The influence of biofilm formation and multidrug resistance on environmental survival of clinical and environmental isolates of Acinetobacter baumannii

Abstract

Background: Acinetobacter baumannii is a gram-negative, opportunistic pathogen. Its ability to form biofilm and increasing resistance to antibiotic agents present challenges for infection control. A better understanding of the influence of biofilm formation and antibiotic resistance on environmental persistence of A baumannii in hospital settings is needed for more effective infection control.

Methods: A baumannii strains isolated from patients and the hospital environment were identified via Matrix Assisted Laser Desorption Ionization Time-of-Flight (MALDI-TOF) mass spectrometry (Bruker Daltonics, Bellerica, MA), repetitive extragenic palindromic polymerase chain reaction genotyped, and antibiotic resistance was determined using Vitek 2 (bioMérieux, Inc, Durham NC). Biofilm mass was quantified via microtiter plate method and desiccation tolerance determined up to 56 days.

Results: High biofilm forming, clinical, multidrug-resistant- (MDR) positive strains were 50% less likely to die of desiccation than low biofilm, non-MDR strains. In contrast, environmental, MDR-positive, low biofilm forming strains had a 2.7 times increase in risk of cell death due to desiccation compared with their MDR-negative counterparts. MDR-negative, high biofilm forming environmental strains had a 60% decrease in risk compared with their low biofilm forming counterparts.

Conclusion: The MDR-positive phenotype was deleterious for environmental strains and the high biofilm phenotype was critical for survival. This study provides evidence of the trade-off between antibiotic resistance and desiccation tolerance, driven by condition-dependent adaptation, and establishes rationale for research into the genetic basis of the variation in fitness cost between clinical and environmental isolates.

Keywords: Condition-dependent adaptation; Desiccation tolerance; Environmental transmission; Fitness cost; Hospital; Infection control.

Figure 1:

Dendogram of 73 dominant rep-type strains consisting of 52 clinical and 21 environmental isolates (1A) and dendogram of 72 sporadic rep-type strains consisting of 63 clinical and 9 environmental isolates (1B) with corresponding gel lanes and antibiotic susceptibility profiles for each isolate. Antibiotic susceptibility is indicated as green=susceptible, yellow=intermediate, red=resistant. *Environmental isolates.

Figure 2:

Percent Multidrug Resistant (MDR), Box plots of biofilm OD₆₀₀ and Survival Curves with standard error bars, hazard ratio (HR) and p-values of A. baumannii isolates collected from a University Hospital between Aug 2012 and Jan 2014, comparing (A) 115 clinical and 30 environmental isolates (B) 73 dominant and 72 sporadic rep-type isolates and (C) clinical and environmental isolates stratified by rep-type. Multidrug resistance was defined as being resistant to the following three drug classes (20): cephalosporins, fluoroquinolones, and aminoglycosides or resistant to two and intermediate to one.

Figure 3:

Likelihood of survival of 115 clinical and 30 environmental isolates of A. baumannii determined using a Cox proportional hazards model, accounting for rep-type, clinical/environmental status, biofilm formation capability and MDR phenotype. Clinical comparison reference group: MDR−, Low BF, clinical isolates. Environmental comparison reference group: MDR−, Low BF, environmental isolates. MDR−, multidrug negative phenotype; MDR+, multidrug positive phenotype; BF, Biofilm.