Development and evaluation of a multiple-locus variable-number tandem-repeats analysis assay for subtyping Salmonella Typhi strains from sub-Saharan Africa

PURPOSE

Molecular epidemiological investigations of the highly clonal Salmonella enterica subspecies enterica serovar Typhi (S. Typhi) are important in outbreak detection and in tracking disease transmission. In this study, we developed and evaluated a multiple-locus variable-number tandem-repeats (VNTR) analysis (MLVA) assay for characterization of S. Typhi isolates from sub-Saharan Africa.

METHODOLOGY

Twelve previously reported VNTR loci were evaluated and an MLVA assay consisting of five polymorphic loci was adopted. The MLVA assay was developed for use on capillary electrophoresis systems by testing a collection of 50 S. Typhi isolates. This S. Typhi strain panel consisted of six outbreak related isolates and 44 epidemiologically unlinked isolates. Amongst these were nine S.Typhi haplotype H58 isolates.

RESULTS

The MLVA assay characterized the 50 isolates into 47 MLVA profiles while PFGE analysis of the same isolates revealed 34 pulsotypes. MLVA displayed higher discriminatory power (Simpson's index of diversity (DI) 0.998 [95 % confidence interval (CI) 0.995-1.000)] as compared to pulsed-field gel electrophoresis [Simpson's DI 0.984 (95 % CI 0.974-0.994)].

CONCLUSION

The MLVA assay presented in this study is a simple, rapid and more accessible tool that serves as a good alternative to other molecular subtyping methods for S. Typhi.

Molecular Subtyping of Salmonella enterica Serovar Typhi by Pulsed-Field Gel Electrophoresis and Multiple-Locus Variable-Number Tandem-Repeat Analysis in India: Their Association with Antimicrobial Resistance Profiles

Molecular subtyping and DNA sequencing-based methods, which are commonly used for discriminating Salmonella enterica serovar Typhi (S. Typhi) isolates, lead to improved molecular epidemiological investigations for prevention and control of typhoid fever. We obtained S. Typhi blood isolates (n = 66) from India during 2007-14 for molecular subtyping by pulsed-field gel electrophoresis (PFGE) and multiple-locus variable-number tandem-repeat (VNTR) analysis (MLVA) in association with antibiotic resistance profiles. Genotypic diversity was observed more by MLVA (Simpson's index of diversity, D value = 0.997) than PFGE (D value = 0.864). Two prevalent pulsotypes containing nalidixic acid-resistant (NAL^R) and NAL^R-ciprofloxacin-resistant (CIP^R) S. Typhi isolates circulated in India. Multidrug-resistant (MDR), NAL^R-CIP^R, and most NAL^R isolates were found to be clonal by PFGE. MLVA could differentiate the clonal isolates. Most of the MDR and NAL^R-CIP^R isolates showed variation in single or double VNTR loci, whereas NAL^R isolates varied in more than 2 loci, reflecting higher genetic diversity among the NAL^R isolates. Of the 6 VNTR loci, TR4,699 (D value = 0.838) and Sal02 (D value = 0.890) loci played important roles as MLVA cluster-supporting alleles. The rapid turnaround time and high-level discriminatory power of MLVA may be useful for tracking and controlling the transmission of S. Typhi isolates during epidemiological investigations.

Metal nanoparticle assisted polymerase chain reaction for strain typing of Salmonella Typhi

Salmonella enterica serotype Typhi (S. Typhi) is the causative agent of typhoid fever and remains a major health threat in most of the developing countries. The prompt diagnosis of typhoid directly from the patient's blood requires high level of sensitivity and specificity. Some of us were the first to report PCR based diagnosis of typhoid. This approach has since then been reported by many scientists using different genomic targets. Since the number of bacteria circulating in the blood of a patient can be as low as 0.3 cfu ml(-1), there is always a room for improvement in diagnostic PCR. In the present study, the role of different types of nanoparticles was investigated to improve the existing PCR based methods for diagnosis and strain typing of S. Typhi (targeting Variable Number of Tandem Repeats [VNTR]) by using optimized PCR systems. Three different types of nanoparticles were used i.e., citrate stabilized gold nanoparticles, rhamnolipid stabilized gold and silver nanoparticles, and magnetic iron oxide nanoparticles. The non-specific amplification was significantly reduced in VNTR typing when gold and silver nanoparticles were used in an appropriate concentration. More importantly, the addition of nanoparticles decreased the non-specificity to a significant level in the case of multiplex PCR thus further validating the reliability of PCR for the diagnosis of typhoid.

