Foodborne human isolates of Salmonella and Shiga toxin -producing Escherichia coli of domestic origin in Finland
Year: 2015
Type: dissertation
Abstract: Salmonella is one of the most commonly reported foodborne pathogens and enterohaemorrhagic Escherichia coli (EHEC) is one of the most dangerous. They both spread by zoonotic and person-to-person transmission routes. Most Salmonella infections are characterized by mild-to-moderate self-limited diarrhea but also serious disease resulting in death has been reported.Bloody diarrhea is a common symptom of human EHEC infection and the infection may lead to severe post-infection disease such as hemolytic uremic syndrome (HUS), thrombotic thrombocytopenic purpura (TTP) and even death. The purpose of this thesis was to investigate the diversity of Salmonella and EHEC strains isolated from domestically acquired infections using several pheno- and genotyping methods for surveillance and outbreak investigation purposes as well as evaluate certain epidemiological typing methods in Finland. All Salmonella and EHEC isolates of domestic origin during 2007-2014 in Finland were studied. Serotyping, phage typing, antimicrobial susceptibility testing, phenotype microarray, pulsed-field gel electrophoresis (PFGE) and multilocus variable-number tandem repeat analysis (MLVA) were applied as epidemiological typing tools for Salmonella isolates. EHEC isolates were analyzed using serotyping, phage typing, antimicrobial susceptibility testing, virulence gene detection (stx1, stx2, eae, hlyA and saa) and PFGE. During the study period, the number of domestically acquired Salmonella infections has decreased about one fifth compared to the previous decade whereas the number of domestic EHEC infections have increased about one third.Th e incidence of Salmonella infections was highest in 2012 (7.5/100,000 population) and lowest in 2014 (5.4/100,000 population). The incidence of EHEC infections was highest in 2013 (0.33/100,000 population) and lowest in 2008 (0.07/100,000 population). 15% of all Salmonella strains and 70% of all EHEC strains were considered domestically acquired. A total of 131 different Salmonella serovars were detected. The most common serovars were Typhimurium (32%), Enteritidis (15%) and group B (6%). Among Typhimurium strains, phage types DT1 (37%), RDNC (18%) and DT104 (9%) were the most common ones. The most frequently detected phage types among the domestically acquired S. Enteritidis infections were PT8 (17%), PT1B (14%) and PT4 (13%). Th e majority of domestic Typhimurium and Enteritidis (60%) strains were susceptible to tested antimicrobials. During the study period, a total of 188 infections caused by EHEC were detected. Most of them were caused by serotype O157:H7 (60%). The majority of O157 strains (63%) were unable to ferment sorbitol. PT8 was the most common phage type among the sorbitol-negative and PT88 among sorbitol positive O157 strains. Among non-O157, 22 distinct O:H serotypes were detected. The most common ones were O26:H11, O103:H2 and O145:H-. The majority of domestic EHEC strains (81%) were susceptible to all tested antimicrobials: 96% of O157:H7/H- and 60% of non-O157 strains. All O157 strains carried stx2 (40% in combination with stx1), eae and hlyA genes. In contrast, 55% of non-O157 stains had stx1 gene and 76% carried eae and hlyA genes. The MLVA method was found to be a powerful epidemiological tool for S. Typhimurium with discriminatory power similar to PFGE. In addition, MLVA was faster, cheaper and the results were easier to compare between laboratories. The domestic S. Typhimurium strains were divided into 170 distinct MLVA types (diversity index 0.891). In MLVA, the three most common profiles (3-16-NA-NA-0311, 3-15-NA-NA-0311 and 3-17-NA-NA-0311) counted for 47% of the strains showing that the lack of locus STTR6 and locus STTR10p was characteristic for domestic S. Typhimurium. However, XbaI-PFGE remains a useful genotyping method for investigations of other Salmonella serovars and EHEC strains. The interpretation of XbaI-PFGE profiles can be challenging as demonstrated by a Finnish nationwide outbreak caused by S. Newport and S. Reading -contaminated iceberg lettuce. The S. Reading strains had four diff erent XbaI-PFGE profiles. Based on epidemiological information, all these different variants of the outbreak causing strains were considered as outbreak-related. The sources of the most EHEC outbreaks remained undetermined. In one out four EHEC O157 outbreaks, unpasteurized milk was found as the source of the infections. Although 40% of the domestic EHEC strains were non-O157, only strains of serogroup O157 caused outbreaks in Finland. However, non-O157 strains caused several family clusters and were linked with HUS. In 2009, a sorbitol-fermenting EHEC O78:H-:stx1c:hlyA was detected in blood and fecal samples of a neonate. This EHEC serotype had not been seen in Finland prior to this family-related outbreak and bacteremia caused by EHEC is exceptionally rare. Taken together, Salmonella and EHEC infections are a major public health concern. This thesis provides new information about the characteristics of Salmonella and EHEC strains isolated from domestically acquired infections in Finland and evidence that effective surveillance is needed for early detection and prevention of the spread of Salmonella and EHEC infections. In particular, typing methods used should be internationally harmonized and the results made comparable.
Author: Taru Lienemann
Language: English
Subfield: Food Science
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W1849970676