acinetobacter infection – an emerging threat to human health
TRANSCRIPT
Critical Review
Acinetobacter Infection – an Emerging Threat to Human Health
Paolo Visca1, Harald Seifert2 and Kevin J. Towner31Department of Biology, University Roma Tre, Rome, Italy2Institute for Medical Microbiology, Immunology and Hygiene, University of Cologne, Germany3Department of Clinical Microbiology, Nottingham University Hospitals, Nottingham, UK
Summary
The genus Acinetobacter comprises a complex and heteroge-neous group of bacteria, many of which are capable of causinga range of opportunistic, often catheter-related, infections inhumans. However, Acinetobacter baumannii, as well as its closerelatives belonging to genomic species 3 (‘‘Acinetobacter pittii’’)and 13TU (‘‘Acinetobacter nosocomialis’’), are important noso-comial pathogens, often associated with epidemic outbreaks ofinfection, that are only rarely found outside of a clinical setting.These organisms are frequently pandrug-resistant and are capa-ble of causing substantial morbidity and mortality in patientswith severe underlying disease, both in the hospital and in thecommunity. Several epidemic clonal lineages of A. baumanniihave disseminated worldwide and seem to have a selectiveadvantage over non-epidemic strains. The reasons for the suc-cess of these epidemic lineages remain to be elucidated, butcould be related to the potential of these organisms to achievevery dynamic reorganization and rapid evolution of their ge-nome, including the acquisition and expression of additional an-tibiotic resistance determinants, under fluctuating environmen-tal and selective conditions. � 2011 IUBMB
IUBMB Life, 63(12): 1048–1054, 2011
Keywords Acinetobacter; clinical importance; epidemiology; infec-
tions; population structure; taxonomy.
INTRODUCTION
The genus Acinetobacter comprises a heterogeneous group
of non-fermentative Gram-negative bacteria that have recently
become a focus of attention for scientists and clinicians, in
terms of both their fundamental biological properties and their
pathogenic potential. Recent developments in the field of mo-
lecular biology have enabled a more detailed understanding of
many of the important aspects of the members of this genus,
including their diversity and taxonomy, clinical importance,
pathogenic mechanisms, genomics, epidemiology and antibiotic
resistance, as well as their potential role as model organisms for
investigating complex topics in laboratory and industrial settings
by using sophisticated metabolic and genetic engineering
approaches. This review provides a general introduction to the
current taxonomy and properties of this genus, an account of
the emergence of certain species within the genus as a threat to
human health, and a summary of recent developments in our
understanding of the population biology and epidemiology of
those species that are of greatest clinical importance.
GENERAL PROPERTIES AND TAXONOMY
The taxonomy of the genus Acinetobacter has a long history
of debate and change (1). According to the most recent taxo-
nomic studies, the genus Acinetobacter belongs to subclass
c-Proteobacteria, family Moraxellaceae, and comprises Gram-
negative, non-motile, oxidase-negative, glucose non-fermenting,
strictly aerobic, catalase-positive bacteria with a G 1 C content
of 39–47% (2). The cells are~1.5 lm in length, with a shape vary-
ing from coccoid to coccobacillary (Fig. 1), depending on the
growth phase. Most Acinetobacter species are metabolically ver-
satile and can be grown easily on simple microbiological media,
forming domed, smooth colonies of~2 mm diameter, with some
species being pigmented pale yellow or grey. The temperature
range is typical of mesophylic bacteria; clinically relevant species
grow optimally at~37 8C, while environmental species may prefer
lower temperatures. Culture in slightly acidic mineral medium
containing acetate and nitrate as carbon and nitrogen sources,
respectively, or in Leeds selective medium (3), can improve the
recovery of Acinetobacter species from complex microbial com-
munities, and can be used for enrichment of clinical or environ-
mental specimens. Haemolytic activity on 5% sheep blood agar
Address correspondence to: Paolo Visca, Department of Biology,
University Roma Tre, Viale G. Marconi 446, Rome I-00146, Italy. Tel:
139-06-5733-6347. Fax: 139-06.5733-6321.
