Abstract
Background
Carbapenem-resistant Acinetobacter baumannii (CRAb) is an important cause of nosocomial infections especially in intensive care units. This study aimed to assess clinical aspects and the genetic background of CRAb among ICU patients at a Brazilian teaching hospital.
Methods
56 critically ill patients colonized or infected by CRAb, during ICU stay, were prospectively assessed. Based on imipenem MIC ≥ 4 μg/mL, 28 CRAB strains were screened for the presence of genes encoding metallo-β-lactamases and OXA-type β-lactamases. The blaOXA-type genes were characterized by PCR using primers targeting ISAba-1 or -3. Genetic diversity of blaOXA-positive strains was determined by ERIC-PCR analysis.
Results
Patient's mean age (±SD) was 61 (±15.1), and 58.9% were male. Eighty-percent of the patients presented risk factors for CRAb colonization, mainly invasive devices (87.5%) and previous antibiotic therapy (77.6%). Thirty-three patients died during hospital stay (59.0%). Resistance to carbapenems was associated with a high prevalence of blaOXA-23 (51.2%) and/or blaOXA-143 (18.6%) genes. ERIC-PCR genotyping identified 10 clusters among OXA-producing CRAb. Three CRAb strains exhibited additional resistance to polymyxin B (MIC ≥ 4 μg/mL), whereas 10 CRAb strains showed tigecycline MICs > 2 μg/mL.
Conclusions
In this study, clonally unrelated OXA-123- and OXA-143-producing A. baumannii strains in ICU patients were strongly correlated to colonization with infected patients being associated with a poor outcome.
Keywords: Acinetobacter baumannii, Carbapenem resistance, blaOXA-23, blaOXA-143
Introduction
Acinetobacter baumannii is a Gram-negative non-fermentative coccobacillus that has gained growing notoriety as a nosocomial pathogen, showing patterns of increasing resistance to antimicrobial drugs and disinfectants. These organisms are implicated in a diverse array of infections, especially in intensive care units (ICUs) associated with countless outbreaks.1, 2 Classically, carbapenems plays a crucial role in the treatment of serious nosocomial infections due to A. baumannii, but this scenario has changed substantially in the last decade, with an increasing prevalence of carbapenem-resistant and multidrug-resistant isolates.3
Carbapenem resistance in A. baumannii is mainly due to the presence of β-lactamases (class B metallo-β-lactamases – MBL, or class D OXA-type β-lactamases – oxacillinases – OXA), to the loss of outer membrane proteins,4 and to altered penicillin-binding proteins (PBP).5 OXA-type β-lactamases are chromosomal enzymes that in A. baumannii can be intrinsic (e.g., OXA-51-like) or, more frequently, acquired (e.g., OXA-23-like, OXA-24-like, OXA-58-like).6 Outbreaks of OXA-23-producing A. baumannii have been reported in some Brazilian hospitals. Moreover, a new oxacillinase, OXA-143-like, was recently described in this country.3, 7, 8, 9
Only few studies concerning the molecular basis of carbapenem resistance in A. baumannii, as well as the clinical impact of these resistant strains on patient outcomes, have been reported, despite the growing importance of these strains in ICUs of Brazilian hospitals.3, 7, 8
Thus, the aim of this study was to investigate the genetic determinants of carbapenem resistance and the clonal relatedness of carbapenem-resistant A. baumannii (CRAb) strains isolated from patients admitted to a Brazilian intensive care unit.
Materials and methods
Study setting and population
This study was conducted in a medical ICU of a 500-bed tertiary care teaching hospital, affiliated to the Federal University of Minas Gerais (UFMG), in Brazil. All adult (≥18 years) patients colonized or infected by CRAb during ICU stay between December 2009 and December 2010 were assessed for potential inclusion in the study. Demographic and clinical data were retrospectively obtained from a data bank prospectively filled, which is used for research and administrative issues. Colonization was defined by the presence of CRAb on skin, on mucous membranes, in open wounds, or in excretions or secretions obtained during routine surveillance for multiresistant bacteria. According to the protocol followed in our ICU, surveillance swabs are obtained upon admission and weekly up to discharge from the unit.10 Infection was defined as the presence of CRAb identified in specimens associated with infection reported according to NHSN criteria. For both cases, colonization and infection, only the first isolate was considered.