Genotyping of Salmonella Typhi using 8-loci multi locus VNTR analysis

Background

Typhoid fever has caused severe epidemics in many Asian and African countries. The early detection of outbreaks and their sources may promote the prevention and control of typhoid fever, for which effective and timely molecular subtyping techniques are required. Pulsed field gel electrophoresis (PFGE) is routinely used as the molecular typing technique for foodborne and waterborne pathogens. However, maneuverable techniques remain necessary to expedite the experimental procedure and obtain more effective subtyping. The multilocus loci of a variable number of tandem repeats (VNTR) analysis (MLVA) is a polymerase chain reaction (PCR)-based subtyping method.

Methods

MLVA method and PFGE based on Xba I enzyme were applied to the 103 Salmonella Typhi (S. Typhi) isolated from different years and regions. Dendrograms were constructed and analyzed to help understand the data. The Simpson’s index of diversity (D value) was calculated to estimate the discriminatory power of MLVA and PFGE. In addition, a set of endogenous 3 bp DNA ladder markers were established to accurately determine the repeat copy number of the VNTR with only a 3 bp repetitive unit, using microfluidics chip-based electrophoresis to generate comparable VNTR data in the public health laboratory network.

Results

The established 8-loci MLVA for S. Typhi subtyping had higher discriminatory power than PFGE. In some cases, PFGE could not distinguish the strains isolated over long intervals and with different epidemic provinces. By contrast, 8-loci MLVA distinctly distinguished these strains, and the strains with the same MLVA patterns were from the same or contiguous years and the same province, showing its significance in epidemiological discrimination. The established set of endogenous 3 bp DNA ladder markers improved the accuracy and reproducibility of VNTR analysis using microfluidics chip-based electrophoresis to 100 %.

Conclusions

Eight VNTRs can be used for the MLVA analysis of the 103 S. Typhi isolates. MLVA based on the 8-loci had higher discriminatory power than PFGE for S. Typhi subtyping. The 8-loci MLVA is easier for the analysis and interpretation of relationships between strains compared to PFGE.

Electronic supplementary material

The online version of this article (doi:10.1186/s13099-016-0094-4) contains supplementary material, which is available to authorized users.

Antimicrobial resistance in Salmonella enterica Serovar Typhi isolates from Bangladesh, Indonesia, Taiwan, and Vietnam

We characterized Salmonella enterica serovar Typhi isolates from Bangladesh, Indonesia, Taiwan, and Vietnam to investigate their genetic relatedness and antimicrobial resistance. The isolates from Bangladesh and Vietnam were genetically closely related but were distant from those from Indonesia and Taiwan. All but a few isolates from Indonesia and Taiwan were susceptible to all antimicrobials tested. The majority of isolates from Bangladesh and Vietnam were multidrug resistant (MDR) and belonged to the widespread haplotype H58 clone. IncHI1 plasmids were detected in all MDR S. Typhi isolates from Vietnam but in only 15% of MDR isolates from Bangladesh. Resistance genes in the majority of MDR S. Typhi isolates from Bangladesh should reside in the chromosome. Among the isolates from Bangladesh, 82% and 40% were resistant to various concentrations of nalidixic acid and ciprofloxacin, respectively. Several resistance mechanisms, including alterations in gyrase A, the presence of QnrS, and enhanced efflux pumps, were involved in the reduced susceptibility and resistance to fluoroquinolones. Intensive surveillance is necessary to monitor the spread of chromosome-mediated MDR and fluoroquinolone-resistant S. Typhi emerging in Bangladesh.

Analysis of Salmonella enterica serovar Typhimurium variable-number tandem-repeat data for public health investigation based on measured mutation rates and whole-genome sequence comparisons

Variable-number tandem repeats (VNTRs) mutate rapidly and can be useful markers for genotyping. While multilocus VNTR analysis (MLVA) is increasingly used in the detection and investigation of food-borne outbreaks caused by Salmonella enterica serovar Typhimurium (S. Typhimurium) and other bacterial pathogens, MLVA data analysis usually relies on simple clustering approaches that may lead to incorrect interpretations. Here, we estimated the rates of copy number change at each of the five loci commonly used for S. Typhimurium MLVA, during in vitro and in vivo passage. We found that loci STTR5, STTR6, and STTR10 changed during passage but STTR3 and STTR9 did not. Relative rates of change were consistent across in vitro and in vivo growth and could be accurately estimated from diversity measures of natural variation observed during large outbreaks. Using a set of 203 isolates from a series of linked outbreaks and whole-genome sequencing of 12 representative isolates, we assessed the accuracy and utility of several alternative methods for analyzing and interpreting S. Typhimurium MLVA data. We show that eBURST analysis was accurate and informative. For construction of MLVA-based trees, a novel distance metric, based on the geometric model of VNTR evolution coupled with locus-specific weights, performed better than the commonly used simple or categorical distance metrics. The data suggest that, for the purpose of identifying potential transmission clusters for further investigation, isolates whose profiles differ at one of the rapidly changing STTR5, STTR6, and STTR10 loci should be collapsed into the same cluster.

