Lactococcus garvieae and Streptococcus iniae infections in rainbow trout Oncorhynchus mykiss: similar, but different diseases

Redefining piscine lactococcosis

Piscine lactococcosis is a significant threat to cultured and wild fish populations worldwide. The disease typically presents as a per-acute to acute hemorrhagic septicemia causing high morbidity and mortality, recalcitrant to antimicrobial treatment or management interventions. Historically, the disease was attributed to the gram-positive pathogen Lactococcus garvieae. However, recent work has revealed three distinct lactococcosis-causing bacteria (LCB)-L. garvieae, L. petauri, and L. formosensis-which are phenotypically and genetically similar, leading to widespread misidentification. An update on our understanding of lactococcosis and improved methods for identification are urgently needed. To this end, we used representative isolates from each of the three LCB species to compare currently available and recently developed molecular and phenotypic typing assays, including whole-genome sequencing (WGS), end-point and quantitative PCR (qPCR) assays, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), API 20 Strep and Biolog systems, fatty acid methyl ester analysis (FAME), and Sensititre antimicrobial profiling. Apart from WGS, sequencing of the gyrB gene was the only method capable of consistent and accurate identification to the species and strain level. A qPCR assay based on a putative glycosyltransferase gene was also able to distinguish L. petauri from L. garvieae/formosensis. Biochemical tests and MALDI-TOF MS showed some species-specific patterns in sugar and fatty acid metabolism or protein profiles but should be complemented by additional analyses. The LCB demonstrated overlap in host and geographic range, but there were relevant differences in host specificity, regional prevalence, and antimicrobial susceptibility impacting disease treatment and prevention.

IMPORTANCE

Lactococcosis affects a broad range of host species, including fish from cold, temperate, and warm freshwater or marine environments, as well as several terrestrial animals, including humans. As such, lactococcosis is a disease of concern for animal and ecosystem health. The disease is endemic in European and Asian aquaculture but is rapidly encroaching on ecologically and economically important fish populations across the Americas. Piscine lactococcosis is difficult to manage, with issues of vaccine escape, ineffective antimicrobial treatment, and the development of carrier fish or biofilms leading to recurrent outbreaks. Our understanding of the disease is also widely outdated. The accepted etiologic agent of lactococcosis is Lactococcus garvieae. However, historical misidentification has masked contributions from two additional species, L. petauri and L. formosensis, which are indistinguishable from L. garvieae by common diagnostic methods. This work is the first comprehensive characterization of all three agents and provides direct recommendations for species-specific diagnosis and management.

Bacterial and host factors involved in zoonotic Streptococcal meningitis

Zoonotic streptococci cause several invasive diseases with high mortality rates, especially meningitis. Numerous studies elucidated the meningitis pathogenesis of zoonotic streptococci, some specific to certain bacterial species. In contrast, others are shared among different bacterial species, involving colonization and invasion of mucosal barriers, survival in the bloodstream, breaching the blood-brain and/or blood-cerebrospinal fluid barrier to access the central nervous system, and triggering inflammation of the meninges. This review focuses on the recent advancements in comprehending the molecular and cellular events of five major zoonotic streptococci responsible for causing meningitis in humans or animals, including Streptococcus agalactiae, Streptococcus equi subspecies zooepidemicus, Streptococcus suis, Streptococcus dysgalactiae, and Streptococcus iniae. The underlying mechanism was summarized into four themes, including 1) bacterial survival in blood, 2) brain microvascular endothelial cell adhesion and invasion, 3) penetration of the blood-brain barrier, and 4) activation of the immune system and inflammatory reaction within the brain. This review may contribute to developing therapeutics to prevent or mitigate injury of streptococcal meningitis and improve risk stratification.

The Characterisation of Lactococcus garvieae Isolated in an Outbreak of Septicaemic Disease in Farmed Sea Bass (Dicentrarchus labrax, Linnaues 1758) in Italy

In aquaculture, Lactococcus garvieae is a common fish pathogen that can cause significant economic losses in several fresh and saltwater species. Despite the extensive range of hosts, L. garvieae infection in sea bass (Dicentrarchus labrax) has rarely been reported. During the summer of 2023, an outbreak occurred in an inland farm in the Gulf of Follonica (Tuscany, Italy). Fish of various sizes were affected, showing apathy, inappetence, erratic swimming and eye lesions, while the mortality was low (2-3% per month). Anatomopathological examinations suggested a septicaemic infection characterised by melanosis, diffuse redness (skin and fins), paleness (gills and internal organs), haemorrhages and splenomegaly. Seventy swabs from the viscera of 14 subjects were collected and colonies similar to Streptococcus spp. grew from all the samples. Lactococcus garvieae was identified via the biochemical tests, API20STREP, MALDI-TOF, 16S rDNA and whole genome sequencing. Genetical characterisation revealed remarkable differences between this isolate and the strains previously isolated in Italian fish farms. Feed treatments with flumequine and erythromycin were ineffective. Considering the limited effects of antimicrobials, preventive measures, such as vaccination and biosecurity, should be implemented.

Temperature-associated virulence, species susceptibility and interspecies transmission of a Lactococcus petauri strain from rainbow trout

Lactococcus petauri is an important emergent aquaculture pathogen in the USA. To better understand environmental conditions conducive to piscine lactococcosis and the susceptibility of fish species, laboratory-controlled challenges were used as models of infection. Rainbow trout Oncorhynchus mykiss maintained at 13 or 18°C were challenged by intracoelomic (ICe) injection with 101, 103 or 105 colony-forming units per fish (CFU fish-1) and monitored for 21 d. At 13°C, trout experienced mortalities of 7, 7 and 0%, and bacterial persistence of 0, 20 and 0% in survivors, respectively. When exposed to the same bacterial doses, trout maintained at 18°C experienced mortalities of 59, 84 and 91%, and bacterial persistence of 60, 66 and 0% in survivors, confirming a significant role of temperature in the pathogenesis of lactococcosis. Additionally, the susceptibility of rainbow trout, Chinook salmon Oncorhynchus tshawytscha, white sturgeon Acipenser transmontanus, Nile tilapia Oreochromis niloticus, and koi Cyprinus carpio to infection by L. petauri was compared using ICe challenges at 18°C. Trout and salmon experienced 96 and 56% cumulative mortality, respectively, and 17% of surviving salmon remained persistently infected. There were no mortalities in the other fish species, and no culturable bacteria recovered at the end of the challenge. However, when surviving fish were used in further cohabitation trials, naïve trout housed with previously exposed tilapia exhibited 6% mortality, demonstrating that non-salmonids can become sub-clinical carriers of this pathogen. The data obtained provide useful information regarding temperature-associated virulence, fish species susceptibility, and potential carrier transmission of L. petauri that can be used in the development of better management practices to protect against piscine lactococcosis.

Detection and virulence of Lactococcus garvieae and L. petauri from four lakes in southern California

OBJECTIVE

The first objective of the study aimed to detect the presence of Lactococcus petauri, L. garvieae, and L. formosensis in fish (n = 359) and environmental (n = 161) samples from four lakes near an affected fish farm in California during an outbreak in 2020. The second objective was to compare the virulence of the Lactococcus spp. in Rainbow Trout Oncorhynchus mykiss and Largemouth Bass Micropterus salmoides.

METHODS

Standard bacterial culture methods were used to isolate Lactococcus spp. from brain and posterior kidney of sampled fish from the four lakes. Quantitative PCR (qPCR) was utilized to detect Lactococcus spp. DNA in fish tissues and environmental samples from the four lakes. Laboratory controlled challenges were conducted by injecting fish intracoelomically with representative isolates of L. petauri (n = 17), L. garvieae (n = 2), or L. formosensis (n = 4), and monitored for 14 days postchallenge (dpc).

RESULT

Lactococcus garvieae was isolated from the brains of two Largemouth Bass in one of the lakes. Lactococcus spp. were detected in 14 fish (8 Bluegills Lepomis macrochirus and 6 Largemouth Bass) from 3 out of the 4 lakes using a qPCR assay. Of the collected environmental samples, all 4 lakes tested positive for Lactococcus spp. in the soil samples, while 2 of the 4 lakes tested positive in the water samples through qPCR. Challenged Largemouth Bass did not show any signs of infection postinjection throughout the challenge period. Rainbow Trout infected with L. petauri showed clinical signs within 3 dpc and presented a significantly higher cumulative mortality (62.4%; p < 0.0001) at 14 dpc when compared to L. garvieae (0%) and L. formosensis (7.5%) treatments.

