Characterization of a high-affinity iron transport system in Acinetobacter baumannii

Abstract

Analysis of a clinical isolate ofAcinetobacter baumannii showed that this bacterium was able to grow under iron-limiting conditions, using chemically defined growth media containing different iron chelators such as human transferrin, ethylenediaminedi-(o-hydroxyphenyl)acetic acid, nitrilotriacetic acid, and 2,2'-bipyridyl. This iron uptake-proficient phenotype was due to the synthesis and secretion of a catechol-type siderophore compound. Utilization bioassays using the SalmoneUla typhimurium iron uptake mutants enb-1 and enb-7 proved that this siderophore is different from enterobactin. This catechol siderophore was partially purified from culture supernatants by adsorption chromatography using an XAD-7 resin. The purified component exhibited a chromatographic behavior and a UV-visible light absorption spectrum different from those of 2,3-dihydroxybenzoic acid and other bacterial catechol siderophores. Furthermore, the siderophore activity of this extraceliular catechol was confirmed by its ability to stimulate energy-dependent uptake of 55Fe(M) as well as to promote the growth of A. baumannii bacterial cells under iron-deficient conditions imposed by 60 ,uM human transferrin. Polyacrylamide gel electrophoresis analysis showed the presence of iron-regulated proteins in both inner and outer membranes of this clinical isolate ofA. baumannii. Some of these membrane proteins may be involved in the recognition and internalization of the iron-siderophore complexes.Acinetobacter baumannii, formerly known asAcinetobacter calcoaceticus subsp. anitratus (15), belongs to a bacterial species that is widely distributed throughout the environment (6). This microorganism can be isolated from the skin (3, 43) and respiratory tract (59) of healthy ambulatory adults as well as from hospital personnel and equipment (36,39,46,67).Lately, numerous outbreaks of nosocomial infections caused by A. baumannii have been reported (7-10, 16, 42, 47, 56, 57, 73), which are of particular concern because of the widespread and increasing antibiotic resistance of the isolated strains (17,30,32,33,40,41,48). Nevertheless, most infections have occurred in patients compromised by either antibiotic therapy, respiratory instrumentation and manipulations, dialysis, or surgery (18,19,39,70). Thus, it was proposed that reduced host defenses and interaction with other bacterial species, rather than the expression of specific bacterial virulence factors (49), are the factors responsible for opportunistic infections caused by Acinetobacter species.Acinetobacter species are able to survive under restricted nutrient conditions, such as those imposed by the host. Iron is one of the essential bacterial nutrients that is tightly controlled by the host, through its chelation by the highaffinity binding glycoproteins transferrin (TF) and lactoferrin (23). Thus, bacteria have developed efficient iron uptake mechanisms that allow them to scavenge iron from these host proteins, either by direct interaction with the ironprotein complexes (34) or by the synthesis an...

Evaluation of adherence, hemagglutination, and presence of genes codifying for virulence factors of Acinetobacter baumannii causing urinary tract infection

“…Smith et al (1990) detected the presence of iron chelator 2,3-dihidroxi-benzoic acid (DHBA) and IROMPS in the culture supernatant of Acinetobacter sp. Besides DHBA, A. baumannii strains present other siderophores type catechol (Echenique et al 1992, Actis et al 1993). The Fur protein of A. baumannii, which regulates the genes involved in iron uptake, was sequenced and indicated that it is 63% identical to that of E. coli (Daniel et al 1999).…”

“…Growth under iron-limiting conditions -The isolates were grown on M9 minimal medium containing the iron chelator 2,2-dipyridyl (DIP) (Sigma) in concentrations of 50 µM and 200 µM and 2% agar (Echenique et al 1992). The plates were incubated at 37ºC for 18 h. DIP was prepared as a 20 mM stock solution in distilled water and kept at -20ºC.…”

Detection and Analysis of Iron Uptake Components Expressed by Acinetobacter baumannii Clinical Isolates

“…This iron uptake system includes a catechol siderophore capable of scavenging iron from high-affinity iron-binding proteins present in the human host. Chemical analyses of a secreted catechol siderophore (14) and the absence of histamine in culture supernatants (3) indicate that the siderophore produced by this isolate is different from acinetobactin. This hypothesis is supported by the recent observation that a chromosomal region of this strain contains a gene cluster that includes a gene encoding a putative outer membrane siderophore receptor protein unrelated to the V. anguillarum and A. baumannii 19606 FatA and FatA-like proteins, respectively (13).…”

“…A previous report characterized a high-affinity iron uptake system expressed by A. baumannii 8399, which was isolated during a nosocomial outbreak of respiratory infections at the Oregon Health Sciences University Hospital, Portland (14). This iron uptake system includes a catechol siderophore capable of scavenging iron from high-affinity iron-binding proteins present in the human host.…”

Recent Advances in the Immunopathogenesis of Acinetobacter baumannii Infection

“…The production of siderophores is one strategy used by the pathogen to grow under ironlimiting conditions (56,57). Siderophores are small secreted molecules that bind iron with high affinity and can be taken up by bacteria as a way to scavenge trace iron from their surroundings.…”