Incidence of an intracellular multiplication niche amongstAcinetobacter baumanniiclinical isolates
Authors: Rubio Tristan , Gagné Stéphanie , Debruyne Charline , Dias Chloé , Cluzel Caroline , Mongellaz Doriane , Rousselle Patricia , Göttig Stephan , Seifert Harald , Higgins Paul G. , Salcedo Suzana P. 
Published: April 2021

The spread of antibiotic resistantAcinetobacter baumanniiposes a significant threat to public health worldwide. This nosocomial bacterial pathogen can be associated with life-threatening infections, particularly in intensive care units.A. baumanniiis mainly described as an extracellular pathogen with restricted survival within cells. This study shows that a subset ofA. baumanniiclinical isolates extensively multiply within non-phagocytic immortalized and primary cells, without the induction of apoptosis, and with bacterial clusters visible up to 48 hours after infection. This phenotype was observed for theA. baumanniiC4 strain associated with high mortality in a hospital outbreak, and theA. baumanniiABC141 strain which wasn’t isolated from an infection site but was found to be hyperinvasive. Intracellular multiplication of theseA. baumanniistrains occurred within spacious single membrane-bound vacuoles, labeled with the lysosomal associate membrane protein (LAMP1). However, these compartments excluded lysotracker, an indicator of acidic pH, suggesting thatA. baumanniican divert its trafficking away from the lysosomal degradative pathway. These compartments were also devoid of autophagy features. A high-content microscopy screen of 43 additionalA. baumanniiclinical strains highlighted various phenotypes: (1) the majority of strains remained extracellular, (2) a significant proportion was capable of invasion and limited persistence, and (3) two strains efficiently multiplied within LAMP1-positive vacuoles, one of which was also hyperinvasive. These data identify an intracellular niche for specificA. baumanniiclinical strains that enables extensive multiplication in an environment protected from host immune responses and out of reach from many antibiotics.ImportanceMultidrug resistantAcinetobacter baumanniistrains are associated with significant morbidity and mortality in hospitals world-wide. Understanding their pathogenicity is critical for improving therapeutics. AlthoughA. baumanniican steadily adhere to surfaces and host cells, most bacteria remain extracellular. Recent studies have shown that a small proportion of bacteria can invade cells but present limited survival. We have found that someA. baumanniiclinical isolates can establish a specialized intracellular niche that sustains extensive intracellular multiplication for a prolonged time without induction of cell death. We propose that this intracellular compartment allowsA. baumanniito escape the cell’s normal degradative pathway, protecting bacteria from host immune responses and potentially hindering antibiotic accessibility. This may contribute toA. baumanniipersistence, relapsing infections and enhanced mortality in susceptible patients. A high-content microscopy-based screen confirmed this pathogenicity trait is present in other clinical isolates. There is an urgent need for new antibiotics or alternative antimicrobial approaches, particularly to combat carbapenem-resistantA. baumannii. The discovery of an intracellular niche for this pathogen as well as hyperinvasive isolates may help guide the development of antimicrobial therapies and diagnostics in the future.