CONCLUSION

The study suggests that qPCR can be used for environmental DNA monitoring of Lactococcus spp. and demonstrates virulence diversity between the etiological agents of piscine lactococcosis.

Immune profiling of rainbow trout (Oncorhynchus mykiss) exposed to Lactococcus garvieae: Evidence in asymptomatic versus symptomatic or vaccinated fish

Lactococcosis, caused by the Gram-positive bacterium Lactococcus garvieae, is a major concern in rainbow trout (Oncorhynchus mykiss) farms, which are regularly affected by outbreaks especially during the summer/fall months. In these farms, unvaccinated healthy and symptomatic fish can coexist with vaccinated fish. In the present study, innate (leukogram, serum lysozyme activity, peroxidase activity, antiprotease activity, bactericidal activity, total IgM and total proteins), and specific immune parameters (serum antibodies to L. garvieae) were assessed in unvaccinated adult rainbow trout naturally exposed to the pathogen, with or without evidence of clinical signs, or subjected to vaccination. Blood was drawn from all three groups, and blood smears were prepared. Bacteria were found in the blood smears of 70% of the symptomatic fish but not in any of the asymptomatic fish. Symptomatic fish showed lower blood lymphocytes and higher thrombocytes than asymptomatic fish (p ≤ .05). Serum lysozyme and bactericidal activity did not vary substantially among groups; however, serum antiprotease and peroxidase activity were significantly lower in the unvaccinated symptomatic group than in the unvaccinated and vaccinated asymptomatic groups (p ≤ .05). Serum total proteins and total immunoglobulin (IgM) levels in vaccinated asymptomatic rainbow trout were significantly higher than in unvaccinated asymptomatic and symptomatic groups (p ≤ .05). Similarly, vaccinated asymptomatic fish produced more specific IgM against L. garvieae than unvaccinated asymptomatic and symptomatic fish (p ≤ .05). This preliminary study provides basic knowledge on the immunological relationship occurring between the rainbow trout and L. garvieae, potentially predicting health outcomes. The approach we proposed could facilitate infield diagnostics, and several non-specific immunological markers could serve as reliable indicators of the trout's innate ability to fight infection.

Mortality in farmed European eel (Anguilla anguilla) in Italy due to Streptococcus iniae

BACKGROUND

Streptococcal infections are one of the main causes of fish disease. During the last decade, Streptococcus iniae has become one of the most important aquatic pathogens worldwide, causing high losses in marine and freshwater finfish. Clinical signs in farmed fish include loss of appetite, lethargy and grouping at the bottom of the tank. Gross changes comprise darkening of the skin and haemorrhage at the basis of fins and opercula. To date, S. iniae has been isolated from several wild and farmed fish species but never in the European eel (Anguilla anguilla). In Europe, eel production from aquaculture is around 4500 tonnes and Italy is the third largest producer. This communication represents the first report of an outbreak of S. iniae infection in European eels.

CASE PRESENTATION

The outbreak occurred at an eel farm in northern Italy between May 2021 and September 2021. The outbreak caused about 2% mortality per month, resulting in the loss of about 10% of the farmed fish. The diseased eels showed apathy, lethargy, inactivity and inappetence. In July 2021, three eels were necropsied. Necropsy revealed skin and branchial hyperaemia, a few skin ulcers, and diffuse peritoneal congestion with a few haemorrhagic-like spot lesions. Swab samples for bacteriology were taken from the kidneys, liver, spleen, and brain. Additionally, four eels were opened and swap samples as above were taken. All the investigated eels were found dead. Bacteriological examination revealed growth of Streptococcus spp. from all samples. Identification of S. iniae was done by biochemical characterization, the API20STREP microsystem, 16S rDNA sequencing, and MALDI-TOF. Antimicrobial therapy (oxytetracycline and erythromycin) was ineffective.

CONCLUSIONS

This is the first report of S. iniae infection in the European eel. Although this may be an isolated outbreak, it is of concern due to the losses associated with this pathogen in fish worldwide and because the European eel is an endangered species. Due to the difficulties of controlling the disease with antimicrobials, it is advisable to plan other effective control measures, such as improving water quality and the environmental conditions, reducing fish density, improving biosecurity, and by using immunostimulants and, when possible, vaccines.

Dietary Thymol Improved Growth, Body Composition, Digestive Enzyme Activities, Hematology, Immunity, Antioxidant Defense, and Resistance to Streptococcus iniae in the Rainbow Trout (Oncorhynchus mykiss)

In this study, thymol (TYM) at dietary levels of 0, 1, 1.5, 2, and 2.5 g/kg diet was used to evaluate its effects on growth, digestive performance, immunity, and resistances to the infection induced by Streptococcus iniae in the rainbow trout, Oncorhynchus mykiss. A number of 450 fish (35.8 ± 4.4 g; Mean ± SD) were distributed to 15 tanks (30 fish/tank) in three replicates and fed TYM for 60 days. After feeding period, Fish fed 1.5-2.5 g TYM showed better growth, higher digestive enzyme activity, and body protein content compared to other diets (P < 0.05). Regression analysis indicated a polynomial relationship between growth parameters and dietary TYM levels. Based upon the varied growth parameters, the optimum dietary TYM level was 1.89% for FCR. TYM at dietary levels of 1.5-2.5 g significantly enhanced liver antioxidant enzyme activity [superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT)], immune components in blood [alternative complement activity (C3), total immunoglobulin (Ig), lysozyme activity, bactericidal activity, and total protein], and in mucus [alkaline phosphatase (ALP), protease activity, lysozyme activity, bactericidal activity, and total protein] compared to other diets (P < 0.05). TYM at dietary levels of 2-2.5 g significantly decreased malondialdehyde (MDA) levels compared to other experimental groups (P < 0.05). In addition, use of TYM at dietary levels of 1.5-2.5 g upregulated the expression of the immune-related genes (C3, Lyz, and Ig) (P < 0.05). In contrast, the expression of inflammatory genes, tumor necrosis factor (TNF-α) and Interleukin-8 (IL-8) significantly were downregulated in response to 2-2.5 g TYM (P < 0.05). The hematology of the fish also altered in response to dietary TYM, where the values of corpuscular hemoglobin concentration (MCHC), hemoglobin (Hb), red blood cell (RBC), hematocrit (Hct), and white blood cell (WBC) significantly increased in fish fed 2-2.5 g TYM compared to other diets (P < 0.05). In addition, MCV significantly decreased in response to 2-2.5 g TYM (P < 0.05). After challenge with Streptococcus iniae, the survival rate was significantly higher in fish fed 2-2.5 g TYM compared to other diets (P < 0.05). The results of the present study concluded that TYM in the diet of rainbow trout can improve the fish growth and immunity and increase the resistance of the fish to Streptococcus iniae infection. The results of this study recommend an optimized dietary level of 2-2.5 g TYM for the fish.

Isolation and identification of four pathogenic bacterial strains from edible snake (Elaphe carinata and Ptyas mucosus) farms with pneumonia in China

This report describes an outbreak and treatment of pneumonia and enteritis in a snake farm with more than 3000 snakes containing Elaphe carinata (one-year-old) and Ptyas mucosus (three-month-old) seedlings in Huanggang, Hubei, China. Gentamicin was used once in the early stage as treatment, administered orally with water or feed by owners, but mortality increased. Lobar pneumonia was confirmed by dissection and histopathology in infected snakes. Four main pathogenic bacteria were isolated and identified with culture and 16S rRNA sequencing: Staphylococcus sciuri, Salmonella enteritis, Vagococcus fluvialis and Providencia vermicola. Drug susceptibility tests were performed, and amikacin, gentamicin and cefitriaxone were chosen accordingly. After two rounds of treatment, the clinical signs for Elaphe carinata were under control, and the mortality was close to 0% after treatment. However, treatments for Ptyas mucosus seedlings did not work well, potentially because of poor administration technique and weak body condition.

Inhibition of microbial pathogens in farmed fish

Aquaculture, also known as aqua farming, is defined as farming fish, crustaceans, mollusks, aquatic plants, algae, and other marine organisms. It includes cultivating fresh- and saltwater populations under controlled conditions compared to commercial fishing or wild fish harvesting. Worldwide, carp, salmon, tilapia, and catfish are the most common fish species used in fish farming in descending order. Disinfectants prevent and/or treat different infections in aquatic animals. The current review indicates the uses of different disinfectants against some important pathogens in aquaculture, with particular reference to tilapia (Oreochromis niloticus) farming. A single review cannot cover all aspects of disinfection throughout aquaculture, so the procedures and principles of disinfection in tilapia farming/aquaculture have been chosen for illustration purposes.

