Articles
GENETIC BASIS OF STREPTOMYCIN RESISTANCE IN PATHOGENIC AND EPIPHYTIC BACTERIA ISOLATED IN APPLE ORCHARDS IN NEW ZEALAND
Two mechanisms resulting in streptomycin resistance have been described in plant pathogenic bacteria.
A mutation in the chromosomal gene rpsL which alters the binding affinity of ribosomal proteins for streptomycin can lead to streptomycin resistance, as well as acquisition of genes whose products modify streptomycin.
Since chromosomal mutations are transmitted almost exclusively during cell division and to daughter cells, streptomycin resistance due to rpsL mutation is not likely to spread rapidly or to spread to other bacterial species.
In contrast, genes coding for enzymes which modify streptomycin are often carried by plasmids or transposons.
Some of these transposons or plasmids are able to move from one bacterium to another even if these two bacteria belong to different species or different genus.
This can result in the rapid widespread of streptomycin resistance.
Mutations in the rpsL gene confer resistance to very high concentrations of streptomycin; whereas genes which product modify streptomycin, usually confer resistance only to low concentrations of this antibiotic.
All streptomycin resistant strains of E. amylovora isolated from New Zealand were resistant to high levels of streptomycin (over 1000 μg/ml). In contrast, 75% of the strains of other species tested were inhibited by concentrations of streptomycin over 250 μg/ml, suggesting that these strains produce one or more enzymes which inactivate streptomycin (Vanneste and Voyle, 1996).
We used a plate assay to confirm the presence of streptomycin modifying enzymes in cells extracts of streptomycin resistant bacteria isolated in New Zealand.
No activity could be detected in any of the cell extracts from E. amylovora strains.
This was in agreement with results presented above and indicated that resistance was probably due to a mutation of the gene rpsL. This was confirmed by A. Jones using PCR (Chiou and Jones 1995). Also in agreement with previous results, streptomycin activity was detected in extracts from 19 of the 20 strains of E. herbicola and in 6 of the 18 strains of Pseudomonas examined, indicating the presence of genes which confer streptomycin resistance in those strains.
In plant pathogenic bacteria, only two genes which confer streptomycin resistance have been identified so far (strA and strB). Total DNA of 40 out of 45 strains of Pseudomonas and total DNA of 43 out of 53 strains of E. herbicola or yellow Pseudomonas hybridised with a DNA probe containing strA and strB. Plasmids carrying the genes strA and strB were detected by DNA/DNA hybridisation in 37 E. herbicola isolates, 26 Pseudomonas syringae and 11 yellow Pseudomonas. Furthermore, total DNA from all but 13 of the E. herbicola strains hybridised with a DNA probe containing tnpA, tnpR and res genes from the transposon Tn5393. The 13 strains whose DNA did not hybridise with this second probe carry a plasmid which has the same size and restriction pattern as RSF1010, a non conjugative but mobilisable plasmid found in a