Bacteriophage-resistant Acinetobacter baumannii are resensitized to antimicrobials

Abstract

Carbapenem-resistant Acinetobacter baumannii is responsible for frequent, hard-to-treat and often fatal healthcare-associated infections. Phage therapy, the use of viruses that infect and kill bacteria, is an approach gaining significant clinical interest to combat antibiotic-resistant infections. However, a major limitation is that bacteria can develop resistance against phages. Here, we isolated phages with activity against a panel of A.baumannii strains and focused on clinical isolates AB900 and A9844 and their phages for detailed characterization. As expected, coincubation of the phages with their hosts in vitro resulted in the emergence of phage-resistant bacterial mutants. Genome sequence analysis revealed that phage-resistant mutants harbored loss-of-function mutations in genes from the K locus, responsible for the biosynthesis of the bacterial capsule.Using molecular biology techniques, phage adsorption assays, and quantitative evaluation of capsule production, we established that the bacterial capsule serves as the primary receptor for these phages. As a collateral phenotype of impaired capsule production, the phage-resistant strains could not form biofilms, became fully sensitized to the human complement system, showed increased susceptibility to beta-lactam antibiotics, and became vulnerable to additional phages. Finally, in a murine model of bacteremia, the phage-resistant A.baumannii demonstrated a diminished capacity to colonize blood and solid tissues. This study demonstrates that phages can be used not only for their lytic activity but, if combined with a posteriori knowledge of their receptors and the mechanism of bacterial resistance, for their potential synergy with other antimicrobial agents, thus providing even broader clinical options for phage therapy.

Antiviral Resistance and Phage Counter Adaptation to Antibiotic-Resistant Extraintestinal Pathogenic Escherichia coli

“…been observed for bacterial pathogens of fish and moths, has been modeled in A. baumannii, and has been inferred with V. cholera in humans (24)(25)(26)(27)(28)(29)(32)(33)(34). To assess virulence in our isolated resisters, we tested their growth ability in media that simulated the host microenvironment.…”

“…Further, these studies are largely limited to pathogens with nonmammalian hosts. One exception is by Altamirano et al (29), who found a loss of virulence in phage-resistant A. baumannii in a murine infection model. The other exception is by Seed et al (27), who found loss of virulence in some Vibrio cholerae strains isolated from cholera patients.…”

“…During the development of resistance, these isolates may be forced to exchange fitness in other environments (9,24,26,27). Although resisters have been developed in culture and found to have attenuated infectivity, phage resisters have yet to be isolated and characterized from a therapeutic setting (9,28,29,(32)(33)(34). Further, these studies are largely limited to pathogens with nonmammalian hosts.…”

“…Consequently, phage-resistant bacteria are likely to develop. Interestingly, studies have found that in some instances, bacteria which develop resistance to phage lose other fitness advantages in exchange (9,(24)(25)(26)(27)(28)(29). Finally, though phage-bacterial coevolution is well studied in vitro, it is poorly understood under in vivo conditions.…”

Genetic Polymorphism Drives Susceptibility Between Bacteria and Bacteriophages

“…For example, Denes et al (2015) revealed that mutations associated with phage resistance in Listeria monocytogenes were found primarily in two loci, which link to phage adsorption. The phage resistant mutants of Acinetobacter baumannii had a single-nucleotide deletion resulting in a frameshift in a gene within the K locus (Gordillo Altamirano et al, 2021). Another lytic phage SPO1 could gain a broader host range against Bacillus subtilis after having mutations in two genes, encoding the baseplate, and the fiber proteins required for host attachment (Habusha et al, 2019).…”