In vitro activity of antibiotic combinations against multidrug-resistant strains of Acinetobacter baumannii and the effects of their antibiotic resistance determinants.

Various combinations of antibiotics are reported to show synergy in treating nosocomial infections with multidrug-resistant (MDR) Acinetobacter baumannii (A. baumannii). Here, we studied hospital-acquired outbreak strains of MDR A. baumannii to evaluate optimal combinations of antibiotics. One hundred and twenty-one strains were grouped into one major and one minor clonal group based on repetitive PCR amplification. Twenty representative strains were tested for antibiotic synergy using Etest(®). Five strains were further analyzed by analytical isoelectric focusing and PCR to identify ß-lactamase genes or other antibiotic resistance determinants. Our investigation showed that the outbreak strains of MDR A. baumannii belonged to two dominant clones. A combination of colistin and doxycycline showed the best result, being additive or synergistic against 70% of tested strains. Antibiotic additivity was observed more frequently than synergy. Strains possessing the same clonality did not necessarily demonstrate the same response to antibiotic combinations in vitro. We conclude that the effect of antibiotic combinations on our outbreak strains of MDR A. baumannii seemed strain-specific. The bacterial response to antibiotic combinations is probably a result of complex interactions between multiple concomitant antibiotic resistance determinants in each strain.

Implications for vancomycin-resistant Enterococcus colonization associated with Clostridium difficile infections.

BACKGROUND: Vancomycin-resistant Enterococcus (VRE) colonization of the gastrointestinal tract shares similar risk factors with Clostridium difficile infection. We sought to elucidate the prevalence and risk factors of VRE colonization associated with C difficile infection. METHODS: All adult inpatients with C difficile infection from July 2006 to October 2006 were prospectively evaluated. All C difficile toxin-positive stool samples were screened for detection of VRE. Risk factors for VRE colonization were compared in patients with C difficile infection with and without VRE colonization. RESULTS: Of the 158 cases of C difficile infection evaluated, 88 (55.7%) involved VRE colonization. Independent risk factors for VRE colonization were admission from long-term care facilities (P = .013), dementia (P = .017), and hospitalization in the previous 2 months (P = .014). No statistically significant difference between C difficile infection cases with and without VRE colonization in terms of previous receipt (within 1 month) of antibiotics, including metronidazole and vancomycin, was found on multivariate analysis. C difficile infection cases with VRE colonization had a higher prevalence of coinfection with methicillin-resistant Staphylococcus aureus (P = .002) and Acinetobacter spp (P = .006). CONCLUSION: VRE colonization was associated with >50% of C difficile infection cases and with a higher rate of coinfection with multidrug-resistant pathogens. Given the high rate of C difficile infection associated with VRE colonization, active surveillance of VRE in patients with C difficile infection is reasonable in high-risk settings.

Comparison of clinical severity score indices for Clostridium difficile infection.

OBJECTIVE: To compare 8 severity score indices for Clostridium difficile infection (CDI). DESIGN: Prospective observational study. METHODS: This study was conducted from July through October 2006. All hospitalized patients in 3 university-affiliated hospitals with a positive fecal Clostridium difficile toxin assay result were evaluated. Infection was considered severe if patients had at least 1 of the following clinical events during their hospitalization: (1) death attributed to CDI within 30 days after diagnosis, (2) colectomy necessitated by CDI, or (3) intensive care unit admission for management of complications attributed to CDI. Severity was assessed on the basis of 8 severity score indices, using published criteria for severe CDI as the benchmark. The 8 severity score indices studied were Beth Israel, University of Pittsburgh Medical Center version 1, University of Pittsburgh Medical Center version 2, Hines VA, modified University of Illinois, University of Calgary version 1, University of Calgary version 2, and University of Temple. RESULTS: Of 184 patients with CDI evaluated, 19 had severe cases and 165 had nonsevere cases, as assessed on the basis of the defined severe CDI criteria. Sensitivities of the 8 severity score indices studied ranged from 63.2% to 84.2%, and specificities ranged from 59.4% to 93.9%. The Hines VA index had the highest kappa score (0.69 [95% confidence interval, 0.54-0.83]), followed by the University of Pittsburgh Medical Center version 1 index. Independent risk factors for severe CDI determined by multivariate analysis were abdominal distention (P = .007), fever (temperature, 38.0°C or above; P = .042), white blood cell count of at least 20,000 cells/mm(3) (P = .035), and hypoalbuminemia (serum albumin level less than 3 mg/dL; P = .029). CONCLUSION: The Hines VA CDI severity score index appeared to display the strongest correlation for predicting more severe forms of CDI.

Resistance to colistin in Acinetobacter baumannii associated with mutations in the PmrAB two-component system.

The mechanism of colistin resistance (Col(r)) in Acinetobacter baumannii was studied by selecting in vitro Col(r) derivatives of the multidrug-resistant A. baumannii isolate AB0057 and the drug-susceptible strain ATCC 17978, using escalating concentrations of colistin in liquid culture. DNA sequencing identified mutations in genes encoding the two-component system proteins PmrA and/or PmrB in each strain and in a Col(r) clinical isolate. A colistin-susceptible revertant of one Col(r) mutant strain, obtained following serial passage in the absence of colistin selection, carried a partial deletion of pmrB. Growth of AB0057 and ATCC 17978 at pH 5.5 increased the colistin MIC and conferred protection from killing by colistin in a 1-hour survival assay. Growth in ferric chloride [Fe(III)] conferred a small protective effect. Expression of pmrA was increased in Col(r) mutants, but not at a low pH, suggesting that additional regulatory factors remain to be discovered.