Pathogenic Acinetobacter: from the Cell Surface to Infinity and Beyond

Bacterial Biofilms on Extracorporeal Membrane Oxygenation Catheters

Despite the advantages of extracorporeal membrane oxygenation (ECMO), secondary catheter infection remains a major concern during ECMO support. In this study, to clarify the mechanism of ECMO catheter-related infection, we evaluated the impact of infection on biofilm formation on the surfaces of ECMO catheters, and we investigated clinical factors associated with biofilm formation. Catheters used for ECMO were prospectively collected aseptically from the femoral vein, internal jugular vein, and femoral artery of 81 patients with acute cardiorespiratory failure between January 2015 and October 2016. Prepared catheter sections were investigated by fluorescence microscopy, confocal scanning laser microscopy, transmission electron microscopy, and using semiquantitative culture methods. Of the 81 patients, 51 were assigned to the infection group and 30 to a control group. Biofilms were identified in 43.1% patients in the infection group, and in 20% controls (p = 0.034). Extracorporeal membrane oxygenation flow, systemic infection, and carbapenem-resistant Acinetobacter baumannii (CRAB) infection were associated with biofilm formation in a univariate analysis (odds ratio [OR]: 1.00, 95% confidence interval [CI]: 1.00-1.00, p = 0.007; OR: 3.03, 95% CI: 1.06-8.69, p = 0.039; OR: 9.60, 95% CI: 2.94-31.30, p < 0.001, respectively). However, of these factors, only CRAB infection was found to independently predict the presence of a biofilm by a multivariate logistic regression analysis (OR: 9.60, 95% CI: 2.94-31.30; p < 0.001). Biofilms were more prevalent in patients with an infection than in uninfected controls. Carbapenem-resistant A. baumannii infection was identified as an independent risk factor for biofilm formation on ECMO catheters.

Contact-dependent growth inhibition systems in Acinetobacter

In bacterial contact-dependent growth inhibition (CDI) systems, CdiA proteins are exported to the outer membrane by cognate CdiB proteins. CdiA binds to receptors on susceptible bacteria and subsequently delivers its C-terminal toxin domain (CdiA-CT) into neighbouring target cells. Whereas self bacteria produce CdiI antitoxins, non-self bacteria lack antitoxins and are therefore inhibited in their growth by CdiA. In silico surveys of pathogenic Acinetobacter genomes have enabled us to identify >40 different CDI systems, which we sorted into two distinct groups. Type-II CdiAs are giant proteins (3711 to 5733 residues) with long arrays of 20-mer repeats. Type-I CdiAs are smaller (1900-2400 residues), lack repeats and feature central heterogeneity (HET) regions, that vary in size and sequence and can be exchanged between CdiA proteins. HET regions in most type-I proteins confer the ability to adopt a coiled-coil conformation. CdiA-CT and pretoxin modules differ significantly between type-I and type-II CdiAs. Moreover, type-II genes only have remnants of genes in their 3' end regions that have been displaced by the insertion of novel cdi sequences. Type-I and type-II CDI systems are equally abundant in A. baumannii, whereas A. pittii and A. nosocomialis predominantly feature type-I and type-II systems, respectively.

Lon Protease Has Multifaceted Biological Functions in Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative opportunistic pathogen that is known to survive harsh environmental conditions and is a leading cause of hospital-acquired infections. Specifically, multicellular communities (known as biofilms) of A. baumannii can withstand desiccation and survive on hospital surfaces and equipment. Biofilms are bacteria embedded in a self-produced extracellular matrix composed of proteins, sugars, and/or DNA. Bacteria in a biofilm are protected from environmental stresses, including antibiotics, which provides the bacteria with selective advantage for survival. Although some gene products are known to play roles in this developmental process in A. baumannii, mechanisms and signaling remain mostly unknown. Here, we find that Lon protease in A. baumannii affects biofilm development and has other important physiological roles, including motility and the cell envelope. Lon proteases are found in all domains of life, participating in regulatory processes and maintaining cellular homeostasis. These data reveal the importance of Lon protease in influencing key A. baumannii processes to survive stress and to maintain viability.IMPORTANCEAcinetobacter baumannii is an opportunistic pathogen and is a leading cause of hospital-acquired infections. A. baumannii is difficult to eradicate and to manage, because this bacterium is known to robustly survive desiccation and to quickly gain antibiotic resistance. We sought to investigate biofilm formation in A. baumannii, since much remains unknown about biofilm formation in this bacterium. Biofilms, which are multicellular communities of bacteria, are surface attached and difficult to eliminate from hospital equipment and implanted devices. Our research identifies multifaceted physiological roles for the conserved bacterial protease Lon in A. baumannii These roles include biofilm formation, motility, and viability. This work broadly affects and expands understanding of the biology of A. baumannii, which will permit us to find effective ways to eliminate the bacterium.

