[Microbial drug resistance and the presence of plasmids in Salmonella strains isolated from different sources].
BACKGROUND
To establish the relationship between the presence of plasmid and their antimicrobial resistance of Salmonella strains.


METHODS
We tested 171 strains of Salmonella isolated from different sources: natural waters (73 strains), food (23 strains), and from clinical samples (75 strains). The disk diffusion method was used to test the antimicrobial susceptibility of the strains to 13 drugs. Plasmid analysis were performed by agarose gel electrophoresis technique.


RESULTS
Antimicrobial resistances of the strains significantly varies according to their primary isolation sources. Strains isolated from the water environment exhibited a full susceptibility to cephalothin and colistin, whilst all the strains isolated from food were sensitive to ampicillin, gentamicin, kanamycin, neomycin, tobramycin and trimethoprim-sulphamethoxazole. On the other hand, resistances to colistin, gentamicin and trimethoprim-sulphamethoxazole were not found in clinical isolates. From the 171 Salmonella strains tested, only 12.2% were sensitive to all the antimicrobials. The most frequently antibiotic resistances detected were to streptomycin (49.3%), tetracycline (33.1%) and nalidixic acid (30.7%). The percentage of strains that harboured plasmids was different depending on the source of isolation, ranging from 41.4% for water isolated, 76% for clinical isolates and 86.9% for food isolates. The relationship between antimicrobial resistance and plasmid presence is very close, since higher percentages of resistance to chloramphenicol, carbenicillin, cephalothin, kanamycin, neomycin, nalidixic acid and trimethoprim-sulphamethoxazole were obtained in strains containing plasmids.


CONCLUSIONS
(a) The most frequently resistance detected in strains of Salmonella was to streptomycin (49.3% of the strains). On the other hand, only 0.6% of the strains were resistant to gentamicin. (b) Percentages of resistance to some antibiotics was higher in strains harbouring plasmids, that implies a relationship between the plasmid presence and the antibiotic resistance in Salmonella. (c) Curing of extrachromosomic elements by acridine orange showed a percentage of resistance lost greater than 70% for cephalothin, kanamycin, neomycin, and nalidixic acid. This indicates that the resistance to those antibiotics is mainly linked to plasmids. In the case of the unusual nalidixic acid-resistance, previously described in Shigella strains, suggests that are needed more studies to demonstrate the direct association between antimicrobial resistance and presence of plasmids.
Departamento de Microbiología, Universidad de Málaga.
A
A
Luque
M A
MA
Moriñigo
C
C
Rodríguez-Avial
J J
JJ
Picazo
J J
JJ
Borrego
D000900:Anti-Bacterial Agents / Q000494:pharmacology; D004352:Drug Resistance, Microbial; D006801:Humans; D011815:R Factors; D012475:Salmonella / Q000145:classification / Q000187:drug effects / Q000235:genetics*
D000900:Anti-Bacterial Agents
Resistencias a antimicrobianos y presencia de plásmidos en cepas de Salmonella aisladas de diferentes orígenes.