The role of the genetic elements bla oxa and IS Aba 1 in the Acinetobacter calcoaceticus-Acinetobacter baumannii complex in carbapenem resistance in the hospital setting

Abstract: INTRODUCTION: Members of the Acinetobacter genus are key pathogens that cause healthcare-associated infections, and they tend to spread and develop new antibiotic resistance mechanisms. Oxacillinases are primarily responsible for resistance to carbapenem antibiotics. Higher rates of carbapenem hydrolysis might be ascribed to insertion sequences, such as the ISAba1 sequence, near bla OXA genes. The present study examined the occurrence of the genetic elements bla OXA and ISAba1 and their relationship with susceptibility to carbapenems in clinical isolates of the Acinetobacter calcoaceticus-Acinetobacter baumannii complex. METHODS: Isolates identified over 6 consecutive years in a general hospital in Joinville, Southern Brazil, were evaluated. The investigation of 5 families of genes encoding oxacillinases and the ISAba1 sequence location relative to bla OXA genes was conducted using polymerase chain reaction. RESULTS: All isolates presented the bla OXA-51-like gene (n = 78), and 91% tested positive for the bla OXA-23-like gene (n = 71). The presence of ISAba1 was exclusively detected in isolates carrying the bla OXA-23-like gene. All isolates in which ISAba1 was found upstream of the bla OXA-23-like gene (n = 69) showed resistance to carbapenems, whereas the only isolate in which ISAba1 was not located near the bla OXA-23-like gene was susceptible to carbapenems. The ISAba1 sequence position of another bla OXA-23-like-positive isolate was inconclusive. The isolates exclusively carrying the bla OXA-51-like gene (n = 7) showed susceptibility to carbapenems. CONCLUSIONS: The presence of the ISAba1 sequence upstream of the bla OXA-23-like gene was strongly associated with carbapenem resistance in isolates of the A. calcoaceticus-A. baumannii complex in the hospital center studied.

Doripenem vs meropenem against Pseudomonas and Acinetobacter.

Recently, doripenem has been approved for the treatment of nosocomial pneumonia (NP), including ventilator-associated pneumonia (VAP). The E-test was performed to determine the MICs of doripenem and meropenem in 203 endotracheal aspirate isolates that consisted of 140 Acinetobacter calcoaceticus-Acinetobacter baumannii complexes and 63 Pseudomonas aeruginosa. Doripenem showed minimum concentration necessary for inhibition of 50% (MIC 50 ) of P. aeruginosa isolates at 0.38 mg/L which is several times (84.2 times) lower than the corresponding MIC 50 value of >32 mg/L for meropenem. The MIC 50 and MIC 90 were similar for both the drugs against A. baumannii. Thus, P. aeruginosa was consistently more susceptible than the A. baumannii.

Sensibilidad de Acinetobacter a la colistina evaluada mediante los métodos de predifusión y de concentración inhibitoria mínima: Detección de aislamientos heterorresistentes / Comparative evaluation of the sensitivity of Acinetobacter to colistin, using the prediffusion and minimum inhibitory concentration methods: Detection of heteroresistant isolates

El objetivo de este estudio fue evaluar comparativamente los métodos de predifusión y de concentración inhibitoria mínima para establecer la sensibilidad de aislamientos del complejo Acinetobacter calcoaceticus-baumannii (ABC) a la colistina y detectar a aquellos que presenten heterorresistencia a dicho antibiótico. Se estudiaron 75 aislamientos de ABC recuperados de materiales clínicamente significativos. Se determinó su sensibilidad a la colistina por el método de predifusión y de concentración inhibitoria mínima. Todos los aislamientos resultaron sensibles, con CIM = 2 µg/ml y halos de inhibición en el ensayo de la predifusión = 20 mm. Mediante el método de eficiencia de plaqueo se evaluó la presencia de heterorresistencia a la colistina. Se encontraron 14 aislamientos que originaron colonias heterorresistentes; sus CIM aumentaron en algunos casos en más de 8 veces. Con estas colonias seleccionadas se repitió el ensayo de predifusión. Finalmente se confeccionaron los gráficos de dispersión y se realizaron los análisis de regresión lineal, tanto para el conjunto inicial de todos los aislamientos clínicos como para el subgrupo de los aislamientos resistentes generados durante la evaluación de la heterorresistencia. Se obtuvieron coeficientes de determinación (r²) de 0,2017 y 0,604, respectivamente, lo que indica correlación entre los métodos sólo al evaluar aislamientos preseleccionados por su resistencia a este agente.

The objective of this study is to perform a comparative evaluation of the prediffusion and minimum inhibitory concentration (MIC) methods for the detection of sensitivity to colistin, and to detect Acinetobacter baumanii-calcoaceticus complex (ABC) heteroresistant isolates to colistin. We studied 75 isolates of ABC recovered from clinically significant samples obtained from various centers. Sensitivity to colistin was determined by prediffusion as well as by MIC. All the isolates were sensitive to colistin, with MIC = 2µg/ml. The results were analyzed by dispersion graph and linear regression analysis, revealing that the prediffusion method did not correlate with the MIC values for isolates sensitive to colistin (r² = 0.2017). Detection of heteroresistance to colistin was determined by plaque efficiency of all the isolates with the same initial MICs of 2, 1, and 0.5 µg/ml, which resulted in 14 of them with a greater than 8-fold increase in the MIC in some cases. When the sensitivity of these resistant colonies was determined by prediffusion, the resulting dispersion graph and linear regression analysis yielded an r² = 0.604, which revealed a correlation between the methodologies used.

Antibiotic resistance pattern and R-plasmids of Acinetobacter calcoaceticus subsp. anitratus.

During a 6 month period from March 1990 to August 1990, a total of 159 strains of Acinetobacter calcoaceticus subsp. anitratus were isolated from various samples and studied for antibiotic resistance pattern to 12 drugs by Kirby-Bauer method. Ceftazidime and Netilmycin were the most sensitive drugs followed by Cefotaxime, Norfloxacin and Augmentin. All the strains were resistant to Chloramphenicol and Tetracycline. Commonest pattern of resistance was ACGKSTSu. Forty eight isolates were tested for R-plasmids by conjugation experiments using Nalidixic acid resistant E. coli K12F-Lac+ as the recipient strain. The incidence of R-plasmids was 81.25%.

Comparative activity of ciprofloxacin, imipenem, amikacin and 16 other agents versus Kuwaiti acinetobacter strains

The susceptibility patterns of Acinetobacter cafcoacetus isolates of Kuwait's Farwaniya Hospital versus ceftazidime, ciprofloxacin, imipenem and 16 other agents were determined using the automated microdilution minimum inhibitory concentration technique of the Vitek system. The identification of isolated strains was performed with the Vitek GNI cards. Very rare [0 - 6%] A. calcoacetus strains of this study displayed a full susceptibility to the first and second generation cephalosporins, ampicillin, nitrofurantoin, chloramphenicol and aztreonam. The highest susceptibility frequency was found for ciprofloxacin and imipenem. The side analysis of susceptibility results to agents which did not display an extreme susceptibility or resistance [cotrimoxazole, amoxicillin /clavulanic acid, ceftazidim, aminoglycosides] has revealed an overall lower suscepiibility frequency for A. calcoacetus stains from the intensive care, medical or surgical wards than from casualty] or the paediatric and maternity wards