Antibiotic resistance and genotyping of clinical group B Salmonella isolated in Accra, Ghana

Antimicrobial resistance national level dialogue and action in Ghana: setting and sustaining the agenda and outcomes

BACKGROUND

Antimicrobial resistance (AMR) has gained national and international attention. The design and launch of national policy on antimicrobial use and resistance and action plan marked a milestone in Ghana's commitment to control AMR. These strategies are some outcomes of getting and sustaining AMR issues prominence on government's agenda. Understanding the agenda setting processes, policy actors involved and policy change is important as this provides insights on how and why policy actors defined and framed AMR issues to sustain its prominence despite the changing priorities of government agenda.

OBJECTIVE

To examine the processes of setting and sustaining AMR issues on government agenda, the policy actors involved and resulting outcomes.

METHODS

A qualitative study was conducted and data collected through interviewing twenty-four respondents and reviewing technical working group meeting reports and health sector documents. Data was analysed drawing on Kingdon's agenda setting framework.

RESULT

Members of a multisectoral technical working group (AMR platform) formed in 2011 constantly built consensus on AMR problem definition, solutions and actively engaged decision makers to mobilise support and interest. The AMR platform members sustained AMR attention and prominence on government's agenda through the following multisectoral coordination mechanisms: (1) institutionalising AMR platform activities (2) gathering evidence, sharing findings, and supporting research (3) creating awareness and training (4) gaining and maintaining political support. The activities of the AMR platform contributed to three remarkable outcomes and these are (1) maintained network of AMR Champions, (2) design of a national policy on antimicrobial use and resistance in Ghana (1st edition) and national action plan (2017-2021), and (3) Ghana's hosting of the second Global call to action on AMR.

CONCLUSION

The AMR platform members as influencers concentrated their efforts to move and sustain AMR issues on government agenda. The identified multisectoral coordination mechanisms collectively contributed to agenda setting processes and policy change. The AMR platform engagements are ongoing and it is important the momentum is maintained. As multisectoral coordination and activities are vital especially for AMR 'One Health' approach, we hope this paper presents lessons for better understanding of how and why multisectoral groups influence national level agenda setting processes.

Assessment of the effects of seasonal changes, urban discharges and leachates on the parasitological and bacteriological qualities of soil and water from Oued Larbaâ (North-eastern, Morocco)

The objective of this study is to evaluate the parasitological and bacteriological qualities of the soil and water of the Oued Larbaâ, through nine stations covering the part of the Oued crossing the Taza city of Morocco (7 km of length). This study was conducted during two dry periods (2017 and 2018) and one wet period (2018). Soil parasitological studies revealed high contamination by helminths belonging to the species Enterobius vermicularis and Toxocara sp., which recorded the highest concentrations in stations S2 and S8, respectively, with values of 11.41 and 29.26 eggs/g. Concerning water, the majority of stations indicated contamination mainly by Ascaris lumbricoides, which reached concentrations of 56.49, 37.39, and 56.15 eggs/l in stations S3, S6, and S8, respectively. Bacteriological examination of the soil showed very high concentrations of bacteria, especially Streptococcus, due to the high retention of this type of bacteria in the soil. The water analysis indicated that stations S3, S4, and S8 are the most impacted, with total coliform and Streptococcus concentrations in station S3 of 2.45 10⁵ and 0.20 10⁷ CFU/ml, respectively, Fecal coliforms and Staphylococci are detected in high concentrations in station S8 with values of 1.35 10⁴ and 8.92 10⁶ CFU/ml, respectively, and total aerobic mesophilic flora in station S4 with a concentration of 1.89 10⁹ CFU/ml. During the study periods, influences of sampling season, nature of discharge existed (wastewater or leachate) on the parasitological and bacteriological composition of the river were established with high concentrations during the dry period and in the stations in which large amounts of liquid discharge were observed.

