Mucoid Acinetobacter baumannii enhances anti-phagocytosis through reducing C3b deposition

Abstract

Background: Multidrug resistant (MDR) Acinetobacter baumannii causes serious infections in intensive care units and is hard to be eradicated by antibiotics. Many A. baumannii isolates are identified as the mucoid type recently, but the biological characteristics of mucoid A. baumannii and their interactions with host cells remains unclear.Methods: The mucoid phenotype, antimicrobial susceptibility, biofilmforming ability, acid resistance ability, peroxide tolerance, and in vivo toxicity of clinical ICUs derived A. baumannii isolates were first investigated. Secondly, the phagocytic resistance and invasive capacity of A. baumannii isolates to macrophages (MH-S, RAW264.7) and epithelial cells (A549) were analyzed. Furthermore, the abundance of C3b (complement factor C3 degradation product) deposition on the surface of A. baumannii was investigated. Last, the relationship between C3b deposition and the abundance of capsule in A. baumannii isolates were analyzed.Results: These A. baumannii strains showed different mucoid phenotypes including hyper mucoid (HM), medium mucoid (MM), and low mucoid (LM). All tested strains were MDR with high tolerance to either acid or hydrogen peroxide exposure. Notably, these mucoid strains showed the increase of mortality in the Galleria mellonella infection models. Besides, the HM strain exhibited less biofilm abundance, higher molecular weight (MW) of capsule, and greater anti-phagocytic activity to macrophages than the LM strain. Together with the increased abundance of capsule, high expression of tuf gene (associated with the hydrolysis of C3b), the HM strain effectively inhibits C3b deposition on bacterial surface, resulting in the low-opsonization phenotype.

Effect of the polysaccharide capsule and its heptose on the resistance of Campylobacter jejuni to innate immune defenses

“…In contrast to the capsule of many pathogens (Gong et al, 2022 ; Ni et al, 2020 ; Wen et al, 2016 ), the C. jejuni capsule only offers limited protection against phagocytosis by macrophages (Kim et al, 2018 ; Myszewski & Stern, 1991 ; Sikic Pogacar et al, 2009 ; Wassenaar et al, 1997 ), including in our study with chicken and human macrophages where we only observed a slight capsule‐mediated decrease in adherence (chicken cells only) or in uptake (chicken and human cells). Furthermore, consistent with many studies that reported killing within ~6 h of C. jejuni after uptake by macrophages (Myszewski & Stern, 1991 ; Šikić Pogačar et al, 2009 ; Wassenaar et al, 1997 ; Wong et al, 2015 ), we recovered live C. jejuni only up to 3 h after phagocytosis.…”

Identification of the blaOXA-23 Gene in the First Mucoid XDR Acinetobacter baumannii Isolated from a Patient with Cystic Fibrosis

“…Hence, mucoid P. aeruginosa, A. xylosoxidans, C. albicans and two strains of A. baumannii with different colony morphologies, one being mucoid and the other with smooth rounded margins, were recovered from patient sputum culture. Both A. baumannii isolates were then cultured overnight on blood agar plates at 37 • C, and bacterial colonies were then stretched with an inoculation loop for the string test to assess their mucoidity [6]. According to Gong and colleagues [6], the two strains were classified as hypermucoid (HM) and low mucoid (LM), with viscous strings of 150 mm and 0 mm in length, respectively (Figure 1).…”

“…Both A. baumannii isolates were then cultured overnight on blood agar plates at 37 • C, and bacterial colonies were then stretched with an inoculation loop for the string test to assess their mucoidity [6]. According to Gong and colleagues [6], the two strains were classified as hypermucoid (HM) and low mucoid (LM), with viscous strings of 150 mm and 0 mm in length, respectively (Figure 1).…”

“…Both A. baumannii isolates were then cultured overnight on blood agar plates at 37 °C, and bacterial colonies were then stretched with an inoculation loop for the string test to assess their mucoidity [6]. According to Gong and colleagues [6], the two strains were classified as hypermucoid (HM) and low mucoid (LM), with viscous strings of 150 mm and 0 mm in length, respectively (Figure 1). Antibiograms for both strains were performed via the broth microdilution method using the Sensititre Gram Negative DMKGN plate (ThermoFisher Scientific, Waltham, MA, USA) and ComASP ® cefiderocol (Liofilchem, Roseto degli Abruzzi, Italy), and were interpreted according to clinical breakpoints based on the European Committee on Antimicrobial Susceptibility Testing (EUCAST) tables (version 13.0) [14].…”

“…To date, among the A. baumannii clones distributed worldwide, the international clone ST2 is the most dominant type [4,5]. Recently, many isolates of A. baumannii have been identified as mucoid-type [6], and in a recent study, most of the mucoid strains belonged to the ST2 clone of A. baumannii [7]. In addition, several studies have demonstrated the prevalence of isolates belonging to this clone that mainly harbor the bla oxa-23 oxacillinase [8].…”