Active screening for multiresistant Enterobacteriaceae

Abstract

The control of multiresistant organisms (MROs) is made difficult by a large reservoir of unrecognised, asymptomatic colonised patients. Hence, active screening is generally used as part of a multifaceted infection control intervention. For MRGNB the ratio of colonised to infected patients has been estimated at~8:1 for extended spectrum beta-lactamases 2 and 9:1 for carbapenem-resistant Enterobacteriaceae (CRE) 3 . In addition to this, the very mechanism by which antibiotic resistance is transferred between common species like Escherichia coli and Klebsiella pneumoniae is such that resistance acquisition can be highly variable in phenotype and often relatively 'silent' 4 . Antibiotic susceptible organisms can acquire resistance mechanisms very quickly 5 , so that the potential for acquisition by gene transfer is itself an important risk to understand.The most appropriate specimen type for screening of MRGNBs depends on the organism's likely habitat on the human body or clinical environment. Thus, the most sensitive specimens for detecting Enterobacteriaceae may be rectal swab, peri-rectal swab or faeces, but wounds and respiratory samples and even environmental sampling (sinks, drains, surfaces) may be more important for the control of outbreaks due to organisms such as Pseudomonas or Acinetobacter 6 .Increased specimen volume and a higher MRO faecal density both increase yield of screening 7,8 . For this reason faecal samples may perform better than rectal swabs 9 , although the latter is more practical. Using more than one screening specimen or specimen type may increase sensitivity 6 , but again for practical purposes one specimen is usual. Concurrent antibiotic use has a variable impact on MRO faecal density 10 but logic dictates that detection of a reservoir of resistance is more likely in the presence of relevant selection pressure.Active screening specimens may be processed in the laboratory using phenotypic (culture based) or direct genotypic (amplification and detection of bacterial DNA) methods. There is no consensus as to the optimal laboratory processing method but factors that should be considered are local molecular epidemiology (including the likely minimum inhibitory concentration (MIC) of relevant antibiotics against target isolates), availability of expertise and equipment, cost, capacity for integration into laboratory workflow and test performance including sensitivity, specificity, limit of detection and turnaround time. Currently, phenotypic methods are more widely used, although we anticipate that with advances in automation, dataIn Focus