Exposure of consumers to substandard antibiotics from selected authorised and unauthorised medicine sales outlets in Ghana

OBJECTIVE

To assess the quality of antibiotics sampled from authorised sales outlets (ATs) (i.e. hospitals/health centres, pharmacies and licensed chemical shops) and unauthorised sales outlets (UATs) (mainly street vendors) in Ghana and to explore the health-seeking behaviour of medicine consumers.

METHODS

The contents of 14 active pharmaceutical ingredients (APIs) in 348 sampled products were determined using a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. Data on health-seeking practices were collected through entry and exit interviews and field observations from ATs and UATs.

RESULTS

It was observed that 66.38% of all sampled antibiotic products were substandard; they either contained less (<90%) or more API (>110%) than the label claim. Medicines from UATs recorded substantially less API contents than those from ATs (F_(2,419)  = 43.01, P < 0.0001). For example, 90.54% of street vendor samples contained < 90% of the APIs. 75.93% of consumers often sought self-treatment with drugs without a prescription from UATs, as they perceived UATs as easily accessible, trustworthy and knowledgeable, and their medicines as inexpensive. These consumers rather thought of the formal healthcare providers as alternative sources.

CONCLUSIONS

Consumers who purchase from UATs are at high risk of receiving substandard medicines. The quality of medicines in the national healthcare system, in the supply chain and in the distribution system needs to be monitored regularly to reduce the incidence of substandard medicines and their impact on antimicrobial resistance. The fight against substandard medicines needs to incorporate a full understanding of socioeconomic factors that drive consumer decisions regarding their health and choice of healthcare providers.

Antibiotic resistance patterns in human, animal, food and environmental isolates in Ghana: a review

Many articles have been published on resistant microorganisms isolated from humans, animals, foods and the environment in Ghana. However, there are no reviews that summarize the information on the isolates and antibiotics tested so far in the country. This literature review was completed through “PubMed” and “Google Scholar” searches. We included publications from the period 1975-2015 with a laboratory-based methodology to determine antibiotic resistance of strains isolated in Ghana. In total, 60 articles were included in the analysis with 10% of the articles carrying out nationwide research on antibiotic resistance. The regions of Ghana with the highest published articles were Greater Accra (40%), Ashanti (21.7%) and Northern Region (10%). Most of the studies (86.7%) were related to isolates collected from human samples followed by environmental (5%), animal (3%) and food samples (2%). Ten different bacteria genera were observed in the studies. The most common was Escherichia coli, followed by Staphylococcus spp., Mycobacterium spp. and Streptococcus spp. The highest mean resistance rate was encountered in Escherichia coli (62.2%) followed by Klebsiella spp. (60.4%) and Pseudomonas spp. (52.1%). High resistance rates have been found in Ghana, however, the data are skewed and some regions of the country have been neglected. There is a need for higher quality research to establish and monitor resistance patterns in Upper West, Brong-Ahafo, Volta and Eastern Regions of Ghana.

A cross-sectional study on antibiotic prescription in a teaching hospital in Ghana

Introduction

Antibiotic misuse is the paramount factor for antibiotic resistance. Tamale Teaching Hospital (TTH), located in Ghana’s Northern Region, is the biggest tertiary hospital in the Northern half of the country and consequently one of the biggest prescribers of antibiotics. Understanding the use of antibiotics in the TTH and providing information that could be inferred to develop strategies for antibiotic prescription is of extreme importance in this era of multiple and pan-resistant strains of pathogenic microorganisms.

Methods

A cross-sectional study on the use of antibiotics at TTH in the Northern region of Ghana was performed. Data were collected by reviewing 10% of patients’ files from January to June 2015 and then assessed for its appropriateness against the criteria based on the British National Formulary (BNF) 2015 and BNF children 2013-2014. Results were expressed in frequencies and percentages.

Results

A total of 617 patients’ records were included in this study. Up to 385 cases of different antibiotic misuse were found, comprising of 335 errors in prescriptions and 50 non-completed treatments. The most common prescription error was made on treatment duration (29.6%). The potential interactions were 16.7%.