Ethical aspects
This study was approved by the Institutional Research Ethical Committee. Privacy was guaranteed and patients were identified by their hospital registration numbers. Only researchers had access to the information of the enrolled patients.
Variables and definitions
Baseline demographic and clinical data were obtained from medical charts. Colonization was defined as the presence of microorganisms on skin, on mucous membranes, in open wounds, or in excretions or secretions, unassociated with clinical signs or symptoms of infection during routine surveillance or specimens not associated to infections.10 Infection was defined as identification of CRAb in clinical specimens associated with infection reported according to NHSN criteria site. Baseline demographic and clinical data were obtained from medical charts.
Laboratory tests
CRAb strains and antimicrobial susceptibility testing
CRAb isolates were identified using the VITEK 2 system (bioMérieux, France®). Antimicrobial screening was carried out using the Kirby-Bauer disk diffusion method.11 The confirmatory susceptibility test was performed using the agar dilution method for imipenem, tigecycline, and polymyxin B, as recommended by the Clinical and Laboratory Standards Institute and the Food and Drug Administration (FDA).11 FDA-approved tigecycline breakpoints for Enterobacteriaceae, were applied to all isolates (susceptible ≤2 μg/mL; resistant ≥8 μg/mL).
Detection of carbapenemases
A screening test for MBL production was performed using the double-disk synergy test (DDST), as recommended by Arakawa et al. Phenotypic detection of MBLs was performed using Etest® MBL strips, following the manufacturer's recommendations (AB bioMérieux, St. Louis, MO, USA).
Polymerase chain reaction (PCR) amplification and DNA sequencing
Multiplex PCR was performed to investigate the genes encoding the following carbapenemases: blaOXA-23-like, blaOXA-51-like, blaOXA-58-like, blaOXA-24-like, blaOXA-143-like, blaVIM-like, blaIMP-1, and blaIMP-like. Specific primers were used to identify the insertion sequences (ISAba1 and ISAba3).12
ERIC-PCR typing
The genetic similarity of A. baumannii isolates was determined using Enterobacterial Repetitive Intergenic Consensus-PCR (ERIC-PCR). A similarity matrix was calculated using the DICE coefficient with 2% tolerance. Percent similarity determination, cluster calculation, and subsequent dendrogram construction were performed using the Bio Numerics software.13
Statistical analysis
Clinical and epidemiological characteristics were described. The distribution of continuous variables was tested, and their normal or non-normal distributions presented as mean ± standard deviation or median (IQR), respectively. Categorical variables were analyzed using frequencies and percentages. Associations between β-lactamases and ICU mortality were tested using chi-square test. The Statistical Package for the Social Sciences (SPSS), version 15.0 (Chicago, IL), was used for all analyses. Statistical significance was determined as p ≤ 0.05 and comparative analyses are presented with the 95% confidence interval (95% CI).
Results
Patients characteristics
Overall, 867 patients were admitted on the ICU during the study period, 56 of whom had positive results for carbapenem-resistant A. baumannii cultures in clinical samples (e.g., blood, tracheal aspirate, urine, or wound secretion) or surveillance swabs. Forty-three out of the 56 strains were subjected to quantitative testing, as well as phenotypic and genotypic resistance evaluation. The characteristics of patients infected or colonized by CRAb are shown in Table 1. From a total of 56 patients, 38 (67.9%) were colonized and 18 (32.1%) were infected with CRAb. Table 2 presents the type of CRAb isolation (colonization or infection), site of infection, and patient outcome. The majority of patients (83.9%) had comorbidities, and the most common condition was diabetes mellitus (33.9%). Eighty-seven percent of the patients had previously received invasive devices and 78.6% had received antimicrobial therapy, mainly cephalosporins (58.9%), carbapenems (33.9%), and glycopeptides (26.8%). Among the 18 infected patients, 10 (55.5%) had pneumonia, six (33.3%) had bloodstream infections, one (5.6%) had urinary tract infection, and one (5.6%) presented an intra-abdominal infection. Overall, 33 out of the 56 patients (59%) died during hospital stay.
