Strain-Specific Transmission in an Outbreak of ESBL-Producing Enterobacteriaceae in a Medical Oncology and Immunology Care Unit

Background Extended-spectrum b-lactamase (ESBL)-producing bacteria are resistant to several types of antibiotics excluding carbapenems. Consequently, infections caused by ESBL-producing bacteria are difficult to treat. Especially for the immunocompromised including hematologic malignancies ESBL-producing bacteria are emergent pathogens. In addition, there are no evidence-based guidelines specifying the infection control for ESBL-producing bacterial outbreak.

Along with the recent trend of wide spreading ESBL genes in the community, carriers ESBL-producing bacteria in the community become common.

Methods An ESBL-producing Enterobacteriaceae (ESBL-E) outbreak observed between July 2012 and August 2012 in Kagawa University Hospital was surveyed using various molecular microbiology techniques. In this study, we attempted to identify the source of an outbreak of ESBL-producing bacteria in a medical oncology and immunology care unit. We used pulsed-field gel electrophoresis (PFGE) and PCR-based ESBL gene analysis as molecular typing methods. DNA extracted from ESBL-E was digested overnight with Xba I. Tenover criteria were used for the separation and identification of each band. ESBL-producing bacteria were screened by PCR/DNA for the detection of blaTEM, blaSHV, and blaCTX-M. Subsequently, ESBL genes with specific CTX-M subtypes (groups 1, 2, 8 and 9) were characterized using multiplex PCR amplification.

The antibiotic susceptibility of strains isolated from clinical samples was assessed by microdilution methods using an IA20MIC mkII system (Koden Industry Co., Japan).

Results In total, 21 isolates (12 K. pneumoniae and 9 E. coli) were obtained from clinical samples, including four control strains (two each of both bacteria), that were physically different from those obtained from other inpatient units in our hospital. PFGE for K. pneumoniae (digested by Xba I) produced similar patterns excluding one control strain. PCR classification of the ESBL gene for K. pneumoniae revealed that all strains other than the control strain carried CTX-M9. On another, ESBL genes derived from E. coli strains were varied in each strain with TEM, SMV, CTX-M-1, CTX-M-2, or CTX-M-9. This result was reconfirmed by direct DNA sequencing. Although the outbreak of K. pneumoniae was considered to be "clonal," PFGE and PCR classification of the ESBL genes for E. coli uncovered at least six different "non-clonal" strains possessing individual ESBL gene patterns. The pattern of antimicrobial susceptibility was more variable for K. pneumoniae than for E. coli.

Conclusion Typing by PFGE and ESBL gene PCR analysis is practical for discriminating various organisms. In our cohort, two outbreaks were concomitantly spread with different transmission strategies, namely clonal and non-clonal, in the same unit. This might represent clinical evidence that transmissibility differs according to the type of strain. Our speculation is that the ability to acquire ESBL genes differs among bacteria, and the penetrance of resistance is strain-dependent. ESBL-E outbreaks can occur concurrently, as shown in this study. We observed that patient-to-patient transmission of ESBL-E occurred according to the properties of each individual strain and strain-specific maneuver could be optimal.