Diversity and Function of Capsular Polysaccharide in Acinetobacter baumannii

The Gram-negative opportunistic bacterium Acinetobacter baumannii is a significant cause of hospital-borne infections worldwide. Alarmingly, the rapid development of antimicrobial resistance coupled with the remarkable ability of isolates to persist on surfaces for extended periods of time has led to infiltration of A. baumannii into our healthcare environments. A major virulence determinant of A. baumannii is the presence of a capsule that surrounds the bacterial surface. This capsule is comprised of tightly packed repeating polysaccharide units which forms a barrier around the bacterial cell wall, providing protection from environmental pressures including desiccation and disinfection regimes as well as host immune responses such as serum complement. Additionally, capsule has been shown to confer resistance to a range of clinically relevant antimicrobial compounds. Distressingly, treatment options for A. baumannii infections are becoming increasingly limited, and the urgency to develop effective infection control strategies and therapies to combat infections is apparent. An increased understanding of the contribution of capsule to the pathobiology of A. baumannii is required to determine its feasibility as a target for new strategies to combat drug resistant infections. Significant variation in capsular polysaccharide structures between A. baumannii isolates has been identified, with over 100 distinct capsule types, incorporating a vast variety of sugars. This review examines the studies undertaken to elucidate capsule diversity and advance our understanding of the role of capsule in A. baumannii pathogenesis.

Surface-Related Features and Virulence Among Acinetobacter baumannii Clinical Isolates Belonging to International Clones I and II

Acinetobacter baumannii currently represents one of the most important nosocomial infection agent due to its multidrug-resistance and a propensity for the epidemic spread. The A. baumannii strains belonging to the international clonal lineages I (IC I) and II (IC II) are associated with the hospital outbreaks and a high virulence. However, the intra and inter lineage-specific features of strains belonging to these most worldwide spread A. baumannii clones are not thoroughly explored. In this study we have investigated a set of cell surface-related features of A. baumannii IC I (n = 20) and IC II (n = 16) lineage strains, representing 30 distinct pulsed-field gel electrophoresis types in the collection of clinical isolates obtained in Lithuanian tertiary care hospitals. We show that A. baumannii IC II strains are non-motile, do not form pellicle and display distinct capsular polysaccharide profile compared with the IC I strains. Moreover, in contrast to the overall highly hydrophobic IC I strains, IC II strains showed a greater variation in cell surface hydrophobicity. Within the IC II lineage, hydrophilic strains demonstrated reduced ability to form biofilm and adhere to the abiotic surfaces, also possessed twofold thicker cell wall and exhibited higher resistance to desiccation. Furthermore, these strains showed increased adherence to the lung epithelial cells and were more virulent in nematode and mouse infection model compared with the hydrophobic IC II strains. According to the polymerase chain reaction-based locus-typing, the reduction in hydrophobicity of IC II strains was not capsule or lipooligosaccharide locus type-dependent. Hence, this study shows that the most widespread A. baumannii clonal lineages I and II markedly differ in the series of cell surface-related phenotypes including the considerable phenotypic diversification of IC II strains at the intra-lineage level. These findings suggest that the genotypically related A. baumannii strains might evolve the features which could provide an advantage at the specific conditions outside or within the host.

