Isolation and identification of a pathogenic bacterium, Exiguobacterium oxidotolerans XP-2, from the abnormal diseased mature sporophytes of a commercially cultivated brown seaweed Saccharina japonica

Abstract

Diseases of the commercially cultivated brown seaweed Saccharina japonica have been noticed since the 1960s in China. However, the usually opportunistic nature of the pathogenic bacteria of seaweeds is still a main obstacle for isolating and identifying pathogenic bacteria responsible for causing diseases of the cultivated S. japonica. In this study, a Gram-positive bacterium XP-2 was isolated from the abnormal diseased mature sporophytes of S. japonica. Pathogenicity test indicated that XP-2 could cause green-rotten disease in healthy juvenile sporophytes and significant algal ultrastructural changes were observed after S. japonica was infected by XP-2. The cells shrank, and the thylakoids of the chloroplasts became blurred after 12 h of re-infection. At 24 h slight deformation was observed in the structure of the mitochondria, chloroplasts, and nucleus. There also were many vacuoles in the infected cells. After 31 h of re-infection, mitochondrial structure and thylakoids became more indistinct. SEM and TEM observations indicated that XP-2 was rod-shaped with no flagella. Analysis of the full length 16S rRNA gene sequence and physiological and chemical characteristics indicated that XP-2 was closely related to Exiguobacterium oxidotolerans 12280 T . Based on these results, XP-2 was designated Exiguobacterium oxidotolerans XP-2. The pathogenicity of E. oxidotolerans XP-2 was identified by Koch's Postulate under laboratory conditions. These results enrich the growing list of pathogenic bacteria in commercially cultivated seaweeds and also provide a foundation for investigating the virulence mechanisms of pathogenic bacteria of commercially cultivated seaweeds in the near future.

An opportunistic pathogenic Pseudomonas isolated from healthy brown alga Saccharina japonica

“…So far, opportunistic pathogenic bacteria have been identified in both farmed and wild seaweeds (see reviews by Egan et al, 2013, Wang et al, 2014. However, although alginic acid-decomposing bacteria are supposed to be the opportunistic pathogenic bacteria for various diseases of farmed S. japonica (Chen et al, 1979(Chen et al, , 1981Ding, 1992;Wang et al, 2008Wang et al, , 2014Ahmad et al, 2021), none of them were identified to species. On the other hand, identification of opportunistic pathogenic bacteria has been well studied in the wild red seaweed D. pulchra, such as Aquimarina sp.…”

“…The identification of pathogens is of great importance to diagnose and to prevent the outbreaks of algal diseases. Currently the cultureindependent methods such as 16S rRNA gene amplicon sequencing, the traditional culture-dependent approaches and infection assays are applied to identify pathogens (Egan et al, 2014;Kumar et al, 2016;Li et al, 2020;Zhang et al, 2020;Ahmad et al, 2021). However, culturing methods cannot adequately reflect the true microbial diversity, because only a very small proportion (about 1%) of the epiphytic bacteria might be isolated with the traditional culture-dependent method (Kumar et al, 2016;Ahmad et al, 2021).…”

“…Opportunistic pathogenic bacteria are those which are present on healthy seaweeds, but can become pathogenic when the environment deteriorates (Chen et al, 1979(Chen et al, , 1984Ding, 1990;Egan et al, 2013Egan et al, , 2014Wang et al, 2014) or when the hosts are compromised (Egan et al, 2013;Zozaya-Valdes et al, 2015;Kumar et al, 2016). Since the successful cultivation on large scale in 1958, Saccharina japonica continues to be threatened by various diseases at both nursery and field cultivation stages, such as green rot disease (Tseng, 1962(Tseng, , 1985Chen et al, 1979Chen et al, , 1981Ahmad et al, 2021), hole-rotten disease (Wang et al, 2008), and falling off disease (Ding, 1992;Wang et al, 2014). Chinese researchers have made efforts to identify the pathogenic bacteria for various diseases of farmed S. japonica (Chen et al, 1979(Chen et al, , 1984Ding, 1990;Wang, 2003, Wang et al, 2014Zhang et al, 2022) since 1979.…”

“…A-1) (Peng and Li, 2013). So far, isolation and identification of pathogenic bacteria from commercially farmed S. japonica has been improved from designating as alginic aciddecomposing bacteria to species level because of the of advances of genomic sequencing technologies (Ahmad et al, 2021;Zhang et al, 2022).…”

Alterations in Epiphytic Bacterial Communities during the Occurrence of Green Rot Disease in Saccharina japonica Seedlings

“…At present, GRD is considered to be caused by both environmental factors (i.e., insufficient sunlight) and bacteria. Although alginic acid decomposing bacteria are usually regarded as opportunistic pathogens during GRD infection owing to their ability to decompose alginate [23][24][25], specific information on GRD pathogenesis is still limited because of the detection of diverse bacterial genera with the ability to degrade alginate and induce similar symptoms to field observation [23,26]. Therefore, there is still a lack of in-depth research on the microbiome of GRD, especially analysis using in situ diseased samples.…”