E-mail: [email protected]
Received 8 June 2011; accepted 9 June 2011
ISSN 1521-6543 print/ISSN 1521-6551 online
DOI: 10.1002/iub.534
IUBMB Life, 63(12): 1048–1054, December 2011
plates is observed occasionally, and hydrolysis of gelatin and urea,
as well as formation of acid from glucose are also variable traits.
Species Identification
Identification of acinetobacters to the individual species level
is difficult. A bacterial species has been defined as a group of
strains that share a number of phenotypic traits. However, phe-
notypic identification schemes have proved to be inadequate for
identification of individual Acinetobacter species (see below).
This holds true even for the new commercially available auto-
matic identification systems that are now used routinely in
many clinical microbiology laboratories. Therefore, previous
clinical and epidemiological studies in which Acinetobacter spe-
cies identification was achieved only by chemotaxonomic sys-
tems should be interpreted with caution.
The advent of molecular techniques in bacterial taxonomy
has allowed the inter- and intra-relatedness of species to be
determined more objectively, particularly through DNA–DNA
homology studies of entire genomes or selected genes that act
as evolutionary clocks. According to genetic criteria, a bacterial
genomic species is defined as a group of strains with [70%
DNA–DNA homology, \5 8C difference of DNA thermal sta-
bility, and [97% identity at the level of the 16S rRNA gene
(4). DNA–DNA hybridization assays formed the original basis
of studies aimed at defining the taxonomy of the genus Acineto-
bacter (5). As at May 2011, the genus Acinetobacter includes
23 species for which a formal name has been assigned
(www.bacterio.cict.fr/), plus at least 11 recognized additional
genomic species without a name, although names have recently
been proposed but are not yet formally assigned for genomic
species 3 and 13TU (Table 1).
Acinetobacter baumannii, Acinetobacter calcoaceticus,
genomic species 3 (‘‘Acinetobacter pittii’’) and genomic spe-
cies 13TU (‘‘Acinetobacter nosocomialis’’) are closely related
according to DNA–DNA hybridization studies, and can hardly
be distinguished according to phenotypic or chemotaxonomic
criteria. For convenience, many laboratories often group these
genomic species together in the so-called. ‘‘A. calcoaceticus –
A. baumannii (Acb) complex’’. From a clinical viewpoint,
such grouping is undesirable as it combines the three most
important species implicated in human disease (A. baumannii
and genomic species 3 and 13TU; see below) with A. calcoa-
ceticus, which is essentially a soil organism. Considerable
effort has therefore been dedicated to the development of new
and user-friendly molecular techniques for precise identifica-
tion of individual Acinetobacter genomic species, in order to
better delineate their ecology, epidemiology, and pathogenicity.
Amplified ribosomal DNA restriction analysis (ARDRA) and
high-resolution amplified fragment length polymorphism
(AFLP) analysis have been used for the construction of large
databases that allow the identification of genetic fingerprints
typical of individual species, clonal lineages and strains (6, 7).
However, the use of these techniques is currently limited to a
few specialized reference laboratories. In the clinical laboratory,
PCR amplification of species–specific DNA regions (e.g., the
blaOXA-51 carbapenemase gene intrinsic to A. baumannii) can be
a valuable tool for confirmatory identification of individual patho-
genic species (8). Similarly, it has proved possible to distinguish
members of the Acb complex by using specific primers to
amplify distinguishing regions of the gyrB gene (9, 10). Other
molecular techniques available for distinguishing individual Aci-
netobacter species have been reviewed previously (5).
CLINICAL IMPORTANCE
As mentioned earlier, Acinetobacter species are receiving
increasing attention as significant opportunistic pathogens,
usually in the context of serious underlying disease (5, 11).
Community-acquired infections (see below) have been reported
mainly from south-east Asia and tropical Australia. In the
hospital setting, Acinetobacter species have been implicated in
a wide range of infections, particularly in critically-ill patients
with impaired host defenses. These infections include pneumo-
nia, skin and soft-tissue infections, wound infections, urinary
tract infections, meningitis, and bloodstream infections (11).