Isolation and Genetic Characterization of Streptococcus iniae Virulence Factors in Adriatic Sturgeon (Acipenser naccarii)

The first case of infection of Streptococcus iniae in Adriatic sturgeon (Acipenser naccarii) was recently reported in a raceway system located in Northern Italy. A second episode of infection in sturgeons with absence of mortality and evident clinical signs, was registered in November 2020 in the same farm and is reported in this study. Histopathological changes observed in infected organs are described. The strains isolated in the two episodes were compared using molecular analysis based on PCR, phylogeny and virulence factors analysis. Not all the major virulence factors were detected for the two strains; in particular the strains 78697, isolated in November, lacks cpsD, compared to the strains 64844, isolated in September. Moreover, genetic variations were reported for lctO and pmg genes. These findings let us hypothesize a different virulence of the strains in accordance with clinical findings related to the sturgeons.

Streptococcus dysgalactiae: A Pathogen of Feral Populations of Silver Carp from a Fish Kill Event

In August 2018, a series of large fish kills involving only Silver Carp Hypophthalmichthys molitrix occurred on the Mississippi River in northern Louisiana. Clinical signs observed in moribund animals included erratic swimming behavior, such as spiraling and spinning at the surface. A moribund specimen was captured by dip net near the surface at Lake Providence Landing in East Carroll Parish, northern Louisiana, and was submitted for analysis. An aseptic necropsy was performed, and diagnostic procedures, including bacteriology, parasitology, histopathology, virology, and electron microscopy, revealed that a gram-positive coccus was the primary pathogen. Pure cultures of the organism were obtained from the brain, and it was the predominant colony type isolated from the spleen, kidney, and liver. Bacterial sepsis caused by the gram-positive coccus and involving multiple organ systems was diagnosed histologically. Bacterial colonization and necrotic lesions were seen in the spleen, liver, kidney, heart, eye, and brain. Numerous cocci were observed dividing intracellularly in phagocytic cells of the kidney and brain by transmission electron microscopy. The organism was identified as Streptococcus dysgalactiae ssp. dysgalactiae by conventional biochemical methods and subsequently by the API 20 Strep system. The identity of the pathogen was later confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and sequencing of the 16S ribosomal RNA gene. Multilocus sequence analysis clustered this isolate along with two other S. dysgalactiae isolates from fish in a divergent phyletic group that was separate from other S. dysgalactiae ssp. dysgalactiae isolates from terrestrial animals, implying a possible novel clade that is pathogenic for fish.

Lactococcosis a Re-Emerging Disease in Aquaculture: Disease Significant and Phytotherapy

Lactococcosis, particularly that caused by Lactococcus garvieae, is a major re-emerging bacterial disease seriously affecting the sustainability of aquaculture industry. Medicinal herbs and plants do not have very much in vitro antagonism and in vivo disease resistance towards lactococcosis agents in aquaculture. Most in vitro studies with herbal extractives were performed against L. garvieae with no strong antibacterial activity, but essential oils, especially those that contain thymol or carvacrol, are more effective. The differences exhibited by the bacteriostatic and bactericidal functions for a specific extractive in different studies could be due to different bacterial strains or parts of chemotypes of the same plant. Despite essential oils being shown to have the best anti-L. garvieae activity in in vitro assays, the in vivo bioassays required further study. The extracts tested under in vivo conditions presented moderate efficacy, causing a decrease in mortality in infected animals, probably because they improved immune parameters before challenging tests. This review addressed the efficacy of medicinal herbs to lactococcosis and discussed the presented gaps.

Isolation and characterization of Lactococcus garvieae from rainbow trout, Onchorhyncus mykiss, from California, USA

Lactococcus garvieae is an emergent bacterial pathogen of salmonid fish in North America that causes acute infections particularly at water temperatures above 15°C. During 2020, L. garvieae was detected in rainbow trout, Onchorhyncus mykiss, cultured in Southern California and the Eastern Sierras. Infected fish exhibited high mortalities and nonspecific clinical signs of lethargy, erratic swimming, dark skin pigmentation, and exophthalmia. Macroscopic changes included external and internal hemorrhages, mainly in the eyes, liver, coelomic fat, intestine, and brain. Histological examination revealed splenitis, branchitis, panophthalmitis, hepatitis, enteritis, and coelomitis, with variable degrees of tissue damage among evaluated fish. Pure colonies of L. garvieae were isolated from infected trout and specific PCR primers for L. garvieae confirmed the preliminary diagnosis. Multilocus sequence analysis showed that the strains recovered from diseased trout represent a novel genetic group. Isolates were able to form biofilms within 24 h that increased their resistance to disinfection by hydrogen peroxide. Laboratory challenge methods for inducing lactococcosis in steelhead trout, O. mykiss, were evaluated by intracoelomic injection with serial dilutions of L. garvieae. The median lethal dose 21 days post challenge was ∼20 colony-forming units/fish. Experimentally infected trout presented similar clinical signs, gross changes, and microscopic lesions as those with natural disease, fulfilling Koch's postulates and demonstrating the high virulence of the recovered strains.

Lactococcus garvieae isolated from Lake Kariba (Zambia) has low invasive potential in Nile tilapia (Oreochromis niloticus)

The pathogenesis of Lactococcus garvieae (L. garvieae) was assessed in Nile tilapia (Oreochromis niloticus) following administration by two different routes of infection (intraperitoneal versus immersion), using 180 fish divided into three groups. The first group of fish was injected intraperitoneally (IP) with 3 × 10⁵ colony-forming units (cfu) of L. garvieae; the second group was infected by immersion (IMM) into water containing 9.6 × 10⁵  cfu/ml L. garvieae, and in group 3 (Control), the fish were injected IP with sterile normal saline. Mortalities were recorded daily, and on 3, 5, 7, and 13 days post-infection (dpi), liver, kidney, spleen, brain and eyes were sampled. The level of infection between groups was assessed by number of mortalities that occurred, pathology/histopathology of internal organs, bacterial re-isolation and presence of bacteria in situ determined using immunohistochemistry. A significant difference (p < .0001) was observed between L. garvieae re-isolation from tilapia following administration by IP injection and IMM. Similarly, more clinical signs and mortalities (p < .001) were observed in the IP group compared to the IMM group where no mortalities were observed. These findings suggest that L. garvieae has a low invasive potential in Nile tilapia with intact skin/external barriers and highlights the importance of maintaining fish without cuts or abrasions under field conditions.

Resistant and susceptible rainbow trout (Oncorhynchus mykiss) lines show distinctive immune response to Lactococcus garvieae

Lactococcosis is one of the main bacterial diseases affecting rainbow trout (Oncorhynchus mykiss), with significant economic and sanitary repercussion. Vaccination and antibiotic treatments are commonly used to prevent and control the infection outbreaks; however, these strategies have some drawbacks including limited coverage, handling costs, induction of antibiotic resistance and chemical residues in the environment. Selective breeding programs represent a promising complementary approach for increasing fish disease resistance in commercial farms and some immunological parameters may be tentatively used as indirect indicators for this purpose. The present study investigated for the first time some innate and adaptive immune responses in two groups of rainbow trout derived from selected lines (susceptible and resistant) showing a different "in field" phenotypical resistance to Yersinia ruckeri, Flavobacterium branchiophilum, F. psychrophilum, and Ichthyophthirius multifiliis, after an immersion-dilution based exposure to Lactococcus garvieae carried out in controlled experimental conditions. Twenty-six resistant and twenty-six susceptible female rainbow trout (mean body weight 80 g, 9 months aged, F5 generation) were obtained from an intensive farm considered L. garvieae free and were exposed to the pathogen. Moreover, 10 resistant and 10 susceptible fish were used as uninfected controls. After 5 days, blood and tissue samples were collected for immunological analyses. A significantly higher serum and mucus lysozyme activity was recorded in resistant rainbow trout compared to susceptible fish (P ≤ 0.05), both before and after exposure to L. garvieae. Similarly, respiratory burst activity of head kidney leukocytes resulted more intense in resistant fish (P ≤ 0.05), suggesting that phagocytes could more quickly activate their microbicidal mechanisms to counteract the bacterial spread. Resistant group displayed also an up-regulation of immunoglobulins M (IgM), major histocompatibility complex II (MHC-II) and interleukin 8 (IL-8) gene expression (P ≤ 0.05) and a significantly higher blood lymphocytes count (P ≤ 0.05), highlighting their potential better ability to trigger the recruitment of defensive cells and the initiation of specific immune processes such as antigen presentation to CD4⁺ T lymphocytes and IgM synthesis. The results herein presented might be useful for the identification of immunological markers to be used as indirect indicators in rainbow trout selective breeding programs.

