Modulation of juvenile brook trout (Salvelinus fontinalis) cellular immune system after Aeromonas salmonicida challenge

Dual-seq transcriptomics of Aeromonas salmonicida infection in Atlantic salmon (Salmo salar) primary macrophages reveals lysosome and apoptosis impairments

A. salmonicida subsp. salmonicida is one of the oldest-known marine pathogens causing furunculosis in freshwater to marine fish species. A. salmonicida causes septicemia and fish death due to a systemic shock. Early stages of A. salmonicida infection, including intracellular macrophage infection, are not fully comprehended. Here, we conducted a dual RNA-seq study and functional analyses in Atlantic salmon primary macrophages infected with A. salmonicida to identify relevant genes for fish cellular immunity and A. salmonicida pathogenesis. At 1-h post-infection (hpi), A. salmonicida modulated the expression of genes associated with inflammation, fatty acids synthesis, and apoptosis. While at 2 hpi A. salmonicida hijacked pathways related to myeloid cell differentiation, cytoskeleton and actin filament organization, lysosome maturation, and apoptosis. In contrast, A. salmonicida upregulated genes encoding for hemolysin, aerolysin, type IVa pili, and T3SS effectors. In conclusion, these results suggest that A. salmonicida induces endocytosis, impairs lysosome maturation, and reduces apoptosis.

Inactivated Aeromonas salmonicida impairs adaptive immunity in lumpfish (Cyclopterus lumpus)

Aeromonas salmonicida, a widely distributed aquatic pathogen causing furunculosis in fish, exhibits varied virulence, posing challenges in infectious disease and immunity studies, notably in vaccine efficacy assessment. Lumpfish (Cyclopterus lumpus) has become a valuable model for marine pathogenesis studies. This study evaluated several antigen preparations against A. salmonicida J223, a hypervirulent strain of teleost fish, including lumpfish. The potential immune protective effect of A. salmonicida bacterins in the presence and absence of the A-layer and extracellular products was tested in lumpfish. Also, we evaluated the impact of A. salmonicida outer membrane proteins (OMPs) and iron-regulated outer membrane proteins (IROMPs) on lumpfish immunity. The immunized lumpfish were intraperitoneally (i.p.) challenged with 104 A. salmonicida cells/dose at 8 weeks-post immunization (wpi). Immunized and non-immunized fish died within 2 weeks post-challenge. Our analyses showed that immunization with A. salmonicida J223 bacterins and antigen preparations did not increase IgM titres. In addition, adaptive immunity biomarker genes (e.g., igm, mhc-ii and cd4) were down-regulated. These findings suggest that A. salmonicida J223 antigen preparations hinder lumpfish immunity. Notably, many fish vaccines are bacterin-based, often lacking efficacy evaluation. This study offers crucial insights for finfish vaccine approval and regulations.

Novel C-type lectin mediated non-specific cytotoxic cells killing activity through NCCRP-1 in nile tilapia (Oreochromis niloticus)

Non-specific cytotoxic cells (NCCs) are vital immune cells involved in teleost's non-specific immunity. As a receptor molecule on the NCCs' surface, the non-specific cytotoxic cell receptor protein 1 (NCCRP-1) is known to play a crucial role in mediating their activity. Nevertheless, there have been limited studies on the signal molecule that transmits signals via NCCRP-1. In this study, a yeast two-hybrid (Y2H) library of tilapia liver and head kidney was constructed and subsequently screened with the bait vector NCCRP-1 of Oreochromis niloticus (On-NCCRP-1) to obtain a C-type lectin (On-CTL) with an interacting protein sequence. Consequently, the full-length sequence of On-CTL was cloned and analyzed. The expression analysis revealed that On-CTL is highly expressed in the liver and is widely distributed in other tissues. Furthermore, On-CTL expression was significantly up-regulated in the brain, intestine, and head kidney following a challenge with Streptococcus agalactiae. A point-to-point Y2H method was also used to confirm the binding between On-NCCRP-1 and On-CTL. The recombinant On-CTL (rOn-CTL) protein was purified. In vitro experiments demonstrated that rOn-CTL can up-regulate the expression of killer effector molecules in NCCs via its interaction with On-NCCRP-1. Moreover, activation of NCCs by rOn-CTL resulted in a remarkable enhancement in their ability to eliminate fathead minnow cells, indicating that rOn-CTL effectively modulates the killing activity of NCCs through the NCC receptor molecule On-NCCRP-1. These findings significantly contribute to our comprehension of the regulatory mechanisms governing NCC activity, paving the way for future research in this field.