Nosocomial infections and hospital outbreaks have been attrib-
uted mainly to A. baumannii, particularly in the intensive care
unit (ICU) setting, and to a lesser extent to genomic species
Figure 1. Morphology and staining properties of Acinetobacter baumannii. (A) Gram stain of stationary phase cells showing coc-
coid and coccobacillary elements, some of which appear blue due to retention of the crystal violet dye. (B) and (C) scanning elec-
tron micrograph of A. baumannii strains AYE and ACICU, respectively (the white bar corresponds to 500 nm). [Color figure can
be viewed in the online issue, which is available at wileyonlinelibrary.com.]
1049ACINETOBACTER INFECTION
13 (‘‘A. nosocomialis’’) and genomic species 3 (‘‘A. pittii’’).
Nosocomial infections caused by other named Acinetobacter
species such as Acinetobacter bereziniae, Acinetobacter
guillouiae, Acinetobacter haemolyticus, Acinetobacter johnso-
nii, Acinetobacter junii, Acinetobacter lwoffii, Acinetobacter
parvus, Acinetobacter radioresistens, Acinetobacter schindleri,
Acinetobacter soli and Acinetobacter ursingii are exceedingly
rare, and are restricted mainly to catheter-related bloodstream
infections (10, 12). These infections usually run a benign
clinical course and their associated mortality is low. Small-
sized outbreaks caused by Acinetobacter species other than
A. baumannii and its close relatives have been observed
occasionally and are often found to be related to contaminated
infusion fluids such as heparin solution.
Risk Factors
Several studies have analyzed risk factors for colonization
and infection with A. baumannii. Major surgery, major trauma,
burns, premature birth, previous hospitalization, stay in an ICU,
length of hospital or ICU stay, mechanical ventilation, indwell-
ing foreign devices (e.g., intravascular catheters, urinary cathe-
ters, and drainage tubes), the number of invasive procedures
performed, and previous antimicrobial therapy have all been
Table 1
Named Acinetobacter species and other recognized unnamed genomic species (May 2011)
Species name Genomic species no. Type or representative strain Major habitat or source
A. baumannii 2 ATCC 19606T Human clinical specimens
A. baylyi DSM 14961T Activated sludge, soil
A. beijerinckii NIPH 838T Soil, water
A. bereziniae 10 ATCC 17924T Human specimens, soil
A. bouvetii DSM 14964T Activated sludge
A. calcoaceticus 1 ATCC 23055T Soil, water
A. gerneri DSM 14967T Activated sludge
A. grimontii DSM 14968T Activated sludge
A. guillouiae 11 ATCC 11171T Human faeces, water, soil
A. gyllenbergii NIPH 2150T Human specimens
A. haemolyticus 4 ATCC 17906T Human specimens
A. johnsonii 7 ATCC 17909T Human skin, water, soil
A. junii 5 ATCC 17908T Human specimens
A. lwoffii 8/9 ATCC 15309T Human skin
A. parvus NIPH384T Humans and animals
A. radioresistens 12 IAM 13186T Human specimens, soil
A. schindleri NIPH1034T Human specimens
A. soli KCTC 22184T Soil
A. tandoii DSM 14970T Activated sludge, soil
A. tjernbergiae DSM 14971T Activated sludge
A. towneri DSM 14962T Activated sludge
A. ursingii NIPH137T Human specimens
A. venetianus ATCC 31012T Marine water
A. pittiia 3 ATCC 19004 Human clinical specimens
6 ATCC 17979 Human specimens
A. nosocomialisa 13TU ATCC 17903 Human clinical specimens
13BJ, 14TU ATCC 17905 Human specimens
14BJ CCUG 14816 Human specimens
15 BJ SEIP 23.78 Human specimens
15TU M 151a Human specimens
16 ATCC 17988 Human specimens
17 SEIP Ac87.314 Human specimens, soil
Between 1 and 3 10095 Human clinical specimens
Close to 13TU 10090 Human clinical specimens
aThis species name has been proposed, but has not yet (May 2011) been assigned formal status in taxonomic nomenclature.
