Mode of Action of Streptomycin Resistance in the Citrus Canker Pathogen (Xanthomonas smithii subsp. citri) in Jeju Island

Abstract

It has been known that streptomycin resistance in bacteria can occur as a results of chromosomal mutation or through gene acquisition or both. Chromosomal mutations for resistances are point mutations in the rpsL gene, which alter ribosomal protein S12. Acquired resistance has occurred when an Sm R plasmid carrying transposon Tn5393 with tandem strA-strB gene is transferred by conjugation. A total of 686 isolates of Xanthomonas smithii subsp. citri causal agent of citrus canker disease were collected from 26 citrus orchards in Jeju Island in 2003 and 2004 seasons. Forty-nine of 111 isolates from streptomycin non-sprayed orchards in 2003 season were resistant to streptomycin. Of 107 isolates from orchards sprayed one time with streptomycin, 58 isolates were resistant, and 166 of 221 isolates from orchards sprayed two times with streptomycin were resistant. In 12 orchards sprayed three or more times with streptomycin, 219 of 247 isolates were resistant to streptomycin. Twenty-five isolates of X. smithii subsp. citri were surveyed to identify the mechanisms of streptomycin resistance in this study. Twenty-one of these 25 isolates were resistant to streptomycin, and it was proven by PCR assay that 18 of the 21 streptomycin resistant isolates have the strB gene. In sixteen of the 21 streptomycin resistant isolates, it was occurred a point mutation altered codon lysine (AAG)-41 of rpsL gene to arginine (AGG). The streptomycin-sensitive isolates easily acquired the resistance by mixed culture with resistant isolates. The strB gene was amplified from the isolates that acquired the resistance by mixed culture, and one isolate of them was also point-mutated in codon 41 of rpsL gene to be resistant. In this study, most of the streptomycin-resistant isolates of X. smithii sub sp. citri in Jeju island expressed the resistance by both chromosomal point mutation and gene acquisition, and the resistance was easily acquired through conjugation by culture mixed with streptomycin resistant and sensitive strains.

Biocontrol of Citrus Canker Disease Caused by Xanthomonas citri subsp. citri Using an Endophytic Bacillus thuringiensis

“…Copper-based bactericides and streptomycin antibiotics have been used in the attempt to control this disease (Behlau et al, 2011; Graham et al, 2008). Nevertheless, they are relatively ineffective on certain Xcc strains (Voloudakis et al, 2005) which have developed resistance to these disease control products (Hyun et al, 2012). Therefore, our aim was to find efficacious alternative citrus canker control measures.…”

“…3). Therefore, it is a promising candidate as a biocontrol agent against the streptomycin-resistant strains Xcc M4-9 (Hyun et al, 2012). The ethyl acetate extract may have contained potent secondary metabolites which suppressed the growth of Xcc .…”

“…Nine Xcc strains, including three wild-type and six streptomycin-resistant mutants (Hyun et al, 2012) were used in this study. The wild-type ( Xcc W1 [ Xcc 19-18], Xcc W2 [ Xcc 10-5], and Xcc W3 [ Xcc 53-4]) and streptomycin-resistant mutant ( Xcc M4 [ Xcc 27-9], Xcc M5 [ Xcc 8-4], Xcc M6 [ Xcc 57-2], Xcc M7 [ Xcc 57-5], Xcc M8 [ Xcc 27-13], and Xcc M9 [ Xcc 27-12]) strains were kindly provided by Dr. Hyun of the Citrus Research Station, National Institute of Horticultural and Herbal Science, Rural Development Administration, Jeju, Korea.…”

“…as well as fire blight, soft rot, bacterial spot, and wildfire diseases on other crops (McManus et al, 2002). Nevertheless, pathogen resistance to streptomycin has developed due to its prolonged and excessive use (Carter et al, 2000; Hyun et al, 2012). In a recent study, 72% of the Xcc strains isolated from Jeju Island, Republic of Korea were found to be resistant to streptomycin (Hyun et al, 2012).…”

Scientific Opinion on the risk to plant health of Xanthomonas citri pv. citri and Xanthomonas citri pv. aurantifolii for the EU territory

“…It has also been shown that copper resistance genes are naturally present within the citrus-associated bacterial microflora in areas exposed to high copper treatment pressure and have the ability to be integrated into the genome of X. citri pv. citri strains were found both in streptomycin-treated citrus orchards and in untreated orchards in Jeju island (South Korea) where streptomycin is registered to control citrus canker (Hyun et al, 2012). Major pathogenicity genes are also plasmid borne and could be exchanged among strains by horizontal gene transfer (El Yacoubi et al, 2007).…”