Characterization of Aeromonas salmonicida mesophilic isolates from Alberta (Canada) allows the development of a more sensitive Dictyostelium discoideum predation test

Aeromonas salmonicida is studied using Dictyostelium discoideum as a model host, with predation resistance measured as a key parameter. Aeromonas salmonicida mesophilic isolates exhibit inconclusive results with the amoebic model. This study focuses on new mesophilic isolates (S24-S38, S26-S10, and S28-S20) from Alberta, Canada, and introduces an improved predation test method. Phylogenetic analysis reveals two subgroups, with S24-S38 and S26-S10 clustering with the subspecies pectinolytica from Argentina, and S28-S20 with strains from India (Y567) and Spain (AJ83), showcasing surprising mesophilic strain diversity across geographic locations. Predation tests were carried out with various mesophilic and psychrophilic strains of A. salmonicida, including Alberta isolates. The amoeba cell lines used were DH1-10 and AX2. Although the mesophilic isolates were very resistant to predation by the amoeba DH1-10, some lost this resistance to the AX2 strain, which appeared more voracious in the conditions tested. In addition, when diluting the culture medium used in a predation test with AX2, a loss of the capacity to predation resistance was observed for all the mesophilic isolates, including the highly resistant S28-S20 isolate. This study provides insights into the predation resistance of A. salmonicida isolates and offers avenues for better characterizing mesophilic isolates.

MQM1, a bacteriophage infecting strains of Aeromonas salmonicida subspecies salmonicida carrying Prophage 3

Aeromonas salmonicida subsp. salmonicida is a Gam-negative bacterium responsible for furunculosis in fish. Because this aquatic bacterial pathogen has a rich reservoir of antibiotic-resistant genes, it is essential to investigate antibacterial alternatives, including the use of phages. Yet, we have previously demonstrated the inefficiency of a phage cocktail designed against A. salmonicida subsp. salmonicida strains due to a phage resistance phenotype associated to a prophage, namely Prophage 3. To bypass this resistance, one of the solutions is to isolate novel phages capable of infecting Prophage 3-bearing strains. Here we report on the isolation and characterization of the new virulent phage vB_AsaP_MQM1 (or MQM1), which is highly specific to A. salmonicida subsp. salmonicida strains. Phage MQM1 inhibited the growth of 01-B516, a strain carrying Prophage 3, including when combined to the previous phage cocktail. MQM1 infected 26 out of the 30 (87%) Prophage 3-bearing strains tested. Its linear dsDNA genome contains 63,343 bp, with a GC content of 50.2%. MQM1 genome can encode 88 proteins and 8 tRNAs, while no integrase or transposase-encoding genes were found. This podophage has an icosahedral capsid and a non-contractile short tail. We suggest that MQM1 may be a good addition to future phage cocktails against furunculosis to resolve the Prophage 3-resistance issue.

Integrated multi-biomarker responses of juvenile rainbow trout (Oncorhynchus mykiss) to an environmentally relevant pharmaceutical mixture

Pharmaceuticals are emerging pollutants of concern for aquatic ecosystems where they are occurring in complex mixtures. In the present study, the chronic toxicity of an environmentally relevant pharmaceutical mixture on juvenile rainbow trout (Oncorhynchus mykiss) was investigated. Five pharmaceuticals (paracetamol, carbamazepine, diclofenac, naproxen and irbesartan) were selected based on their detection frequency and concentration levels in the Meuse river (Belgium). Fish were exposed for 42 days to three different concentrations of the mixture, the median one detected in the Meuse river, 10-times and 100-times this concentration. Effects on the nervous, immune, antioxidant, and detoxification systems were evaluated throughout the exposure period and their response standardized using the Integrated Biomarker Response (IBRv2) index. IBRv2 scores increased over time in the fish exposed to the highest concentration. After 42 days, fish exposed to the highest concentration displayed significantly higher levels in lysozyme activity (p < 0.01). The mixture also caused significant changes in brain serotonin turnover (p < 0.05). In short, our results indicate that the subchronic waterborne exposure to a pharmaceutical mixture commonly occurring in freshwater ecosystems may affect the neuroendocrine and immune systems of juvenile rainbow trout.