Table 1.
Demographic data of ICU patients with CRAb.
| Parameters | n (±SD) | % |
|---|---|---|
| Patient characteristics | ||
| Age (years) | 61.0 ± 15.1 | – |
| Male/female | 33/23 | – |
| ICU stay (days) | 18.7 ± 16.5 | – |
| APACHE II score | 14.3 ± 6.0 | – |
| Co-morbidities (%) | 47 | 83.9 |
| Underlying pathology | ||
| Hematological malignancies | 3 | 5.3 |
| Solid tumor | 2 | 3.6 |
| Steroid use | 5 | 8.9 |
| Diabetes mellitus | 19 | 33.9 |
| Chronic kidney disease | 13 | 23.2 |
| Cardiovascular disease | 14 | 25.0 |
| Chronic obstructive pulmonary disease | 10 | 17.8 |
| Liver failure (Child B or C) | 6 | 10.7 |
| Previous invasive devices | 49 | 87.5 |
| Central venous catheter | 38 | 67.9 |
| Foley catheter | 46 | 82.1 |
| Mechanical ventilation | 46 | 82.1 |
| Previous antimicrobial use | 44 | 78.6 |
| Carbapenems | 19 | 33.9 |
| Cephalosporins 3rd Gen. | 18 | 32.1 |
| Cephalosporins 4th Gen. | 15 | 26.8 |
| Quinolones | 9 | 16.0 |
| β-Lactamase inhibitors | 12 | 21.4 |
| Glycopeptides | 15 | 26.8 |
Table 2.
Classification of the patients according to CRAb colonization or infection.
| Parameters | n | % |
|---|---|---|
| Type of isolation | ||
| Colonization | 38 | 67.9 |
| Nosocomial infection | 18 | 32.1 |
| Site of infection | ||
| Intra-abdominal | 1 | 5.6 |
| Lungs (pneumonia) | 10 | 55.5 |
| Bloodstream (catheter-associated) | 5 | 27.7 |
| Bloodstream (primary) | 1 | 5.6 |
| Urinary tract | 1 | 5.6 |
| Progression of infection | ||
| Discharge | 23 | 41.0 |
| Death | 33 | 59.0 |
Phenotypic antimicrobial susceptibility of isolates
The antimicrobial susceptibility profiles of the CRAb isolates, defined by disk diffusion and agar dilution methods, are presented in Table 3. Gentamicin turned out to be the best antimicrobial in terms of susceptibility (33.9%), followed by sulfamethoxazole/trimethoprim (21.4%). Resistance to imipenem was confirmed for only 58.1% of all A. baumannii strains. Polymyxin and tigecycline resistance was seen in three (7.0%) and 10 (23.3%) out of all the 43 strains studied, respectively.
Table 3.
Clinical and microbiological characteristics of OXA-producing Acinetobacter baumannii strains isolated from ICU patients.