Conclusion

The study revealed a high burden of antibiotics misuse in TTH. This suggests a need for the development of an antibiotic stewardship programme for the hospital.

The Use of a Combined Bioinformatics Approach to Locate Antibiotic Resistance Genes on Plasmids From Whole Genome Sequences of Salmonella enterica Serovars From Humans in Ghana

In the current study, we identified plasmids carrying antimicrobial resistance genes in draft whole genome sequences of 16 selected Salmonella enterica isolates representing six different serovars from humans in Ghana. The plasmids and the location of resistance genes in the genomes were predicted using a combination of PlasmidFinder, ResFinder, plasmidSPAdes and BLAST genomic analysis tools. Subsequently, S1-PFGE was employed for analysis of plasmid profiles. Whole genome sequencing confirmed the presence of antimicrobial resistance genes in Salmonella isolates showing multidrug resistance phenotypically. ESBL, either blaTEM52-B or blaCTX-M15 were present in two cephalosporin resistant isolates of S. Virchow and S. Poona, respectively. The systematic genome analysis revealed the presence of different plasmids in different serovars, with or without insertion of antimicrobial resistance genes. In S. Enteritidis, resistance genes were carried predominantly on plasmids of IncN type, in S. Typhimurium on plasmids of IncFII(S)/IncFIB(S)/IncQ1 type. In S. Virchow and in S. Poona, resistance genes were detected on plasmids of IncX1 and TrfA/IncHI2/IncHI2A type, respectively. The latter two plasmids were described for the first time in these serovars. The combination of genomic analytical tools allowed nearly full mapping of the resistance plasmids in all Salmonella strains analyzed. The results suggest that the improved analytical approach used in the current study may be used to identify plasmids that are specifically associated with resistance phenotypes in whole genome sequences. Such knowledge would allow the development of rapid multidrug resistance tracking tools in Salmonella populations using WGS.

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.

A retrospective audit of antibiotic prescriptions in primary health-care facilities in Eastern Region, Ghana

Resistance to antibiotics is increasing globally and is a threat to public health. Research has demonstrated a correlation between antibiotic use and resistance development. Developing countries are the most affected by resistance because of high infectious disease burden, limited access to quality assured antibiotics and more optimal drugs and poor antibiotic use practices. The appropriate use of antibiotics to slow the pace of resistance development is crucial. The study retrospectively assessed antibiotic prescription practices in four public and private primary health-care facilities in Eastern Region, Ghana using the WHO/International Network for the Rational Use of Drugs rational drug use indicators. Using a systematic sampling procedure, 400 prescriptions were selected per facility for the period April 2010 to March 2011. Rational drug use indicators were assessed in the descriptive analysis and logistic regression was used to explore for predictors of antibiotic prescription. Average number of medicines prescribed per encounter was 4.01, and 59.9% of prescriptions had antibiotics whilst 24.2% had injections. In total, 79.2% and 88.1% of prescribed medicines were generics and from the national essential medicine list, respectively. In the multivariate analysis, health facility type (odds ratio [OR] = 2.05; 95% confidence interval [CI]: 1.42, 2.95), patient age (OR = 0.97; 95% CI: 0.97, 0.98), number of medicines on a prescription (OR = 1.85; 95% CI: 1.63, 2.10) and 'no malaria drug' on prescription (OR = 5.05; 95% CI: 2.08, 12.25) were associated with an antibiotic prescription. A diagnosis of upper respiratory tract infection was positively associated with antibiotic use. The level of antibiotic use varied depending on the health facility type and was generally high compared with the national average estimated in 2008. Interventions that reduce diagnostic uncertainty in illness management should be considered. The National Health Insurance Scheme, as the main purchaser of health services in Ghana, offers an opportunity that should be exploited to introduce policies in support of rational drug use.

