Burkholderia glumae

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1. Classification

a. Higher order taxa

Bacteria; Pseudomonadota; Betaproteobacteria; Burkholderiales; Burkholderiaceae; Burkholderia

Species

NCBI: [1]

Burkholderia glumae

2. Description and significance

Burkholderia glumae, formerly known as Pseudomonas glumae, is a bacterial species recognized primarily for causing bacterial panicle blight (BPB) of rice (Oryza sativa) in tropical and subtropical rice-growing countries. BPB can reduce rice yields by as much as 75% [2] and can also cause wilting in tomato, eggplant, pepper, and sesame plants [3]. Recognized as an emerging threat to global rice production [4], B. glumae grows optimally at elevated temperature and humidity, suggesting that its prevalence will increase as global temperatures rise [5]. While B. glumae is not considered a human pathogen, one case of infection was reported in an immunodeficient infant in 2007, suggesting the potential for opportunistic pathogenicity in humans [10]. B. glumae also secretes the lipase LipA, which is used in the pharmaceutical industry to produce enantiopure compounds [7]. Despite advances in understanding B. glumae’s genome and virulence factors, gaps remain in understanding regulation of its pathogenicity, as well as mitigation strategies in the context of evolving climate conditions.

3. Genome structure

There are complete genomes of several B. glumae strains available. These include BGR1 [6], PG1 [7], BD_21G [8], and Chinese strains HN1/HN2 [9]. The genome size is roughly 6.7 to 7.0 Mbp (million base pairs) across strains, with a relatively high G+C % (approximately 68%), which is typical for the genus Burkholderia. B. glumae’s genomic structure is usually multipartite, with the genome of strain BGR1 containing two chromosomes and four plasmids, though these numbers often vary between strains [6], [25]. Protein-coding genes range from 6,000-7,000 per genome. Non-coding regions, including regulatory RNAs and intergenic spacers, are less well-characterized [6], [25].

Some genes of interest include quorum sensing (QS) regulons. All strains of B. glumae possess at least one QS system mediated by acyl-homoserine lactone (AHL). In particular, strain BGR1 uses a LuxI/LuxR homolog (designated TofI/TofR) with an AHL synthase encoded by the tofI gene, while strain BGPG1 is the only known strain of B. glumae to possess three distinct AHL synthase genes [10].

The toxoflavin biosynthetic gene cluster also houses genes that are responsible for producing the phytotoxin toxoflavin. This compound plays a crucial role in BPB, as it is one of the main virulence factors in B. glumae [12]. The toxABCDE operon (consisting of genes toxA to E) codes for toxoflavin biosynthesis, while the toxFGHI operon (genes toxF to I) codes for a system of resistance-nodulation-division (RND)-like efflux transporters [20]. Both operons are regulated by the LysR-type regulatory protein ToxR, which uses toxoflavin as a coinducer, and by the transcriptional activator ToxJ, which is regulated by TofI/TofR quorum sensing [20].

4. Cell structure

“Burkholderia glumae” is a Gram-negative, rod-shaped, motile bacterium with two to four polar flagella [4]. Cells typically range from 0.5 to 0.7 µm in width and 1.5 to 2.5µm in length. “B. glumae” does not form spores [16] but produces surface structures such as pili and flagella, regulated in part by quorum sensing [10]. Scanning electron microscopy studies demonstrate cytoplasmic leakage and cell lysis due to increased permeability of the cell membrane upon exposure to ginger essential oil, despite the presence of lipopolysaccharides in the outer membrane, which provide increased protection against hydrophobic molecules such as essential oils [17].