Iron limitation enhances acyl homoserine lactone (AHL) production and biofilm formation in clinical isolates ofAcinetobacter baumannii

Abstract

Acinetobacter baumannii is an important source of infections in intensive care units (ICUs) of our hospitals in Kerman, Iran and the most frequently isolated strains produce biofilm. There is a little information about role of iron (Fe) levels on acyl homoserine lactone (AHL) production and biofilm formation in this microorganism. In the present study, we investigated the influence of iron-III limitation on AHL, siderophore, catechol and virulence factors in the biofilm forming clinical strains of A. baumannii. A total of 65 non-duplicated multidrug resistance (MDR) strains of A. baumannii were isolated from patients in ICUs of 2 hospitals in Kerman, Iran. Antibiotic susceptibility, siderophore and other iron chelators, hemolysis, cell twitching motility, capsule, gelatinase and DNase were studied. Presence of quorum sensing, LuxI and LuxR genes was detected by multiplex-PCR. AHL activity quantified by colorimetric method and the functional groups were determined by Fourier Transform Infra-Red Spectroscopy (FT-IR). Biofilm formation was detected by microtiter plate technique. All of the isolates were resistant to third generation of cephalosporins, ciprofloxacin, levofloxacin, tetracycline, whereas, 78% and 81% were resistant to amikacin and carbapenems, respectively. The siderophore activity was highest at 20 mM Fe 3C (70%); however, it decreased to 45% as concentration of Fe 3C increased to 80 mM. Furthermore, screening of the isolates for LuxI and LuxR genes showed that presence of both genes required in the isolates with high AHL activity. FT-IR analysis indicated CDO bond of the lactone ring and primary amides. Significantly, a higher amount of AHL (70%) was detected in the presence of low concentration of iron-III (20 mM); as iron concentration increased to 80 mM, the AHL activity was reduced to 40% (P 0.05). All the isolates exhibited twitching motility and had a capsule. No any gelatinase or DNase activity was detected. Quantification of the biofilm formation introduced 23 isolates with efficient attachment to microplate wells and strong biofilm. We found that both the AHL production and biofilm formation were regulated by iron concentration in a dose dependent manner. These findings provide evidence that iron limitation plays an important regulatory role in AHL and siderophore production resulting in strong or weak biofilm, thereby helping the organism to persist in less available micronutrient environment.

Cloning and characterization of short‐chain N‐acyl homoserine lactone‐producing Enterobacter asburiae strain L1 from lettuce leaves

“…Besides, the study by Modarresi et al. () showed that during iron limitation stage, the activities of AHL and siderophore production of Acinetobacter baumannii were enhanced. This allows the bacterial cells to survive under inappropriate environment conditions (e.g., low concentration of micronutrients) and form strong biofilm.…”

Global Dynamic Proteome Study of a Pellicle-forming Acinetobacter baumannii Strain

“…Some works even concluded that iron availability did not have a significant role in the ability of the bacterium to form a biofilm at the S-L interface (54,66), or on the contrary generated an inhibitory effect (10,58). The present results may suggest 2 hypotheses: (1) in contrast to S-L biofilm, iron is an essential inducer for A. baumannii pellicle formation; (2) as suggested for static culture growth conditions, a gradient of nutrient from the bottom to the top, including iron, was established (67) and A-L biofilms, because of their location, might be more starved.…”

“…This operon could be involved in the synthesis of a biosurfactant lipopeptide, or an N-Acyl homoserine lactone (AHL) (25,81). It is independently regulated by three different factors: (1) by the AHL, N-(3-hydroxydodecanoyl)-homoserine lactone (3-OH-C 12 -HSL), itself in a abaI-dependent manner (AbaI, A1S_0109, being the AHL synthase (81,83), (2) by the cAMP via the CpdA regulator (82), and (3) via an iron limitation (58). A1S_0112 and A1S_0115 proteins are an AcylCoA synthetase and an AMP-dependent synthetase, respectively.…”

Modeling Acinetobacter baumannii wound infections

“…A. baumannii ’s most potent virulence activation system relies on its ability to respond to iron limitation through iron chelation (35, 36). We therefore hypothesized that the ability of A. baumannii to cause traumatic wound infection in Injury/Ischemia mouse model relies on its ability to scavenge iron from the wound milieu.…”