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

Safe and efficacious inactivated immersion vaccine against KG+ phenotype of emerging bacterial pathogen Lactococcus garvieae for early developmental stage of rainbow trout (Oncorhynchus mykiss) in India

Rainbow trout (Oncorhynchus mykiss) farming in India faces a significant threat from the bacterium Lactococcus garvieae strain RTCLI10 (KG+ phenotype). Thus, this study aims to develop an inactivated immersion vaccine (Lg IIV) against this pathogen. The biosafety of LgIIV was assessed in vitro using epithelioma papulosum cyprini (EPC) cell line, and in vivo in rainbow trout (body weight: 0.5 ± 0.01 g) challenge model by immersion (2.4 × 109 cells mL-1). For efficacy assessment of Lg IIV, rainbow trout (body weight: 1.26 ± 0.43 g) received primary (0 days) and booster vaccination (7 days post booster vaccination) (1.2 × 109cells mL-1), followed by challenge with homologous strain, L. garvieae RTCLI10 (KG+ phenotype; LD50: 2.6 × 105 CFU mL-1). EPC cell survivability was above 90 % and cytotoxicity were not observed in these cells, when exposed to LgIIV for 120 h at different dilutions. Lg IIV did not show any adverse effects on survival, behavior or health of exposed rainbow trout post booster vaccination, and no histological changes were observed in gill, liver, kidney, spleen, eye and muscle. The relative percentage survival (RPS) of rainbow trout was 83.78 %, with 96.15 % survival in vaccinated and challenge (VC) group, compared to 76.28 % in non-vaccinated and challenge (NVC) group. Bactericidal activity, lysozyme activity and serum antibody titre were significantly elevated in VC group in between 3 and 5 days post challenge (dpc). Quantitative PCR (qPCR) analysis of 12 immune-related genes in the spleen and anterior kidney showed significantly elevated inflammatory gene expression in VC rainbow trout, in comparison to NVC. Therefore, our study confirms the biosafety and protective efficacy of Lg IIV against L. garvieae infection for sustainable aquaculture in India.

Diving Into the Depths: Unveiling the Main Etiologies of Piscine Lactococcosis With a Novel Multiplex qPCR Assay

Piscine lactococcosis poses a significant threat to a wide range of cultured and wild fish populations worldwide, typically presenting as acute haemorrhagic septicemia with high morbidity and mortality. Although Lactococcus garvieae was historically considered the sole causative agent of piscine lactococcosis, recent studies have identified L. petauri and L. formosensis as additional, highly pathogenic species. In this study, we developed a novel TaqMan-based multiplex qPCR assay for the simultaneous detection and differentiation of L. garvieae, L. petauri and L. formosensis, following a pangenome analysis of the publicly available genomes of these bacterial species. The assay demonstrated high sensitivity and specificity across 156 bacterial isolates obtained from various cultured fish species and geographical locations between 2008 and 2024, as well as against a panel of non-target bacteria. It also successfully detected target pathogens in 146 field tissue samples, including tissues preserved in 70% ethanol, formalin-fixed paraffin-embedded (FFPE) tissues and tissues fixed on FTA cards. Compared to classical bacteriology, the multiplex qPCR assay yielded higher detection rates and enabled precise identification of the causative species of piscine lactococcosis. Overall, the multiplex qPCR assay developed in this study provides a reliable, rapid, highly sensitive and species-specific molecular approach for diagnosing piscine lactococcosis, contributing to better surveillance and management of the disease in aquaculture.

Antimicrobial Resistance in Lactococcus spp. Isolated from Native Brazilian Fish Species: A Growing Challenge for Aquaculture

Lactococcus spp. has emerged as a pathogen that is affecting global aquaculture, with L. garvieae, L. petauri, and L. formosensis causing piscine lactococcosis. While antimicrobials are commonly used to treat diseases in aquaculture, reports of antimicrobial resistance in fish isolates are increasing. However, little is known about the susceptibility patterns of Lactococcus spp. strains isolated from native fish species in Brazil. This study aimed to assess the antimicrobial susceptibility of these strains and establish a provisional epidemiological cutoff value for L. garvieae using the normalized resistance interpretation approach. A total of 47 isolates were tested: 17 L. garvieae, 24 L. petauri, and 6 L. formosensis. The isolates were classified as wild-type (WT) or non-wild-type (NWT) based on inhibition zone diameters. Isolates classified as NWT for three or more antimicrobial classes were considered multidrug-resistant, and the multiple antibiotic resistance (MAR) index was calculated. The results revealed heterogeneity in antimicrobial resistance profiles, with higher resistance to trimethoprim/sulfamethoxazole and norfloxacin. Resistance to other antimicrobials, including florfenicol and oxytetracycline (approved for use in Brazil), varied according to the bacterial species. Lactococcus petauri (87.5%) and L. formosensis (66.7%) showed the highest multidrug resistance, compared to L. garvieae (11.7%), along with higher MAR index values. These findings suggest that multidrug-resistant strains could pose future challenges in the production of native species, underscoring the need for ongoing monitoring of antimicrobial resistance and responsible use of antimicrobials in aquaculture.