Multiple-locus variable number of tandem repeats analysis of Salmonella enterica serotype paratyphi A from Yuxi and comparison with isolates from the Chinese Medical Culture Collection Center

The aim of the present study was to genotype Salmonella enterica serotype paratyphi A (SPA) isolated from Yuxi, China, in a multiple-locus variable number of tandem repeats (VNTRs) analysis (MLVA) and to compare them with isolates from the Chinese Medical Culture Collection Center (CMCC). Potential VNTRs were screened from the genomes of ATCC9150 and AKU_12601 using the Tandem Repeats Finder program. Nine VNTRs were established for MLVA typing of 195 SPA isolates from Yuxi and 20 isolates from CMCC. The dendogram for MLVA profiles and minimum spanning tree (MST) were drawn using the categorical coefficient calculated by BioNumerics software. A total of 23 MLVA types were identified in 215 SPA isolates and were grouped into six distinct cluster groups A, B, C, D, E and F. A total of 195 Yuxi SPA isolates were exclusively grouped into cluster C with nine MLVA genotypes. A total of 20 CMCC isolates were grouped in clusters A B, D, E and F with the other 14 MLVA types. The MLVA with nine VNTR loci, which was exploited in the present study, represents a successful strategy for genotyping SPA. Furthermore, the 195 Yuxi isolates appear to be closely related to each other and distinct from the 20 CMCC strains.

Diversity of Salmonella enterica serovar Typhi strains collected from india using variable number tandem repeat (VNTR)-PCR analysis

BACKGROUND

Typhoid fever is endemic in India, and a seasonal increase of cases is observed annually. In spite of effective therapies and the availability of vaccines, morbidity is widespread owing to the circulation of multiple genetic variants, frequent migration of asymptomatic carriers, unhygienic food practices and the emergence of multidrug resistance and thus continues to be a major public health problem in developing countries, particularly in India. Classical methods of strain typing such as pulsed-field gel electrophoresis, ribotyping, random amplification of polymorphic DNA and amplified fragment length polymorphism are either laborious and technically complicated or less discriminatory.

METHODS

We investigated the molecular diversity of Indian strains of Salmonella enterica serovar Typhi (S. Typhi) isolated from humans from different parts of India to establish the molecular epidemiology of the organism using the variable number tandem repeat (VNTR)-PCR analysis. The electrophoretic band pattern was analysed using the GelCompar II software program.

RESULTS

Of the 94 strains tested for three VNTRs loci, 75 VNTR genotypes were obtained. Of the three VNTRs tested in this study, VNTR1 was amplified in all the strains except one and found to be predominant. VNTR2 was amplified only in 57 strains with a Simpson diversity index of 0.93 indicating the high variability of this region within the strains. VNTR3 was amplified in 90 strains. The discriminatory power of this typing tool has been greatly enhanced by this VNTR2 region as the other two regions could not discriminate strains significantly. In our study, about 55 % of the strains amplified all three VNTR regions and 39 % of the strains lacked the VNTR2 region. Among the three VNTR regions tested, the majority of the strains produced similar banding pattern for any two regions grouped into a cluster. The strains grouped as a genotype were from the same geographical location. Strains collected from each geographical region were also highly heterogeneous.

CONCLUSION

Such analysis is important to identify the genetic clones of the pathogen associated with sporadic infections and disease outbreak to identify the common source and implement public health measures.

Salmonella source attribution based on microbial subtyping

Source attribution of cases of food-borne disease represents a valuable tool for identifying and prioritizing effective food-safety interventions. Microbial subtyping is one of the most common methods to infer potential sources of human food-borne infections. So far, Salmonella microbial subtyping source attribution models have been implemented by using serotyping and phage-typing data. Molecular-based methods may prove to be similarly valuable in the future, as already demonstrated for other food-borne pathogens like Campylobacter. This review assesses the state of the art concerning Salmonella source attribution through microbial subtyping approach. It summarizes the available microbial subtyping attribution models and discusses the use of conventional phenotypic typing methods, as well as of the most commonly applied molecular typing methods in the European Union (EU) laboratories in the context of their potential applicability for Salmonella source attribution studies.