First Identification and Characterization of Lactococcus garvieae Isolated from Rainbow Trout (Oncorhynchus mykiss) Cultured in Mexico

Lactococcosis is a hyperacute hemorrhagic septicemia disease caused by Lactococcus garvieae, which is an emerging pathogen in global fish farming. Between 2016 and 2018, rainbow trout (Oncorhynchus mykiss) from five farms that presented outbreaks were sampled as part of a Mexican surveillance program for the detection of fish diseases. Fourteen L. garvieae isolates were recovered from sampled fish, as confirmed by biochemical tests, 16S rRNA gene sequencing, and clinical and histological insights. The biochemical and protein profiles of the isolates obtained were homogeneous. Repetitive extragenic palindromic-(REP)-and enterobacterial repetitive intergenic consensus sequence PCR (ERIC-PCR) analyses established weak genetic heterogeneity. Rainbow trout challenged with two of the isolates used at different bacterial concentrations (10-2 and 10-4 CFU/mL) showed melanosis, and hemorrhages were observed in the fins, liver, kidney, and spleen. Isolates were obtained from all of the organs sampled, including from surviving fish, as either pure or mixed cultures. The present study is the first to confirm the presence of L. garvieae as the agent of severe lactococcosis outbreaks in the two primary Mexican states for trout farming.

Low lethal doses of Streptococcus iniae caused enteritis in Siberian sturgeon (Acipenser baerii)

In aquaculture, the incidence of enteritis due to Streptococcus iniae infection in Siberian sturgeon (Acipenser baerii) has increased in recent years. The pathogenesis of S. iniae is largely unknown due to the paucity of experimental studies on fish intestinal inflammation. In this study, S. iniae infection of A. baerii juveniles was induced by anal intubation of 0.15 mL at a low lethal dose (2 × 10⁷ CFU/mL). Intestinal pathology and gene expression studies were conducted within 10 days of the experiment. Histopathological examination showed severe intestinal lesions, inflammatory cell infiltration, intestinal submucosa edema, epithelial cell shedding and necrosis. Predominant symptoms of exudative inflammation, metamorphic inflammation and proliferative inflammation on days 1-3, 4-6, and 7-10 post infection were shown, respectively. Ultrastructural observations also revealed fractured microvilli and shedding on days 4-6. Intestinal villi gradually repaired during the subsequent 7-10 days post infection. Expression of the pro-inflammatory cytokines, tumor necrosis factor and interleukin 1β were up-regulated on days 1-3 followed by a significant decrease on day 5, ultimately reaching control levels on day 10 post infection. A similar pattern was shown in mucus cells, involving mucin secretion and expression of the mucin encoding gene, Mucin-2. These results showed the cellular response to S. iniae infection associated with inflammatory genes expression in the Siberian sturgeon.

Antibacterial and antioxidant activity of clove oil against Streptococcus iniae infection in Nile tilapia (Oreochromis niloticus) and its effect on hepatic hepcidin expression

This study was designed to evaluate the modulating effect dietary clove essential oil (CL) has on the antioxidant and immunological status of Nile tilapia following Streptococcus iniae (Si) infection. Fish were placed on either control or (1.5 and 3%) CL-supplemented diets for 4 weeks. After sampling, the remaining fish in the control group were divided into 2 groups: an unchallenged (negative control) and an Si-challenged positive control. On the other hand, the remaining fish in CL-supplemented groups were challenged with Si, and mortality was checked for two weeks before the final sampling. Serum immunological parameters, tissue antioxidants, and oxidative stress markers were determined. Moreover, hepatic hepcidin expression was also measured in different groups. The obtained results showed improvements in blood phagocytic, bactericidal, lysozyme, and respiratory burst activities in CL-supplemented fish before and after the Si challenge. Si-challenge caused a remarkable increase in tissue malondialdehyde (MDA) levels that was inhibited by CL supplementation. The activities of glutathione peroxidase (GPx) and superoxide dismutase (SOD) in tissues were significantly elevated in a dose-dependent manner in CL-supplemented groups in both pre- and post-challenge experiments; renal SOD did not show any differences. Hepatic nitric oxide (NO) level was significantly decreased in CL-supplemented fish in a dose-dependent manner. In the post-challenge experiment, nitrosative stress was apparent in the liver and kidney; however, CL supplementation was sufficient to reverse it. Interestingly, a remarkable induction of the hepatic hepcidin expression was observed in all CL-supplemented groups in the pre-challenge experiment and Si-challenged fish, underscoring the role of CL as an antibacterial through inducing hepatic hepcidin expression to combat S. iniae infection. CL-supplementation was associated with lower mortality rates after Si-challenge, which was more pronounced in CL-3% supplemented fish. In conclusion, our results demonstrate that CL has a potent antioxidant role via increasing antioxidant enzymes' activities and antagonizing lipid peroxidation. Moreover, CL has an immune-stimulant effect by inducing the hepatic hepcidin expression and immunological markers in response to S. iniae infection.

Diversity of non-starter lactic acid bacteria in autochthonous dairy products from Western Balkan Countries - Technological and probiotic properties

The aim of this review was to summarize the data regarding diversity of non-starter lactic acid bacteria (NSLAB) isolated from various artisanal dairy products manufactured in Western Balkan Countries. The dairy products examined were manufactured from raw cow's, sheep's or goat's milk or mixed milk, in the traditional way without the addition of commercial starter cultures. Dairy products such as white brined cheese, fresh cheese, hard cheese, yogurt, sour cream and kajmak were sampled in the households of Serbia, Croatia, Slovenia, Bosnia and Herzegovina, Montenegro, and North Macedonia. It has been established that the diversity of lactic acid bacteria (LAB) from raw milk artisanal dairy products is extensive. In the reviewed literature, 28 LAB species and a large number of strains belonging to the Lactobacillus, Lactococcus, Enterococcus, Streptococcus, Pediococcus, Leuconostoc and Weissella genera were isolated from various dairy products. Over 3000 LAB strains were obtained and characterized for their technological and probiotic properties including: acidification and coagulation of milk, production of aromatic compounds, proteolytic activity, bacteriocins production and competitive exclusion of pathogens, production of exopolysaccharides, aggregation ability and immunomodulatory effect. Results show that many of the isolated NSLAB strains had one, two or more of the properties mentioned. The data presented emphasize the importance of artisanal products as a valuable source of NSLAB with unique technological and probiotic features important both as a base for scientific research as well as for designing novel starter cultures for functional dairy food.

Identification, expression profiling and functional characterization of interleukin 11a ortholog from redlip mullet Liza haematocheila: Insight into its roles in the inflammation and apoptosis regulation

Interleukin 11 (IL-11) is a secretory cytokine with pleotropic properties, including anti-inflammatory and anti-apoptotic functions. This study aimed to functionally characterize a teleostean IL-11a ortholog from redlip mullet (LhIL-11a) through bioinformatic analysis, transcriptional expression profiling and protein function assays. The deduced LhIL-11a protein sequence is 200 amino acids long, with a predicted molecular weight of 23.168 kDa. Multiple sequence alignment indicates that LhIL-11a has a typical four-bundle architecture of α-helixes as observed in other IL-11s. The identity-similarity matrix show a higher identity between LhIL-11a and other fish IL-11a sequences. Phylogenetic analysis demonstrated that LhIL-11a falls within a clade including other fish counterparts. In the tissue distribution analysis, the highest constitutive expression of LhIL-11a mRNA was observed in the mullet gastrointestinal tract and brain tissues. Following the challenges with LPS, poly I:C and Lactococcus garvie, the transcription levels of LhIL-11a were significantly upregulated in both PBCs and liver. In the biological functional assay, recombinant LhIL-11a protein showed strong activities of suppressing pro-inflammatory cytokines and apoptotic gene expression in mullet kidney cells and reducing LPS stimulated NO production in murine macrophage cells. Overall, the findings in this study provide the experimental clues to understanding the functional roles of fish IL-11a in inflammation and apoptosis regulation during host defense against invading microbial pathogens.

