Host fate is rapidly determined by innate effector-microbial interactions during Acinetobacter baumannii bacteremia

CipA of Acinetobacter baumannii Is a Novel Plasminogen Binding and Complement Inhibitory Protein

Acinetobacter baumannii is an emerging opportunistic pathogen, responsible for up to 10% of gram-negative, nosocomial infections. The global increase of multidrug-resistant and pan-resistant Acinetobacter isolates presents clinicians with formidable challenges. To establish a persistent infection,A. baumannii must overcome the detrimental effects of complement as the first line of defense against invading microorganisms. However, the immune evasion principles underlying serum resistance inA. baumannii remain elusive. Here, we identified a novel plasminogen-binding protein, termed CipA. Bound plasminogen, upon conversion to active plasmin, degraded fibrinogen and complement C3b and contributed to serum resistance. Furthermore, CipA directly inhibited the alternative pathway of complement in vitro, irrespective of its ability to bind plasminogen. A CipA-deficient mutant was efficiently killed by human serum and showed a defect in the penetration of endothelial monolayers, demonstrating that CipA is a novel multifunctional protein that contributes to the pathogenesis ofA. baumannii.

Cryopreservation of virulent Acinetobacter baumannii to reduce variability of in vivo studies

Background

Microbiological assays require accurate and reproducible preparation of bacterial inocula. Inocula prepared on different days by different individuals can vary significantly from experiment to experiment. This variance is particularly problematic for Gram-negative bacterial infections, for which threshold effects can result in marked variations in host outcome with minor differences in inocula.

Results

We compared the accuracy of traditional methods versus using frozen stocks for preparing Acinetobacter baumannii inocula for infection in mice. Standard inoculum preparation resulted in substantial variability, both with respect to the actual inocula achieved compared to the targeted inocula, and with respect to the in vivo outcome resulting from similar inocula. Cryopreservation of the bacteria resulted in no significant decrement in growth of the bacteria. Furthermore, preparation of multiple infectious inocula from a frozen stock significantly improved the accuracy of the achieved inocula, and resulted in more reproducible in vivo outcomes from infection. Frozen stocks reduced inter-experiment variability associated with inoculum preparation, displayed no significant loss of growth capacity, and maintained virulence, increasing the reliability of infection.

Conclusions

Frozen stocks require considerably less time to prepare and enhance reproducibility of in vivo experimental results when infecting with A. baumannii.

Neutropenia exacerbates infection by Acinetobacter baumannii clinical isolates in a murine wound model

The Gram negative coccobacillus Acinetobacter baumannii has become an increasingly prevalent cause of hospital-acquired infections in recent years. The majority of clinical A. baumannii isolates display high-level resistance to antimicrobials, which severely compromises our capacity to care for patients with A. baumannii disease. Neutrophils are of major importance in the host defense against microbial infections. However, the contribution of these cells of innate immunity in host resistance to cutaneous A. baumannii infection has not been directly investigated. Hence, we hypothesized that depletion of neutrophils increases severity of bacterial disease in an experimental A. baumannii murine wound model. In this study, the Ly-6G-specific monoclonal antibody (mAb), 1A8, was used to generate neutropenic mice and the pathogenesis of several A. baumannii clinical isolates on wounded cutaneous tissue was investigated. We demonstrated that neutrophil depletion enhances bacterial burden using colony forming unit determinations. Also, mAb 1A8 reduces global measurements of wound healing in A. baumannii-infected animals. Interestingly, histological analysis of cutaneous tissue excised from A. baumannii-infected animals treated with mAb 1A8 displays enhanced collagen deposition. Furthermore, neutropenia and A. baumannii infection alter pro-inflammatory cytokine release leading to severe microbial disease. Our findings provide a better understanding of the impact of these innate immune cells in controlling A. baumannii skin infections.

Immunological evaluation of OMV(PagL)+Bap(1-487aa) and AbOmpA(8-346aa)+Bap(1-487aa) as vaccine candidates against Acinetobacter baumannii sepsis infection

Acinetobacter baumannii is an important nosocomial pathogen that causes a high morbidity and mortality rate in infected patients with sepsis form. The surface exposed virulence proteins and serum resistance factors helping to dissemination of this bacterium to bloodstream are the most promising vaccine candidates against this microorganism. In this project we immunologically evaluated OMV(PagL)+Bap(1-487aa) and AbOmpA (8-346aa)+Bap(1-487aa) as combination forms as well as Bap(1-487aa), AbOmpA(8-346aa) and OMV(PagL) singly, with addition of alum adjuvant as vaccine candidates. The titers of total IgG, IgG1 and IgG2c as well as concentration of IL-4 and IFN-γ and survival rates were measured in a C57BL/6 murine model with disseminated sepsis. The ratio of IgG1/IgG2c and profile of IL-4/IFN-γ in OMV (PagL)+Bap (1-487aa) formulation shows the humoral and cellular immune responses have been induced robustly and have created a full protection against A. baumannii ATCC 19606 and MDR AB-44 strains. We found that the two combination vaccine candidates were protective and induced both Th1 and Th2 responses.

