Virulence and genotype stability of Salmonella enterica serovar Berta during a natural outbreak

Genetic surveillance of endemic bovine Salmonella Infantis infection

Background

Salmonella serovar Infantis is endemic in Finnish food-producing animals since the 1970s. The purpose of this study was to describe the molecular epidemiology of the infection in cattle during 1985–2005, to follow the persistence of the feed-related outbreak strain from 1995 in the cattle population, and to analyse the stability of XbaI-banding patterns in individual herds during long-lasting infections.

Methods

Salmonella Infantis isolates from 478 cattle herds (n = 588), covering 73% of the subclinically or clinically infected herds, were typed by pulsed-field gel electrophoresis (PFGE) using XbaI. DNA fragments larger than 125 kb were counted in PFGE types because of high plasmid background. Ribotyping and IS200-typing with BanI-digested DNA were done on 57 selected isolates.

Results

The isolates associated with the infection consisted of 51 PFGE types with genetic similarity (F value) between 0.58 and 0.95. From 1985 to 2003, the major type appeared on 68% of the farms. The three most common types, with F values of 0.90 to 0.95, accounted for 80% of the isolates. Only 17% of the isolates had F values below 0.80, and 1% below 0.70. Ribotyping and IS200-typing classified 89% of the analysed isolates into the major ribotype and IS200 type combination, and the rest fell into closely related types. Analysis of successive isolates from 142 herds revealed changes in XbaI-banding patterns in 21% of the herds with two analysed isolates and in 38% of the herds from which three or more isolates were analysed. The feed-related S. Infantis genotype from the 1995 outbreak had disappeared by 1999, at the time when the incidence of bovine salmonella, and S. Infantis in particular, strongly decreased.

Conclusion

The study showed how genetic surveillance, as part of salmonella control, provides tools to follow the persistence of particular infections, and to assess the efficacy of control measures. Testing of several isolates from a herd in outbreak investigations is advisable, because minor changes in PFGE banding patterns frequently occur during long-lasting infections.

Transmission routes of Salmonella Typhimurium DT 104 between 14 cattle and pig herds in Denmark demonstrated by molecular fingerprinting

AIMS

Salmonella Typhimurium DT 104 is generally assumed to be spread by contact between live animals, e.g. by trading. The aim of the present study was to assess the importance of other routes of transmission in the dissemination of this bacterium.

METHODS AND RESULTS

An outbreak among 14 cattle and pig herds located in a geographically narrow area in Denmark was investigated. Epidemiological information and disease history of the herds was obtained through interviews. Based on this, the hypothesis for horizontal spread was proposed, and these were confirmed by comparison of the pulsed field gel electrophoresis (PFGE) and the plasmid profiles of isolates obtained by continuous sampling over a period of almost 3 years.

CONCLUSIONS

The study indicated that other routes might play an important role, than the trading of live animals, in the spread of S. Typhimurium DT 104 among livestock.

SIGNIFICANCE AND IMPACT OF THE STUDY

Salmonella Typhimurium DT 104 infected herd might pose a significant risk to herds located within the same geographic area. In advising on how to avoid the spread of this bacterium, factors like person contacts, sharing of equipment and contaminated slurry should be focussed on in addition to infected animals.

Quantitative comparison of intestinal invasion of zoonotic serotypes of Salmonella enterica in poultry

The aim of the present study was to compare the invasion of selected zoonotic Salmonella serotypes of poultry in an in vivo chicken intestinal loop model and also in vitro in epithelial cell cultures. Invasion was measured relative to a reference strain, Salmonella Typhimurium 4/74 invH201::TnphoA. Two serotypes demonstrated intracellular log(10) counts that differed significantly from all other serotypes tested: Salmonella Enteritidis PT4 being 1.5 log(10) colony forming units (CFU) (31-fold) higher, and Salmonella Tennessee being 0.7 log(10) CFU (fivefold) lower than the reference strain (P < or = 0.0001). A group of serotypes, which can be vertically transmitted, showed significantly higher intracellular counts (fourfold to eightfold) than the reference strain. The group included S. Typhimurium 4/74, S. Typhimurium DT104 (poultry and porcine isolates), S. Enteritidis PT1, S. Enteritidis PT6, S. Enteritidis PT8, and Salmonella Berta. The serotypes Salmonella Hadar, Salmonella Virchow, S. 4,12:b:-, S. Typhimurium DT41, and Salmonella Infantis, most of which are considered horizontally transmitted, did not show significantly different intracellular counts from the reference strain. Results from the cell culture invasion studies agreed with the in vivo data, with the exception of S. Berta and the poultry isolate of S. Typhimurium DT104.

Molecular epidemiological analysis of Salmonella enterica serotype Derby infections in Hong Kong

OBJECTIVES

We aimed to study the antimicrobial susceptibilities and molecular epidemiology of Salmonella enterica serotype Derby, a unique and common salmonella serotype in Hong Kong.

METHODS

Salmonella Derby strains isolated from stools of patients in a large general hospital in Hong Kong from 1989 to 1994 and from food samples isolated in the Public Health Laboratory were randomly selected and investigated for the antimicrobial susceptibilities by determining the minimal inhibitory concentrations of 19 antimicrobial agents and their relatedness using plasmid analysis, ribotyping, pulsed-field gel electrophoresis (PFGE) and total DNA fingerprinting.

RESULTS

About 50% of the 127 isolates studied were susceptible to all the 19 antibiotics tested, although resistance to tetracycline (49%) and sulfamethoxazole (38%) was high. Only 12% did not harbour any detectable plasmids, while the rest contained plasmids in 51 profiles. There were two predominant clones, one comprising of 35% of isolates that could not be pulsotyped because discrete bands were not discernible after PFGE and another comprising 34% of isolates that could be pulsotyped. The remaining 31% belonged to a variety of types.

CONCLUSIONS

Approximately 70% of S. Derby belonging to two clones were endemic in the community, while the remaining isolates belonged to a variety of types which were probably a result of sporadic infection. The sources of human infections were foods, since most isolates from foods also belonged to the two endemic clones. Typing of S. Derby isolates from other sources such as animals or the environment would help elucidate how foods were contaminated. PFGE might not be universally applicable to all salmonella strains.