The characterization of Danish isolates of Salmonella enterica serovar Enteritidis by phage typing and plasmid profiling: 1980-1990

Prevalence and characterization of Salmonella among humans in Ghana

Background

Non-typhoidal Salmonella (NTS) is a public health problem worldwide and particularly in Africa with high disease burden. This study characterized Salmonella isolates from humans in Ghana to determine serovar distribution, phage types, and antimicrobial resistance. Further, the clonal relatedness among isolates was determined.

Methods

One hundred and thirty-seven Salmonella isolates (111 clinical and 26 public toilet) were characterized using standard serotyping, phage typing, and antimicrobial susceptibility testing methods. The molecular epidemiology of common serovars (Salmonella Typhimurium and Salmonella Enteritidis) was established by pulsed field gel electrophoresis (PFGE).

Results

Twenty-two serovars were identified with S. Enteritidis, S. Typhimurium, and Salmonella Derby as the most dominant. One hundred and twelve isolates showed resistance to more than one antimicrobial. Fifty-eight (n = 58/112; 54.5%) strains were multi-resistant with low resistance to cephalosporins ceftazidime (8.0%), cefotaxime (4.5%), and cefoxitin (2.7%) with synergy to clavulanic acid indicating possible ESBLs. Isolates showed high resistance to trimethoprim (66.1%), tetracycline (61.6%), ampicillin (57.1%), sulfamethoxazole (46.4%), chloramphenicol (33.9%), and ciprofloxacin (25.0%). The most common resistance pattern of multi-resistant serovars was to ampicillin, chloramphenicol, sulphonamide, and trimethoprim. S. Enteritidis (18/43) strains reacted with typing phages but did not conform to any phage type with PT14B and PT4 as predominant definitive phage types. Six S. Typhimurium strains reacted but did not conform to any recognized phage type while seven were non-typable. The predominant definitive phage types were DT1 and DT22. PFGE patterns of human S. Enteritidis were closely related to patterns of poultry isolates obtained in a previous study in Ghana.

Conclusions

Cephalosporin resistance is uncommon among Salmonella from humans in Ghana. Poultry may be an important source of human salmonellosis. There is an urgent need for the implementation of routine surveillance of antimicrobial use and bacterial resistance among humans in Ghana.

Evolution of antimicrobial resistance of Salmonella enteritidis (1972-2005)

With the extensive use of antibiotics in livestock production, surveillance revealed an increase in Salmonella resistance to the commonly used antimicrobials in veterinary and public health. This serious threat to health care is further exacerbated by the limited epidemiological information about the common zoonotic agent, Salmonella enteritidis, required to determine antibiotic therapy. The aim was to characterise the antimicrobial resistance patterns of S. enteritidis isolates across different timelines (1972-2005) with accompanying genetic changes being investigated. Thirty-seven stored S. enteritidis isolates were collected from the Central Veterinary Laboratory, Harare, with antimicrobial susceptibility determined against eight antibiotics. Plasmids were isolated to analyse any genetic variation. An overall significant increase in resistance (p < 0.05) to nalidixic acid (0% - 10%), ampicillin (14.3% - 50%), tetracycline (14.3% - 30%) and erythromycin (71.4% - 100%) was observed across the timeline. However, the highest rates of susceptibility were maintained for gentamicin, sulphamethoxazole-trimethoprim, kanamycin and chloramphenicol. We report an increase in multidrug resistance (MDR) of 14.2% - 50% with an increase in resistotypes and plasmid profiles across the timeline. Eleven plasmid profiles were obtained in the 37 isolates studied with a minority of isolates (21.6%, 8/37) harbouring a 54 kb plasmid, commonly serovar-specific. A concerning increase in antimicrobial resistance to commonly administered drugs was observed across the timeline. The surge in MDR is of great concern and implies the need for consistent antimicrobial stewardship. No correlation was observed between the plasmid and antibiotic profiles.