| CRAb strain | Sample | Resistance profile |
blaOXA-type genea | ERIC profileb | S/Age (y) | Admission (m/d/y) | Apache II | Clinical status/ATM | Isolation data | Clinical outcome | Invasive devices | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| MIC (μg/mL) |
Kirby-Bauer |
|||||||||||||
| IMP | POL | TIG | Antimicrobial drugs | |||||||||||
| 288 | Nasal | >32 | 1.0 | 1.0 | AK, SAM, CFP, CTZ, CIP, MER, PTZ | blaOXA-51, blaOXA-143 | G2 | M/71 | 12/02/09 | 23 | Colonized/No | 12/13/09 | Death | CVC + IUC + MV |
| 295 | Nasal | >32 | 1.0 | 2.0 | AK, SAM, CFP, CFZ, CIP, MER, PTZ | blaOXA-51, blaOXA-143 | G4 | M/56 | 01/03/10 | 15 | Colonized/Yes | 01/19/10 | Survival | CVC + IUC + MV |
| 296 | Perianal | >32 | 1.0 | 2.0 | AK, SAM, CFP, CFZ, CIP, MER, PTZ | blaOXA-51, blaOXA-143 | H | M/49 | 01/19/10 | 16 | Colonized/No | 01/19/10 | Death | – |
| 297 | Perianal | >32 | 1.0 | 1.0 | AK, SAM, ATM, CFP, CTZ, CIP, GEN MER | blaOXA-51, blaOXA-143 | I | F/77 | 12/08/09 | 12 | Colonized/Yes | 01/19/10 | Death | CVC + IUC + MV |
| 356 | Nasal | 4.0 | 0.5 | <0.5 | AK, SAM, ATM CFP, CTZ, CIP, MER, PTZ, SUT | blaOXA-51, blaOXA-143, blaOXA-23 | F | M/63 | 05/19/10 | 15 | Colonized/Yes | 05/19/10 | Survival | MV |
| 384 | Nasal | >32 | 2.0 | 2.0 | AK, SAM, CFP, CTZ, CIP, GEN, MER, PTZ, SUT | blaOXA-51, blaOXA-143, blaOXA-23 | F | M/58 | 05/13/10 | 9 | Colonized/Yes | 06/10/10 | Survival | CVC + IUC + MV |
| 385 | Blood | >32 | 2.0 | 4.0 | AK, SAM, CFP, CTZ, CIP, GEN, MER, PTZ, SUT | blaOXA-51, blaOXA-143, blaOXA-23 | F | M/75 | 06/11/10 | 10 | NI-sepse/Yes | 06/23/10 | Death | CVC + IUC + MV |
| 390 | Perianal | 8.0 | 4.0 | 4.0 | SAM, ATM, CFP, CTZ, CIP, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | J1 | F/45 | 07/20/10 | 15 | Colonized/Yes | 07/29/10 | Survival | CVC + IUC + MV |
| 394 | Nasal | 16 | 1.0 | 4.0 | AK, SAM, CFP, CTZ, CTX, CIP, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | J1 | F/83 | 07/10/10 | 15 | Colonized/Yes | 07/16/10 | Death | CVC + IUC + MV |
| 395 | Perianal | 16 | 0.5 | 2.0 | AK, SAM, CFP, CTZ, CIP, MER, PTZ | blaOXA-51, blaOXA-143, blaOXA-23 | I | M/59 | 07/16/10 | 15 | Colonized/Yes | 07/28/10 | Death | CVC + IUC + MV |
| 399 | Perianal | >32 | 8.0 | 4.0 | SAM, ATM, CFP, CTZ, CIP, GEN, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | F | F/76 | 07/23/10 | 15 | Colonized/No | 07/24/10 | Survival | IUC |
| 407 | Perianal | 16 | 2.0 | 4.0 | AK, SAM, CFP, CTZ, CIP, GEN, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | B | M/20 | 07/27/10 | 10 | Colonized/Yes | 08/02/10 | Survival | CVC + IUC |
| 408 | Nasal | 16 | 1.0 | <0.5 | AK, SAM, CFP, CTZ, CIP, GEN, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | H | M/68 | 07/28/10 | 18 | Colonized/Yes | 08/02/10 | Death | CVC + IUC + MV |
| 414 | Nasal | >32 | 1.0 | 2.0 | AK, SAM, ATM, CFP, CTZ, CIP, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | H | F/71 | 08/02/10 | 11 | NI-sepse/No | 08/07/10 | Death | CVC + IUC + MV |