Vaccination with a single CD4 T cell peptide epitope from a Salmonella type III-secreted effector protein provides protection against lethal infection

Salmonella infections affect millions worldwide and remain a significant cause of morbidity and mortality. It is known from mouse studies that CD4 T cells are essential mediators of immunity against Salmonella infection, yet it is not clear whether targeting CD4 T cell responses directly with peptide vaccines against Salmonella can be effective in combating infection. Additionally, it is not known whether T cell responses elicited against Salmonella secreted effector proteins can provide protective immunity against infection. In this study, we investigated both of these possibilities using prime-boost immunization of susceptible mice with a single CD4 T cell peptide epitope from Salmonella secreted effector protein I (SseI), a component of the Salmonella type III secretion system. This immunization conferred significant protection against lethal oral infection, equivalent to that conferred by whole heat-killed Salmonella bacteria. Surprisingly, a well-characterized T cell epitope from the flagellar protein FliC afforded no protection compared to immunization with an irrelevant control peptide. The protective response appeared to be most associated with polyfunctional CD4 T cells raised against the SseI peptide, since no antibodies were produced against any of the peptides and very little CD8 T cell response was observed. Overall, this study demonstrates that eliciting CD4 T cell responses against components of the Salmonella type III secretion system can contribute to protection against infection and should be considered in the design of future Salmonella subunit vaccines.

Studies on the bacteriological qualities of the Buffalo River and three source water dams along its course in the Eastern Cape Province of South Africa

The Buffalo River and its dams are major surface water sources used for fresh produce irrigation, raw water abstraction and recreation in parts of the Eastern Cape Province in South Africa. Over a 12-month period (August 2010 to July 2011), we assessed the bacteriological qualities of water from the river and 3 source water dams along its course. Faecal indicator bacteria (FIB), including total coliform (TC), faecal coliform (FC) and enterococci (ENT) counts, were high and ranged as follows: 1.9 × 10(2)-3.8 × 10(7), 0-3.0 × 10(5) and 0-5.3 × 10(5) cfu/100 ml for TC, FC and ENT, respectively. Significantly (P<0.05) higher concentrations of FC and ENT were observed at the sampling sites located at the lower reaches of the river compared to the upper reaches, and at Bridle Drift Dam compared to the other two dams. FIB counts mostly exceeded the recommended maximum values suggested by national and international guidelines for safe fresh produce irrigation, domestic applications, full-contact recreation and livestock watering. These results show that the bacteriological qualities of the Buffalo River and dams were poor, and suggest that sewage was dumped into the Buffalo River during the study period. Urban runoffs and effluents of wastewater treatment plants appear to be important sources of faecal contamination in the river. We conclude that these water bodies represent significant public health hazards. Provision of adequate sanitary infrastructure will help prevent source water contamination, and public health education aimed at improving personal, household and community hygiene is imperative.

Incidence and characteristics of bacteremia among children in rural Ghana

The objective of the study was to describe systemic bacterial infections occurring in acutely ill and hospitalized children in a rural region in Ghana, regarding frequency, incidence, antimicrobial susceptibility patterns and associations with anthropometrical data.

Blood cultures were performed in all children below the age of five years, who were admitted to Agogo Presbyterian Hospital (APH), Asante Region, Ghana, between September 2007 and July 2009. Medical history and anthropometrical data were assessed using a standardized questionnaire at admission. Incidences were calculated after considering the coverage population adjusted for village-dependent health-seeking behavior.

