Redefining Piscine Lactococcosis

Authors: HECKMAN, TAYLOR - University Of California, Davis; YAZDI, ZEINAB - University Of California, Davis; Older, Caitlin; GRIFFIN, MATT - Mississippi State University; Waldbieser, Geoffrey - Geoff; CHOW, ALEXANDER - University Of California, Davis; MEDINA SILVA, ISABELLA - University Of California, Davis; ANENSON, KELSEY - University Of California, Davis; Garcia, Julio; Lafrentz, Benjamin

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2024
Publication Date: 4/10/2024
Citation: Heckman, T.I., Yazdi, Z., Older, C.E., Griffin, M.J., Waldbieser, G.C., Chow, A.M., Medina Silva, I., Anenson, K.M., Garcia, J.C., Lafrentz, B.R. 2024. Redefining Piscine Lactococcosis. Applied and Environmental Microbiology. 90:e02349-23. https://doi.org/10.1128/aem.02349-23.
DOI: https://doi.org/10.1128/aem.02349-23

Interpretive Summary: Lactococcosis is a bacterial disease that affects a broad range of host species including fish from cold, temperate, and warm environments in freshwater or marine systems, as well as humans and other terrestrial animals. It exists in European and Asian aquaculture and is rapidly encroaching on economically important fish populations across the Americas. The disease was previously thought to be caused by a single bacterial species, Lactococcus garvieae, but recent research has implicated two additional species, L. petauri and L. formosensis, that can be misidentified as L. garvieae. Scientists at the University of California Davis, in collaboration with scientists from multiple institutions including the USDA ARS Warmwater Aquaculture Research Unit in Stoneville, MS, and the USDA ARS Aquatic Animal Health Research Unit in Auburn, AL, developed appropriate methods to diagnose the disease and differentiate between the species using DNA sequencing of the entire genome or only the gyrB gene. The host and geographic range of the lactococcosis-causing pathogens overlapped, but there were relevant differences in host-specificity, regional prevalence, and antimicrobial susceptibility. Accurate identification of the specific bacteria causing lactococcosis is necessary to monitor the emergence of this disease in US fish populations and to inform effective treatment.

Technical Abstract: Piscine lactococcosis is a significant emerging threat to cultured and wild fish worldwide. A broad range of host species are susceptible, including fish from cold, temperate, and warm environments in freshwater or marine systems, as well as humans and other terrestrial animals. The disease is an established issue in European and Asian aquaculture but is rapidly encroaching on ecologically and economically important fish populations across the Americas. Lactococcosis typically presents as a hemorrhagic septicemia causing high mortality rates. Antimicrobial treatment often fails with disease progression, and development of carrier fish or biofilm formation may lead to recurrent outbreaks. Historically, the disease was attributed to the gram-positive pathogen Lactococcus garvieae. However, recent work has revealed there are three distinct lactococcosis causing bacteria (LCB) – L. garvieae, L. petauri and L. formosensis – which are phenotypically and genetically similar, leading to widespread misdiagnosis. 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. Our recently developed 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, fatty acid metabolism or protein profiles, but should be complimented by additional analysis. The LCB demonstrated overlap in host and geographic range, but there were relevant differences in host-specificity, regional prevalence, and antimicrobial susceptibility. This work provides a comprehensive characterization of the etiologic agents of piscine lactococcosis, with direct recommendations for species-specific diagnosis and disease management.