Plasmid composition in Aeromonas salmonicida subsp. salmonicida 01-B526 unravels unsuspected type three secretion system loss patterns

Background

Aeromonas salmonicida subsp. salmonicida is a ubiquitous psychrophilic waterborne bacterium and a fish pathogen. The numerous mobile elements, especially insertion sequences (IS), in its genome promote rearrangements that impact its phenotype. One of the main virulence factors of this bacterium, its type three secretion system (TTSS), is affected by these rearrangements. In Aeromonas salmonicida subsp. salmonicida most of the TTSS genes are encoded in a single locus on a large plasmid called pAsa5, and may be lost when the bacterium is cultivated at a higher temperature (25 °C), producing non-virulent mutants. In a previous study, pAsa5-rearranged strains that lacked the TTSS locus on pAsa5 were produced using parental strains, including 01-B526. Some of the generated deletions were explained by homologous recombination between ISs found on pAsa5, whereas the others remained unresolved. To investigate those rearrangements, short- and long-read high-throughput sequencing technologies were used on the A. salmonicida subsp. salmonicida 01-B526 whole genome.

Results

Whole genome sequencing of the 01-B526 strain revealed that its pAsa5 has an additional IS copy, an ISAS5, compared to the reference strain (A449) sequence, which allowed for a previously unknown rearrangement to occur. It also appeared that 01-B526 bears a second large plasmid, named pAsa9, which shares 40 kbp of highly similar sequences with pAsa5. Following these discoveries, previously unexplained deletions were elucidated by genotyping. Furthermore, in one of the derived strains a fusion of pAsa5 and pAsa9, involving the newly discovered ISAS5 copy, was observed.

Conclusion

The loss of TTSS and hence virulence is explained by one consistent mechanism: IS-driven homologous recombination. The similarities between pAsa9 and pAsa5 also provide another example of genetic diversity driven by ISs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3921-1) contains supplementary material, which is available to authorized users.

Aeromonas salmonicida type III secretion system-effectors-mediated immune suppression in rainbow trout (Oncorhynchus mykiss)

Aeromonas salmonicida subsp. salmonicida, the etiologic agent of furunculosis, is a major pathogen in aquaculture. Together with other pathogens, it is characterized by the presence of a type 3 secretion system (T3SS). The T3SS is the main virulence mechanism of A. salmonicida. It is used by the bacterium to secrete and translocate several toxins and effector proteins into the host cell. Some of these factors have a detrimental impact on the integrity of the cell cytoskeleton, likely contributing to impair phagocytosis. Furthermore, it has been suggested that effectors of the T3SS are able to modulate the host's immune response. Here we present the first partial characterization of the immune response in rainbow trout (Oncorhynchus mykiss) infected with distinct strains of A. salmonicida either carrying (i) a fully functional T3SS or (ii) a functionally impaired T3SS or (iii) devoid of T3SS ("cured" strain). Infection with an A. salmonicida strain either carrying a fully functional or a secretion-impaired T3SS was associated with a strong and persistent immune suppression. However, the infection appeared to be fatal only in the presence of a fully functional T3SS. In contrast, the absence of T3SS was neither associated with immune suppression nor fish death. These findings suggest that the T3SS and T3SS-delivered effector molecules and toxins of A. salmonicida do not only impair the host cells' cytoskeleton thus damaging cell physiology and phagocytosis, but also heavily affect the transcription of critical immune mediators including the shut-down of important warning signals to recognize infection and induce immune defense.

Increasing genomic diversity and evidence of constrained lifestyle evolution due to insertion sequences in Aeromonas salmonicida

BACKGROUND

Aeromonads make up a group of Gram-negative bacteria that includes human and fish pathogens. The Aeromonas salmonicida species has the peculiarity of including five known subspecies. However, few studies of the genomes of A. salmonicida subspecies have been reported to date.

RESULTS

We sequenced the genomes of additional A. salmonicida isolates, including three from India, using next-generation sequencing in order to gain a better understanding of the genomic and phylogenetic links between A. salmonicida subspecies. Their relative phylogenetic positions were confirmed by a core genome phylogeny based on 1645 gene sequences. The Indian isolates, which formed a sub-group together with A. salmonicida subsp. pectinolytica, were able to grow at either at 18 °C and 37 °C, unlike the A. salmonicida psychrophilic isolates that did not grow at 37 °C. Amino acid frequencies, GC content, tRNA composition, loss and gain of genes during evolution, pseudogenes as well as genes under positive selection and the mobilome were studied to explain this intraspecies dichotomy.