Among 1,196 hospitalized children, 19.9% (n = 238) were blood culture positive. The four most frequent isolated pathogens were nontyphoidal salmonellae (NTS) (53.3%; n = 129), Staphylococcus aureus (13.2%; n = 32), Streptococcus pneumoniae (9.1%; n = 22) and Salmonella ser. Typhi (7.0%; n = 17). Yearly cumulative incidence of bacteremia was 46.6 cases/1,000 (CI 40.9–52.2). Yearly cumulative incidences per 1,000 of the four most frequent isolates were 25.2 (CI 21.1–29.4) for NTS, 6.3 (CI 4.1–8.4) for S. aureus, 4.3 (CI 2.5–6.1) for S. pneumoniae and 3.3 (CI 1.8–4.9) for Salmonella ser. Typhi. Wasting was positively associated with bacteremia and systemic NTS bloodstream infection. Children older than three months had more often NTS bacteremia than younger children. Ninety-eight percent of NTS and 100% of Salmonella ser. Typhi isolates were susceptible to ciprofloxacin, whereas both tested 100% susceptible to ceftriaxone. Seventy-seven percent of NTS and 65% of Salmonella ser. Typhi isolates were multi-drug resistant (MDR). Systemic bacterial infections in nearly 20% of hospitalized children underline the need for microbiological diagnostics, to guide targeted antimicrobial treatment and prevention of bacteremia. If microbiological diagnostics are lacking, calculated antimicrobial treatment of severely ill children in malaria-endemic areas should be considered.

Resistance to antimicrobial drugs in Ghana

Background

Antimicrobial drug resistance is a global issue that affects health, economic, and social development. The problem has been attributed to misuse of antimicrobial agents.

Purpose

To identify the agents of bacterial infection in Ghana, determine their antibiogram, and the possibility of setting up a surveillance program.

Patients and methods

A prospective quantitative study set in various hospitals including two teaching hospitals, seven regional hospitals, and two district hospitals in Ghana. A total of 5099 bacterial isolates from various clinical specimens were collected over a period of 1 year, including data related to the patients. Susceptibility of the isolates was determined by the Kirby–Bauer method. In addition, the minimum inhibitory concentration (MIC) of multidrug-resistant isolates of epidemiological significance was also determined using the E-test.

Results

A wide range of bacterial isolates were identified in both teaching and regional hospitals. High percentage of resistance was observed for tetracycline (82%), cotrimoxazole (73%), ampicillin (76%), and chloramphenicol (75%). Multidrug resistance was observed to a combination of ampicillin, tetracycline, chloramphenicol, and cotrimoxazole. On the other hand, a lower percentage of resistance was observed for ceftriaxone (6.3%), ciprofloxacin (11%), and amikacin (9.9%).

Conclusion

Generally, the prevalence of multidrug resistance was widespread among the various isolates. Some multidrug-resistant strains of Staphylococcus aureus, Salmonella typhi, and non-typhoidal Salmonella (NTS) had high MIC to cefuroxime (>256), gentamicin (>256), and ciprofloxacin (>32).

Systemic bacteraemia in children presenting with clinical pneumonia and the impact of non-typhoid salmonella (NTS)

Background

The diagnosis and antimicrobial treatment of pneumonia in African children in the absence of diagnostic means such as x-ray facilities or microbiological laboratories relies primarily on clinical symptoms presented by the patients. In order to assess the spectrum of bacterial pathogens, blood cultures were performed in children fulfilling the clinical criteria of pneumonia.

Methods

In total, 1032 blood cultures were taken from children between 2 months and 5 years of age who were admitted to a rural hospital in Ghana between September 2007 and July 2009. Pneumonia was diagnosed clinically and according to WHO criteria classified as "non-severe pneumonia" and "severe pneumonia" ("severe pneumonia" includes the WHO categories "severe pneumonia" and "very severe pneumonia").

Results

The proportion of bacteriaemia with non-typhoid salmonella (NTS) was similar in children with pneumonia (16/173, 9.2%) compared to children hospitalized for other reasons (112/859, 13%). NTS were the predominant organisms isolated from children with clinical pneumonia and significantly more frequent than Streptococcus pneumoniae (8/173, 4.6%). Nine percent (9/101) of children presenting with severe pneumonia and 10% (7/72) of children with non-severe pneumonia were infected with NTS. Nineteen out of 123 NTS isolates (15%) were susceptible to aminopenicillins (amoxycillin/ampicillin), 23/127 (18%) to chlorampenicol, and 23/98 (23%) to co-trimoxazole. All NTS isolates were sensitive to ceftriaxone and ciprofloxacin.