High efficacy and economical procedure of oral vaccination against Lactococcus garvieae/Streptococcus iniae in rainbow trout (Oncorhynchus mykiss)

The present study was aimed to examine the efficacy of chitosan-alginate coated vaccines against pathogenicity of Lactococcus garvieae and Streptococcus iniae in rainbow trout. Fish were divided into four groups including: Group A: fish immunized by chitosan-alginate coated vaccine, Group B: fish immunized by non-coated vaccine, Group C: fish feed by chitosan-alginate coated pellets without vaccine and Group D: fish feed by basic diet (non-coated and without vaccine). In groups A and B, the vaccination was carried out for 14 days and after that supplemented with fundamental diet (control diet). Comparable to groups A and B, fish of group C were also fed 14 days with test diets and after that fed control food. On day 0, 20, 40 and 60 of the experiment, serum samples were given. Fish have been challenged with live L. garvieae and S. iniae after 60 days. The levels of bactericidal activity and complement activity among innate immunity components extended on day 20 of the research and after that decreased in group A and B (P < 0.05) all through the examination. The relative expression of IL-6 and IgM in groups A and B extended on examination day 20. The expression of these genes illustrated no advancements in different groups in during the examination (P > 0.05). In group A, the serum antibody titer against L. garvieae and S. iniae broadly raised on day 40 and 60 of examination, whereas in group B, the immune response titer against S. iniae and L. garvieae illustrated a significant elevation on day 60 of the trial (P < 0.05). After challenge with live bacteria, survival rate of 83 ± 9.1%(challenged with S. iniae) and 72.18 ± 9.8% (challenged with L. garvieae) were gotten independently in group A, which were higher than survival of other exploratory groups (P < 0.05). In conclusion, the results of the present examination appear that the orally vaccination of rainbow trout with chitosan-alginate covered vaccine stimulates immunity system and also efficiently protects rainbow trout against Lactococcus garvieae and Streptococcus iniae.

Pharmacodynamics of Ceftiofur Selected by Genomic and Proteomic Approaches of Streptococcus parauberis Isolated from the Flounder, Paralichthys olivaceus

We employed an integrative strategy to present subtractive and comparative metabolic and genomic-based findings of therapeutic targets against Streptococcus parauberis. For the first time, we not only identified potential targets based on genomic and proteomic database analyses but also recommend a new antimicrobial drug for the treatment of olive flounder (Paralichthys olivaceus) infected with S. parauberis. To do that, 102 total annotated metabolic pathways of this bacterial strain were extracted from computational comparative metabolic and genomic databases. Six druggable proteins were identified from these metabolic pathways from the DrugBank database with their respective genes as mtnN, penA, pbp2, murB, murA, coaA, and fni out of 112 essential nonhomologous proteins. Among these hits, 26 transmembrane proteins and 77 cytoplasmic proteins were extracted as potential vaccines and drug targets, respectively. From the FDA DrugBank, ceftiofur was selected to prevent antibiotic resistance as it inhibited our selected identified target. Florfenicol is used for treatment of S. parauberis infection in flounder and was chosen as a comparator drug. All tested strains of fish isolates with S. parauberis were susceptible to ceftiofur and florfenicol with minimum inhibitory concentrations (MIC) of 0.0039-1 μg/mL and 0.5-8 μg/mL, IC50 of 0.001-0.5 μg/mL and 0.7-2.7 μg/mL, and minimum biofilm eradication concentrations (MBEC) of 2-256 μg/mL and 4-64 μg/mL, respectively. Similar susceptibility profiles for ceftiofur and florfenicol were found, with ceftiofur observed as an effective and potent antimicrobial drug against both planktonic and biofilm-forming strains of the fish pathogen Streptococcus parauberis, and it can be applied in the aquaculture industry. Thus, our predictive approach not only showed novel therapeutic agents but also indicated that marketed drugs should also be tested for efficacy against newly identified targets of this important fish pathogen.

A whole-cell Lactococcus garvieae autovaccine protects Nile tilapia against infection

The autovaccine was produced in-house using a bacterial isolate from a diseased fish from the target farm. Three groups of 150 fish each were injected with either 1) an oil-adjuvanted, inactivated whole cell autovaccine, 2) adjuvant only or 3) PBS (negative control). Approximately 660 degree days post vaccination, the fish were challenged with 9x105 cfu bacteria/fish by intraperitoneal injection and monitored for a further 28 days. Protection against infections was measured by lack of/reduced bacterial loads both by bacterial re-isolation and immunohistochemistry as well as absence of clinical signs/pathology. Significantly less L. garvieae (p<0.03) was re-isolated from either the adjuvant only or control groups compared to the vaccinated group. Furthermore, a significantly high amount (p<0.001) of anti-L. garvieae specific antibodies were observed in the vaccinated group compared to the adjuvant only or control groups at time of challenge. This coincided with protection against infection measured by absence/reduced L. garvieae re-isolation from internal organs, reduced clinical signs and lack of pathology in this group. In the adjuvant only and control groups, bacteria were re-isolated from the kidney, liver, spleen, brain and eyes during the first 14 days. The findings suggest that oil-based vaccines can protect tilapia against L. garvieae infection through an antibody mediated response.

Characterization of Streptococcus iniae-induced microRNA profiles in Paralichthys olivaceus and identification of pol-3p-10740_175 as a regulator of antibacterial immune response

MicroRNAs (miRNAs) are involved in many biological activities including immune defense against pathogens. In this study, we applied high-throughput sequencing technology to examine miRNAs in Japanese flounder (Paralichthys olivaceus) infected with Streptococcus iniae at different times. A total of 1038 miRNAs were identified, of which, 249 were novel miRNAs, and 81 showed differential expression (named DEmiRNAs) after S. iniae infection. Of the 81 DEmiRNAs identified, 34 and 58 occurred at 6 h and 24 h post-infection, respectively; most DEmiRNAs were strongly time-specific, and only 13.6% of the DEmiRNAs were shared between the two time points. A total of 9582 target genes were predicted for the 81 DEmiRNAs. The putative target genes were enriched in various GO and KEGG pathways of biological processes and molecular/cellular functions, in particular endocytosis, regulation of transcription, lysososme, and the signaling pathways of MAPK, ErbB, and AMPK. One of the DEmiRNAs, pol-3p-10740_175, was found to target dual specificity phosphatase 6 (Dusp6) and repress the expression of the latter. Transfection of flounder FG cells with pol-3p-10740_175 caused a significant inhibition on S. iniae invasion. The results of this study provided the first S. iniae-induced miRNA profile in Japanese flounder and indicated that flounder miRNAs play an important role in antibacterial immunity.

A Summer Mortality Outbreak of Lactococcosis by Lactococcus garvieae in a Raceway System Affecting Farmed Rainbow Trout (Oncorhynchus mykiss) and Brook Trout (Salvelinus fontinalis)

Simple Summary

Lactococcus garvieae is the etiological agent of lactococcosis, a bacterial disease affecting many species of fish and causing major economic losses in aquaculture. In this study we described, for the first time, the isolation of L. garvieae in brook trout farmed in northwestern Italy by performing a molecular and epidemiological characterization. Results confirmed water as vehicle of infection, favoring the transmission of the pathogen between rainbow trout farmed in the upstream compartments of a raceways system and the brook trout located in downstream tanks.

Abstract

Lactococcosis is a fish disease of major concern in Mediterranean countries caused by Lactococcus garvieae. The most susceptible species is the rainbow trout (Oncorhynchus mykiss), suffering acute disease associated with elevated mortalities compared to other fish species. References reported that other salmonids are also susceptible to the disease, but no mortality outbreak has been described to date. The aim of this study was to present a mortality outbreak that occurred in brook trout (Salvelinus fontinalis) farmed in northwestern Italy during the summer of 2018. Fish exhibited clinical signs, such as exophthalmos, diffused hemorrhages localized in the ocular zone, hemorrhagic enteritis, and enlarged spleen. L. garvieae was isolated in all fish. Molecular and epidemiological characterization of the isolates, through Pulsed Field Gel Electrophoresis (PFGE), confirmed the initial hypothesis of water as vehicle of infection favoring transmission between rainbow trout farmed in upstream compartments and brook trout located in downstream tanks. Moreover, several environmental conditions affected and promoted the outbreak, among them the high-water temperature, which probably induced a physiological stress in brook trout, being way above the optimal temperature for this species, increasing the susceptibility to infection.

