Carbapenem-resistant Acinetobacter baumannii outbreak at university hospital.

Nineteen clonally related imipenem-resistant Acinetobacter baumannii isolates were recovered from eight intensive care unit patients. All isolates harboured bla OXA-51-like β-lactamase genes and showed the absence of 22 kDa fraction in outer membrane porin profile analysis. It suggests a combination of two mechanisms as responsible for carbapenem-resistant phenotypes.

INTRODUCTION

In Brazilian hospitals, multidrug-resistant (MDR) Acinetobacter baumannii constitute a serious cause of nosocomial infection, comprising 8.8% of the total nosocomial bacterial isolates that cause infections in ICU patients, according to the MYSTIC Program Brazil (11). In this respect, carbapenems remain as the widest spectrum therapeutic option for treatment of such infections. However, resistance to these antimicrobial agents has increased, resulting in the use of potentially more toxic agents such as the polymyxins (7). Although high carbapenem resistance rates have been reported among Acinetobacter spp. isolated in Brazil, very little is known about their mechanisms of resistance. Recently, it has been reported that IMP-1 metallo-beta lactamase-producing Acinetobacter strains emerged in 1998 in some Brazilian hospitals (15). Regarding OXA-type carbapenemases, only the blaOXA-23-like gene has been associated with imipenem resistance in Brazil (6). We hereby report the combination of the naturally intrinsic harboured blaOXA-51-like gene and impermeability as mechanism responsible for imipenem–resistant phenotype in clonally related A. baumannii recovered from an outbreak, in a Brazilian teaching hospital.

From September 2005 to February 2006 eleven MDR, Acinetobacter baumannii isolates were recovered from six ICU patients hospitalized at the Hospital Universitário da Universidade de São Paulo (HU-USP). Species identification and antimicrobial susceptibility (Table 1) were evaluated using the Vitek system (BioMérieux, Hazlewood, Mo.) and the disk diffusion method, respectively. Molecular typing was performed by Pulsed Field Gel Electrophoresis (PFGE) of ApaI-digested genomic DNA of A. baumannii isolates (17). PFGE band profiles were identical for all carbapenem-resistant strains. Minimum inhibitory concentrations (MICs) for all isolates were determined by the agar dilution method (4). Additionally, some combinations antibiotic/β-lactamase inhibitors were tested as follows: ceftazidime/clavulanic acid (4.0 mg/L) (4), imipenem/EDTA (320 mg/L) (22), imipenem/NaCl (200 mM) (16). All strains were found to be resistant to more than 3 antimicrobial groups (Table 1), presenting MICs ≥ 32 and ≥ 64 mg/L for imipenem and ceftazidime, respectively. The inhibitors tested did not affect MIC´s values when associated with imipenem or ceftazidime.

Table 1

Antibiotic susceptibilities of the Acinetobacter baumannii strains in this study.

Case	Origin (Specimen)	Isolation date	Hospital unit	Antimicrobial susceptibility
Case 1	Urine	05/09/2005	Adult ICU	none
Case 2	Tracheal secretion	11/01/2006	Adult ICU	ART
	Blood	12/01/2006	Adult ICU	SAM, ART
Case 3	Blood	19/01/2006	Adult ICU	SAM
	Catheter tip	19/01/2006	Adult ICU	SAM
	Catheter tip	19/01/2006	Adult ICU	SAM
Case 4	Abdominal secretion	20/01/2006	Adult ICU	SAM
	Tracheal secretion	24/01/2006	Adult ICU	SAM
Case 5	Tracheal secretion	31/01/2006	Adult ICU	SAM
	Blood	02/02/2006	Adult ICU	SAM, FEP
Case 6	Vaginal secretion	10/02/2006	Adult ICU	SAM, FEP

Table captions: All strains were tested by Kirby Bauer method for: PIP, piperacillin; TZP, piperacillin/tazobactam; CAZ, ceftazidime; CTX, cefotaxime; FEP, cefepime; ATM, aztreonam; IMP, imipenem; MEM, meropenem; CIP, ciprofloxacin; AMK, Amikacin; GEN, Gentamicin; SXT, Trimethoprim/Sulfamethoxazole; ART, aztreonam; SAM, ampicillin/sulbactam; ICU, intensive care unit.

Research on carbapenemase production and outer membrane porin profile was performed using imipenem-susceptible and non-susceptible isolates recovered from HU-USP.

Carbapenemase activity was evaluated using a bioassay (9). This test involved satellite growth of Staphylococcus aureus ATCC 25923 around the putative carbapenemaseproducing A. baumannii strains growing on Muller-Hinton agar plates containing 108 CFU of ATCC strain/mL and imipenem at a concentration of 0.06 or 0.12 mg/L. Imipenemase activity was confirmed in all imipenem resistant isolates.

Metallo-β-lactamase (MBL) production was then screened by a double disk synergy test using ceftazidime and imipenem as substrates and EDTA and thiol compounds as β-lactamase inhibitors (1,12). Imipenemase activity was not inhibited by EDTA or thiol compounds, suggesting that a serine-type β-lactamase was responsible for the hydrolysis of imipenem.

Imipenem-susceptible and resistant A. baumannii were screened by PCR for the blaIMP, blaVIM, blaSPM-1, blaOXA-23-like, blaOXA-24-like, blaOXA-51-like and blaOXA-58-like genes, as previously described (21,23). The blaOXA-51-like gene was the only one detected, even in imipenem-susceptible strain, confirming global reports of the intrinsic presence of class D carbapenemase in A. baumannii (10,20). ISAba1 was also found by PCR, but the insertion sequence was not upstream of the blaOXA-51-like gene in any isolate (21).

Alterations in permeability were evaluated by outer membrane porin (OMP) analysis in imipenem susceptible (MIC 0.5 mg/L) and resistant A. baumannii isolates. The OMP fractions were prepared by the N-Lauryl-sarcosinate method (5). Total OMP concentration was measured according to Bradford (3). OMP profiles were analyzed by SDS-PAGE and showed absence of an expected 22 kDa fraction in the imipenemresistant isolates.

A. baumannii is recognized as playing a significant role in the colonization and infection of hospitalized patients, especially those in critical care environments. The carbapenems, such as imipenem, have been widely used to treat infections caused by MDR A. baumannii clinical isolates, nevertheless, regrettably, carbapenem-resistant A. baumannii clinical isolates have become more prevalent. In this respect, impermeability or drug inactivation by carbapenemases belonging to metallo-betalactamase class B or some class D OXA-type enzyme subgroups have been described as major causes of resistance.

Additionally, a decrease in outer membrane permeability has been associated with resistance to carbapenems in A. baumannii clinical strains (15). It was associated to with the loss of 29 kDa OMP (13), 31-36kDa (5), 25/29 kDa corresponding to the so-called CarO (2,5,18,14). Thus, isolates with weak OXA carbapenemases could be required to bear additional codeterminants of resistance, in particular, the absence of outer-membrane proteins as demonstrated by Costa et al. (5), whose resistant isolates had acquired two β-lactamases and had also lost a protein of 31-36 kDa. Bou et al. (2) report a multiresistant isolate that produce OXA-24 with reduced expression of two proteins 22kDa, the same lacked protein in our isolate, and 33kDa.

At the ICU from HU-USP, the outbreak involved eight cases of infection by a single RAPD-PCR clone. Carbapenem resistant phenotype was related to the lack of a 22 kDa OMP and the presence of blaOXA-51-like β-lactamase genes. Although blaOXA51-like β-lactamase genes were the only ones identified, further studies are necessary to understand the role of these genes like resistance mechanism.