CONCLUSION

Insertion sequences appeared to be an important driving force that locked the psychrophilic strains into their particular lifestyle in order to conserve their genomic integrity. This observation, based on comparative genomics, is in agreement with previous results showing that insertion sequence mobility induced by heat in A. salmonicida subspecies causes genomic plasticity, resulting in a deleterious effect on the virulence of the bacterium. We provide a proof-of-concept that selfish DNAs play a major role in the evolution of bacterial species by modeling genomes.

Diversity and Homogeneity among Small Plasmids of Aeromonas salmonicida subsp. salmonicida Linked with Geographical Origin

Furunculosis, which is caused by Aeromonas salmonicida subsp. salmonicida, is a major salmonid disease in fish farms worldwide. Several plasmids found in this bacterium confer phenotypes such drug resistance and virulence. Small plasmids (pAsa1, pAsa2, pAsa3, and pAsal1) related to ColE1- and ColE2-type replicons are usually present in its normal plasmidome. In the present study, with the objective to investigate if these plasmids display particularities related to the origin of the isolates bearing them, a total of 153 isolates, including 78 new and 75 previously described, were analyzed for the presence of small plasmids by PCR and DNA restriction fragment profiling. A geographical dichotomy between Canadian and European isolates for their propensity to do not have pAsa3 or pAsal1 was found. In addition, the genotyping analysis led to the identification of two European isolates harboring an unusual pAsal1. An investigation by next-generation sequencing (NGS) of these two isolates shed light on two pAsal1 variants (pAsal1C and pAsal1D). As with pAsal1B, another pAsal1 variant previously described, these two new variants bore a second insertion sequence (ISAS5) in addition to the usual ISAS11. The characterization of these variants suggested that they could predominate over the wild-type pAsal1 in stressful conditions such as growth at temperatures of 25°C and above. To obtain a comprehensive portrait of the mutational pressure on small plasmids, 26 isolates whose DNA had been sequenced by NGS were investigated. pAsa3 and pAsal1 were more prone to mutations than pAsa1 and pAsa2, especially in the mobA gene, which encodes a relaxase and a primase. Lastly, the average copy number of each plasmid per cell was assessed using raw sequencing data. A clear trend with respect to the relative proportion per cell of each plasmid was identified. Our large-scale study revealed a geographical dichotomy in small plasmid repertoire in addition to a clear trend for pAsa3 and pAsal1 to be more frequently altered. Moreover, we present the discovery of two new variants of pAsal1: pAsal1C and pAsal1D.

Effects of aluminium and bacterial lipopolysaccharide on oxidative stress and immune parameters in roach, Rutilus rutilus L

Aluminium is used in diverse anthropogenic processes at the origin of pollution events in aquatic ecosystems. In the Champagne region (France), high concentrations of aluminium (Al) are detected due to vine-growing practices. In fish, little is known about the possible immune-related effects at relevant environmental concentrations. The present study analyzes the simultaneous effects of aluminium and bacterial lipopolysaccharide (LPS), alone and in combination, on toxicological biomarkers in the freshwater fish species Rutilus rutilus. For this purpose, roach treated or not with LPS were exposed to environmental concentrations of aluminium (100 μg/L) under laboratory-controlled conditions for 2, 7, 14 and 21 days. After each exposure time, we assessed hepatic lipoperoxidation, catalase activity, glutathione reductase activity and total glutathione content. We also analyzed cellular components related to the LPS-induced inflammatory response in possible target tissues, i.e. head kidney and spleen. Our results revealed a significant prooxidant effect in the liver cells and head kidney leukocytes of roach exposed to 100 μg of Al/L for 2 days. In liver, we observed more lipoperoxidation products and lower endogenous antioxidant activity levels such as glutathione reductase activity and total glutathione content. These prooxidant effects were associated with a higher oxidative burst in head kidney leukocytes, and they were all the more important in fish stimulated by LPS injection. These findings demonstrate that environmental concentrations of Al induce oxidative and immunotoxic effects in fish and are associated to an immunomodulatory process related to the inflammatory response.