Healthcare-associated infections: bacteriological characterization of the hospital surfaces in the University Hospital of Abomey-Calavi/so-ava in South Benin (West Africa)

Background

Healthcare-associated infections have become a public health problem, creating a new burden on medical care in hospitals. The emergence of multidrug-resistant bacteria poses a difficult task for physicians, who have limited therapeutic options. The dissemination of pathogens depends on “reservoirs”, the different transmission pathways of the infectious agents and the factors favouring them. Contaminated environmental surfaces are an important potential reservoir for the transmission of many healthcare-associated pathogens. Pathogens can survive or persist in the environment for months and be a source of infection transmission when appropriate hygiene and disinfection procedures are inefficient. The aim of this study was to identify bacterial species from hospital surfaces in order to effectively prevent healthcare-associated infections.

Methods

Samples were taken from surfaces at the University Hospital of Abomey-Calavi/So-Ava in South Benin (West Africa). To achieve the objective of this study, 160 swab samples of hospital surfaces were taken as recommended by the International Organization for Standardization (ISO 14698-1). These samples were analysed in the bacteriology section of the National Laboratory for Biomedical Analysis. All statistical analyses were performed using SPSS Statistics 21 software. A Chi Square Test was used to test the association between the Results of culture samples and different care units.

Results

Of the 160 surface samples, 65% were positive for bacteria. The frequency of isolation was predominant in Paediatrics (87.5%). The positive samples were 64.2% Gram-positive bacteria and 35.8% of Gram-negative bacteria. Staphylococcus aureus predominated (27.3%), followed by Bacillus spp. (23.3%).

The proportion of other microorganisms was negligible. S. aureus and Staphylococcus spp. were present in all care units. There was a statistically significant association between the Results of culture samples and different care units (χ2 = 12.732; p = 0.048).

Conclusion

The bacteria found on the surfaces of the University Hospital of Abomey-Calavi/So-Ava’s care environment suggest a risk of healthcare-associated infections. Adequate hospital hygiene measures are required. Patient safety in this environment must become a training priority for all caregivers.

Extraction and Visualization of Capsular Polysaccharide from Acinetobacter baumannii

Methods to determine the presence of a bacterial capsule typically involve the use of stains specific for capsule, the exclusion of a dye by the capsule or by electron microscopy. However, these procedures require equipment that may not be readily available to all researchers. Here we describe a method for extraction, visualization, and quantification of extracellular capsular polysaccharide from Acinetobacter baumannii with commonly used reagents and equipment.

Distinct amino acid residues confer one of three UDP-sugar substrate specificities in Acinetobacter baumannii PglC phosphoglycosyltransferases

Acinetobacter baumannii is an opportunistic human pathogen with the highest reported rates of multidrug resistance among Gram-negative pathogens. The capsular polysaccharide of A. baumannii is considered one of its most significant virulence factors providing resistance against complemented-mediated killing. Capsule synthesis in A. baumannii is usually initiated by the phosphoglycosyltransferase PglC. PglC transfers a phosphosugar from a nucleotide diphosphate-sugar to a polyprenol phosphate generating a polyprenol diphosphate-linked monosaccharide. Traditionally, PglC was thought to have stringent specificity towards UDP-N-N'-diacetylbacillosamine (UDP-diNAcBac). In this work we demonstrate that A. baumannii PglC has the ability to utilize three different UDP-sugar substrates: UDP-N-acetylglucosamine (UDP-GlcNAc), UDP-N-acetylgalactosamine (UDP-GalNAc) or UDP-diNAcBac. Using phylogenetic analyses, we first demonstrate that A. baumannii PglC orthologs separate into three distinct clades. Moreover, all members within a clade are predicted to have the same preference for one of the three possible sugar substrates. To experimentally determine the substrate specificity of each clade, we utilized in vivo complementation models and NMR analysis. We demonstrate that UDP-diNAcBac is accommodated by all PglC orthologs, but some orthologs evolved to utilize UDP-GlcNAc or UDP-GalNAc in a clade-dependent manner. Furthermore, we show that a single point mutation can modify the sugar specificity of a PglC ortholog specific for UDP-diNAcBac and that introduction of a non-native PglC ortholog into A. baumannii can generate a new capsule serotype. Collectively, these studies begin to explain why A. baumannii strains have such highly diverse glycan repertoires.

