-leafy vegetables. Further studies are required to improve our understanding of the interaction between human microbial pathogens and surfaces of raw vegetables.

Biofilm formation in bacterial pathogens of veterinary importance

Bacterial biofilms are structured communities of bacterial cells enclosed in a self-produced polymer matrix that is attached to a surface. Biofilms protect and allow bacteria to survive and thrive in hostile environments. Bacteria within biofilms can withstand host immune responses, and are much less susceptible to antibiotics and disinfectants when compared with their planktonic counterparts. The ability to form biofilms is now considered a universal attribute of micro-organisms. Diseases associated with biofilms require novel methods for their prevention, diagnosis and treatment; this is largely due to the properties of biofilms. Surprisingly, biofilm formation by bacterial pathogens of veterinary importance has received relatively little attention. Here, we review the current knowledge of bacterial biofilms as well as studies performed on animal pathogens.

Protection of Carassius auratus Gibelio against infection by Aeromonas hydrophila using specific immunoglobulins from hen egg yolk

Specific immunoglobulin (IgY) from egg yolk against Aeromonas hydrophila was produced by immunization of White Leghorn hens with formalin-killed whole cells of A. hydrophila. ELISA test using A. hydrophila as the coating antigen revealed that the specific antibody titer started to increase in the egg yolk at the 13th day post-immunization (P/N=2.18), reached the peak at the 56th day (P/N=13.82), and remained at high level until day 133 (P/N=7.03). The antibody was purified by saturated ammonium sulphate with a recovery rate of 63.5%. The specific IgY inhibited the growth of A. hydrophila at a concentration of 1.0 mg/ml during the 18 h incubation. Pre-treatment of polyploid gibel carps Carassius auratus Gibelio with specific IgY had a protection rate of 60% (6/10) against challenge with A. hydrophila, while none of the fishes in the control groups receiving sterile phosphate buffered saline (PBS) or non-specific IgY survived the challenge. Treatment of fishes with the specific IgY 4 h after the challenge also had lower mortality (70%, 7/10), a 30% reduction against the control PBS or non-specific IgY groups (10/10). These results indicate that specific IgY antibodies could be obtained easily from hens immunized with an inactivated A. hydrophila and could provide a novel alternative approach to control of diseases in fishes caused by this organism.