Genetic diversity and evolution of Salmonella enterica serovar Enteritidis strains with different phage types

Phage typing has been used for the epidemiological surveillance of Salmonella enterica serovar Enteritidis for over 2 decades. However, knowledge of the genetic and evolutionary relationships between phage types is very limited, making differences difficult to interpret. Here, single nucleotide polymorphisms (SNPs) identified from whole-genome comparisons were used to determine the relationships between some S. Enteritidis phage types (PTs) commonly associated with food-borne outbreaks in the United States. Emphasis was placed on the predominant phage types PT8, PT13a, and PT13 in North America. With >89,400 bp surveyed across 98 S. Enteritidis isolates representing 14 distinct phage types, 55 informative SNPs were discovered within 23 chromosomally anchored loci. To maximize the discriminatory and evolutionary partitioning of these highly homogeneous strains, sequences comprising informative SNPs were concatenated into a single combined data matrix and subjected to phylogenetic analysis. The resultant phylogeny allocated most S. Enteritidis isolates into two distinct clades (clades I and II) and four subclades. Synapomorphic (shared and derived) sets of SNPs capable of distinguishing individual clades/subclades were identified. However, individual phage types appeared to be evolutionarily disjunct when mapped to this phylogeny, suggesting that phage typing may not be valid for making phylogenetic inferences. Furthermore, the set of SNPs identified here represents useful genetic markers for strain differentiation of more clonal S. Enteritidis strains and provides core genotypic markers for future development of a SNP typing scheme with S. Enteritidis.

Molecular study of Salmonella enteritidis in poultry samples by PCR, plasmid curing, antibiotic resistance and protein pattern analysis

In this study two control isolates of Salmonella enteritidis, RTCC1623 and RTCC1624, were obtained from the institute ofRazi (Karaj-Iran) and 14 strains were isolated from poultry samples in Kermanshah province of Iran, according to a standard protocol. These isolates were confirmed by PCR amplification of SefA gene fragments. Results showed that, 6 isolates of 14 isolates of Salmonella which their biochemical tests were positive contain 511 bp amplified fragments of the SefA gene. In other purpose, to correlating the presence of plasmids with antibiotic resistance and protein pattern, plasmid DNA was isolated before and after plasmid curing by using the alkaline lysis method. Strains of S. enteritidis contain seven different plasmid profiles (P1-P7) which were characterized by antibiotic resistance and protein pattern. Our observed showed, there was a high molecular weight plasmid with Rf 0.17 in all strains and the frequency of other plasmids was low. The plasmid with Rf about 0.2 is responsible for resistance to Cephalothin and the isolates that lost it were susceptible to this antibiotic. All strains, 100%, were resistant to ampicillin before and after curing of strains. According to present findings, PCR is a rapid and sensitive method for typing of S. enteritidis and plasmid profiling; antibiotic resistance and protein pattern are suitable methods for subtyping of S. enteritidis isolates. No direct correlation was found between plasmid contents, antibiotic resistance patterns and protein profiles of local S. enteritidis isolates.

Relationship of pulsed-field profiles with key phage types of Salmonella enterica serotype Enteritidis in Europe: results of an international multi-centre study

Salmonella is one of the most common causes of foodborne infection in Europe with Salmonella enterica serovar Enteritidis (S. Enteritidis) being the most commonly identified serovar. The predominant phage type for S. Enteritidis is phage type (PT) 4, although PT 8 has increased in incidence. Within these phage types, pulsed-field gel electrophoresis (PFGE) provides a method of further subdivision. The international project, Salm-gene, was established in 2001 to develop a database of PFGE profiles within nine European countries and to establish criteria for real-time pattern recognition. It uses DNA fingerprints of salmonellas to investigate outbreaks and to evaluate trends and emerging issues of foodborne infection within Europe. The Salm-gene database contains details of about 11 700 S. Enteritidis isolates, demonstrating more than 65 unique PFGE profiles. The clonal nature of S. Enteritidis is evidenced by the high similarity and distribution of PFGE profiles. Over 56% (6603/11 716) of the submitted isolates of several different phage types were profile SENTXB.0001, although this profile is most closely associated with PT 4. The next most common profiles, SENTXB.0002 and SENTXB.0005, were closely associated with PT 8 and PT 21 respectively. Studies to investigate the relationship of profile types with outbreaks and possible vehicles of infection suggest that the incidence of PFGE profile SENTXB.0002, and thus PT 8, in some countries may be due to importation of foods or food production animals from Eastern Europe, where PT 8 is amongst the most frequently identified phage types. Collation of subtyping data, especially in the commonly recognized phage types, is necessary in order to evaluate trends and emerging issues in salmonella infection.