Optimization of a plasmid electroporation protocol for Aeromonas salmonicida subsp. salmonicida

Aeromonas salmonicida subsp. salmonicida is a major fish pathogen. Molecular tools are required to study the virulence and genomic stability of this bacterium. An efficient electroporation-mediated transformation protocol for A. salmonicida subsp. salmonicida would make genetic studies faster and easier. In the present study, we designed the 4.1-kb pSDD1 plasmid as a tool for optimizing an electroporation protocol for A. salmonicida subsp. salmonicida. We systematically tested the electroporation conditions to develop a protocol that generates the maximum number of transformants. Under these optimal conditions (25 kV/cm, 200 Ω, 25 μF), we achieved an electroporation efficiency of up to 1×10(5) CFU/μg DNA. The electroporation protocol was also tested using another plasmid of 10.6-kb and three different strains of A. salmonicida subsp. salmonicida. The strains displayed significant differences in their electro-transformation competencies. Strain 01-B526 was the easiest to electroporate, especially with the pSDD1 plasmid. This plasmid was stably maintained in the 01-B526 transformants, as were the native plasmids, but could be easily cured by removing the selection conditions. This is the first efficient electroporation protocol reported for A. salmonicida subsp. salmonicida, and offers new possibilities for studying this bacterium.

Aeromonas salmonicida subsp. salmonicida in the light of its type-three secretion system

Aeromonas salmonicida subsp. salmonicida is an important pathogen in salmonid aquaculture and is responsible for the typical furunculosis. The type-three secretion system (T3SS) is a major virulence system. In this work, we review structure and function of this highly sophisticated nanosyringe in A. salmonicida. Based on the literature as well as personal experimental observations, we document the genetic (re)organization, expression regulation, anatomy, putative functional origin and roles in the infectious process of this T3SS. We propose a model of pathogenesis where A. salmonicida induces a temporary immunosuppression state in fish in order to acquire free access to host tissues. Finally, we highlight putative important therapeutic and vaccine strategies to prevent furunculosis of salmonid fish.

Insertion sequence AS5 (IS AS5 ) is involved in the genomic plasticity of Aeromonas salmonicida

The genome of the fish pathogen Aeromonas salmonicida subsp salmonicida harbors a large number of insertion sequences (ISs), many of which are located on plasmids. In the present study, we analyzed the small plasmid profile of A. salmonicida strains to identify evidences of plasmid alterations. Ten out of 78 strains analyzed displayed an unconventional plasmid profile. However the HER1104 strain was unique, having a positive PCR signal for pAsal1 plasmid despite not carrying this plasmid. Instead, HER1104 was bearing a plasmid at higher molecular weight than pAsal1. We characterized this new larger plasmid, which we called pAsal1B since it is a derivative of pAsal1 containing one more complete IS (ISAS5) than the parental plasmid. An additional 96 bp relic of ISAS5 was also present in pAsal1B. These results propose that ISAS5 is another active mobile genetic element in A. salmonicida subsp salmonicida and provided further proof of the genomic plasticity of this bacterium.

Aeromonas spp. whole genomes and virulence factors implicated in fish disease

It is widely recognized that Aeromonas infections produce septicaemia, and ulcerative and haemorrhagic diseases in fish, causing significant mortality in both wild and farmed freshwater and marine fish species that damage the economics of the aquaculture sector. The descriptions of the complete genomes of Aeromonas species have allowed the identification of an important number of virulence genes that affect the pathogenic potential of these bacteria. This review will focus on the most relevant information derived from the available Aeromonas genomes in relation to virulence and on the diverse virulence factors that actively participate in host adherence, colonization and infection, including structural components, extracellular factors, secretion systems, iron acquisition and quorum sensing mechanisms.

An insertion sequence-dependent plasmid rearrangement in Aeromonas salmonicida causes the loss of the type three secretion system

Aeromonas salmonicida, a bacterial fish pathogen, possesses a functional Type Three Secretion System (TTSS), which is essential for its virulence. The genes for this system are mainly located in a single region of the large pAsa5 plasmid. Bacteria lose the TTSS region from this plasmid through rearrangements when grown in stressful growth conditions. The A. salmonicida genome is rich in insertion sequences (ISs), which are mobile DNA elements that can cause DNA rearrangements in other bacterial species. pAsa5 possesses numerous ISs. Three IS11s from the IS256 family encircle the rearranged regions. To confirm that these IS11s are involved in pAsa5 rearrangements, 26 strains derived from strain A449 and two Canadian isolates (01-B526 and 01-B516) with a pAsa5 rearrangement were tested using a PCR approach to determine whether the rearrangements were the result of an IS11-dependent process. Nine out of the 26 strains had a positive PCR result, suggesting that the rearrangement in these strains were IS-dependent. The PCR analysis showed that all the rearrangements in the A449-derived strains were IS11-dependent process while the rearrangements in 01-B526 and 01-B516 could only be partially coupled to the action of IS11. Unidentified elements that affect IS-dependent rearrangements may be present in 01-B526 and 01-B516. Our results suggested that pAsa5 rearrangements involve IS11. This is the first study showing that ISs are involved in plasmid instability in A. salmonicida.

