Burkholderia pseudomallei multi-centre study to establish EUCAST MIC and zone diameter distributions and epidemiological cut-off values

OBJECTIVES

Melioidosis, caused by Burkholderia pseudomallei, requires intensive antimicrobial treatment. However, standardized antimicrobial susceptibility testing (AST) methodology based on modern principles for determining breakpoints and ascertaining performance of methods are lacking for B. pseudomallei. This study aimed to establish MIC and zone diameter distributions on which to set epidemiological cut-off (ECOFF) values for B. pseudomallei using standard EUCAST methodology for non-fastidious organisms.

METHODS

Non-consecutive, non-duplicate clinical B. pseudomallei isolates (9-70 per centre) were tested at eight study centres against eight antimicrobials by broth microdilution (BMD) and the EUCAST disc diffusion method. Isolates without and with suspected resistance mechanisms were deliberately selected. The EUCAST Development Laboratory ensured the quality of study materials, and provided guidance on performance of the tests and interpretation of results. Aggregated results were analysed according to EUCAST recommendations to determine ECOFFs.

RESULTS

MIC and zone diameter distributions were generated using BMD and disc diffusion results obtained for 361 B. pseudomallei isolates. MIC and zone diameter ECOFFs (mg/L; mm) were determined for amoxicillin-clavulanic acid (8; 22), ceftazidime (8; 22), imipenem (2; 29), meropenem (2; 26), doxycycline (2; none), tetracycline (8; 23), chloramphenicol (8; 22) and trimethoprim-sulfamethoxazole (4; 28).

CONCLUSIONS

We have validated the use of standard BMD and disc diffusion methodology for AST of B. pseudomallei. The MIC and zone diameter distributions generated in this study allowed us to establish MIC and zone diameter ECOFFs for the antimicrobials studied. These ECOFFs served as background data for EUCAST to set clinical MIC and zone diameter breakpoints for B. pseudomallei.

Outbreak of tularaemia connected to a contaminated well in the Västra Götaland region in Sweden

Tularaemia, is a zoonotic disease caused by the bacterium Francisella tularensis. This disease has been reported in Sweden since 1931 and its wide distribution in the country poses a challenge for understanding the transmission, ecology and epidemiology of the disease. In Sweden, the disease is usually transmitted by mosquitoes, but in this study we could show that consumption of well water was epidemiologically linked to the outbreak, by isolating F. tularensis from the water. In this article, we describe an outbreak of tularaemia in the region of Västra Götaland in the southwest of Sweden in spring of 2013.

Francisella tularensis subspecies holarctica occurs in Swedish mosquitoes, persists through the developmental stages of laboratory-infected mosquitoes and is transmissible during blood feeding

In Sweden, mosquitoes are considered the major vectors of the bacterium Francisella tularensis subsp. holarctica, which causes tularaemia. The aim of this study was to investigate whether mosquitoes acquire the bacterium as aquatic larvae and transmit the disease as adults. Mosquitoes sampled in a Swedish area where tularaemia is endemic (Örebro) were positive for the presence of F. tularensis deoxyribonucleic acid throughout the summer. Presence of the clinically relevant F. tularensis subsp. holarctica was confirmed in 11 out of the 14 mosquito species sampled. Experiments performed using laboratory-reared Aedes aegypti confirmed that F. tularensis subsp. holarctica was transstadially maintained from orally infected larvae to adult mosquitoes and that 25% of the adults exposed as larvae were positive for the presence of F. tularensis-specific sequences for at least 2 weeks. In addition, we found that F. tularensis subsp. holarctica was transmitted to 58% of the adult mosquitoes feeding on diseased mice. In a small-scale in vivo transmission experiment with F. tularensis subsp. holarctica-positive adult mosquitoes and susceptible mice, none of the animals developed tularaemia. However, we confirmed that there was transmission of the bacterium to blood vials by mosquitoes that had been exposed to the bacterium in the larval stage. Taken together, these results provide evidence that mosquitoes play a role in disease transmission in part of Sweden where tularaemia recurs.

Identification and subtyping of Francisella by pyrosequencing and signature matching of 16S rDNA fragments

AIMS

To analyse the V1 region of the 16S rDNA gene by a universal pyrosequencing protocol to identify and subtype Francisella in 31 strains from a repository collection and 96 patient isolates.

METHODS AND RESULTS

Pyrosequencing was used to determine the nucleotide sequence of PCR amplification products of the variable region (V1) of the 16S rDNA from 31 repository strains and 96 isolates from Swedish patients with ulceroglandular tularaemia. Pyrosequencing resulted in a 37 nucleotide sequence, specific for Francisella sp., for all repository strains and patient samples analysed. In addition, the isolates could be divided into two groups based on the analysis of a single nucleotide polymorphism in the sequence: one group included Francisella tularensis ssp. tularensis, ssp. holarctica and ssp. mediasiatica, whereas the other group included Francisella tularensis ssp. novicida and other species of Francisella. The analysis of samples taken from patients suffering from ulceroglandular tularaemia revealed that all isolates belonged to the first group comprising subspecies of F. tularensis virulent for humans.

CONCLUSIONS

The pyrosequencing analysis of the 16S rDNA V1 is a useful molecular tool for the rapid identification of suspected isolates of Francisella sp. in clinical or environmental samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

Virulent F. tularensis ssp. causing ulceroglandular tularaemia, or those with a potential to be used in a bioterrorism event, could rapidly be discriminated from subspecies less virulent for humans.