Molecular characterization of trimethoprim resistance in Shigella sonnei in Sicily

Comparison of phenotypic and WGS-derived antimicrobial resistance profiles of Shigella sonnei isolated from cases of diarrhoeal disease in England and Wales, 2015

Objectives

Phenotypic and genotypic methods for the detection of antimicrobial resistance (AMR) in Shigella sonnei in England and Wales were compared and evaluated.

Methods

WGS data from 341 isolates of S. sonnei isolated between June 2015 and January 2016 were mapped to genes known to be associated with phenotypic AMR. Antimicrobial susceptibility testing was performed on all viable isolates (n = 335).

Results

Fifteen of 335 isolates had a discrepancy between phenotypic and genotypic testing for 1 of the 10 antimicrobial classes tested, equating to 15 (0.45%) discordant results out of a possible 3350 isolate/antimicrobial combinations. All 15 mismatched results were genotypically resistant but phenotypically susceptible. Eleven of the 15 discrepancies were observed in streptomycin resistance profiles. The most common resistance profile was trimethoprim, sulphonamides, tetracyclines and streptomycin, occurring in 97 (28.4%) isolates. Resistances to ciprofloxacin and the third-generation cephalosporins, not detected in England and Wales prior to 2002, were identified in 18.2% and 12% of isolates, respectively. Three hundred and four (89.1%) isolates were MDR. There was no significant association between any of the AMR determinants tested and recent foreign travel in male or female cases. The number of isolates of S. sonnei harbouring blaTEM-1 and ermB/mphA was significantly higher in men who reported no recent travel outside the UK.

Conclusions

The use of WGS for routine public health surveillance is a reliable method for rapid detection of emerging AMR in isolates of S. sonnei.

Molecular epidemiology of antibiotic resistance

Molecular typing methods based on the analysis of the genetic structure of bacteria, are used to address many different problems such as the study of genomic organisation and evolution, the identification of patterns of infection, the identification of sources of transmission, the epidemiological surveillance of infectious diseases and for investigations into outbreaks. Of particular interest is the application of these techniques for acquiring information on the spread of micro-organisms that have become resistant to many clinically important antibiotics. The emergence of antibiotic resistance is one of the most dangerous phenomena of the last 20 years and knowledge of the mechanisms of resistant-gene exchange means fully understanding their spread into all environments. Studies on the molecular epidemiology of antibiotic-resistance in micro-organisms should make it easier to distinguish clonality with respect to horizontal transfer of the determinants of resistance.

Multiresistant Shigella species isolated from pediatric patients with acute diarrheal disease

A total of 57 strains of Shigella (36 S sonnei, 21 S flexneri), isolated from children with acute diarrheal disease who presented for treatment at the Andes University Hospital, Merida, Venezuela, from June 1993 to June 1995, were tested for their susceptibility to trimethoprim, sulfamethoxazole, ampicillin, cefamandole, ceftriaxone, streptomycin, fleroxacin, and nalidixic acid, by the agar dilution method. Twenty-seven strains (75%) of S sonnei and eight strains of S flexneri (38.1%) isolates showed high-level resistance to trimethoprim (MIC90 > 1024 microg/mL), which was also associated with other resistance patterns. The most common resistant phenotype associated with trimethoprim-resistance among S sonnei isolates was sulfamethoxazole-streptomycin (63%); among S flexneri isolates, it was sulfamethoxazole-ampicillin-streptomycin (87.5%). Individual resistance was only observed for ampicillin, mainly in four isolates of S flexneri, and in one isolate of S sonnei. Most Shigella strains were resistant to three or more antimicrobial agents. These results confirmed that multiresistant strains of Shigella are present in Merida, and emphasize the importance to maintain these under surveillance in order to assess local susceptibility patterns and empiric therapy.

Transferable trimethoprim resistance in Shigella strains

The susceptibility patterns of 35 Shigella isolates (16 S. flexneri, 14 S. dysenteriae and 5 S. sonnei) to trimethoprim (Tp) and various antibiotics including amoxycillin, amoxycillin-clavulanic acid, nalidixic acid, ciprofloxacin, ceftazidime and ceftriaxone, were investigated. Twenty-two (62.8%) strains were resistant to Tp with a minimal inhibitory concentration (MIC50) value of 512 mg/L. Only six isolates were amoxycillin resistant, to which clavulanic acid restored sensitivity in all of them. None of the isolates were resistant either to extended spectrum cephalosporins or to quinolones. Resistance to Tp was transferred from 7 of the 22 isolates (31.8%) to the recipient Escherichia coli K12. Tp MIC values of the transconjugants were 512 mg/L. In no strain could amoxycillin resistance be transferred. Our results indicate that as the prevalence of transferable Tp resistance in Shigella isolates in Izmir is substantially high, alternative antimicrobial agents should be considered for empirical antibiotic therapy.

