Natural history of Acinetobacter baumannii infection in mice

Abstract

In 2017, the WHO identified Acinetobacter baumannii as the top priority for the development of new antibiotics. Despite the need for new antibiotics, there remains a lack of well validated preclinical tools for A . baumannii . Here, we characterize and validate a mouse model for A . baumannii translational research. Antibiotic sensitivity for meropenem, amikacin, and polymyxin b was determined by the broth microdilution MIC assay. LD 100 inoculums, in both blood and lung infection models, were determined in male and female C3HeB/FeJ mice that were challenged with various A . baumannii clinical isolates. Blood (blood infection model) or blood and lung tissue (lung infection model) were collected from infected mice at 2 and 18 hours and the bacterial burden was determined by quantitative culture. Blood chemistry was analyzed using the iStat system. Cytokines (IL-1ß, TNF, IL-6, and IL-10) were measured in the blood and lung homogenate by ELISA assay. Lung sections (H&E stains) were scored by a pathologist. In the blood infection model, the cytokines and physiological data indicate that mice become moribund due to sepsis (low blood pH, falling bicarbonate, and a rising base deficit), whereas mice become moribund due to respiratory failure (low blood pH, rising bicarbonate, and a falling base deficit) in the oral aspiration pneumonia model. We also characterized the timing of changes in various clinical and biomarker endpoints, which can serve as a basis for future interventional studies. Susceptibility was generally similar across gender and infection route. However, we did observe that female mice were approximately 2-fold more sensitive to LAC-4 Col R in the blood infection model. We also observed that female mice were more than 10-fold more resistant to VA-AB41 in the oral aspiration pneumonia model. These results establish parameters to follow in order to assess efficacy of novel preventative and therapeutic approaches for these infections.

Capsule carbohydrate structure determines virulence in Acinetobacter baumannii

“…Capsule carbohydrate analysis of ATCC 17978 Δgtr6 revealed the loss of the N-acetyl-β-D-glucosamine residue seen in the wild type strain (residue E above) as well as the retention of 50% acetylation of residue A consistent with the absence of a pgt1 gene in the mutant strain. As previously published, HUMC1 is intrinsically resistant to phagocytosis by neutrophils and macrophages, resulting in increased virulence in intravenous and intratracheal mouse infection models [14]. As for ATCC 17978 and NIH1, newly constructed strains with disrupted gtr6 exhibited similar degrees of marked reduction in phagocytic uptake compared to their isogenic strains with intact gtr6 (Fig 2A).…”

“…ATCC 17978, a lab-adapted avirulent reference strain originally isolated from cerebrospinal fluid more than 50 years ago, is a KL3-type strain. Only two differences were found in the capsule loci of these strains, which exhibit vastly different in vivo virulence [14]. First was the presence of an extra gene (pgt1) near the end of the capsule locus in the KL22 type strain (HUMC1), and not in the KL3 strain (ATCC 17978).…”

Synergistic Rifabutin and Colistin Reduce Emergence of Resistance When Treating Acinetobacter baumannii

“… Efficacy of rifabutin colistin combination treatment in vivo . (A) C3HeB/FeJ mice ( n = 10 per group) were infected with 1.2 × 10 7 to 3.9 × 10 7 CFU of the hypervirulent (100% lethal dose [LD 100 ] < 2 × 10 7 CFU) carbapenem-resistant A. baumannii HUMC1 ( 9 , 10 ) and treated with PBS, 0.05 mg/kg RBT, 0.005 mg/kg COL, or a combination of RBT and COL. There was a significant difference comparing the RBT+COL group to the PBS (<0.001, log rank) and COL monotherapy groups (0.0113, log rank).…”

In vivo Fitness of Acinetobacter baumannii Strains in Murine Infection Is Associated with International Lineage II-rep-2 and International Lineage III Clones Showing High Case Fatality Rates in Human Infections

“…These models must be relevant to human infection. The limitation of many animal models is that it is unclear how well they correlate with clinical outcome, since routes of infection are often non-physiologic (for instance, intra-peritoneal injection), or mice are rendered immunocompromised [ 26 ]. Our study provides evidence that murine infection models can be used to stratify CRAB clonal groups according to risk.…”