Comparative genomic analysis of three lytic Lactococcus garvieae phages, novel phages with genome architecture linking the 936 phage species of Lactococcus lactis

To date, a number of bacteriophages that infect Lactococcus garvieae isolated from marine fish have been identified. However, the evolutionary insight between L. garvieae phages and other viral community have not yet been immersedly investigated. In this study, completed genomic sequence of phage PLgY-30 was obtained, a comparative analysis of three lytic phages, which have been using for phage typing and treatment of L. garvieae infecting marine fish, is conducted. The results revealed that the genomes of lytic phages specific for L. garvieae isolated from diseased marine fish share a high level of homology and almost all proteins are conserved. At genome level, no similarity was detected for either PLgY-30 or PLgY-16, while PLgW-1 shares only very limited homology (1%) with other sequences in Genbank database. In addition, the function of only 35% of ORFs in the PLgY-30 phage genomes could be predicted, demonstrating that it is novel phage. At protein level, lytic phage proteins shared a significant similarity to various proteins of global phage species isolated from dairy fermentation facilities that utilize L. lactis as a primary starter culture, called the 936 phage group. Genome organization and architecture of three lytic phages are also similar to that of the 936 phage group. To our knowledge, this is the first time lytic bacteriophages infecting L. garvieae from marine fish were characterized to genome level.

Valuable method for production of oral vaccine by using alginate and chitosan against Lactococcus garvieae/Streptococcus iniae in rainbow trout (Oncorhynchus mykiss)

The effectiveness of ionotropic gelation method (by combining alginate and chitosan) vaccine against Lactococcus garvieae and Streptococcus iniae was examined in rainbow trout. Fish were separated into four groups and fed the distinctive examined feeds. Our groups were included: A) fish immunized by chitosan-alginate coated vaccine, B) fish immunized by non-coated vaccine, C) fish feed by chitosan-alginate coated pellets without vaccine and D) fish feed by basic diet (non-coated and without vaccine). In groups A and B, the vaccination was carried out for 14 days. Fish of group C, like groups A and B were fed 14 days with pellets covered with chitosan-alginate without vaccine and a short time later they were fed with control diet. On day 0, 20, 40 and 60 of the trial, serum samples were extracted. Fish were challenged with L. garvieae and S. iniae after 60 days of research. Innate immunity components containing complement activity, total protein and IgM appeared no significant changes nearly in all groups during the 60 days that the examination finished. Although, bactericidal activity and lysozyme activity demonstrated a significant increase on days 20, 40 and 60 in group A compared to control groups (C and D) (P < 0.05) and similar results about the blood respiratory burst activity just on days 20 and 40 were obtained. Also, the relative expression of IL-6 of group A, was significantly higher compared to all of other groups (B, C and D) on days 20 and 60 of experiment (P < 0.05). The same results were obtained about the relative expression of IgM. The serum ELISA antibody titer against L. garvieae, increased significantly on days 20 and 40 of experiment in fish immunized by chitosan-alginate coated vaccine (Group A) compared to control groups (C and D)(P < 0.05) while the result of ELISA test against S. iniae was significantly higher on days 40 and 60 of experiment in group A compared to groups B, C and D (P < 0.05). After challenge with these two live bacteria (S. iniae and L. garvieae), a survival rates of 76.67 ± 5.77% (challenged with S. iniae) and 66.67 ± 5.77% (challenged with L. garvieae) were seen in group immunized with chitosan-alginate coated vaccine (Group A), which were higher than survival rates gotten in other trial groups (P < 0.05). The consequences of the present experiment show that the oral vaccination of rainbow trout with improved chitosan-alginate (via ionotropic procedure) (group A) properly secures this important fish against Lactococcus garvieae and Streptococcus iniae.

Immunomodulatory effects of orally administrated florfenicol in rainbow trout (Oncorhynchus mykiss) following experimental challenge with streptococcosis/lactococcosis

The purpose of the present work was to determine whether florfenicol (FFC) as the prominent broad-spectrum antibiotic could affect serum biochemical and immunological parameters, as well as immune-related genes expression in rainbow trout (55 ± 7.6 g) challenged with the Lactococcus garvieae and Streptococcus iniae. In the first trial, the doses of the pathogens for challenge test were determined based on LD50. The therapeutic dosage of the drug (15 mg.kg^-1 BW for 10 consecutive days) was administrated as medicated feed. After anesthesia, blood and kidney samples were collected from individual fish and were kept in deep freezing mode until the beginning of the measurements. Serum biochemical and enzymatic indices were measured using commercial kits. Immune parameters including total immunoglobulin level, lysozyme, ACH50, respiratory burst (RB), and phagocytic activities (PA) and the expression of immune genes namely TNF-α, IL-1β, IL-8, and IgM was evaluated. The levels of lysozyme and RB activities, as well as the expression of TNF-α and IL-1β genes, showed a significant increase in the FFC treated/infected fish compared to untreated diseased fish (P < 0.05). In contrast, serum total immunoglobulin and IgM-related genes expression were suppressed following drug administration represented by a significant reduction in untreated streptococcal infected fish compared to other treatments (P < 0.05). However, no significant effect of FFC was observed on serum ACH50 activity, PA values and IL-8-related gene expression (P > 0.05). These results demonstrated that FFC treatment could improve some physiological status including stress resistance and some liver function parameters, and much innate immunity was invigorated, but at the same time, the suppressive effects of FFC on acquainted immunity cannot be ignored.

Contemporary Threats of Bacterial Infections in Freshwater Fish

Changes occurring in freshwater ecosystems seem to be fundamental in the development of all microorganisms, including those pathogenic to fish. This has been especially evident in recent years during which dynamic variations in bacterial fish pathology have been observed. Gram-negative bacteria commonly known to be pathogenic to fish, like Aeromonas spp., Flavobacterium spp., Pseudomonas spp., and Shewanella putrefaciens are replaced by other species, which until now have not been known to be virulent or even conditionally pathogenic to fish. Nowadays, among these other species Acinetobacter spp., Plesiomonas shigelloides, Sphingomonas paucimobilis, and Stenotrophomonas maltophilia are the most frequently isolated from fish exhibiting clinical signs of disease. Two Gram-positive bacteria have become pathogens of particular importance in fish pathology in Poland: Lactococcus garviae and Streptococcus iniae. In addition, infections caused by the Gram-positive bacterium Kocuria rhizophila have appeared in recent years. This bacterium has not been known until now to be pathogenic to fish. Therefore, this infection could be called an emergent disease.

Dialogue between Staphylococcus aureus SA15 and Lactococcus garvieae strains experiencing oxidative stress

Background

Staphylococcus aureus is an important foodborne pathogen. Lactococcus garvieae is a lactic acid bacterium found in dairy products; some of its strains are able to inhibit S. aureus growth by producing H₂O₂. Three strains of L. garvieae from different origins were tested for their ability to inhibit S. aureus SA15 growth. Two conditions were tested, one in which H₂O₂ was produced (high aeration) and another one in which it was not detected (low aeration). Several S. aureus genes related to stress, H₂O₂-response and virulence were examined in order to compare their level of expression depending on the inoculated L. garvieae strain. Simultaneous L. garvieae H₂O₂ metabolism gene expression was followed.

Results

The results showed that under high aeration condition, L. garvieae strains producing H₂O₂ (N201 and CL-1183) inhibited S. aureus SA15 growth and impaired its ability to deal with hydrogen peroxide by repressing H₂O₂-degrading genes. L. garvieae strains induced overexpression of S. aureus stress-response genes while cell division genes and virulence genes were repressed. A catalase treatment partially or completely restored the SA15 growth. In addition, the H₂O₂ non-producing L. garvieae strain (Lg2) did not cause any growth inhibition. The SA15 stress-response genes were down-regulated and cell division genes expression was not affected. Under low aeration condition, while none of the strains tested exhibited H₂O₂-production, the 3 L. garvieae strains inhibited S. aureus SA15 growth, but to a lesser extent than under high aeration condition.

Conclusion

Taken together, these results suggest a L. garvieae strain-specific anti-staphylococcal mechanism and an H₂O₂ involvement in at least two of the tested L. garvieae strains.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1340-3) contains supplementary material, which is available to authorized users.

Comparative genomics inferred two distinct populations of piscine pathogenic Streptococcus agalactiae, serotype Ia ST7 and serotype III ST283, in Thailand and Vietnam

The genomes of Streptococcus agalactiae (group B streptococcus; GBS) collected from diseased fish in Thailand and Vietnam over a nine-year period (2008-2016) were sequenced and compared (n = 21). Based on capsular serotype and multilocus sequence typing (MLST), GBS isolates are divided into 2 groups comprised of i) serotype Ia; sequence type (ST)7 and ii) serotype III; ST283. Population structure inferred by core genome (cg)MLST and Bayesian clustering analysis also strongly indicated distribution of two GBS populations in both Thailand and Vietnam. Deep phylogenetic analysis implied by CRISPR array's spacer diversity was able to cluster GBS isolates according to their temporal and geographic origins, though ST7 has varying CRISPR1-spacer profiles when compared to ST283 strains. Based on overall genotypic features, Thai ST283 strains were closely related to the Singaporean ST283 strain causing foodborne illness in humans in 2015, thus, signifying zoonotic potential of this GBS population in the country.