Study of a melanic pigment of Proteus mirabilis

The present study sought to determine whether the pigment produced by Proteus mirabilis from the L-forms of various aromatic amino acids under aerobic conditions is melanic in nature. It is a black-brown pigment which behaves like a melanin in many respects, namely solubility features, bleaching by oxidizing agents and positive response to the Fontana-Masson assay. In the present study, for the first time, it was shown by electron spin resonance analysis that a bacterial melanin is able to act as a free radical trap, as was previously demonstrated for other melanins. Scanning electron microscopy studies showed a specific organized structure of the pigment as rounded aggregates of spherical bodies. DNA hybridization data did not reveal, in the P. mirabilis genome, any nucleotide sequence related to Shewanella colwelliana mel A, one of the two melanogenesis systems already defined at the molecular level in bacteria. Results obtained from experiments on pigment production inhibition suggest a possible role of tyrosinase in P. mirabilis melanogenesis. In conclusion, from the bulk of our results, it appears that the pigment produced by P. mirabilis is melanic in nature.

Molecular characterization of trimethoprim resistance in salmonellas isolated in Sicily, 1985-1988

The occurrence of trimethoprim (Tp) resistance in salmonellas isolated from humans and water samples in Sicily between 1985 and 1988 has been investigated and the Tp resistance mechanisms have been further characterized on the basis of hybridization with probes for the dihydrofolate reductase (DHFR) genes types I, II, IV and V. Of 765 strains examined, high level (> 1000 mg/l) resistance to Tp was identified in 23 strains (3%). In 22 of these strains, such resistance was associated with resistance to sulphonamides. Six serovars with Tp-resistant strains were identified, Salmonella typhimurium (14 strains), S. enteridis (2), S. agona (2), S. mbandaka (2), S. virchow (2), S. indiana (1). In all strains with high level Tp resistance, resistance to this antimicrobial was plasmid-encoded, in most strains by plasmids with MWs ranging from 70-100 MDa. On the basis of restriction endonuclease analysis, four different categories of Tp resistance plasmids were identified in Tp-resistant strains of S. typhimurium. Hybridization with the DHFR I probe was observed in three strains of Tp-resistant S. typhimurium and two strains of Tp-resistant S. enteritidis; in contrast, in none of the strains tested was there any detectable hybridization with the probes for DHFR types II, IV and V. It is concluded that the DHFR type I resistance mechanism, common in Tp-resistant enterobacteria in many European countries, is relatively uncommon in Tp-resistant salmonellas isolated in Sicily. Furthermore, the DHFR V resistance mechanism, previously identified in strains of Shigella sonnei isolated in Sicily and associated with travellers from Sri Lanka, has not yet appeared in salmonellas in Sicily.

Trimethoprim resistant dihydrofolate reductases in normal faecal flora isolated in India

A high incidence of resistance to trimethoprim has been shown in the normal faecal flora in a population in south India. The dihydrofolate reductase (dhfr) genes mediating transferable resistance to trimethoprim have been identified. Unusually, in this study, the dhfrV was shown to be the predominant resistance gene (dhfrV 50% of transconjugants, dhfrIa 30%), the dhfrIb was also detected being distinguished from the dhfrV by an oligo-probe. However, when non-transferable resistance was considered, the dhfrIa was the most prevalent of the dhfrs identified. All those plasmids harbouring the dhfrIa were shown to possess Tn7. All the plasmids that probed positive for the dhfrV and the dhfrIb were shown to be associated with the integrase of the Tn21-like transposons, but 8 of the dhfrV genes were not associated with the Tn21 resolvase. The dhfrIV was shown to be present in all seven plasmids that produced low level trimethoprim-resistance. The dhfrV, first characterized in Sri Lanka, would seem to have a local distribution in this region of Asia but is distinguishable from the dhfrIb only by the use of an oligo-probe.