Salmonella enterica serotypes isolated from imported frozen chicken meat in the Canary islands

To determine the prevalence of Salmonella enterica serotypes in imported frozen chicken meat, 406 samples (whole chicken, legs, and breast meat) were analyzed for Salmonella according to ISO6579 rules, serotypes were assigned, and phage typing was conducted for Salmonella serotypes Enteritidis, Typhimurium, and Heidelberg. The overall frequency of Salmonella isolation was 16.5%. By country of origin, the highest percentage of cases was found among the samples from France followed by samples from Brazil. The differences between legs and breast meat were significant. The most frequently isolated serotype of Salmonella was Enteritidis, followed by Salmonella Heidelberg, Salmonella Typhimurium, and Salmonella Virchow. By country of origin, we identified a large percentage of serotype Salmonella Enteritidis in the samples imported from Brazil. There was a greater diversity of serotypes isolated from the French samples, and Salmonella Enteritidis was not the dominant strain. In the samples from the United States, the only serotype isolated was Salmonella Kentucky, although a smaller number of samples was analyzed. The Salmonella Enteritidis phage type that prevailed in both France and Brazil was 4. Phage types 204c and 204 were identified for Salmonella Typhimurium, and phage types 8, 31, and 37 were identified for Salmonella Virchow.

Application of molecular methods for identification of strains classified as Salmonella enterica serovar 6, 7:-:- by conventional serotyping

An increased prevalence of Salmonella enterica serovar Tennessee (6, 7: z(29):-) was observed in broiler flocks in Denmark in 1994 and a parallel increase in the prevalence of Salmonella enterica serovar 6, 7:-:- was demonstrated, albeit at a lower level. Plasmid profiling and ribotyping revealed similar genotypes and it was speculated that serovar 6, 7:-:- could represent a non-motile variant of Salmonella Tennessee. Re-testing of the Salmonella 6, 7:-:- isolates demonstrated the presence of flagella through positive motility. All isolates but one demonstrated motility using both tube tests and light microscopy of overnight broth cultures. Molecular characterization indicated that all but two isolates previously classified as Salmonella 6, 7:-:, were isolates of Salmonella Tennessee and Salmonella Infantis, exhibiting reduced motility. Re-serotyping and multiplex polymerase chain reaction analysis for the phase 2 gene fljB demonstrated variants of Salmonella Infantis (6, 7: r: z(49)) expressing the R-phase antigen (Rz(49)) and possessing the gene for normal phase 2 antigen H: 1, 5. One of the two undefined strains demonstrated genotypic identity with a Salmonella Livingstone reference strain. The remaining putative 6, 7:-:- strain could not be identified and was genuinely non-motile. Diagnostic procedures performed initially were thus insufficient to differentiate between the different levels of motility and also resulted in mis-serotyping. As similar observations were made with two of 14 isolates received from a foreign laboratory, this may represent a general diagnostic problem.

Characterization of south african isolates of Salmonella enteritidis by phage typing, numerical analysis of RAPD-PCR banding patterns and plasmid profiles

Eleven of the 33 strains of Salmonella enteritidis (S.E.) included in this study belonged to phage type 34. Six strains belonged to phage type 14, six strains to phage type 4 and four strains to phage type 7. The remaining six strains belonged to phage types 35, 1, 24var (a variation of phage type 24), 9a, 1b and an unknown phage type. The majority of S.E. phage type 34 strains (eight of the 11) grouped at R2 > or =0.45 into one RAPD-PCR cluster with two strains of phage types 4, a strain of phage type 24var and a strain of phage type 9a, indicating that they consist of a genetically heterogeneous collection of strains. Two of the remaining three phage type 34 strains grouped into two different clusters, well separated from the other phage type 34 strains. One strain of phage type 34 was genetically diverse and did not cluster with any of the strains included in this study. Three of the phage type 14 strains grouped into cluster 11 at R2 > or =0.72, suggesting that they are genetically closely related. However, the remaining three strains of phage type 14 grouped into two separate clusters. Strains of phage types 7, 35, and 1 grouped in one cluster at R2 > or = 0.55. Our results clearly indicated that S.E. strains of the same phage type are not always genetically related. On the other hand, strains of a high genetic relatedness classified as different phage types. No specific plasmid profile could be linked to any of the phage types. Based on results obtained by LD50 virulence tests, strains containing the 38 MDa plasmid are more virulent compared to strains which do not contain the plasmid.