First Detection of Lactococcus petauri in Domestic Dogs in Italy

Lactococcus garvieae has been considered for a long time the only causal agent of lactococcosis. In recent years, different papers reported the involvement of other two bacterial species: Lactococcus petauri and Lactococcus formosensis. A different host tropism has been described for these species where L. garvieae and L. petauri are predominant species in fish and humans' infections, while L. formosensis in bovine. L. garvieae has been reported as rare infectious agent in dog. This paper represents the first isolation of L. petauri in two domesticated dog cases from urine and skin samples, respectively. The recovered L. petauri has been identified using PCR and sequencing based on Internal Transcribe Spacer (ITS) and phylogenetic analysis showed that it belongs to the L. petauri cluster with a 100% of identity with sequences previously reported from fish isolates while there were differences with L. petauri isolated from urinary tract infection from humans. L. petauri in human infection has been considered not necessarily deriving from the ingestion of contaminated food but rather as an opportunistic pathogen colonization intestinal tract. Differences among virulotypes have been reported for humans and dogs, and a comparison was also made between the virulotyping of L. petauri and L. garvieae in dogs. The antimicrobial pattern showed susceptibility for the election treatment molecules. These data contribute to our understanding of the host trophism of this species which was misclassified for long time and provide new data on its virulence factors and antimicrobial resistance.

Multiplex PCR assay for the accurate and rapid detection and differentiation of Lactococcus garvieae and L. petauri

Lactococcosis is a common bacterial fish disease caused by Lactococcus garvieae, L. petauri and L. formosensis. Although there are different PCR-based techniques to identify the etiological agent, none of these can differentiate these two bacteria without sequencing PCR-amplified fragments. In the present study, we developed a multiplex PCR assay for simultaneous detection and differentiation of L. garvieae and L. petauri. The specificity of the primers was validated against the bacterial DNA of the targeted and non-targeted bacteria. The sizes of the PCR amplicons were obtained as 204 bp for the DUF1430 domain-containing protein gene of L. garvieae, 465 bp for the Lichenan permease IIC component gene of L. petauri, and 302 bp for the teichoic acid biosynthesis protein F gene of both L. garvieae and L. petauri. The PCR amplicons were clearly separated by agarose gel electrophoresis. The multiplex PCR assay did not produce any amplification products with the DNA of the non-targeted bacteria. The multiplex PCR detection limits for L. garvieae and L. petauri were 5 and 4 CFU in pure culture and 50 and 40 CFU/g in spiked tissue samples, respectively. It takes less than 2 h from plate-cultured bacteria and 3 h from tissue samples to get results. In conclusion, the developed multiplex PCR assay is a rapid, specific, accurate, and cost-effective method for the detection and differentiation of L. garvieae and L. petauri and is suitable to be used for routine laboratory diagnosis of L. garvieae and L. petauri.

Changes in blood serum parameters in farmed rainbow trout (Oncorhynchus mykiss) during a piscine lactococcosis outbreak

The aquaculture sector plays a vital role in global food security, yet it grapples with significant challenges posed by infectious diseases. Piscine lactococcosis is one of the significant threats in rainbow trout aquaculture due to its potential to cause severe economic losses through mortalities, reduced growth rates, and increased susceptibility to other pathogens. It poses challenges in disease management strategies, impacting the sustainability and profitability of rainbow trout farming. The current study focuses on the variations in serum blood parameters of farmed rainbow trout Oncorhynchus mykiss during a lactococcosis outbreak caused by Lactococcus garvieae. Blood samples were collected for biochemical analysis, fish were examined for parasites and bacteria, and DNA from bacterial colonies was PCR-amplified and sequenced for identification. Overall, 13 biochemical parameters, including proteins, enzymes, lipids, chemicals, and minerals, were measured in serum blood samples from both diseased and healthy fish. The results indicate significant alterations in the levels of these parameters during the outbreak, highlighting the impact of infections on the blood profile of farmed rainbow trout. Urea levels were significantly higher in diseased fish compared to controls, and creatinine, phosphorus, and magnesium also showed similar trends. Alanine aminotransferase and total protein levels were higher in control fish. Chloride levels differed significantly between groups. Iron levels were higher in controls and lower in diseased fish. No significant differences were found in other parameters. This study reveals significant changes in serum blood parameters of rainbow trout during a lactococcosis outbreak caused by L. garvieae. These changes highlight the potential of these parameters as tools for monitoring health status, stress, and aquaculture management. Continuous monitoring can provide valuable insights into disease severity and overall fish health, aiding in the development of improved management practices. The presented data contribute to understanding the pathophysiology of piscine lactococcosis and developing effective mitigation strategies for farmed rainbow trout.