| 416 | Perianal | 16 | 1.0 | 4.0 | AK, CFP, CTZ, CIP, GEN, MER, PTZ | blaOXA-51, blaOXA-23 | B | F/68 | 08/02/10 | 20 | Colonized/Yes | 08/06/10 | Death | CVC + IUC + MV |
| 417 | Catheter | 16 | 2.0 | 4.0 | AK, SAM, ATM, CFP, CTZ, CIP, PTZ | blaOXA-51, blaOXA-23 | A | M/47 | 08/03/10 | 20 | NI-PNU/Yes | 08/09/10 | Survival | CVC + IUC + MV |
| 421 | Catheter | 16 | 1.0 | 4.0 | AK, SAM, ATM, CFP, CTZ, CIP, GEN, MER, PTZ | blaOXA-51, blaOXA-23 | B | M/41 | 06/16/10 | 17 | NI-sepse/Yes | 08/17/10 | Death | CVC + IUC + MV |
| 436 | Nasal | 16 | 0.5 | 2.0 | AK, ATM, CFP, CTZ, CIP, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | K | M/85 | 08/25/10 | 13 | NI-PNU/No | 09/01/10 | Death | CVC + IUC + MV |
| 461 | Nasal | 16 | 0.5 | <0.5 | AK, SAM, CFP, CTZ, CIP, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | K | M/79 | 09/16/10 | 13 | NI-PNU/Yes | 13/10/10 | Death | CVC + IUC + MV |
| 464 | Nasal | 16 | 1.0 | 2.0 | AK, SAM, CFP, CTZ, CTX, CIP, MER, PTZ | blaOXA-51, blaOXA-23 | K | F/85 | 08/27/10 | – | Colonized/Yes | 10/03/10 | Death | CVC + IUC + MV |
| 470 | Nasal | >32 | 0.5 | 2.0 | AK, SAM, ATM, CFP, CTZ, CTX, CIP, GEN, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | D | M/47 | 10/08/10 | 16 | Colonized/Yes | 10/15/10 | Death | CVC + IUC + MV |
| 479 | Catheter | 16 | 4.0 | 2.0 | AK, ATM, CFP, CTZ, CTX, CIP, GEN, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | O | F/65 | 09/30/10 | 13 | Colonized/Yes | 10/20/10 | Survival | CVC + IUC + MV |
| 487 | Perianal | 16 | 0.5 | 4.0 | AK, SAM, ATM, CFP, CTZ, CIP, GEN, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | G | M/53 | 10/17/10 | 9 | Colonized/Yes | 10/23/10 | Survival | CVC + IUC + MV |
| 489 | Nasal | 4.0 | 1.0 | 4.0 | AK, SAM, ATM, CFP, CTZ, CIP, GEN, MER, PTZ | blaOXA-51, blaOXA-23 | O | M/64 | 10/22/10 | 10 | NI-sepse/No | 10/26/10 | Death | CVC + IUC + MV |
| 503 | Nasal | 16 | 0.5 | 2.0 | AK, SAM, ATM, CFP, CTZ, CIP, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | O | M/77 | 09/29/10 | 21 | Colonized/Yes | 11/18/10 | Death | CVC + IUC + MV |
| 511 | Nasal | 16 | 1.0 | 2.0 | SAM, ATM, CFP, CTZ, CIP, GEN, MER, PTZ | blaOXA-51 | O | M/56 | 11/08/10 | 13 | Colonized/No | 11/21/10 | Death | – |
| 515 | Perianal | 16 | 0.5 | 2.0 | AK, SAM, ATM, CFP, CTZ, CIP, MER, PTZ, SUT | blaOXA-51, blaOXA-23 | K1 | F/72 | 11/12/10 | 11 | NI-PNU/Yes | 11/22/10 | Death | – |
| 544 | Nasal | 16 | 1.0 | 2.0 | AK, SAM, ATM, CFP, CTZ, CIP, GEN, MER, PTZ, SUT | blaOXA-51 | D1 | F/20 | 12/06/10 | 9 | Colonized/No | 12/24/10 | Survival | CVC + IUC + MV |
IMP, imipenem; POL, polymyxin; TIG, tigecycline; AK, amikacin; SAM, ampicillin/sulbactam; ATM, aztreonam; CFP, cefepime; CTZ, ceftazidime; CTX, ceftriaxone; CIP, ciprofloxacin; GEN, gentamicin; MER, meropenem; PTZ, piperacillin/tazobactam; SUT, sulfamethoxazole/trimethoprim; NI, nosocomial infection; PNU, pneumonia; ATM, previous antimicrobial therapy; CVC, central venous catheter; IUC, indwelling urinary catheter; MV, mechanical ventilation.
