Rat pneumonia and soft-tissue infection models for the study of Acinetobacter baumannii biology

Insights into acinetobacter war-wound infections, biofilms, and control

OBJECTIVE

Multidrug-resistant Acinetobacter baumannii is associated with a wide spectrum of infectious diseases ranging from nosocomial, community-acquired infections to those acquired following war or natural disaster. Especially to military personnel with war wounds, Acinetobacter infection is a formidable threat. The treatment has become exceedingly difficult, not only because the bacterium can develop extensive antimicrobial resistance but because it also forms biofilms that are resistant to host defense and antimicrobial treatment. Such causative factors as biofilm formation and resistance are highly likely to lead to nonhealing wounds. This review, although focused on A baumannii infections, aims to summarize what is known about immunization protection against wound biofilm infections and to apply such understanding in exploring the unknown area of war-wound infection protection.

DATA SOURCES AND STUDY SELECTION

Publications were searched and selected through http://www.pubmed.gov by using the key words Acinetobacter baumannii, bacteria, war wounds, burn wounds, wound infections, biofilms, vaccines, and immunization. The literature selected was categorized according to the subheadings within this article.

CONCLUSIONS

It is imperative to develop such effective measures as active and passive immunization to control multidrug-resistant and tenacious A baumannii infections and to prevent nonhealing wounds. The authors' understanding in immunization against burn wound-related infections by the model bacteria will facilitate research progress in the poorly explored area of immunization against war-wound biofilm infections.

Identification and characterization of a glycosyltransferase involved in Acinetobacter baumannii lipopolysaccharide core biosynthesis

Although Acinetobacter baumannii has emerged as a significant cause of nosocomial infections worldwide, there have been few investigations describing the factors important for A. baumannii persistence and pathogenesis. This paper describes the first reported identification of a glycosyltransferase, LpsB, involved in lipopolysaccharide (LPS) biosynthesis in A. baumannii. Mutational, structural, and complementation analyses indicated that LpsB is a core oligosaccharide glycosyl transferase. Using a genetic approach, lpsB was compared with the lpsB homologues of several A. baumannii strains. These analyses indicated that LpsB is highly conserved among A. baumannii isolates. Furthermore, we developed a monoclonal antibody, monoclonal antibody 13C11, which reacts to an LPS core epitope expressed by approximately one-third of the A. baumannii clinical isolates evaluated to date. Previous studies describing the heterogeneity of A. baumannii LPS were limited primarily to structural analyses; therefore, studies evaluating the correlation between these surface glycolipids and pathogenesis were warranted. Our data from an evaluation of LpsB mutant 307::TN17, which expresses a deeply truncated LPS glycoform consisting of only two 3-deoxy-d-manno-octulosonic acid residues and lipid A, suggest that A. baumannii LPS is important for resistance to normal human serum and confers a competitive advantage for survival in vivo. These results have important implications for the role of LPS in A. baumannii infections.

The virulence variability of different Acinetobacter baumannii strains in experimental pneumonia

OBJECTIVES

Our objective was to compare the virulence of 5 strains of Acinetobacter baumannii by using a mouse model of pneumonia.

METHODS

Six-week old female C3H/HeN mice were used. The pneumonia was inducted by intra-tracheal inoculation of 5.10(6) bacteria. Spontaneous outcome was evaluated by mortality, mice weight variations, and a clinical score. Bacterial counts in lungs, spleen and blood, and inflammatory response in lungs (dosages of tumor necrosis factor-alpha and macrophage inflammatory protein-2) were also measured. Lastly, a histological examination of lungs was performed for 3 strains, giving a histological score.

RESULTS

Global mortality varied from 13% to 79% (P<10(-4)). Bacterial counts in lungs within the 4 days following inoculation varied significantly according to different strains. The evolution curves of bacterial counts were also different. There was a significant correlation between the clinical score and mortality (P<0.05) but not between bacterial counts in lungs and mortality. The increase of pro-inflammatory mediator production in lungs and the histological score also varied according to strains.

CONCLUSIONS

These results demonstrate the variability of the virulence between strains, and suggest that bacterial proliferation is not the only virulence factor responsible for the pathogenesis in A. baumannii pneumonia.

Penicillin-binding protein 7/8 contributes to the survival of Acinetobacter baumannii in vitro and in vivo

BACKGROUND

Acinetobacter baumannii is a bacterial pathogen of increasing medical importance. Little is known about genes important for its survival in vivo.

METHODS AND RESULTS

Screening of random transposon mutants of the model pathogen AB307-0294 identified the mutant AB307.27. AB307.27 contained its transposon insertion in pbpG, which encodes the putative low-molecular-mass penicillin-binding protein 7/8 (PBP-7/8). AB307.27 was significantly killed in ascites (P<.001), but its growth in Luria-Bertani broth was similar to that of its parent, AB307-0294 (P=.13). The survival of AB307.27 was significantly decreased in a rat soft-tissue infection model (P<.001) and a rat pneumonia model (P=.002), compared with AB307-0294. AB307.27 was significantly killed in 90% human serum in vitro, compared with AB307-0294 (P<.001). Electron microscopy demonstrated more coccobacillary forms of AB307.27, compared with AB307-0294, suggesting a possible modulation in the peptidoglycan, which may affect susceptibility to host defense factors.

CONCLUSIONS

These findings demonstrate that PBP-7/8 contributes to the pathogenesis of A. baumannii. PBP-7/8 either directly or indirectly contributes to the resistance of AB307-0294 to complement-mediated bactericidal activity. An understanding of how PBP-7/8 contributes to serum resistance will lend insight into the role of this low-molecular-mass PBP whose function is poorly understood.