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Revista do Instituto de Medicina Tropical de São Paulo

On-line version ISSN 1678-9946

Rev. Inst. Med. trop. S. Paulo vol.51 no.2 São Paulo Mar./Apr. 2009 



Indications of carbapenem resistance evolution through heteroresistance as an intermediate stage in Acinetobacter baumannii after carbapenem administration


Indicações da evolução da resistência ao carbapenem através da heteroresistência como estágio intermediário no Acinetobacter baumannii após administração de carbapenem



Silvana Vargas SupertiI; Daniela de Souza MartinsI; Juliana CaierãoI; Fabiana da Silva SoaresI; Taísa ProchnowI; Alexandre Prehn ZavasckiII

ILaboratory of Microbiology, Hospital São Lucas da Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, RS, Brazil
IIDept. Infectology, Hospital São Lucas da Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, RS, Brazil

Correspondence to




We describe an in vivo evolution of an antimicrobial profile from susceptibility to full-resistance to carbapenems, with heteroresistance as an intermediate stage, in an Acinetobacter baumannii strain. Heteroresistance was characterized by the growth of sub-populations within the susceptibility halo in both disk-diffusion and Etest. PCRs for the main A. baumannii carbapenemases were negative. The exact resistance mechanism, diagnostic methods and clinical relevance of heteroresistance in A. baumannii warrant further investigations. This is the first description of such phenomenon in vivo and the second report of heteroresistance to carbapenems in A. baumannii.

Keywords: Acinetobacter baumannii; Carbapenem-heteroresistance; Meropenem treatment.


Descrevemos a evolução in vivo, de um perfil de sensibilidade aos antimicrobianos, passando de sensibilidade a resistência total aos antibióticos carbapenêmicos, com um estágio intermediário de heteroresistência em isolado de Acinetobacter baumannii. A heteroresistência foi caracterizada pelo crescimento de sub-população na zona de inibição pelo método de disco-difusão e pelo Etest. PCRs para as principais carbapenemases envolvidas com resistência neste microrganismo foram negativas. O exato mecanismo de resistência envolvido, método diagnóstico e relevância clínica justificam investigação adicional. Esta é a primeira descrição deste fenômeno in vivo e o segundo relato de heteroresistência em A. baumannii.




Acinetobacter baumannii, which is associated with a wide range of infections, particularly pneumonia among intensive care unit (ICU) patients, has emerged as an important nosocomial pathogen in many parts of the world4. The carbapenems, imipenem and meropenem, are among the drugs of choice used to treat nosocomial infections due to multidrug-resistant A. baumannii isolates4.

As other Gram-negative rods, resistance in these bacteria is known to be homogeneous within a culture although a report has described the spread of hetero-carbapenem-resistant A. baumannii isolates7. In the present report, we describe the recovery from blood of an A. baumannii isolate presenting heteroresistance to carbapenems in a patient receiving meropenem due to an intra-abdominal abscess.



In March 2006, a first A. baumannii resistant to both imipenem and meropenem was identified in our institution12. Two isolates were recovered from an intra-abdominal abscess secretion and from the blood of a 67-year-old female patient at ICU, respectively. Identification of the isolates has been based on biochemical tests, including mobility, oxidase activity, oxidation of glucose on OF-medium; and API 20 NE method (6.0 version, bio-Merieux, Marcy L'Etoile, France). This patient was receiving I.V. meropenem (1g IV every eight hours) for exactly 31 days, for the treatment of the abscess after an esophagectomy for a carcinoma of the esophagus. The therapy was changed for polymyxin B (75 mg IV every 12 hours) when the culture results became available (three days after collection), but the patient ultimately died of septic shock seven days after starting new antimicrobial treatment. 

The phenotypic profile of this isolate showed heteroresistance to both imipenem and meropenem, as indicated by the growth of sub-populations within the clear zone of inhibition, which demonstrated susceptibility (20 and 19 mm, respectively) in the disk-diffusion test (Fig. 1A), performed according to Clinical Laboratory Standards Institute guidelines3. The test was repeated with the sub-populations and the same phenotypic profile was observed, i.e. the growth of resistant sub-populations within the zone of inhibition with a diameter indicating susceptibility. These findings were confirmed using the Etest (AB Biodisk, Solna, Sweden) (Fig. 1B). The same phenotypic phenomenon was observed when the sub-populations were re-tested.





Phenotypic tests for MBL detection using a disk approximation test with imipenem (Oxoid) in the presence of EDTA were negative for both isolates. Additionally, PCR for carbapenemases genes (blaIMP, blaVIM, blaSPM, bla OXA-23, bla OXA-24, bla OXA-51 and blaOXA-58 was carried out using specific primers as described elsewhere1-5, but there was no product of amplification for any primers, except for the blaOXA-51.

Forty-three days latter, a carbapenem-resistant A. baumannii was recovered from a tracheal aspirate of a 44-years male ICU patient, who was under mechanical ventilation due to severe tetanus. This patient received I.V. meropenem (1g t.i.d.) during the previous 12 days as empirical treatment for a ventilator-associated pneumonia. This isolate was fully resistant to imipenem and meropenem (MIC >32 µg/mL for both) by Etest method.

Both hetero and full-resistant isolates were submitted to molecular typing with other A. baumannii isolate previously recovered from patients at ICU and proved to be the same strain of a carbapenem-susceptible isolate recovered 102 days before the recovery of the heteroresistant one. This latter patient was a 65-year-old woman with erythematous systemic lupus, who was hospitalized at ICU and presented the A. baumannii recovered from the tracheal aspirate and blood. This patient had not received carbapenem.



We presented a rare case of a carbapenem-heteroresistant A. baumannii recovered from blood and abscess secretion of a patient receiving meropenem therapy. A previous report has described heteroresistance to carbapenems in eight A. baumannii isolates which were also neither MBL nor oxacillinase producers7. As in this previous study, the exact mechanism of resistance in our strain could not be determined. Although the main A. baumannii carbapenemases have been ruled out, the isolate carried a gene encoding an OXA-51-like group β-lactamase, a gene present in most A. baumannii strains, but with a non-clearly defined role in carbapenem resistance11.

Heteroresistance, which may be defined as a phenotypic manifestation of resistance within a genetically homogeneous strain10, has only rarely been described in Gram-negative bacteria6-9. Although authors are not certain about the clinical impact of this resistance phenotype in Gram-negative rods, our report suggests that heteroresistance may adversely affect the outcome of patients, since the A. baumannii isolate was recovered from the blood of a patient receiving meropenem therapy.

In conclusion, heteroresistance in nonfermentative Gram-negative bacteria is a rare and poorly understood phenomenon, the exact mechanism determining this phenotype, the best diagnostic methods and its clinical relevance warrant further investigations.



We are very grateful to Líbera Dalla Costa, who had kindly performed PCR tests.



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12. ZAVASCKI, A.P.; SOARES, F.C.; SUPERTI, S.V. et al. - Stable carbapenem susceptibility rates among multidrug-resistant Acinetobacter spp. strains in a setting of high prevalence of carbapenem-resistant Pseudomonas aeruginosa. Int. J. Antimicrob. Agents, 30: 187-189, 2007.         [ Links ]



Correspondence to:
Silvana Vargas Superti
Hospital São Lucas da Pontifícia Universidade Católica do Rio Grande do Sul
Laboratório de Microbiologia
Av. Ipiranga 6690, 90610-000
Porto Alegre, RS, Brasil
Tel./Fax: +55.51.33203145

Received: 9 September 2008
Accepted: 5 March 2009

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