Phage types, antibiotic susceptibilities and plasmid profiles of Salmonella typhimurium and Salmonella enteritidis strains isolated in Istanbul, Turkey

OBJECTIVE

To examine 13 Salmonella typhimurium and 22 S. enteritidis strains isolated from individual cases of gastroenteritis for their phage types, antibiotic susceptibilities and plasmid profiles.

METHODS

The phage typing of S. typhimurium strains was done according to the method of Anderson et al, and the phage typing scheme of Ward et al was used for phage typing of S. enteritidis strains. Antibiotic susceptibility testing was performed by the Kirby-Bauer disk diffusion method. Extended-spectrum beta-lactamase production of the strains was determined by the three-dimensional method. Plasmid profiles of the strains were examined using the method described by Kado and Liu with some modification by Graeber et al.

RESULTS

Two S. typhimurium strains were DT 193 and one was DT 22, whereas 10 strains were untypable. PT 4 was the predominant phage type among S. enteritidis strains. Four S. enteritidis strains were DT 6a, three strains were PT 1 and one strain was PT 8, whereas only one strain was untypable. Eleven of 13 S. typhimurium and three of 22 S. enteritidis strains were found to be multiresistant. Ten different resistance patterns among S. typhimurium and four different resistance patterns among S. enteritidis strains were detected. Extended-spectrum beta-lactamase production was detected in 10 of 13 S. typhimurium and in three of 22 S. enteritidis strains. All S. typhimurium strains but one were found to contain at least one plasmid, with molecular masses varying between 4 and 107 MDa, and 11 different plasmid patterns were determined. Plasmid pattern analysis permitted further differentiation of the S. enteritidis strains into nine groups. A serovar-specific virulence plasmid of 36 MDa was detected in 13 of 22 S. enteritidis strains.

CONCLUSIONS

The results suggest that the majority of S. typhimurium strains were closely related.

Genotypic characterisation by PFGE of Salmonella enterica serotype Enteritidis phage types 1, 4, 6, and 8 isolated from animal and human sources in three European countries

A total of 101 strains of Salmonella Enteritidis phage types (PT) 1, 4, 6, and 8 from Denmark, England and Spain were studied by PFGE to elucidate genetic relationships among strains isolated from animal, human and environmental sources between 1983 and 1997. Analysis with Xba I, Bln I and Spe I enzymes showed that the power of discrimination of this method was increased by the combination of the three enzymes (D=0.802), subdividing the strains into 28 genomic groups or genotypes. Many of the PT1, PT4, and PT6 strains from the three countries shared the same PFGE combination profile A1-A1-A1, confirming the close relationship among these phage types and the protracted spread of a single clone over a large geographical area. In general, strains from Denmark showed more variation in their PFGE profiles than those from England and Spain. PT4 strains exhibited genetic homogeneity in the three countries independently of their sources and period of isolation. Spe I gave the highest index of discrimination among PT6 strains as evidenced by a variety of PFGE profiles. The data clearly confirmed that PT8 strains isolated in the three countries were of a unique clonal origin, and the PFGE combination profile A10-A10-A1 was predominant and specific for this phage type. It is concluded that PFGE, in combination with phage typing, represents a suitable tool for the epidemiological typing of Salmonella Enteritidis strains which could be used for investigations or surveillance of the international spread of these clones.

Subdivision of Salmonella enterica serovar enteritidis phage types PT14b and PT21 by plasmid profiling

We have shown that plasmid profiling is a sensitive method for further identification of strains of Salmonella enterica serovar enteritidis (S. enteritidis) phage type PT21 and to a lesser extent the strains of phage type PT14b. Five and three plasmid types were identified within 33 strains of phage type PT21 and 19 strains of phage type PT14b, respectively. Plasmid types in strains of phage type PT21 showed significant correlation with geographical origin of the strain. In strains of phage type PT14b a single isolate predominated suggesting that the plasmid designated as 'C' can be directly linked with S. enteritidis PT14b strains. Application of IS200 fingerprinting did not reveal any other differences and showed just one copy of IS200 in all the 52 analysed strains. All the strains were tested for antibiotic resistance and only four strains were resistant to ampicillin, cefotaxime, cefuroxime and cotrimoxazole. This indicates that low molecular weight plasmids in Salmonella enteritidis are not responsible for the spread of antibiotic resistance.