The role of compatible solutes in desiccation resistance of Acinetobacter baumannii

Acinetobacter baumannii is a nosocomial pathogen which can persist in the hospital environment not only due to the acquirement of multiple antibiotic resistances, but also because of its exceptional resistance against disinfectants and desiccation. A suitable desiccation assay was established in which A. baumannii ATCC 19606^T survived for ca. 1 month. The growth medium slightly influenced survival after subsequent desiccation. A significant effect could be attributed to the growth phase in which bacteria were dried: In exponential phase, cells were much more desiccation sensitive. The main focus of the present study was the elucidation of the role of compatible solutes, which are known to protect many bacteria under low water activity conditions, in desiccation survival of A. baumannii. Exogenous trehalose was shown to efficiently protect A. baumannii on dry surfaces, in contrast to other compatible solutes tested such as mannitol or glycine betaine. To analyze the importance of intracellularly accumulated solutes, a double mutant lacking biosynthesis pathways for mannitol and trehalose was generated. This mutant accumulated glutamate as sole solute in the presence of high NaCl concentrations and showed severe growth defects under osmotic stress conditions. However, no effect on desiccation tolerance could be seen, neither when cells were dried in water nor in the presence of NaCl.

Acinetobacter in veterinary medicine, with an emphasis on Acinetobacter baumannii

Acinetobacter spp. are aerobic, rod-shaped, Gram-negative bacteria belonging to the Moraxellaceae family of the class Gammaproteobacteria and are considered ubiquitous organisms. Among them, Acinetobacter baumannii is the most clinically significant species with an extraordinary ability to accumulate antimicrobial resistance and to survive in the hospital environment. Recent reports indicate that A. baumannii has also evolved into a veterinary nosocomial pathogen. Although Acinetobacter spp. can be identified to species level using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF/MS) coupled with an updated database, molecular techniques are still necessary for genotyping and determination of clonal lineages. It appears that the majority of infections due to A. baumannii in veterinary medicine are nosocomial. Such isolates have been associated with several types of infection such as canine pyoderma, feline necrotizing fasciitis, urinary tract infection, equine thrombophlebitis and lower respiratory tract infection, foal sepsis, pneumonia in mink, and cutaneous lesions in hybrid falcons. Given the potential multidrug resistance of A. baumannii, treatment of diseased animals is often supportive and should preferably be based on in vitro antimicrobial susceptibility testing results. It should be noted that animal isolates show high genetic diversity and are in general distinct in their sequence types and resistance patterns from those found in humans. However, it cannot be excluded that animals may occasionally play a role as a reservoir of A. baumannii. Thus, it is of importance to implement infection control measures in veterinary hospitals to avoid nosocomial outbreaks with multidrug-resistant A. baumannii.

Acinetobacter nosocomialis: Defining the Role of Efflux Pumps in Resistance to Antimicrobial Therapy, Surface Motility, and Biofilm Formation

Acinetobacter nosocomialis is a member of the Acinetobacter calcoaceticus-Acinetobacter baumannii (ACB) complex. Increasingly, reports are emerging of the pathogenic profile and multidrug resistance (MDR) phenotype of this species. To define novel therapies to overcome resistance, we queried the role of the major efflux pumps in A. nosocomialis strain M2 on antimicrobial susceptibility profiles. A. nosocomialis strains with the following mutations were engineered by allelic replacement; ΔadeB, ΔadeJ, and ΔadeB/adeJ. In these isogenic strains, we show that the ΔadeJ mutation increased susceptibility to beta-lactams, beta-lactam/beta-lactamase inhibitors, chloramphenicol, monobactam, tigecycline, and trimethoprim. The ΔadeB mutation had a minor effect on resistance to certain beta-lactams, rifampicin and tigecycline. In addition, the ΔadeJ mutation resulted in a significant decrease in surface motility and a minor decrease in biofilm formation. Our results indicate that the efflux pump, AdeIJK, has additional roles outside of antibiotic resistance in A. nosocomialis.

