Modern Acinetobacter baumannii clinical isolates replicate inside spacious vacuoles and egress from macrophages

Abstract

Multidrug-resistant Acinetobacter baumannii infections are increasing at alarming rates. Therefore, novel antibiotic-sparing treatments to combat these A. baumannii infections are urgently needed. The development of these interventions would benefit from a better understanding of this bacterium’s pathobiology, which remains poorly understood. A. baumannii is regarded as an extracellular opportunistic pathogen. However, research on Acinetobacter has largely focused on common lab strains, such as ATCC 19606, that have been isolated several decades ago. These strains exhibit reduced virulence when compared to recently isolated clinical strains. In this work, we demonstrate that, unlike ATCC 19606, several modern A. baumannii clinical isolates, including the recent clinical urinary isolate UPAB1, persist and replicate inside macrophages within spacious vacuoles. We show that intracellular replication of UPAB1 is dependent on a functional type I secretion system (T1SS) and pAB5, a large conjugative plasmid that controls the expression of several chromosomally-encoded genes. Finally, we show that UPAB1 escapes from the infected macrophages by a lytic process. To our knowledge, this is the first report of intracellular growth and replication of A. baumannii. We suggest that intracellular replication within macrophages may contribute to evasion of the immune response, dissemination, and antibiotic tolerance of A. baumannii.

Incidence of an Intracellular Multiplication Niche among Acinetobacter baumannii Clinical Isolates

“…Interestingly, other studies have identified clinical strains able to multiply in cultured macrophages by bacterial CFU counts ( 21 ), suggesting these phenotypes could be extended to phagocytic cells. Consistently, while this work was being submitted, a study has shown that A. baumannii urinary tract infection isolates were capable of intracellular replication in cultured macrophages ( 29 ) by both CFU and microscopy counts, strengthening the hypothesis that an intracellular replication niche may be clinically relevant for a subset of A. baumannii isolates.…”

“…After 24 h of replication, in the case of the A. baumannii ABC141 strain, we observed a reduction in the presence of large vacuoles. The recent work of the Feldman lab showed that A. baumannii urinary tract infection strains capable of intramacrophage multiplication ( 29 ) escape from the cell. Therefore, it is likely that a similar event is taking place for A. baumannii C4 and ABC141 in epithelial cells, allowing bacteria to multiply without killing the host cell and then egress from infected cells to disseminate within the tissue or systemically.…”

InvL, an Invasin-Like Adhesin, Is a Type II Secretion System Substrate Required for Acinetobacter baumannii Uropathogenesis

“…Bacterial culture conditions, secreted protein enrichment, digestion of secretome samples, and liquid chromatography-mass spectrometry analysis of secretome samples were as previously described ( 101 ). Details are provided in the supplemental material.…”

InvL, an invasin-like adhesin, is a type II secretion system substrate required for Acinetobacter baumannii uropathogenesis

“…Proteomic analysis of the T2SS-dependent secretome. Bacterial culture conditions, secreted protein enrichment, digestion of secretome samples, and liquid chromatography-mass spectrometry analysis of secretome samples was performed as previously described (98). Details are provided in the supplementary materials.…”