1050 VISCA ET AL.
identified as risk factors predisposing to the acquisition of and
infection with A. baumannii (13).
Nosocomial Infections
Ventilator-associated pneumonia (VAP) is the most frequent
clinical manifestation of hospital-acquired A. baumannii infec-
tion, although it is sometimes difficult to distinguish upper re-
spiratory tract colonization from true infection. Recent data
from the National Nosocomial Surveillance System (NNIS)
showed a substantial increase in the number of cases of A. bau-
mannii-associated nosocomial pneumonia, with currently 5–10%
of cases of ICU-acquired pneumonia in the USA being caused
by A. baumannii (14). Bacteremic pneumonia carries a particu-
lar poor prognosis (15).
A. baumannii ranks 10th among the most frequent organisms
causing nosocomial bloodstream infections in the USA, being
responsible for 1.3% of all monomicrobial nosocomial blood-
stream infections (12). A recent study has revealed that about
30% of bloodstream infections attributed to A. baumannii were
in fact caused by ‘‘A. nosocomialis’’ and ‘‘A. pittii’’, but that the
organisms involved were misidentified by commercial identifi-
cation systems (16). A. baumannii bloodstream infection may
be associated with considerable morbidity and (overall) mortal-
ity as high as 58% (16). Risk factors for a fatal outcome are se-
verity-of-illness markers, such as septic shock at onset of infec-
tion, elevated APACHE II score, and ultimately fatal underlying
disease.
It has long been known that A. baumannii may cause wound
colonization and infection in patients with severe burns or
trauma. Characteristically, nosocomial A. baumannii wound
infection is associated with natural catastrophes or man-made
disasters (e.g., earthquakes, floods, the tsunami catastrophe of
2004, terrorist attacks and military campaigns) when hospitals’
capacities for patient care are overloaded and standard hygiene
procedures can no longer be enforced (17, 18). A. baumannii
received public attention when severe wound infections, burn
wound infections and osteomyelitis were reported in soldiers
who had suffered major injuries during military operations in
Iraq or Afghanistan and who were then repatriated to the USA
or the UK (17–19). The isolates from these infections were of-
ten multidrug resistant. Based on a widespread misinterpretation
that ‘‘A. baumannii is a ubiquitous organism’’, it was speculated
that the organism might have been inoculated at the time of
injury, either from previously colonized skin or from contami-
nated dust or soil. However, it is now widely accepted that the
soldiers acquired their infecting organism during emergency
care at field hospitals or following cross-transmission during
their hospitalization in military hospitals (17).
A. baumannii only occasionally causes urinary tract infection
related to indwelling Foley catheters. These infections are usu-
ally benign and occur more frequently in rehabilitation centers
than in ICUs (11). A distinct clinical entity is cerebrospinal
shunt-related meningitis in neurosurgical patients (20).
Community-Acquired Infections
Acinetobacter spp. have been reported occasionally as causa-
tive agents of community-acquired infections such as wound
infection, urinary tract infection, otitis media, eye infections,
meningitis and endocarditis. However, identification to the spe-
cies level was not performed with reference methods in most of
these reports, leaving doubts about the exact Acinetobacter spe-
cies involved. In addition, Acinetobacter spp. other than A. bau-
mannii and its close relatives are normal commensals, often col-
onizing the skin and mucous membranes of humans, and their
recovery may therefore have been misinterpreted as being indic-
ative of agents causing infection. Nevertheless, A. baumannii is
recognized as a rare but important cause of severe community-
acquired pneumonia in tropical areas of Asia and Australia.
Such patients typically had severe underlying disease, such as
chronic obstructive pulmonary disease, as well as diabetes mel-
litus or a history of excessive alcohol consumption or heavy
smoking. These cases often run a fulminant clinical course with
a high incidence of bacteremia and a high mortality rate of 40–
64% (21).
