Enterobacter hormaechei from isolate to epidemic

Abstract

A large outbreak of a multidrug-resistant Enterobacter hormaechei (EHOS) occurred at the University Medical Centre Utrecht (UMCU). The EHOS disseminated throughout the hospital despite adequate implementation of internationally accepted infection prevention guidelines, and caused invasive infections in more than 100 patients. The EHOS was also detected in 11 of the 15 Dutch hospitals investigated. In at least four hospitals it also caused an outbreak. The aim of this thesis was to gain an understanding of the mechanisms that contributed to the epidemic behaviour of the EHOS. The EHOS belongs to the Enterobacter cloacae complex (ECC) and is its most frequently member in the hospital. Genetic studies showed that the ECC consists of two genetically distinct clades. The younger clade is more homogenous and contains E. hormaechei. The second, older clade consists of several (sub)species that are more heterogeneous at the genetic level. An approximately 307 kb conjugative plasmid (pQC) was found to be present in the EHOS. Bacteria with pQC were less susceptible to sulfamethoxazole, tetracyclines, aminoglycosides, fluoroquinolones, extended-spectrum ?-lactamases (ESBLs), and probably heavy metals. pQC could transfer to many other Enterobacteriaceae by conjugation resulting in a multidrug resistant phenotype for these bacteria. The EHOS also contained a functional High Pathogenicity Island (HPI). This enables the strain to obtain iron from saturated lactoferrin in an iron-depleted environment, which is beneficial during infection. Supporting evidence was obtained that the HPI can be transferred in vivo and may have contributed to the epidemic behaviour of the EHOS. Genome sequence analysis showed that a relatively high percentage of genes are involved in motility and cell-wall biogenesis. Possibly 14 different pili or flagella may be produced by the EHOS. It is not certain whether the encoded pili contributed to colonization of humans but some homologues of other bacterial species have been show to contirbute to ocllonization. The EHOS had a propensity for recombination, which was illustrated by a recombination hotspot (RHS). Five putative genomic islands were detected in this RHS. Four of the five genomic islands could be excised and formed circular extrachromosomal DNA, which are possibly transferable to other ECC. Transferability of the HPI or pQC is worrisome. Incorporation of the HPI or other genomic islands into the chromosome or uptake of pQC or other resistance conferring plasmids by other isolates may potentially generate more pathogenic and antibiotic-resistant strains that, due to their different genetic backgrounds, are able to adapt to new niches. In conclusion, the genetic plasticity allowed the parent of the EHOS, which apparently already encoded a large number of colonization factors, to acquire additional resistance and virulence genes. This resulted in a strain which became a serious healthcare threat for patients. Super bugs like the EHOS are increasingly discovered. This stresses the need for new strategies and antibiotics active against these bacteria to combat this new threat.