Genomic investigation of Lactococcus formosensis, Lactococcus garvieae, and Lactococcus petauri reveals differences in species distribution by human and animal sources

Lactococcus garvieae is a fish pathogen that can cause diseases in humans and cows. Two genetically related species, Lactococcus formosensis and Lactococcus petauri, may be misidentified as L. garvieae. It is unclear if these species differ in host specificity and virulence genes. This study analyzed the genomes of 120 L. petauri, 53 L. formosensis, and 39 L. garvieae isolates from various sources. The genetic diversity and virulence gene content of these isolates were compared. The results showed that 77 isolates previously reported as L. garvieae were actually L. formosensis or L. petauri. The distribution of the three species varied across different collection sources, with L. petauri being predominant in human infections, human fecal sources, and rainbow trout, while L. formosensis was more common in bovine isolates. The genetic diversity of isolates within each species was high and similar. Using a genomic clustering method, L. petauri, L. formosensis, and L. garvieae were divided into 45, 22, and 13 clusters, respectively. Most rainbow trout and human isolates of L. petauri belonged to different clusters, while L. formosensis isolates from bovine and human sources were also segregated into separate clusters. In L. garvieae, most human isolates were grouped into three clusters that also included isolates from food or other sources. Non-metric multidimensional scaling ordination revealed the differential association of 15 virulence genes, including 14 adherence genes and a bile salt hydrolase gene, with bacterial species and certain collection sources. In conclusion, this work provides evidence of host specificity among the three species.

IMPORTANCE

Lactococcus formosensis and Lactococcus petauri are two newly discovered bacteria, which are closely related to Lactococcus garvieae, a pathogen that affects farmed rainbow trout, as well as causes cow mastitis and human infections. It is unclear whether the three bacteria differ in their host preference and the presence of genes that contribute to the development of disease. This study shows that L. formosensis and L. petauri were commonly misidentified as L. garvieae. The three bacteria showed different distribution patterns across various sources. L. petauri was predominantly found in human infections and rainbow trout, while L. formosensis was more commonly detected in cow mastitis. Fifteen genes displayed a differential distribution among the three bacteria from certain sources, indicating a genetic basis for the observed host preference. This work indicates the importance of differentiating the three bacteria in diagnostic laboratories for surveillance and outbreak investigation purposes.

Characterization of Lactococcus garvieae and Streptococcus agalactiae in cultured red tilapia Oreochromis sp. in Thailand

OBJECTIVE

Acute mortality with clinical symptoms of streptococcal-like infections was observed in red tilapia Oreochromis sp. cultured in floating cages in Prachin Buri Province, Thailand, during May 2023. Herein, we identified an emerging pathogen, Lactococcus garvieae, as the etiological agent.

METHODS

After bacterial isolation from the brain and kidney of diseased fish, identification was performed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and the VITEK 2 system. Sequencing of the 16S ribosomal RNA (rRNA) gene and phylogenetic analysis were applied to confirm bacterial species. Antimicrobial susceptibility testing was conducted. Histopathological findings in the brain, kidney, spleen, liver, and heart were evaluated.

RESULT

From 20 fish samples, L. garvieae (n = 18 isolates) and Streptococcus agalactiae (n = 2 isolates) were identified. A phylogenetic tree of the 16S rRNA gene revealed that Thai isolates of either L. garvieae or S. agalactiae clustered with reference piscine isolates from intercontinental locations. Our isolates showed resistance against quinolones while being susceptible to other antimicrobials. Histopathological changes demonstrated severe septicemic conditions, with more invasive lesions-especially in the heart and liver-being apparent in L. garvieae-infected fish compared to S. agalactiae-infected fish.

CONCLUSION

This study represents the first reported outbreak of L. garvieae with a concurrent S. agalactiae infection in farmed red tilapia in Thailand.

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.