To the Editor—Carbapenem resistance among enterobacterial species has increased alarmingly and is a major worldwide threat. Two distinct paths evidence this phenotype: (1) resistance to carbapenems by any mechanism, including the production of an acquired carbapenemase or the production of extended spectrum β-lactamase (ESBL) or AmpC combined with porin-loss (carbapenem-resistant Enterobacteriaceae [CRE]) and (2) resistance to carbapenems by means of an acquired carbapenemase (carbapenemase-producing Enterobacteriaceae—CPE).Reference Drew, Turton and Hill ¹

A prevalence survey monitoring carbapenem resistance among Enterobacteriaceae, including rectal screens and clinical specimens, was performed in a tertiary hospital in southern Brazil between April 2013 and May 2015. Rectal swabs were collected at admission and weekly from all patients in an intensive care unit (ICU) as described previously.Reference Perez, Rodrigues and Dias ²

Identification and prior carbapenem susceptibility were performed using a MicroScan Walkaway system (Siemens). Minimum inhibitory concentration (MIC) for ertapenem and meropenem were assessed by Etest. A synergistic test was applied using phenyl-boronic acid and ethylenediaminetetraacetic acid for detecting Klebsiella pneumoniae carbapenemase (KPC) and New Delhi metallobetalactamase, respectively. Enzymatic inhibition using clavulanic acid and cloxacillin was used to detect ESBLs and AmpC enzymes, in that order. All isolates were submitted to polymerase chain reaction (PCR) for carbapenemase gene detection.Reference Monteiro, Widen and Pignatari ³

Statistical analyses were conducted using SPSS version 13.0 (IBM, Inc., Chicago, IL, USA). Prevalence ratio (PR), odds ratio (OR), and 95% confidence intervals (CIs) were calculated. P value was calculated using χ² or Fisher’s exact test.

In total, 3,975 rectal swabs were obtained from 1,334 distinct patients, of whom 294 patients (PR, 22%; 95% CI, 19.9–24.3) had a rectal swab with CPE and the remaining 21 patients (PR, 1.6%; 95% CI, 1.0–2.4) had rectal swabs with non-carbapenemase producers. The prevalence of CPE was significantly higher than non-carbapenemase producers (OR, 5.7%; 95% CI, 3.6–8.9; P<.001).

Distribution and identification of species are shown in Table 1. Among the 497 enterobacteriaceae isolates with reduced susceptibility to any carbapenem detected in rectal swabs, 475 (95.6%; 95% CI, 93.4–97.1) were KPC-2 producers, as identified by polymerase chain reaction (PCR) assay (3 isolates with a negative phenotypic test). Another 22 isolates (4.4%; 95% CI, 2.9–6.6) were negative for carbapenemase production (phenotypic and PCR).

TABLE 1

Distribution and Identification of Enterobacterial Species Characterized as Carbapenem-Resistant Enterobacteriaceae among the 497 Samples (Rectal Screen and Clinical Specimens)

^a bla _KPC-2 was the sole carbapenemase gene detected;

^b A randomly selected sample of isolates (rectal swab, n=10 and clinical specimen, n=13) was typed by pulsed-field gel electrophoresis and showed the same macrorestriction profile of DNA.

In total, 211 isolates from 145 distinct patients presenting reduced susceptibility to carbapenems were found in blood (93.9%), respiratory secretions (91.3%), urine (84.5%), and other sites (75%). Of these 211 isolates, in 184 (87.2%; 95% CI, 82–91) the bla _KPC-2 gene was detected (although 3 would not have been detected using only phenotypic testing). The remaining 27 isolates (12.8%; 95% CI, 8.9–18) from urine (15.5%), respiratory secretions (8.7%), blood (6.1%), and others (25%), were negative for carbapenemase production. For all isolates in which a carbapenemase gene was not detected, AmpC and/or ESBL production by phenotypic tests were positive.

Among KPC producers, MIC₅₀ and MIC₉₀ were >32 mg/L for meropenem and ertapenem. Among the nonproducers, MIC₅₀ and MIC₉₀ were 8 µg/mL and 16 mg/L for meropenem and 32 mg/L and >32 mg/L for ertapenem, respectively.

Carbapenem resistance due to the production of an ESBL or AmpC associated with impermeability may be related to further reductions in carbapenem susceptibility during therapeutic treatment. On the other hand, carbapenem resistance is often unstable as it uses up a lot of energy, meaning that these strains rarely spread.Reference Findlay, Hamouda and Dancer ⁴

In contrast with the findings of Drew et al,Reference Drew, Turton and Hill ¹ in which the carbapenem resistance for most isolates was due to AmpC or ESBL combined with impermeability in a UK pediatric population, an acquired carbapenemase gene seems to be the issue in our institution. Undoubtedly, high prevalence of CRE is due to the rapid spread of a specific carbapenemase (KPC-2 in this study) and a bacterial species with a high capacity to adapt and survive (Klebsiella pneumoniae in this study).

A potential limitation of this study is that a denominator for clinical specimens was not given; therefore, the prevalence in these sites cannot be expressed. However, the fact that ~20% of ICU patients were colonized with CRE is a concern, as >93% were due to KPC-2-producing K. pneumoniae, subsequently reflecting on the development of infections (Table 1).

Although K. pneumoniae was the main species recovered in rectal swabs in both carbapenemase producers and nonproducers, it is noteworthy that Enterobacter cloacae appears to be the protagonist (66.7%) among the nonproducers in clinical specimens. However, these latter isolates were successfully treated with meropenem, as AmpC plus impermeability conferred only a low-level of carbapenem resistance, often with ertapenem MICs just above the cutoff; while for the KPC-2 producers only polymyxin B exhibited a good in vitro activity.Reference Perez ⁵

Because KPC producers show a higher prevalence than nonproducers, early appropriate therapy is necessary, mainly in bloodstream infections for which high mortality rates have been attributed.Reference Tumbarello, Viale and Viscoli ⁶ In addition, control measures should be implemented to avoid further spread of this resistance mechanism.

In conclusion, this study demonstrates a major prevalence of carbapenemase producers compared with nonproducers as a CRE etiological agent in our institution. This finding is based on the high performance of a dominant clone identified as KPC-2-producing K. pneumoniae being the prevalent agent and being responsible for the current endemic level. Nevertheless, the prevalence of CRE, no matter the mechanism, must be further evaluated, since its occurrence may greatly impact infection control practices.