An international multicenter retrospective study of Pseudomonas aeruginosa nosocomial pneumonia: impact of multidrug resistance

INTRODUCTION

Pseudomonas aeruginosa nosocomial pneumonia (Pa-NP) is associated with considerable morbidity, prolonged hospitalization, increased costs, and mortality.

METHODS

We conducted a retrospective cohort study of adult patients with Pa-NP to determine 1) risk factors for multidrug-resistant (MDR) strains and 2) whether MDR increases the risk for hospital death. Twelve hospitals in 5 countries (United States, n = 3; France, n = 2; Germany, n = 2; Italy, n = 2; and Spain, n = 3) participated. We compared characteristics of patients who had MDR strains to those who did not and derived regression models to identify predictors of MDR and hospital mortality.

RESULTS

Of 740 patients with Pa-NP, 226 patients (30.5%) were infected with MDR strains. In multivariable analyses, independent predictors of multidrug-resistance included decreasing age (adjusted odds ratio [AOR] 0.91, 95% confidence interval [CI] 0.96-0.98), diabetes mellitus (AOR 1.90, 95% CI 1.21-3.00) and ICU admission (AOR 1.73, 95% CI 1.06-2.81). Multidrug-resistance, heart failure, increasing age, mechanical ventilation, and bacteremia were independently associated with in-hospital mortality in the Cox Proportional Hazards Model analysis.

CONCLUSIONS

Among patients with Pa-NP the presence of infection with a MDR strain is associated with increased in-hospital mortality. Identification of patients at risk of MDR Pa-NP could facilitate appropriate empiric antibiotic decisions that in turn could lead to improved hospital survival.

Antibiotic modulation of capsular exopolysaccharide and virulence in Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic pathogen of increasing importance due to its propensity for intractable multidrug-resistant infections in hospitals. All clinical isolates examined contain a conserved gene cluster, the K locus, which determines the production of complex polysaccharides, including an exopolysaccharide capsule known to protect against killing by host serum and to increase virulence in animal models of infection. Whether the polysaccharides determined by the K locus contribute to intrinsic defenses against antibiotics is unknown. We demonstrate here that mutants deficient in the exopolysaccharide capsule have lowered intrinsic resistance to peptide antibiotics, while a mutation affecting sugar precursors involved in both capsule and lipopolysaccharide synthesis sensitizes the bacterium to multiple antibiotic classes. We observed that, when grown in the presence of certain antibiotics below their MIC, including the translation inhibitors chloramphenicol and erythromycin, A. baumannii increases production of the K locus exopolysaccharide. Hyperproduction of capsular exopolysaccharide is reversible and non-mutational, and occurs concomitantly with increased resistance to the inducing antibiotic that is independent of the presence of the K locus. Strikingly, antibiotic-enhanced capsular exopolysaccharide production confers increased resistance to killing by host complement and increases virulence in a mouse model of systemic infection. Finally, we show that augmented capsule production upon antibiotic exposure is facilitated by transcriptional increases in K locus gene expression that are dependent on a two-component regulatory system, bfmRS. These studies reveal that the synthesis of capsule, a major pathogenicity determinant, is regulated in response to antibiotic stress. Our data are consistent with a model in which gene expression changes triggered by ineffectual antibiotic treatment cause A. baumannii to transition between states of low and high virulence potential, which may contribute to the opportunistic nature of the pathogen.

Acinetobacter baumannii has gained notoriety as a cause of hospital-acquired infections that are difficult to treat due to extensive antibiotic resistance. While the microorganism rarely causes disease in the community, it commonly infects patients receiving antibiotics. The factors intrinsic to the bacterium that enable growth in the presence of antibiotics are not well characterized. Furthermore, the consequences of subinhibitory antibiotic concentrations on A. baumannii disease are unknown. Here we examined the K locus, a bacterial disease determinant responsible for the production of protective surface polysaccharides, and asked whether this determinant also contributes to antibiotic resistance. We found that K locus polysaccharides facilitate resistance to multiple antibiotics, and, unexpectedly, that the bacterium responds to certain antibiotics at subinhibitory concentrations by increasing production of capsule, the principal K locus polysaccharide. This augmented production of capsule, which is mediated by upregulation of K locus gene expression, increased the ability of the bacterium to overcome attack by the complement system, an important anti-pathogen host defense, and result in lethal disease during experimental bloodstream infection in mice. Our studies indicate that A. baumannii increases its disease-causing potential in the setting of inadequate antibiotic treatment, which may promote the development of opportunistic infections.

Fulminant endocarditis and disseminated infection caused by carbapenem-resistant Acinetobacter baumannii in a renal-pancreas transplant recipient

Acinetobacter baumannii is an important cause of healthcare-associated infections, and is particularly problematic among patients who undergo organ transplantation. We describe a case of fulminant sepsis caused by carbapenem-resistant A. baumannii harboring the blaOXA-23 carbapenemase gene and belonging to international clone II. This isolate led to the death of a patient 6 days after simultaneous kidney-pancreas transplantation. Autopsy findings revealed acute mitral valve endocarditis, myocarditis, splenic and renal emboli, peritonitis, and pneumonia. This case highlights the severe nature of certain A. baumannii infections and the vulnerability of transplanted patients to the increasingly intractable "high-risk" clones of multidrug-resistant organisms.