Clinical isolates of Enterobacter species can persist in human macrophages without bacterial replication and minimal cellular cytotoxicity
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Background
The Enterobacter cloacae complex (Ecc) encompasses opportunistic Gram-negative bacteria demonstrating considerable phenotypic and genotypic diversity. Bloodstream, respiratory and urinary tract infections by Ecc bacteria are associated with morbidity and mortality worldwide. These infections are often difficult to treat since Ecc bacteria are resistant to penicillins, quinolones, aminoglycosides, and third-generation cephalosporins. Resistance also extends to carbapenems, leaving only polymyxins, such as colistin, as a last resort antibiotic for treatment. However, colistin resistance in Ecc isolates is also unexpectedly frequent. Despite extensive information on antibiotic resistance by Enterobacter species, much less is known about their infection biology. There are few reports on the survival and persistence of selected Enterobacter species in macrophages and epithelial cells, but how Enterobacter isolates interact with innate immune host cells upon engulfment remains unexplored. In this study, we have investigated the intracellular trafficking of a subset of antimicrobial resistant Ecc clinical isolates, including colistin-resistant strains, within human macrophages, and determined the macrophage response to the intracellular infection.
Methods
Phagocytosis of 11 clinical Ecc isolates, including E. cloacae, E. bugandensis, E. kobei, E. xiangfangensis, E. roggenkampii, E. hoffmannii , and E. ludwigii was investigated in THP-1 and human monocyte derived macrophages (HMDMs). Confocal fluorescence microscopy was used to ascertain intracellular trafficking via co-localisation of cell markers with fluorescent bacteria. Intracellular bacterial replication was assessed by bacterial enumeration in cell lysates after killing extracellular bacteria and by a fluorescence dilution approach to follow the synthesis of the bacterial cell wall over time. Macrophage cell cytotoxicity was investigated by quantifying the release of lactate dehydrogenase during infection with all isolates. Two prototypic isolates, the E. cloacae ATCC13047 type strain and the E. bugandensis 104107, were used to explore in more detail the response of macrophages to the intracellular infection by determining cleavage of the proinflammatory markers caspase-1, gasdermin D and pro-interleukin-1β.
Findings
We found that Ecc isolates do not replicate in human macrophages but survive within a modified late phagolysosome compartment. Survival occurred in all species investigated and did not correlate with colistin resistance, lipopolysaccharide modifications, or bacterial pathogenicity in the Galleria mellonella infection model. All isolates induced macrophage cell cytotoxicity at significantly lower levels than controls treated with lipopolysaccharide and nigericin treatment (to induce a proinflammatory response). Low cytotoxicity also correlated with absence of cleavage of proinflammatory markers in infected macrophages.
Interpretation
Ecc species can survive without replication inside human macrophages with minimal effects on cell viability and inflammation. These observations could have implications in the clinical outcome of patients that cannot readily clear infecting Ecc bacteria. This can potentially lead to prolonged intracellular survival and infection relapse.
Funding
Biotechnology and Biological Sciences Research Council grants BB/T005807/1 and BB/S006281/1.
Research In context
Evidence before this study
We searched PubMed with the terms “Enterobacter” AND “macrophage”, “Enterobacter” AND “monocytes”, and “Enterobacter” AND “intracellular” for original articles published in English up to July 9, 2024. The search excluded terms “sakazakii” and “aerogenes” since E. aerogenes and E. sakazakii have been moved to the genus Klebsiella and Cronobacter , respectively. Of the 55, 15, and 181 studies we identified, respectively, only one reported testing Enterobacter cloacae phagocytosis. Another study reported intracellular bacterial communities in uroepithelial cells, which serve as a reservoir during urinary tract infection. One other study reported the isolation of E. hormaechei from human atherosclerotic tissue and described studies in THP-1 monocytic macrophages. A few earlier studies also reported Enterobacter cytotoxins affecting immune cells, and an E. cloacae polysaccharide capable of inducing apoptosis in epithelial cells. These studies did not investigate mechanisms and there have been no more recent follow-ups; importantly, it remains unclear if the strains employed in these studies were properly identified as Enterobacter species. Therefore, despite isolated observations of survival of Enterobacter isolates surviving in immune and epithelial cells, there is an overall knowledge gap in our understanding of these pathogens concerning intracellular survival compartments, kinetics of survival, and induction of macrophage cytotoxicity and inflammatory responses.
Added value of this study
Our study is the first to investigate in detail how clinical isolates of various Enterobacter species can survive intracellularly in human macrophages. All isolates display multidrug antimicrobial resistance, including some with colistin resistance, and can survive intracellularly in human macrophages. Our data demonstrate that intracellular Enterobacter resides in vacuoles for up to 44 hours without replication. Colocalization experiments with various fluid phase and membrane cellular markers revealed that bacteria-containing vacuoles are modified late phagolysosomes, which do not accumulate the autophagosome marker LC3B. Intracellular bacterial survival did not associate to any Enterobacter species tested, the presence of colistin resistance, lipopolysaccharide modifications, or virulence in the Galleria mellonella infection model. Moreover, intracellular infection caused minimal cytotoxicity in macrophages without evidence of macrophage proinflammatory cell death by pyroptosis.
Implications of all the available evidence
Our findings underscore the capacity of Enterobacter species, traditionally viewed as extracellular bacteria, to hideout in macrophages without inducing a significant inflammatory response. These properties may further complicate the treatment of antibiotic-resistant Enterobacter infections in susceptible populations such as the elderly and neonates. These findings open a door to the development of host-directed therapeutics to enhance bacterial clearance by macrophage-mediated killing.