ISAba-1 was found upstream of the blaOXA-23 gene.
ERIC-PCR fingerprint patterns were analyzed using the Dice similarity coefficient and the unweighted-pair group method using average linkages cluster method (BioNumeric software, Applied Maths, Kortrijk, Belgium).
The MBL enzyme was not found in any of the 43 CRAb strains by the double-disk synergy test (DDST).
Molecular characterization of CRAb strains
The blaOXA-51 gene was detected in all studied strains, whereas blaOXA-23 was found in 22 (51.2%) and blaOXA-143was detected in eight (18.6%) isolates. Of the 17 strains susceptible to imipenem in the confirmatory phenotypic tests, i.e., agar dilution method, two presented blaOXA-23 and one presented blaOXA-143. Table 3 depicts the main clinical and microbiological characteristics of the 56 patients harboring (colonized or infected by) OXA producing CRAb strains. Other resistance-encoding genes such as VIM and IMP-like were not found among the tested strains. Finally, two imipenem-resistant strains did not display any of the acquired oxacillinases or metallo-β-lactamases during the investigation.
Molecular typing
Genotypic analysis of the CRAb isolates from 43 patients, using the ERIC-PCR, identified 15 different patterns, with 90% similarity according to dendrogram analysis. The main clusters (B, F, H, J, K, O) were observed in 25 (58.1%) CRAb isolates. Sensitive and resistant strains were found for the same genotype (Table 3).
Discussion
Carbapenem-resistant A. baumannii has emerged over the last decades as an important infection-causing microorganism in intensive care patients.1, 2 Despite being considered opportunistic, many authors underline the negative impacts stemming from CRAb infections, probably due to the delay to initiate appropriate antimicrobial therapy.14 However, in spite of the high prevalence of this microorganism in hospitals around the world, especially in Brazil, little is known about the relationship between the presence of resistance genes and the outcome of CRAb infected patients.
CRAb is an important microorganism that colonizes and infects patients in our intensive care unit. During this study CRAb was isolated from ∼10% of the patients and the main determinant of carbapenem resistance among A. baumannii isolates in this population was the production of oxacillinases OXA-23 and OXA-143. Moreover, a polyclonal pattern was observed among the studied strains.
This study presents limitations that are inherent to retrospective analysis and the quality of data depended on the clinical records. Patient enrollment occurred during a limited period of time, with small sample of patients evaluated. Additionally, molecular tests were done in ∼77% of cases (43/54) and even though carbapenem resistance was an inclusion criteria for the study, only ∼60% of A. baumannii samples had this resistance confirmed by microdilution tests. The disk diffusion is a method of screening, with potential limitations as inadequate drug concentration and lability of the drug on the disk. Thus, transportation and disk storage in less than ideal conditions may interfere in the results.15
In a study conducted in Brazil showed that none of the evaluated brands distributed in Brazil presented satisfactory performance.15 It is worth mentioning that some strains even presenting a sensible profile, but with elevated minimum inhibitory concentration, had already genetic mutations (e.g. OXA 23) related to resistance. In this case series only enzymatic mechanisms of resistance were investigated.