Novel phages of healthy skin metaviromes from South Africa

Recent skin metagenomic studies have investigated the harbored viral diversity and its possible influence on healthy skin microbial populations, and tried to establish global patterns of skin-phage evolution. However, the detail associated with the phages that potentially play a role in skin health has not been investigated. While skin metagenome and -metavirome studies have indicated that the skin virome is highly site specific and shows marked interpersonal variation, they have not assessed the presence/absence of individual phages. Here, we took a semi-culture independent approach (metaviromic) to better understand the composition of phage communities on skin from South African study participants. Our data set adds over 130 new phage species of the skin to existing databases. We demonstrated that identical phages were present on different individuals and in different body sites, and we conducted a detailed analysis of the structural organization of these phages. We further found that a bacteriophage related to the Staphylococcus capitis phage Stb20 may be a common skin commensal virus potentially regulating its host and its activities on the skin.

Association of virulence gene expression with colistin-resistance in Acinetobacter baumannii: analysis of genotype, antimicrobial susceptibility, and biofilm formation

Background

Acinetobacter baumannii causes difficult-to-treat nosocomial infections, which often lead to morbidity due to the development of antimicrobial drug resistance and expression of virulence genes. Data regarding the association of resistance to colistin, a last treatment option, and the virulence gene expression of A. baumannii is scarce.

Methods

We evaluated the MLVA genotype, antimicrobial resistance, and biofilm formation of 100 A. baumannii isolates from burn patients, and further compared the in vitro and in vivo expression of four virulence genes among five colistin-resistant A. baumannii (Cst-R-AB) isolates. Five Cst-R-AB isolates were tested; one from the present study, and four isolated previously.

Results

Our results showed that reduced expression of recA, along with increased in vivo expression of lpsB, dnaK, and blsA; are associated with colistin resistance among Cst-R-AB isolates. Differences in virulence gene expressions among Cst-R-AB isolates, may in part explain common discrepant in vitro vs. in vivo susceptibility data during treatment of infections caused by Cst-R-AB.

Conclusions

Our findings highlight the intricate relationship between colistin-resistance and virulence among A. baumannii isolates, and underscore the importance of examining the interactions between virulence and antimicrobial resistance toward efforts to control the spread of multidrug-resistant A. baumannii (MDR-AB) isolates, and also to reduce disease severity in burn patients with MDR-AB infection.

Electronic supplementary material

The online version of this article (10.1186/s12941-018-0277-6) contains supplementary material, which is available to authorized users.

iCN718, an Updated and Improved Genome-Scale Metabolic Network Reconstruction of Acinetobacter baumannii AYE

Acinetobacter baumannii has become an urgent clinical threat due to the recent emergence of multi-drug resistant strains. There is thus a significant need to discover new therapeutic targets in this organism. One means for doing so is through the use of high-quality genome-scale reconstructions. Well-curated and accurate genome-scale models (GEMs) of A. baumannii would be useful for improving treatment options. We present an updated and improved genome-scale reconstruction of A. baumannii AYE, named iCN718, that improves and standardizes previous A. baumannii AYE reconstructions. iCN718 has 80% accuracy for predicting gene essentiality data and additionally can predict large-scale phenotypic data with as much as 89% accuracy, a new capability for an A. baumannii reconstruction. We further demonstrate that iCN718 can be used to analyze conserved metabolic functions in the A. baumannii core genome and to build strain-specific GEMs of 74 other A. baumannii strains from genome sequence alone. iCN718 will serve as a resource to integrate and synthesize new experimental data being generated for this urgent threat pathogen.

Characterization of Two Novel Bacteriophages Infecting Multidrug-Resistant (MDR) Acinetobacter baumannii and Evaluation of Their Therapeutic Efficacy in Vivo