Risk factors associated with Salmonella enterica serovar typhimurium infection in Danish broiler flocks

A retrospective longitudinal study was conducted to identify risk factors associated with Salmonella enterica serovar typhimurium (S. typhimurium) infection in Danish broiler flocks. The data included all broiler flocks slaughtered in 1995, and the epidemiological unit was the individual broiler flock. The S. typhimurium status was determined by microbiological examination of 60 fresh fecal samples. This procedure should detect an infected flock with a probability above 95%, if the prevalence is above 5%, and given that the sensitivity of the test is 100%. Nineteen variables were selected for analysis. Five factors and an interaction term were found significant by multivariate logistic regression analysis. An increased risk for S. typhimurium infection was associated with two parent flocks, one confirmed infected and one suspected of being infected with S. typhimurium, with two of the hatcheries, and with five houses on the farm. An interaction between season and the previously mentioned hatcheries, and a random effect at farm level was also found to be statistically significant. Twelve variables were not found to be associated with S. typhimurium infection: medication, growth promoters, breed of the laying flock, animal density, size of the flock, area of the house, age of the house, geographical location of the farm, observation of beetles, number of days between disinfection and replacement, visual appearance of the bedding, and age of the chickens when they were tested for Salmonella. Three variables (feed mill, slaughterhouse, and Salmonella status of the preceding flock) were not evaluated in the multivariate analysis due to collinearity with other included variables.

Distribution of serotypes of Campylobacter jejuni and C. coli from Danish patients, poultry, cattle and swine

The number of human cases of enteritis caused by Campylobacter jejuni and C. coli is increasing in Denmark and other European countries. No systemic typing has earlier been performed on Campylobacter isolates of Danish origin. The primary purpose of this study was to provide a serotype distribution of Campylobacter isolates from Danish patients and the major food production animals. In addition, the occurrence of intestinal carriers of thermophilic campylobacters among these food production animals was examined. In a nationwide survey, the individual isolation rate was 36% for broiler chickens, 47% for cattle and 46% for swine when sampled at the slaughterhouse. C. jejuni accounted for 83-91% of the thermophilic Campylobacter spp. in broiler chickens and cattle, whereas 95% of the isolates from swine was C. coli. In human patients with Campylobacter enteritis, 94% of the isolates were C. jejuni and 6% were C. coli. Heat-stable serotyping (the 'Penner scheme') was performed on a total of 398 isolates from the four sources: human patients (n = 145), broiler chickens (n = 94), swine (n = 111) and cattle (n = 48). Among human isolates, serotype O:1,44, O:2 and the O:4-complex accounted for 62% of the C. jejuni isolates. These serotypes were also common in samples from broilers and cattle. In swine, C. coli O:30 and O:46 were most common. The serotype distribution of human clinical isolates showed large overlap with the serotype distribution of campylobacters in cattle and chickens, and on this basis both could be major sources of human campylobacteriosis.

Molecular epidemiology of antibiotic resistance of Salmonella enteritidis during a 7-year period in Greece

A significant increase in the frequency of isolation of Salmonella enteritidis has been observed during recent years in Greece, parallelled by an increasing rate of resistance of this organism to antibiotics. A substantial proportion of ampicillin- and doxycycline-resistant isolates exhibited cross-resistance to drugs of other classes, such as sulfonamides and streptomycin. Isolates of human origin were overall less resistant than those of animal or food-feed origin. Indeed, strains associated with animal infections were characterized by the highest rates of resistance to several antibiotics. These phenotypic data were correlated with genotypic information concerning two distinct populations: isolates from all sources that were resistant only to ampicillin, the drug toward which resistance rates were highest, and a control group of sensitive isolates. Ampicillin resistance was due to a 34-MDa conjugative plasmid. DNA fingerprinting by macrorestriction of genomic DNA revealed two types, A and B, common to both ampicillin-resistant and -sensitive strains, with 80 to 90% of strains being of type A. However, a third type, C, was specific for the sensitive population, representing 17% of those strains. Therefore, although the majority of resistant isolates were genetically related to sensitive ones, there existed a susceptible clone which had not acquired any resistance traits.

Correlation of conversion of Salmonella enterica serovar enteritidis phage type 1, 4, or 6 to phage type 7 with loss of lipopolysaccharide

Studies of pairs of Salmonella enterica serovar Enteritidis isolates from different poultry flocks showed that phage type (PT) 7 may be derived from PT 1, 4, and 6. The conversion appeared to be associated with loss of lipopolysaccharide. It is concluded that PT 7 may be of little value as an epidemiological marker of S. enterica serovar Enteritidis.