Multiple Drug Resistant Salmonella Newport Strains In Pennsylvania

Shelley Rankin*, Donald Munro, Helen Aceto and Charles E Benson

University of Pennsylvania, School of Veterinary Medicine, New Bolton Center, 382 West Street Road
Kennett Square, PA 19348

In July 2000 the Salmonella Reference Center (SRC) at the University of Pennsylvania received an isolate of Salmonella Newport that was resistant to multiple antibiotics. Since that time, the laboratory has received 54 multiple drug resistant (MDR) isolates and two additional Newport isolates; one of which was resistant only to sulphonamides and one that was fully susceptible to all of the antibiotics tested at SRC. From 1997 to date, the SRC database has accumulated over 11,500 salmonella isolates. A search of this database revealed only 5 isolates of S. Newport obtained prior to 2000, none of which showed the MDR phenotype.

Antibiotic susceptibility testing at the Salmonella Reference Center revealed the predominant MDR profile: ampicillin, chloramphenicol, tetracycline, streptomycin, sulfamethoxazole, cephalothin, and ceftiofur (ACTSSuCfXnl). In some cases, additional resistance markers were observed; most commonly spectinomycin, gentamicin and kanamycin. Additionally, all isolates were tested by disk diffusion for resistance to ceftriaxone and were defined as "sensitive" by NCCLS criteria. However, the zone diameter was considerably smaller than the fully susceptible strains and MIC testing should be performed.

Regardless of antibiotic phenotype, Plasmid Profile Analysis (PPA) was performed on all 61 isolates received at the SRC since 1997. PPA revealed 10 different plasmid profiles. A single plasmid of approximately 140 kb was observed in 43 of 61 isolates (69%). A plasmid of the same size was also seen in a further 12 isolates, but was present in combination with one or more additional plasmids. Two isolates were plasmid free, two isolates had the profile 150: 8 kb, and the final 2 isolates had unrelated profiles. It is important to note that similarity of molecular weight does not indicate genetic identity and plasmid restriction endonuclease fragmentation pattern analysis should always be performed to confirm whether or not these plasmids are structurally related.

Pulsed-field gel electrophoresis (PFGE) was carried out on all 61 isolates after digestion with the restriction enzyme XbaI. Twelve distinct PFGE patterns were observed. Newport-PFP1 was found in 42 of 61 isolates and Newport-PFP5 was seen in 9 of 61 isolates. All other PFPs were represented by a single strain.

Of the 61 isolates, some were multiple submissions from a single farm or animal. When the data were sorted to take account of epidemiological relationships, we found that 38 isolates came from only 6 sources. The remaining 23 isolates have no known epidemiological relationship. We occasionally observed multiple phenotypes from a single source and, in two instances, more than one genotype from a common source.

Plasmid REFP analysis is currently underway as is an analysis for the presence of integrons and gene cassettes, preliminary results from these investigations will be presented.

Pennsylvania Animal Diagnostic Laboratory System
PADLS Conferences Spring 2001 10. Drug Resistant Salmonella
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