Draft genome sequence of the virulent strain 01-B526 of the fish pathogen Aeromonas salmonicida

Aeromonas salmonicida is an important fish pathogen, mainly of salmonids. This bacterium causes a disease named furunculosis, which is particularly detrimental for the aquaculture industry. Here, we present the draft genome sequence of A. salmonicida 01-B526, a strain isolated from a brook trout that is more virulent than A. salmonicida reference strain A449, for which a genome sequence is available.

Short-term infection of striped bass Morone saxatilis with Mycobacterium marinum

Striped bass Morone saxatilis were studied in order to characterize their immune responses over the short term following challenge with Mycobacterium marinum. The expression of immunity-related genes (IL-1beta, TNF-alpha, Nramp and TGF-beta) quickly increased following infection with M. marinum, but these genes were subsequently down-regulated despite the fact that bacterial counts remained high. The number of monocytes and neutrophils also initially increased at 1 d postinfection. This confirms the importance of these types of cells in initial inflammation and mycobacterial infection in striped bass. The phagocytic index of splenic leukocytes over these same time frames did not change significantly following infection. The discrete window in which inflammatory mechanisms were stimulated in striped bass may be related to the intracellular nature of this pathogen.

Evolution of tolerance to PCBs and susceptibility to a bacterial pathogen (Vibrio harveyi) in Atlantic killifish (Fundulus heteroclitus) from New Bedford (MA, USA) harbor

A population of the non-migratory estuarine fish Fundulus heteroclitus (Atlantic killifish) resident to New Bedford (NB), Massachusetts, USA, an urban harbor highly contaminated with polychlorinated biphenyls (PCBs), demonstrates recently evolved tolerance to some aspects of PCB toxicity. PCB toxicology, ecological theory, and some precedence supported expectations of increased susceptibility to pathogens in NB killifish. However, laboratory bacterial challenges of the marine pathogen Vibrio harveyi to wild fish throughout the reproductive season and to their mature laboratory-raised progeny demonstrated comparable survival by NB and reference killifish, and improved survival by NB males. These results are inconsistent with hypothesized trade-offs of adaptation, and suggest that evolved tolerance in NB killifish may include mechanisms that minimize the immunosuppressive effects of PCBs. Compensatory strategies of populations persisting in highly contaminated environments provide a unique perspective for understanding the long-term ecological effects of toxic chemicals.

Adjuvant and immunostimulatory effects of beta-glucan administration in combination with lipopolysaccharide enhances survival and some immune parameters in carp challenged with Aeromonas hydrophila

Combined effects of beta-glucan and lipopolysaccharide (LPS) on survival and immune response were studied in Cyprinus carpio that were challenged with the pathogen Aeromonas hydrophila. beta-Glucan from Saccharomyces cervisiae and LPS from a virulent strain of A. hydrophila were used in this study. Different concentrations of beta-glucan+LPS mixture were administered on days 1, 7, and 14 through different routes (intraperitoneal injection, bathing, and oral administration). Control and test fish were challenged by intraperitoneal injection of LD50 concentration of A. hydrophila on day 16 and subsequently, mortality and relative percent survival (RPS) were recorded. Intraperitoneal injection elicited 100% RPS even at the lowest concentration (100 microg beta-glucan+10 microg LPS); whereas, oral administration improved RPS rate of carps at higher concentration (1% beta-glucan+0.25% LPS). Bathing did not improve the RPS. Test animals injected with even the minimum dose of the immunomodulators (100 microg beta-glucan+10 microg LPS/fish) had a significant increase in total blood leukocyte counts and an increase in the proportion of neutrophils and monocytes. Superoxide anion production by macrophages was also elevated, which presumably aided the efficient killing of bacterial pathogen. Lower concentration of beta-glucan+LPS had an adjuvant effect on antibody production as pretreatment by injection of 100 microg beta-glucan+10 microg LPS/fish resulted in higher antibody titer against A. hydrophila following vaccination. RT-PCR analyses showed that the expression of interleukin-1beta mRNA did not increase in test fish when compared with the control. Classical and alternative complement pathways were not affected by either the dose or the route of administration of the compounds. It may be concluded that intraperitoneal injection and oral administration, and not the bathing, of beta-glucan+LPS mixture in carp could enhance resistance to challenge by A. hydrophila through changes in several non-specific and specific immune responses.