Use of Antibiotic Susceptibility Patterns and Pulsed-Field Gel Electrophoresis To Compare Historic and Contemporary Isolates of Multi-Drug-Resistant Salmonella enterica subsp. enterica Serovar Newport

Abstract

Recently, multi-drug-resistant (MDR) Salmonella enterica subspecies enterica serovar Newport reemerged as a public and animal health problem. The antibiotic resistance of 198 isolates and the pulsed-field gel electrophoresis patterns (PFGE) of 139 isolates were determined. Serovar Newport isolates collected between 1988 and 2001 were included in the study. One hundred seventy-eight isolates were collected from the San Joaquin valley in California and came from dairy cattle clinical samples, human clinical samples, bulk tank milk samples, fecal samples from preweaned calves, and waterways. Twenty clinical isolates from humans from various regions of the United States were also included in the study. Resistance to 18 antibiotics was determined using a disk diffusion assay. PFGE patterns were determined using a single enzyme (XbaI). The PFGE and antibiogram patterns were described using cluster analysis. Although the antibiotic resistance patterns of historic (1988 to 1995) and contemporary (1999 to 2001) isolates were similar, the contemporary isolates differed from the historic isolates by being resistant to cephalosporins and florfenicol and in their general sensitivity to kanamycin and neomycin. With few exceptions, the contemporary isolates clustered together and were clearly separated from the historic isolates. One PFGE-antibiogram cluster combination was predominant for the recent isolates, which were taken from human samples from all parts of the United States, as well as in the isolates from California, indicating a rapid dissemination of this phenotypic strain. The data are consistent with the hypothesis that the reemergence of MDR serovar Newport is not simply an acquisition of further antibiotic resistance genes by the historic isolates but reflects a different genetic lineage.

Comparison of Multilocus Sequence Typing, Pulsed-Field Gel Electrophoresis, and Antimicrobial Susceptibility Typing for Characterization of Salmonella enterica Serotype Newport Isolates

“…In contrast, 85% (17/20) of S. enterica serovar Newport isolates from ill cattle displayed the MDR-AmpC phenotype. These data support previous studies of the epidemiology of S. enterica serovar Newport MDRAmpC infections in humans and links with cattle or beef products (3,22,45,50,59).…”

“…For example, the majority of MDR-AmpC strains grouped together in cluster A (n ϭ 34/39), whereas the majority of pansusceptible strains grouped in cluster C (n ϭ 22/33). The demarcation of PFGE patterns and clusters between the MDR-AmpC isolates and pansusceptible isolates has been previously noted (3,22,59) and suggests that the recent emergence of MDR-AmpC S. enterica serovar Newport is due to the clonal expansion of a limited number of genetically related strains that have acquired plasmid-mediated resistance genes. A similar finding was recently reported by Alcaine et al, who postulated that plasmid-mediated ceftiofur-resistant Salmonella evolved by independent emergence and clonal spread (2).…”

Molecular Epidemiology of Ceftiofur-Resistant Escherichia coli Isolates from Dairy Calves

“…PFGE profiles correlated well with O serogroup and antibiotic resistance. A similar correlation between antibiotic resistance and PFGE profile has been seen in multidrug-resistant Salmonella enterica serotype Newport (4,31).…”

“…Similar cross-resistance has also been reported for Salmonella spp. (2,4). All E. coli strains were sensitive to the fluoroquinolone antibiotic Eno.…”

Salmonella enterica Serotype 4,5,12:i:−, an Emerging Salmonella Serotype That Represents Multiple Distinct Clones

“…These findings provide another example of a Salmonella serotype of considerable public health relevance that represents at least two independent genetic lineages. For example, Salmonella serotype Newport has previously been shown to represent two distinct genetic lineages, including one lineage that contains predominantly pansusceptible isolates and one that contains predominantly multidrug-resistant isolates (2,8,22). In addition, the multiple independent emergences of Salmonella serotype 4,5,12:i:Ϫ and subsequent ecological success of multiple lineages (as evidenced by common isolation from human clinical cases in both Spain and the United States) suggest a strong selective pressure for loss of phase 2 flagella or a closely linked genotype.…”