Acinetobacter baumannii is emerging as a challenging nosocomial pathogen due to its rapid evolution of antibiotic resistance. We report characterization of two novel bacteriophages, PBAB08 and PBAB25, infecting clinically isolated, multidrug-resistant (MDR) A. baumannii strains. Both phages belonged to Myoviridae of Caudovirales as their morphology observed under an electron microscope. Their genomes were double stranded linear DNAs of 42,312 base pairs and 40,260 base pairs, respectively. The two phages were distinct from known Acinetobacter phages when whole genome sequences were compared. PBAB08 showed a 99% similarity with 57% sequence coverage to phage AB1 and PBAB25 showed a 97% similarity with 78% sequence coverage to phage IME_AB3. BLASTN significant alignment coverage of all other known phages were <30%. Seventy six and seventy genes encoding putative phage proteins were found in the genomes of PBAB08 and PBAB25, respectively. Their genomic organizations and sequence similarities were consistent with the modular theory of phage evolution. Therapeutic efficacy of a phage cocktail containing the two and other phages were evaluated in a mice model with nasal infection of MDR A. baumannii. Mice treated with the phage cocktail showed a 2.3-fold higher survival rate than those untreated in 7 days post infection. In addition, 1/100 reduction of the number of A. baumannii in the lung of the mice treated with the phage cocktail was observed. Also, inflammatory responses of mice which were injected with the phage cocktail by intraperitoneal, intranasal, or oral route was investigated. Increase in serum cytokine was minimal regardless of the injection route. A 20% increase in IgE production was seen in intraperitoneal injection route, but not in other routes. Thus, the cocktail containing the two newly isolated phages could serve as a potential candidate for therapeutic interventions to treat A. baummannii infections.

A pathway-directed positive growth restoration assay to facilitate the discovery of lipid A and fatty acid biosynthesis inhibitors in Acinetobacter baumannii

Acinetobacter baumannii ATCC 19606 can grow without lipooligosaccharide (LOS). Lack of LOS can result from disruption of the early lipid A biosynthetic pathway genes lpxA, lpxC or lpxD. Although LOS itself is not essential for growth of A. baumannii ATCC 19606, it was previously shown that depletion of the lipid A biosynthetic enzyme LpxK in cells inhibited growth due to the toxic accumulation of lipid A pathway intermediates. Growth of LpxK-depleted cells was restored by chemical inhibition of LOS biosynthesis using CHIR-090 (LpxC) and fatty acid biosynthesis using cerulenin (FabB/F) and pyridopyrimidine (acetyl-CoA-carboxylase). Here, we expand on this by showing that inhibition of enoyl-acyl carrier protein reductase (FabI), responsible for converting trans-2-enoyl-ACP into acyl-ACP during the fatty acid elongation cycle also restored growth during LpxK depletion. Inhibition of fatty acid biosynthesis during LpxK depletion rescued growth at 37°C, but not at 30°C, whereas rescue by LpxC inhibition was temperature independent. We exploited these observations to demonstrate proof of concept for a targeted medium-throughput growth restoration screening assay to identify small molecule inhibitors of LOS and fatty acid biosynthesis. The differential temperature dependence of fatty acid and LpxC inhibition provides a simple means by which to separate growth stimulating compounds by pathway. Targeted cell-based screening platforms such as this are important for faster identification of compounds inhibiting pathways of interest in antibacterial discovery for clinically relevant Gram-negative pathogens.

Crystal structure of a UDP-GlcNAc epimerase for surface polysaccharide biosynthesis in Acinetobacter baumannii

With new strains of Acinetobacter baumannii undergoing genomic analysis, it has been possible to define regions of genomic plasticity (RGPs), encoding specific adaptive elements. For a selected RGP from a community-derived isolate of A. baumannii, we outline sequences compatible with biosynthetic machinery of surface polysaccharides, specifically enzymes utilized in the dehydration and conversion of UDP-N-acetyl-D-glucosamine (UDP-D-GlcNAc). We have determined the crystal structure of one of these, the epimerase Ab-WbjB. This dehydratase belongs to the ‘extended’ short-chain dehydrogenase/reductase (SDR) family, related in fold to previously characterised enzymes CapE and FlaA1. Our 2.65Å resolution structure of Ab-WbjB shows a hexamer, organised into a trimer of chain pairs, with coenzyme NADP+ occupying each chain. Specific active-site interactions between each coenzyme and a lysine quaternary group of a neighbouring chain interconnect adjacent dimers, so stabilising the hexameric form. We show UDP-GlcNAc to be a specific substrate for Ab-WbjB, with binding evident by ITC (K_a = 0.23 μmol^-1). The sequence of Ab-WbjB shows variation from the consensus active-site motifs of many SDR enzymes, demonstrating a likely catalytic role for a specific threonine sidechain (as an alternative to tyrosine) in the canonical active site chemistry of these epimerases.