Conclusion

In Sub-saharan Africa, sepsis with NTS should be considered in children with symptoms of pneumonia and aminopenicillins might often not be the adequate drugs for treatment.

Molecular typing of Salmonella enterica serovar Sofia in Australia by pulsed-field gel electrophoresis and repetitive element PCR typing

AIMS

In this study, we used two molecular fingerprinting methods to investigate the genetic and clonal relationship shared by Australian Salmonella Sofia isolates.

METHODS AND RESULTS

A total of 84 Australian Salm. Sofia isolates from various states in Australia were typed using pulsed-field gel electrophoresis (PFGE) (XbaI and SpeI) and repetitive element PCR (REP1R-I primer). The previous problem of DNA degradation of Salm. Sofia strains was solved by modifying the lysis solution used to treat the bacterial plugs, allowing Salm. Sofia to be subtyped using PFGE. Molecular typing of isolates resulted in the generation of eight XbaI, six SpeI and five REP1 pattern profiles. Individual typing methods showed low discrimination index values (<0·5), indicating the poor discriminatory ability of the methods. However, the combination of the typing methods was able to improve the discrimination of isolates, further dividing them into 16 subtypes and raising the index value to 0·721.

CONCLUSIONS

The combination of typing methods was shown to be the best approach to fingerprint Salm. Sofia. The Australian Salm. Sofia isolates only showed limited genetic diversity and probably share a clonal relationship. A majority of the Salm. Sofia isolates were not geographically restricted with the predominant pattern subtype observed amongst the isolates from various states.

SIGNIFICANCE AND IMPACT OF THE STUDY

We have successfully devised a PFGE protocol that counteracts DNase activity of Salm. Sofia, enabling typing of this serovar.

High prevalence of markers for sulfadoxine and pyrimethamine resistance in Plasmodium falciparum in the absence of drug pressure in the Ashanti region of Ghana

Markers of Plasmodium falciparum resistance to chloroquine (CQ) and pyrimethamine-sulfadoxine (PYR-SDX) are widespread in areas where malaria is endemic. In an area where the use PYR-SDX is negligible, the Ashanti Region of Ghana, West Africa, adult individuals were enrolled in an analysis of CQ- and PYR-SDX-associated molecular resistance markers in 2001 (n = 177) and 2003 (n = 180). Parasite prevalence, as assessed by PCR assays, were 56.5 and 48.8% in 2001 and 2003, respectively. A high frequency of CQ, PYR, and SDX resistance markers was observed, whereby, as a weak trend, the frequency was higher in 2003. The quintuple combination of three pfdhfr mutations and two pfdhps mutations has previously been recognized to be the most important determinant of PYR-SDX resistance. Approximately 60% of parasite carriers harbored fourfold mutated parasites, indicative of a considerable risk for a switch to high-level PYR-SDX resistance in an area where the rate of PYR-SDX use is low. Among the factors contributing to the high frequency of PYR-SDX resistance-associated mutations are background use of PYR-SDX, past use of PYR for malaria prophylaxis, cross-resistance of trimethoprim with PYR, and the sufficient biological fitness of resistant parasites in the absence of drug pressure.

Subtracted restriction fingerprinting--a new typing technique using magnetic capture of tagged restriction fragments

Molecular typing of bacterial pathogens is an important issue in the epidemiological analysis of emerging infections in humans and animals. Numerous methods have been developed for and applied to a wide variety of bacteria of medical, veterinary and zoonotic importance. The present minireview provides a description of a new typing approach designated subtracted restriction fingerprinting (SRF), its use for typing of Salmonella isolates and a comparison with the most widely used typing techniques for these bacteria. SRF is based on double restriction endonuclease digestion of whole cell DNA, followed by a fill-in reaction with specifically tagged nucleotides and subtractive capture of selected restriction fragments. This results in a reduced number of fragments optimal for separation in standard agarose gels.