Clinical Impact
A. baumannii infections mainly affect patients with severe
underlying disease, and are associated with major surgery, burns
or trauma, concomitant infections, high APACHE II scores, and
a poor prognosis. Most studies report high overall mortality
rates in patients with A. baumannii bacteremia or pneumonia.
The true clinical impact of nosocomial A. baumannii infection
in these patients is difficult to assess and has been a matter of
continuous debate in the literature. While previously many
researchers claimed that patients died with A. baumannii (i.e.,
from their underlying disease) rather than from A. baumannii
infection, a recent review of matched cohort and case-control
studies concluded that A. baumannii infections are indeed asso-
ciated with increased attributable mortality of 8–32%, but that
the methodological heterogeneity among the studies reviewed
did not allow a meta-analysis to be performed to enable a defin-
itive conclusion to be reached (22).
Unfortunately, the clinical impact of A. baumannii is coupled
with increasing resistance of A. baumannii to the major antimi-
crobial drugs. This is a cause of serious concern, particularly as
this organism is also known for its propensity for nosocomial
cross-transmission, perhaps because of its multidrug resistance
and its capacity for long-term survival in the hospital environ-
ment. These characteristics have lead to the designation of A.
baumannii as the Gram-negative ‘‘methicillin-resistant Staphylo-
coccus aureus’’ (MRSA). An ever-increasing number of hospital
outbreaks caused by A. baumannii has been reported from
numerous countries around the world. In addition, inter-hospital
spread of multidrug resistant A. baumannii has been observed
as well as spread between countries. Recent studies have there-
fore focused on developing methods for understanding the pop-
ulation structure of A. baumannii in order to gain new insights
1051ACINETOBACTER INFECTION
into the epidemiology of this organism and to develop new
strategies for coping with its epidemic spread.
POPULATION STRUCTURE AND EPIDEMIOLOGYOF A. BAUMANNII
The development of novel multilocus sequence typing
(MLST) methods for clinical isolates of A. baumannii has indi-
cated that this is a genetically compact species that can be
clearly demarcated from other Acinetobacter species (23). There
is only limited information concerning the population structure
of other Acinetobacter species, but A. baumannii itself appears
to have a star-like phylogeny and a restricted amount of genetic
diversity, suggesting that A. baumannii is a species that has suf-
fered a severe evolutionary bottleneck in the recent past, possi-
bly linked to a restricted ecological niche (23). Alternatively, it
is possible that clinical isolates of A. baumannii do not fully
represent the diversity of the species, and simply constitute a re-
stricted subset of the population that has acquired the ability to
colonize and infect humans. No natural source or habitat for A.
baumannii has been identified outside of the hospital environ-
ment, although occasional isolates have been obtained from
non-clinical sources. An important outstanding task will involve
assessing the diversity of isolates from non-clinical sources in
order to better understand and clarify the population structure
and ecology of A. baumannii.
Application of various molecular typing methods, including
cell-envelope protein profiling, ribotyping, pulsed-field gel elec-
trophoresis (PFGE), randomly amplified polymorphic DNA
(RAPD) analysis and AFLP genomic fingerprinting, to epidemic
and non-epidemic clinical isolates has led to the recognition
that a limited number of widespread clonal lineages of A. bau-
mannii are responsible for hospital outbreaks in many countries.
Three major groups of isolates involved in epidemics, originally
named European clones I, II and III, have been distinguished.
These three ‘‘European clones’’ are now more appropriately
called ‘‘worldwide clonal lineages’’, as they have been associ-
ated with infection and epidemic spread in hospitals worldwide
(24–28).
More detailed information concerning the population struc-
ture has become available from MLST analysis, which has dem-
onstrated that the three original worldwide lineages correspond
to three MLST clonal complexes, each comprising a central,
predominant genotype and a few single locus variants (a hall-
mark of recent clonal expansion) (23). At least two other major
clonal complexes have been identified, which may represent the
emergence of new epidemic lineages (23). One of these may
correspond with a recently identified ‘‘pan-American clone’’
(25). The A. baumannii MLST database (http://pubmlst.org/
abaumannii/) currently contains more than 250 unique sequence
types (STs), and it seems that the A. baumannii population is
now undergoing a phase of rapid clonal expansion, possibly
driven by the major selective advantage associated with the
widespread antibiotic resistance found in A. baumannii.