Efficacy of a Eudragit L30D-55 encapsulated oral vaccine containing inactivated bacteria (Lactococcus garvieae/Streptococcus iniae) in rainbow trout (Oncorhynchus mykiss)

The efficacy of a Eudragit L30D-55 encapsulated vaccine against Lactococcus garvieae and Streptococcus iniae was investigated in rainbow trout. Fish were divided into four groups and fed the different experimental feeds. Groups were: A) fish immunized by Eudragit-coated pellets containing vaccine, B) fish immunized by vaccine-coated pellets without Eudragit, C) fish fed Eudragit-coated pellets without vaccine and D) fish fed pellets without vaccine orEudragit (control group). In groups A and B, the vaccination was conducted for 14 days. Similar to groups A and B, fish of group C were fed 14 days with pellets coated with Eudragit and afterwards they were fed control diet. Serum samples were taken on day 0, 20, 40 and 60 of the experiment. After 60 days, fish were challenged with L. garvieae and S. iniae. In almost all groups, innate immunity components including alternative complement activity, lysozyme activity, bactericidal activity, IgM and total protein showed no significant changes during the 60 days that the experiment lasted. However, the blood respiratory burst activity and lysozyme activity showed a significant increase on day 20 of experiment in groups B and D respectively (P < 0.05). The relative expression of immune-related genes including IL-6 and IgM genes was higher in vaccinated fish, with the highest expression in those immunized by Eudragit-coated pellets (Group A). In addition, the relative expression of IL-6 and IgM peaked on day 20 but decreased on day 60 in vaccinated groups. The ELISA antibody titer against L. garvieae increased from day 20 and peaked on day 60 of experiment (P < 0.05). Also, the antibody titer against L. garvieae was higher in fish immunized by Eudragit-coated pellets (Group A) compared to fish of group C and control. After bacterial challenge, a survival percentages of % 85 ± 7.07% (challenged with S. iniae) and % 72.21 ± 7.8% (challenged with L. garvieae) were observed respectively in groups immunized with pellets coated with Eudragit L30D-55 (Group A), which were higher than survival percentages obtained in other experimental groups (P < 0.05). The results of the present study demonstrate that the oral administration of Eudragit L30D-55-encapsulated vaccine appropriately protects rainbow trout against Lactococcus garvieae and Streptococcus iniae.

Molecular characterization and virulence gene profiling of pathogenic Streptococcus agalactiae populations from tilapia ( Oreochromis sp.) farms in Thailand

Streptococcus spp. were recovered from diseased tilapia in Thailand during 2009-2010 ( n = 33), and were also continually collected from environmental samples (sediment and water) from tilapia farms for 9 months in 2011 ( n = 25). The relative percent recovery of streptococci from environmental samples was 13-67%. All streptococcal isolates were identified as S. agalactiae (group B streptococci [GBS]) by a species-specific polymerase chain reaction. In molecular characterization assays, 4 genotypic categories comprised of 1) molecular serotypes, 2) the infB allele, 3) virulence gene profiling patterns ( cylE, hylB, scpB, lmb, cspA, dltA, fbsA, fbsB, bibA, gap, and pili backbone-encoded genes), and 4) randomly amplified polymorphic DNA (RAPD) fingerprinting patterns, were used to describe the genotypic diversity of the GBS isolates. There was only 1 isolate identified as molecular serotype III, while the others were serotype Ia. Most GBS serotype Ia isolates had an identical infB allele and virulence gene profiling patterns, but a large diversity was established by RAPD analysis with diversity tending to be geographically dependent. Experimental infection of Nile tilapia ( Oreochromis niloticus) revealed that the GBS serotype III isolate was nonpathogenic in the fish, while all 5 serotype Ia isolates (3 fish and 2 environmental isolates) were pathogenic, with a median lethal dose of 6.25-7.56 log₁₀ colony-forming units. In conclusion, GBS isolates from tilapia farms in Thailand showed a large genetic diversity, which was associated with the geographical origins of the bacteria.

How does temperature influences the development of lactococcosis? Transcriptomic and immunoproteomic in vitro approaches

Lactococcus garvieae is the aetiological agent of lactococcosis, a haemorrhagic septicaemia that affects marine and freshwater fish, with special incidence and economic relevance in farmed rainbow trout. Water temperature is one of the most important predisposing factors in the development of lactococcosis outbreaks. Lactococcosis in trout usually occur when water temperatures rise to about 18 °C, while fish carriers remain asymptomatic at temperatures below 13 °C. The aim of this work was to analyse the differences in the complete transcriptome response of L. garvieae grown at 18 °C and at 13 °C and to identify the immunogenic proteins expressed by this bacterium at 18 °C. Our results show that water temperature influences the expression of L. garvieae genes involved in the lysis of part of the bacterial cell population and in the cold response bacterial adaptation. Moreover, the surface immunogenic protein profile at 18 °C suggests an important role of the lysozyme-like enzyme, WxL surface proteins and some putative moonlighting proteins (proteins with more than one function, usually associated with different cellular locations) as virulence factors in L. garvieae. The results of this study could provide insights into the understanding of the virulence mechanisms of L. garvieae in fish.

New Insights into the Anti-pathogenic Potential of Lactococcus garvieae against Staphylococcus aureus Based on RNA Sequencing Profiling

The bio-preservation potential of Lactococcus garvieae lies in its capacity to inhibit the growth of staphylococci, especially Staphylococcus aureus, in dairy products and in vitro. In vitro, inhibition is modulated by the level of aeration, owing to hydrogen peroxide (H₂O₂) production by L. garvieae under aeration. The S. aureus response to this inhibition has already been studied. However, the molecular mechanisms of L. garvieae underlying the antagonism against S. aureus have never been explored. This study provides evidence of the presence of another extracellular inhibition effector in vitro. This effector was neither a protein, nor a lipid, nor a polysaccharide, nor related to an L-threonine deficiency. To better understand the H₂O₂-related inhibition mechanism at the transcriptome level and to identify other mechanisms potentially involved, we used RNA sequencing to determine the transcriptome response of L. garvieae to different aeration levels and to the presence or absence of S. aureus. The L. garvieae transcriptome differed radically between different aeration levels mainly in biological processes related to fundamental functions and nutritional adaptation. The transcriptomic response of L. garvieae to aeration level differed according to the presence or absence of S. aureus. The higher concentration of H₂O₂ with high aeration was not associated with a higher expression of L. garvieae H₂O₂-synthesis genes (pox, sodA, and spxA1) but rather with a repression of L. garvieae H₂O₂-degradation genes (trxB1, ahpC, ahpF, and gpx). We showed that L. garvieae displayed an original, previously undiscovered, H₂O₂ production regulation mechanism among bacteria. In addition to the key factor H₂O₂, the involvement of another extracellular effector in the antagonism against S. aureus was shown. Future studies should explore the relation between H₂O₂-metabolism, H₂O₂-producing LAB and the pathogen they inhibit. The nature of the other extracellular effector should also be determined.

Lactococcus garvieae: an emerging bacterial pathogen of fish

Lactococcus garvieae is the causative agent of lactococcosis, a hyperacute, haemorrhagic septicaemia of fish. This bacterium is also considered an emerging zoonotic pathogen, as reports of human infection are increasing. Significant economic loss in aquaculture is suffered as a result of lactococcosis, as numerous freshwater and marine species of commercial interest are affected. Development of antibiotic resistance in L. garvieae to several chemotherapeutic agents complicates and restricts treatment options. Effective, sustainable treatment and prevention options are thus needed, but progress is impeded by the lack of knowledge concerning several aspects of the disease and the pathogen. This review aims to present the latest research on L. garvieae, with specific focus on pathogenesis, virulence factors, risks associated with chemotherapeutic administration and possible control options.

