The Capsule Depolymerase Dpo48 Rescues Galleria mellonella and Mice From Acinetobacter baumannii Systemic Infections

Abstract

The emergence of multidrug- and extensively drug-resistant Acinetobacter baumannii has made it difficult to treat and control infections caused by this bacterium. Thus, alternatives to conventional antibiotics for management of severe A. baumannii infections is urgently needed. In our previous study, we found that a capsule depolymerase Dpo48 could strip bacterial capsules, and the non-capsuled A. baumannii were significantly decreased in the presence of serum complement in vitro. Here, we further explored its potential as a therapeutic agent for controlling systemic infections caused by extensively drug-resistant A. baumannii. Prior to mammalian studies, the anti-virulence efficacy of Dpo48 was first tested in a Galleria mellonella infection model. Survival rate of Dpo48-pretreated bacteria or Dpo48 treatment group was significantly increased compared to the infective G. mellonella without treatment. Furthermore, the safety and therapeutic efficacy of Dpo48 to mice were evaluated. The mice treated with Dpo48 displayed normal serum levels of TBIL, AST, ALT, ALP, Cr, BUN and LDH, while no significant histopathology changes were observed in tissues of liver, spleen, lung, and kidney. Treatment with Dpo48 could rescue normal and immunocompromised mice from lethal peritoneal sepsis, with the bacterial counts in blood, liver, spleen, lung, and kidney significantly reduced by 1.4–3.3 log colony-forming units at 4 h posttreatment. Besides, the hemolysis and cytotoxicity assays showed that Dpo48 was non-homolytic to human red blood cells and non-toxic to human lung, liver and kidney cell lines. Overall, the present study demonstrated the promising potential of capsule depolymerases as therapeutic agents to prevent antibiotic-resistant A. baumannii infections.

K2 Capsule Depolymerase Is Highly Stable, Is Refractory to Resistance, and Protects Larvae and Mice from Acinetobacter baumannii Sepsis

“…Other studies have also demonstrated that a single administration of capsular depolymerases could significantly protect mice infected with E. coli (K1, K2, K5, and K30) (13,14) and K. pneumoniae (K1, K5, K64, and KN2) (15)(16)(17)(18). A recent study also showed that a capsule depolymerase administered intraperitoneally could rescue 100% of mice with A. baumannii systemic infections (19). In that study, the A. baumannii strain used has an undefined capsule type, which enabled us to correlate the activity with our K2 depolymerase.…”

“…Overall, since capsule removal by the K2 depolymerase attenuated the pathogenicity of the cells and prolonged the larval life span, it was demonstrated that the capsule K2 type is a major virulence factor of A. baumannii. Generally, other researchers have also shown success in preventing and treating the lethal effects of encapsulated K. pneumoniae (K3, K21, and K36) (31,35) and A. baumannii (undefined capsule type) (19) with single injections of capsular-specific depolymerases. However, these models have administered enzymes immediately after infection (5 min or less), which may raise the question of whether they are only providing a protective effect rather than a therapeutic one.…”

“…These enzymes are typically displayed at the tips of bacteriophage (phage) tails or baseplates to "shave" bacterial capsules, exposing their receptors for phage binding and further cell infection (12). Phage-derived capsular depolymerases have been shown to reduce the virulence of Gram-negative bacteria, namely, Escherichia coli (K1, K2, K5, and K30) (13,14), Klebsiella pneumoniae (K1, K5, K64, and KN2) (15)(16)(17)(18), and A. baumannii (undefined capsule type) (19), in murine models of sepsis by enhancing killing by complement systems, neutrophils, and macrophages. Our research group has also recently shown that capsular depolymerases are widespread in Acinetobacter phage genomes, being responsible for the activity and binding to specific Acinetobacter bacterial capsules (20), and that they can be heterologously expressed and digest A. baumannii capsules, making bacteria susceptible to serum killing (21).…”

Genomic and Phenotypic Analyses of Acinetobacter baumannii Isolates From Three Tertiary Care Hospitals in Thailand

“…In addition to the phylogenetic diversity (almost 50% non-GC2 isolates) and the even spread across the three hospitals, we show that there is a high degree of strainto-strain capsule variability, and development of depolymerase therapeutics will need to account for the challenge of a wide range of capsule types. Nevertheless, a recent study has demonstrated the potential of capsule depolymerase against A. baumannii in a Galleria mellonella (wax moth) larvae infection model and protection of both normal and immunocompromised mice from lethal peritoneal sepsis (Liu et al, 2019a).…”

Development of antimicrobial therapy methods to overcome the antibiotic resistance of Acinetobacter baumannii

“…Destruction of the bacterial capsule reduces biofilm formation and, as a result, antibiotic resistance: so, using bacteriophage depolymerases to eliminate the biofilm in the treatment of bacterial infections was proposed [ 53 , 54 , 55 ]. Various isolated phages against A. baumannii were shown to encode depolymerase, which successfully eliminates the capsular exopolysaccharide of the bacterium [ 53 , 56 , 57 ]. Thus, endolysin (LysAB3) of phage φAB3 specific to A. baumannii effectively eliminates the biofilm associated with A. baumannii in vitro [ 58 ].…”