Reasons for the Success of the Epidemic Clonal Lineages
The evolutionary success of the worldwide clonal lineages
currently remains unexplained. Investigation of characteristics
(e.g., resistance to disinfectants or drying, biofilm formation, ad-
herence to human cells, presence of iron-uptake systems) that
could favor particular lineages as colonizers or pathogens in
hospitals has so far failed to distinguish isolates belonging to
successful epidemic lineages from other genotypes (29–33).
However, multidrug resistance to antimicrobial agents appears
to be strongly associated with the international epidemic line-
ages (23), and may therefore represent one of the main reasons
for their evolutionary success. Nevertheless, it remains to be
determined whether susceptible strains belonging to these line-
ages are more likely to acquire resistance genes in the form of
foreign genetic material, or whether the association of multi-
drug-resistance with these lineages is a result of the epidemic
spread of a relatively small number of already established mul-
tidrug-resistant strains. Replacement of one epidemic clonal lin-
eage by another has been observed in several centers (34–36),
but the reason(s) why one epidemic lineage might have an
advantage over another epidemic lineage also remains
unknown.
As suggested previously (23), different selection pressures
and genetic pools of resistance genes, as well as instability of
some resistance determinants, could contribute to the observed
intra-clonal diversity. There is growing evidence that members
of even a single ST can differ by the presence or absence of re-
sistance genes, resistance islands and mobile elements. For
example, several resistance island variants have been identified
among members of worldwide clonal lineage I (28). There is
also evidence that clinical isolates of A. baumannii share a com-
mon pool of intrinsic plasmids, among which can be found a
general intrinsic system for plasmid mobilization and conse-
quent horizontal transmission of foreign plasmids and their
associated antibiotic resistance genes (37). It therefore appears
that clinical isolates of A. baumannii have the potential for very
dynamic reorganization and flexibility of plasmid architecture
under fluctuating environmental and selective conditions. Thus,
individual MLST STs can be regarded as having a stable core
genome, while their accessory genome, including resistance
determinants, can undergo rapid evolution.
Molecular Typing of A. baumannii Clinical Isolates
Fine typing of A. baumannii isolates that belong to wide-
spread clonal lineages is necessary for studies of hospital epide-
miology. A wide range of comparative molecular fingerprinting
methods is available for local epidemiological studies, including
ribotyping, AFLP, RAPD, PFGE, and repetitive extragenic pal-
indromic (REP) PCR fingerprinting (38). MLST is a portable
typing method that allows inter-laboratory comparisons and the
large-scale monitoring of national and international clones
involved in nosocomial outbreaks, but it is unsuitable for use in
routine hospital microbiology laboratories. However, PFGE and
1052 VISCA ET AL.
RAPD have both been shown to produce concordant results to
MLST when used in clonality studies at the local level (39, 40).
For local epidemiology purposes, AFLP appears to be more dis-
criminatory than MLST (23). A novel multiple-locus variable-
number tandem-repeat analysis (MLVA) assay involving simple
PCR- and agarose gel-based electrophoresis steps has also been
developed for A. baumannii typing. This MLVA assay is com-
patible with the use of high-throughput automated methods and
is capable of discriminating between isolates with identical
PFGE types (41). A database containing information and
MLVA profiles for A. baumannii strains has been constructed
(http://mlva.u-psud.fr/). In the future, improvement of the num-
ber of isolates typed by MLVA and the design of multicenter
studies for interlaboratory comparisons will be required for fur-
ther validation of this method. The use of validated molecular
typing methods should enable major insights to be gained into
the hospital epidemiology of these organisms, their mode of
spread, the role of hospital personnel in their transmission, and
the significance of the survival of these organisms on environ-
mental surfaces.
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