Importance of Core Genome Functions for an Extreme Antibiotic Resistance Trait

Extreme antibiotic resistance in bacteria is associated with the expression of powerful inactivating enzymes and other functions encoded in accessory genomic elements. The contribution of core genome processes to high-level resistance in such bacteria has been unclear. In the work reported here, we evaluated the relative importance of core and accessory functions for high-level resistance to the aminoglycoside tobramycin in the nosocomial pathogen Acinetobacter baumannii. Three lines of evidence establish the primacy of core functions in this resistance. First, in a genome scale mutant analysis using transposon sequencing and validation with 594 individual mutants, nearly all mutations reducing tobramycin resistance inactivated core genes, some with stronger phenotypes than those caused by the elimination of aminoglycoside-inactivating enzymes. Second, the core functions mediating resistance were nearly identical in the wild type and a deletion mutant lacking a genome resistance island that encodes the inactivating enzymes. Thus, most or all of the core resistance determinants important in the absence of the enzymes are also important in their presence. Third, reductions in tobramycin resistance caused by different core mutations were additive, and highly sensitive double and triple mutants (with 250-fold reductions in the MIC) that retained accessory resistance genes could be constructed. Core processes that contribute most strongly to intrinsic tobramycin resistance include phospholipid biosynthesis, phosphate regulation, and envelope homeostasis.

The inexorable increase in bacterial antibiotic resistance threatens to undermine many of the procedures that transformed medicine in the last century. One strategy to meet the challenge antibiotic resistance poses is the development of drugs that undermine resistance. To identify potential targets for such adjuvants, we identified the functions underlying resistance to an important class of antibiotics for one of the most highly resistant pathogens known.

Interplay between Colistin Resistance, Virulence and Fitness in Acinetobacter baumannii

Acinetobacter baumannii is an important opportunistic nosocomial pathogen often resistant to multiple antibiotics classes. Colistin, an “old” antibiotic, is now considered a last-line treatment option for extremely resistant isolates. In the meantime, resistance to colistin has been reported in clinical A. baumannii strains. Colistin is a cationic peptide that disrupts the outer membrane (OM) of Gram-negative bacteria. Colistin resistance is primarily due to post-translational modification or loss of the lipopolysaccharide (LPS) molecules inserted into the outer leaflet of the OM. LPS modification prevents the binding of polymyxin to the bacterial surface and may lead to alterations in bacterial virulence. Antimicrobial pressure drives the evolution of antimicrobial resistance and resistance is often associated with a reduced bacterial fitness. Therefore, the alterations in LPS may induce changes in the fitness of A. baumannii. However, compensatory mutations in clinical A. baumannii may ameliorate the cost of resistance and may play an important role in the dissemination of colistin-resistant A. baumannii isolates. The focus of this review is to summarize the colistin resistance mechanisms, and understand their impact on the fitness and virulence of bacteria and on the dissemination of colistin-resistant A. baumannii strains.

Acinetobacter baumannii Virulence Traits: A Comparative Study of a Novel Sequence Type with Other Italian Endemic International Clones

Carbapenem-resistant Acinetobacter baumannii (CRAb) have emerged in recent decades as major causes of nosocomial infections. Resistance is mainly due to overexpression of intrinsic and/or acquired carbapenemases, especially oxacillinases (OXA). In Italy, although the sequence type (ST) 2 and the ST78 are the most frequently detected, we recently reported ST632, a single locus variant of ST2. Therefore, this study was aimed at unraveling common bacterial surface virulence factors involved in pathogenesis and antibiotic resistance in representative CRAb of these ST genotypes. Outer membrane protein (OMP) composition together with motility, biofilm formation, in vitro adherence to, invasion of, and survival within pneumocytes were analyzed. Differently from the carbapenem-susceptible reference strain ATCC 17978, either overexpressed OXA-51 or both OXA-23 and OXA-51 co-purified with OMPs in CRAb. This tight association ensures their maximal concentration on the inner surface of the outer membrane to provide the best protection against carbapenems. These findings led us to propose for the first time a common behavior of OXA enzymes in CRAb. Despite the presence of both OmpA and phosphorylcholine-porinD and the ability of all the strains to adhere to cells, invasion, and survival within pneumocytes was shown only by ST2 and ST78 isolates, sharing the highest number of identified OMPs. Conversely, notwithstanding genetic and OMPs similarities with ST2, ST632 was unable to invade and survive within epithelial cells. Overall, our study shows that different STs share a specific OMP composition, also shaped by overexpressed OXA, that is needed for invasiveness and survival of CRAb.