Antimicrobial resistance in invasive non-typhoid Salmonella from the Democratic Republic of the Congo: emergence of decreased fluoroquinolone susceptibility and extended-spectrum beta lactamases

Background

Co-resistance against the first-line antibiotics ampicillin, chloramphenicol and trimethoprim/sulphamethoxazole or multidrug resistance (MDR) is common in non typhoid Salmonella (NTS). Use of alternative antibiotics, such as fluoroquinolones or third generation cephalosporins is threatened by increasing resistance, but remains poorly documented in Central-Africa.

Methodology/Principal findings

As part of a microbiological surveillance study in DR Congo, blood cultures were collected between 2007 and 2011. Isolated NTS were assessed for serotype and antimicrobial resistance including decreased ciprofloxacin susceptibility and extended-spectrum beta-lactamase (ESBL) production. In total, 233 NTS isolates (representing 23.6% of clinically significant organisms) were collected, mainly consisting of Salmonella Typhimurium (79%) and Salmonella Enteritidis (18%). The majority of NTS were isolated in the rainy season, and recovered from children ≤2 years old. MDR, decreased ciprofloxacin susceptibility, azithromycin and cefotaxime resistance were 80.7%, 4.3%, 3.0% and 2.1% respectively. ESBL production was noted in three (1.3%) isolates. Decreased ciprofloxacin susceptibility was associated with mutations in codon 87 of the gyrA gene, while ESBLs all belonged to the SHV-2a type.

Conclusions/Significance

Presence of almost full MDR among NTS isolates from blood cultures in Central Africa was confirmed. Resistance to fluoroquinolones, azithromycin and third generation cephalosporins is still low, but emerging. Increased microbiological surveillance in DR Congo is crucial for adapted antibiotic therapy and the development of treatment guidelines.

Invasive non typhoid Salmonella spp. (NTS) are an important cause of bloodstream infection in sub-Saharan Africa and associated with a high mortality. Levels of multidrug resistance have become alarmingly high. Treatment therefore increasingly relies on the oral fluoroquinolones such as ciprofloxacin, with third generation cephalosporins such as cefotaxime as alternatives for parenteral treatment. Azithromycin represents another alternative antimicrobial drug. Worldwide, increased use of these drugs is associated with spread of resistance as well, a phenomenon poorly documented in Central-Africa. In the present study, 233 NTS isolates were collected from blood cultures sampled between 2007 and 2011 in DR Congo, mainly from children ≤2 years of age. Most isolates were recovered during the rainy season. Widespread multidrug resistance was confirmed as well as decreased susceptibility to ciprofloxacin, resistance to azithromycin and resistance to third generation cephalosporins. Our findings demonstrate emergence of antibiotic resistance among NTS in DR Congo and underline the need for increased microbiological surveillance, being a prerequisite for rational antibiotic therapy and the development of standard treatment guidelines.

Salmonella enterica serovar Typhi variants in long-term carriers

Long-term typhoid carriers can simultaneously excrete Salmonella enterica serovar Typhi variants with considerable genetic differences, a situation that complicates the interpretation of the subtyping data used in outbreak investigations and disease surveillance.

Salmonella typhi in the democratic republic of the congo: fluoroquinolone decreased susceptibility on the rise

BACKGROUND

Drug resistance of Salmonella enterica serovar Typhi (Salmonella Typhi) to first-line antibiotics is emerging in Central Africa. Although increased use of fluoroquinolones is associated with spread of resistance, Salmonella Typhi with decreased ciprofloxacin susceptibility (DCS) has rarely been reported in Central Africa.

METHODOLOGY/PRINCIPAL FINDINGS

As part of a microbiological surveillance study in the Democratic Republic of the Congo (DR Congo), Salmonella Typhi isolates from bloodstream infections were collected prospectively between 2007 and 2011. The genetic relationship of the Salmonella Typhi isolates was assessed by pulsed-field gel electrophoresis (PFGE). The antimicrobial resistance profile of the isolates was determined and mutations associated with DCS were studied. In total, 201 Salmonella Typhi isolates were collected. More than half of the Salmonella Typhi isolates originated from children and young adults aged 5-19. Thirty different PFGE profiles were identified, with 72% of the isolates showing a single profile. Multidrug resistance, DCS and azithromycin resistance were 30.3%, 15.4% and 1.0%, respectively. DCS was associated with point mutations in the gyrA gene at codons 83 and 87.

CONCLUSIONS/SIGNIFICANCE

Our study describes the first report of widespread multidrug resistance and DCS among Salmonella Typhi isolates from DR Congo. Our findings highlight the need for increased microbiological diagnosis and surveillance in DR Congo, being a prerequisite for rational use of antimicrobials and the development of standard treatment guidelines.