Patients colonized or infected by CRAb strains presented a high hospital mortality rate (59%), although mortality could not be attributed only to the presence of CRAb. This feature, however, was not the focus of our study. Recently, Lemos et al. (2013) observed in a meta-analysis that CRAb patients had an increased mortality risk (OR-2.22; 95% CI: 1.66–2.98). These authors included 16 studies and most of them referred to infections due to CRAb, mainly bacteremia. Only one study considered infected and colonized patients. Nevertheless, most patients also presented a severe underlying illness and received inappropriate empirical antimicrobial treatment.16
Treatment options for CRAb infections are scarce and generally employ polymyxins or tigecycline.17, 18 In our study, 76.7% of the strains were susceptible to tigecyline, a lower percentage than the 97.1% found by Rossi et al. in samples from Latin America.19 Regarding polymyxins (polymyxin B and E), resistance to polymyxin B was observed in three (7%) of the analyzed strains in this study (C> 4 μg/mL, CLSI 2011). SENTRY – 2006 found 2.1% resistance to polymyxin in Latin America.20 This finding is worrisome since polymyxins are regarded as the major therapy options for treating CRAB infections, despite their toxicity.21 Another important issue is the variable therapeutic effectiveness of polymyxin on CRAB (50–80%).18, 22
In the present study, we found that the blaOXA-51 gene was present in all strains studied, confirming the A. baumannii species. OXA-51 has weak carbapenemase activity, but potentially increases the minimum inhibitory concentration (MIC) when overproduced.23 Carbapenem resistance in A. baumannii frequently results from the production of acquired oxacillinases, such as OXA-23, OXA-24, and OXA-58, besides the less frequent OXA-143 and OXA-72.24 The production of the carbapenemase OXA-23 has been frequently identified in CRAB strains from patients admitted to Brazilian hospitals.25 In this study, OXA-23 was the most commonly acquired carbapenemase (51.2%), followed by OXA-143, which was identified in eight strains (18.6%). It is worth noting that OXA-143 was described in 2009 by Higgins et al., and has so far only been found in Brazilian hospitals.3, 7, 8
In our study, almost all CRAB strains presented at least one acquired oxacillinase (OXA-23 or OXA-143), and only two resistant strains did not display any of the investigated enzyme mechanisms. In this case, other resistance mechanisms may be involved, such as efflux pumps, changes in outer membrane proteins (OMPs) or changes in affinity or expression of penicillin-binding proteins (PBPs).1
Among the analyzed strains, 15 distinct clones were identified in six main related clusters, which characterizes a polyclonal distribution but includes groupings with >90% similarity. This fact is probably related to cross-transmission. CRAb strain was first identified in our institution in 2003, and since then there has been an exponential increase in number of cases. HC/UFMG is a hospital of high complexity and public reference center. Thus, the hospital always receives many patients who are possibly colonized/infected with CRAb from many other services.
In the present study, three out of the four CRAb patients with the K clone ultimately passed away. Moreover, the blaOXA-23 gene was identified in all these CRAb strains. Due to the small sample of patients and the heterogeneity of their clinical and microbiological presentations, associations regarding the production of oxacillinases and patient mortality could not be performed in this study.
Conclusions
Despite the limitations, this study showed that CRAb colonized or infected patients presented a high frequency of comorbidities, with long hospital stays and increased hospital morbidity. The strains studied showed a wide genetic diversity with polyclonal dissemination pattern. The most frequent resistance mechanism was the production of carbapenemases, notably OXA-23, but with cases of OXA-143 expression. Although the production of metallo-β-lactamases was not identified, other mechanisms may be involved, as there were resistant strains that did not express any gene related to carbapenemases. Strains drug resistance profile was discordant in 40% of cases when using disk diffusion and agar dilution methods. We also need to underscore that polymyxin B resistance was higher than described in literature and this may be a challenge for patient management, due to the limitations of therapeutic options. In our view, prevention for MDR infection is the key for better assisting patients in intensive care unit and antimicrobial susceptibility tests need to present more rigorous quality control. The increasing of polymixin resistance also warrants a better approach.
Funding
Foundation for Research Support of the State of São Paulo (FAPESP), Higher Education Personnel Improvement Coordination (CAPES) and Nacional Council of Scientific and Technological Development (CNPq).
Conflicts of interest
The authors declare no conflicts of interest.
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