Streptomycin resistance survey
Claudia Nischwitz
Extension Plant Pathologist
Utah State University
Streptomycin resistance
• Observed in orchards across the U.S.
• In CA, streptomycin resistant E. amylovora
detected in 1971
• Resistant strains were still detected 10 years after streptomycin applications stopped
Resistance mechanism
• Two mechanisms:
– Acquisition of a plasmid with resistance genes from other bacteria
– Mutation in gene that encodes for the enzyme streptomycin binds to
Example: AGCTGGTTAAAAAAATTGCTA
AGCTGGTTAAGAAAATTGCTA
• Mutation changes the enzyme structure and streptomycin can no longer bind to it
Streptomycin resistance
• In CA, OR, WA, NY resistance is due to a mutation in gene
• In Midwest it is often due to acquisition of plasmid
Resistance development
Before streptomycin application
Year 1
S
R SS
S S S
S
S S
S
S
S S S
S
Year 2
S
R S R
R
S S R
S
R
RS S
S
R S RS
S
R
S
Year 3
R
R
R R
R R
R
R
R
R
R
R R
R
R
R
R
R R
R
R
R
R
R
R R
R
R
After streptomycin application
Survey ‐ 2011
County
# orchards sampled resistant
Sensitive
mixed
No Erwinia
detected
Utah
20
13
2
3
2
Box Elder
1
0
0
0
1
Cache
2
0
0
0
2
Davis
1
0
1
0
0
Survey ‐ 2011
• Resistance mechanism:
– All resistant isolates have the mutation in the gene encoding for the protein that streptomycin binds to
– There is one isolate we are still working on that potentially has the plasmid.
0ppm 100ppm 1000ppm
• Fitness of resistant bacteria:
– Tested at three concentrations of streptomycin
• 0, 100 and 1,000 ppm
– A few resistant ones tested at 200,000ppm
Summary
• Resistant isolates are highly resistant • No cost to their fitness
• Alternatives for fire blight management:
– Kasumin (kasugamycin)
– Non‐antibiotic chemical is tested in Kaysville
– Cultural strategies
Summary
– Cultural strategies:
• Prune infected branches 8‐12 inches below symptoms
• Clean pruning tools after cutting infected branches
1min 5 min 1 hour 2 hours
Summary
– Cultural strategies cont.
• Remove infected branches from orchards as soon as possible
• Canker collected in May 2011
• Sprayed occasionally with water and E. amylovora was isolated more than six weeks
Acknowledgments
•
•
•
•
USDA SCBG
USHA
Apple and pear growers
Christine Dhiman