Molecular epidemiology of Salmonella typhimurium DT160 in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies in Public Health at Massey University, Palmerston North, New Zealand
Salmonellosis is a zoonotic bacterial disease of national and international importance. In New Zealand (NZ), the most common foodborne notifiable disease is campylobacteriosis, which is followed by salmonellosis. In 1998, Salmonella enterica subsp. enterica serotype Typhimurium Definitive Type 160 (DT160) was identified in NZ. Since first reported, S. Typhimurium DT160 has caused several epidemics in the country but has not produced significant illness worldwide.
Therefore, the objectives of the project were to investigate the molecular epidemiology of S. Typhimurium DT160 and the association between isolates from human and animal origin. Ninety Salmonella isolates obtained in the period between 1999 and 2009 from the Institute of Environmental Science and Research, NZ were assessed for colony morphology, serotype, susceptibility to 11 antimicrobials, virulotyped using Polymerase Chain Reaction (PCR) and the Pulsed Field Gel Electrophoresis (PFGE) patterns were also determined. In addition, 4 isolates were further assessed with Triple Sugar Agar, API20E biochemical and motility tests.
All 90 isolates were confirmed as Salmonella spp. with no indications for resistance to multiple antimicrobials. All isolates were susceptible to the antimicrobials used in this study with the exception of 26 and 8 isolates that had intermediate susceptibility against tetracycline and oxytetracycline, respectively. In an attempt to discriminate between potentially pathogenic and pathogenic Salmonella isolates, PCR-based virulotyping was performed based on 12 potential virulence genes. Results revealed that all isolates were positive for at least 10 of the 12 virulence genes. Two of the six isolates negative for one of the virulence genes (invA, iroN, pefA or sifA) were of human origin and the remaining four were sparrow
isolates. The PFGE patterns determined with restriction enzymes XbaI and SpeI demonstrated that the genotype profile AA1 accounted for 78/90 (86.7%) of the isolates, whilst the second most common profile, AA2, was found in only three isolates (3.3%), comprising two isolates from sparrows and one from a human. The remaining nine profiles were found in single isolates. All isolates of AA2 profile were PCR negative for sifA.
In conclusion, no obvious correlation was observed between the phenol- and geno-type and the isolates, year and month of isolation, and source of the samples. There was no obvious evidence for multidrug resistance among DT160 isolates. The PFGE and virulotyping profiles suggest close relation among majority of isolates with predominant and epidemiologically important genotype persistent in multiple hosts. Finally, the few genotypes with low prevalence in multiple hosts may indicate emergence of sporadic genomic variants in the population.