Effects of host genetics and environment on egg-associated microbiotas in brown trout (Salmo trutta)

Recent studies found fish egg-specific bacterial communities that changed over the course of embryogenesis, suggesting an interaction between the developing host and its microbiota. Indeed, single-strain infections demonstrated that the virulence of opportunistic bacteria is influenced by environmental factors and host immune genes. However, the interplay between a fish embryo host and its microbiota has not been studied yet at the community level. To test whether host genetics affects the assemblage of egg-associated bacteria, adult brown trout (Salmo trutta) were sampled from a natural population. Their gametes were used for full-factorial in vitro fertilizations to separate sire from dam effects. In total, 2520 embryos were singly raised under experimental conditions that differently support microbial growth. High-throughput 16S rRNA amplicon sequencing was applied to characterize bacterial communities on milt and fertilized eggs across treatments. Dam and sire identity influenced embryo mortality, time until hatching and composition of egg-associated microbiotas, but no link between bacterial communities on milt and on fertilized eggs could be found. Elevated resources increased embryo mortality and modified bacterial communities with a shift in their putative functional potential. Resource availability did not significantly affect any parental effects on embryo performance. Sire identity affected bacterial diversity that turned out to be a significant predictor of hatching time: embryos associated with high bacterial diversity hatched later. We conclude that both host genetics and the availability of resources define diversity and composition of egg-associated bacterial communities that then affect the life history of their hosts.

cpsJ gene of Streptococcus iniae is involved in capsular polysaccharide synthesis and virulence

The capsular polysaccharides are an important virulence factor of Streptococcus iniae, protecting the bacterium from destruction and clearance by the immune system. The cpsJ gene encodes a putative UDP-glucose epimerase involved in the capsule synthesis system. To determine the role of the CpsJ protein in the production of the capsule, a ΔcpsJ mutant was generated and analyzed by comparing its growth performances and virulence with those of the wild type (WT) strain. The ΔcpsJ mutant had longer chains, smaller colonies, and a slower growth rate and decreased optical density than the WT, suggesting that the ΔcpsJ mutant produces less capsular polysaccharide. The ΔcpsJ mutant was more able to adhere to and invaded epithelioma papulosum cyprinid cells (EPCs) when its virulence in vitro was compared with that of the WT, but survived less well in the whole blood of channel catfish. When a channel catfish infection model was used to determine the virulence of the ΔcpsJ mutant in vivo, the mutant caused an increase in survival with the mutant (53.33 %) versus the WT (26.67 %). In summary, mutation of the cpsJ gene influenced both the capsule synthesis and virulence of S. iniae.

Draft Genome Sequence of Lactococcus garvieae Strain PAQ102015-99, an Outbreak Strain Isolated from a Commercial Trout Farm in the Northwestern United States

We announce the draft genome assembly of Lactococcus garvieae strain PAQ102015-99, a recently isolated strain from an outbreak of lactococcosis at a commercial trout farm in the northwestern United States. The draft genome comprises 14 contigs totaling 2,068,357 bp with an N₅₀ of 496,618 bp and average G+C content of 38%.

Induction and characterization of a lysogenic bacteriophage of Lactococcus garvieae isolated from marine fish species

This study investigated the presence of prophages in Lactococcus garvieae isolated from several marine fish species in Japan. Representative strains of 16 bacterial genotypes (S1-S16) selected from more than 400 L. garvieae isolates were used to induce lysogenic bacteriophages. These strains were treated with 500 ng mL(-1) freshly prepared mitomycin C. A cross-spotting assay was performed to validate the lysogenic and indicator strains. The lysogenic strains were selected for isolation and concentration of the phages. Phage DNA was digested with EcoRI for biased sinusoidal field gel electrophoresis analysis. Polymerase chain reaction (PCR) was used to detect integrated prophage DNA. Of the 16 representative bacterial genotypes, 12 strains integrated prophages as indicated by the PCR assay, and 10 phages were detected and isolated using two indicator bacterial strains. Analysis of genomic DNA showed that these phages were homologous and named as PLgT-1. Transmission electron microscopy revealed that the morphology of PLgT-1 was consistent with the virus family Siphoviridae. PCR analysis of the prophage DNA revealed that all of the S1 genotype strains were lysogenic (30/30), but none of the S16 genotype strains were lysogenic (0/30). This is the first study to investigate lysogenic bacteriophages from L. garvieae.

Immunohistochemical evaluation of experimental Vagococcus salmoninarum infection in rainbow trout (Oncorhynchus mykiss, Walbaum 1792)

The aim of this study was to investigate the pathogenesis and histopathological and immunohistochemical findings in rainbow trout (Oncorhynchus mykiss) following experimental vagococcosis. For this purpose, 60 rainbow trout were used. The experimental study used the pathogen Vagococcus salmoninarum. The fish were intraperitoneally (IP) administered with an inoculate containing 0.1 mL of the bacteria, resulting in a dose of 1.2 × 10(9) cfu mL(-1) per fish. For histopathological observations, tissue samples were taken from fish that died during the experiment and fish that survived until the end of the trial (60th day). All the tissue samples were immunohistochemically stained by the avidin-biotin-peroxidase complex and immunofluorescence methods using polyclonal antibody to detect V. salmoninarum antigens. In immunoperoxidase staining, positive reactions to bacterial antigens were most commonly seen in the kidney, heart and liver. In the immunofluorescence analysis, the distribution of antigens in the tissue and organs was similar to that observed with the immunoperoxidase staining. The results reveal an important correlation between histochemical and immunohistochemical staining in demonstrating the distribution of V. salmoninarum antigens in the affected tissues.

Experimental early pathogenesis of Streptococcus agalactiae infection in red tilapia Oreochromis spp

Streptococcus agalactiae causes a severe systemic disease in fish, and the routes of entry are still ill-defined. To address this issue, two groups of 33 red tilapia Oreochromis spp. each of 10 g were orally infected with S. agalactiae (n = 30), and by immersion (n = 30), six individuals were control-uninfected fish. Three tilapias were killed at each time point from 30 min to 96 h post-inoculation (pi); controls were killed at 96 h. Samples from most tissues were examined by haematoxylin-eosin (H&E), indirect immunoperoxidase (IPI) and periodic acid-Schiff; only intestine from fish infected by gavage was evaluated by transmission electron microscopy. The results of both experiments suggest that the main entry site of S. agalactiae in tilapia is the gastrointestinal epithelium; mucus seems to play an important defensive role, and environmental conditions may be an important predisposing factor for the infection.

Effects of dietary Bacillus licheniformis on growth performance, immunological parameters, intestinal morphology and resistance of juvenile Nile tilapia (Oreochromis niloticus) to challenge infections

The effects of oral administration of Bacillus licheniformis on growth performance, immunity, intestinal morphology and disease resistance of juvenile tilapia were investigated. Six experimental diets supplemented with different concentrations of B. licheniformis (0%, 0.02%, 0.04%, 0.06%, 0.08% and 0.1% of AlCare(®), containing live germ 2 × 10(10) CFU/g) were formulated, viz. control, T1, T2, T3, T4 and T5. Each diet was randomly assigned to triplicate groups of 30 fishes (3.83 ± 0.03 g). After 10 weeks of feeding trial, weight gain (WG), final body wet weight (FBW) and specific growth rate (SGR) increased significantly in groups T2, T3, T4 and T5 compared with control and T1 (p < 0.05). However, survival rate and feed conversion ratio (FCR) were not found to be significantly affected (P > 0.05). Compared with control, dietary B. licheniformis supplementation increased the content of complement C3 in serum significantly (P < 0.05). The lysozyme activity was observed to be highest in T2 (P < 0.05) without differences among other groups. However, SOD activity was not affected by B. licheniformis supplementation (P > 0.05). When tilapia were challenged against Streptococcus iniae, survival rate improved significantly when tilapia fed with T2, T3, T4 and T5 (P < 0.05). Although there was no significant differences in villi length and muscular layer thickness of anterior intestinal among the treatments, intestinal villi of fish fed with higher concentrations of B. licheniformis (T2, T3, T4, T5) tended to be regularly arranged and exhibited less exfoliation, twist and fusion. These results indicated that dietary supplementation of B. licheniformis not only increased the growth, immune response and disease resistance of juvenile tilapia, but also influenced anterior intestinal development and integrity. Furthermore, in our study, the optimal concentration of B. licheniformis in diets for tilapia was greater than or equal to 4.4 × 10(6) CFU/g.

Complete Genome Sequence of Streptococcus iniae YSFST01-82, Isolated from Olive Flounder in Jeju, South Korea

Streptococcus iniae is associated with morbidity in commercial fish species, especially in olive flounders (Paralichthys olivaceus), and was recently identified as an emerging human pathogen. Here, we report the complete 2.09-Mb genome sequence of S. iniae strain YSFST01-82, isolated from an olive flounder with streptococcosis disease in Jeju, South Korea.