Temporal dynamics in microbial soil communities at anthrax carcass sites

BACKGROUND

Anthrax is a globally distributed disease affecting primarily herbivorous mammals. It is caused by the soil-dwelling and spore-forming bacterium Bacillus anthracis. The dormant B. anthracis spores become vegetative after ingestion by grazing mammals. After killing the host, B. anthracis cells return to the soil where they sporulate, completing the lifecycle of the bacterium. Here we present the first study describing temporal microbial soil community changes in Etosha National Park, Namibia, after decomposition of two plains zebra (Equus quagga) anthrax carcasses. To circumvent state-associated-challenges (i.e. vegetative cells/spores) we monitored B. anthracis throughout the period using cultivation, qPCR and shotgun metagenomic sequencing.

RESULTS

The combined results suggest that abundance estimation of spore-forming bacteria in their natural habitat by DNA-based approaches alone is insufficient due to poor recovery of DNA from spores. However, our combined approached allowed us to follow B. anthracis population dynamics (vegetative cells and spores) in the soil, along with closely related organisms from the B. cereus group, despite their high sequence similarity. Vegetative B. anthracis abundance peaked early in the time-series and then dropped when cells either sporulated or died. The time-series revealed that after carcass deposition, the typical semi-arid soil community (e.g. Frankiales and Rhizobiales species) becomes temporarily dominated by the orders Bacillales and Pseudomonadales, known to contain plant growth-promoting species.

CONCLUSION

Our work indicates that complementing DNA based approaches with cultivation may give a more complete picture of the ecology of spore forming pathogens. Furthermore, the results suggests that the increased vegetation biomass production found at carcass sites is due to both added nutrients and the proliferation of microbial taxa that can be beneficial for plant growth. Thus, future B. anthracis transmission events at carcass sites may be indirectly facilitated by the recruitment of plant-beneficial bacteria.

Clinical and Pathophysiological Overview of Acinetobacter Infections: a Century of Challenges

Acinetobacter is a complex genus, and historically, there has been confusion about the existence of multiple species. The species commonly cause nosocomial infections, predominantly aspiration pneumonia and catheter-associated bacteremia, but can also cause soft tissue and urinary tract infections. Community-acquired infections by Acinetobacter spp. are increasingly reported. Transmission of Acinetobacter and subsequent disease is facilitated by the organism's environmental tenacity, resistance to desiccation, and evasion of host immunity. The virulence properties demonstrated by Acinetobacter spp. primarily stem from evasion of rapid clearance by the innate immune system, effectively enabling high bacterial density that triggers lipopolysaccharide (LPS)-Toll-like receptor 4 (TLR4)-mediated sepsis. Capsular polysaccharide is a critical virulence factor that enables immune evasion, while LPS triggers septic shock. However, the primary driver of clinical outcome is antibiotic resistance. Administration of initially effective therapy is key to improving survival, reducing 30-day mortality threefold. Regrettably, due to the high frequency of this organism having an extreme drug resistance (XDR) phenotype, early initiation of effective therapy is a major clinical challenge. Given its high rate of antibiotic resistance and abysmal outcomes (up to 70% mortality rate from infections caused by XDR strains in some case series), new preventative and therapeutic options for Acinetobacter spp. are desperately needed.

Virulence profiles and innate immune responses against highly lethal, multidrug-resistant nosocomial isolates of Acinetobacter baumannii from a tertiary care hospital in Mexico

Virulence profiles and innate immune responses were studied in Acinetobacter baumannii from nosocomial infections collected over one year in a tertiary care hospital in Mexico. A. baumannii were identified by VITEK 2 System followed by susceptibility tests. Carbapenemase genes, active efflux mechanism to imipenem and meropenem and outer membrane proteins profile were analyzed to evaluate their role on the activity of carbapenem resistance. All isolates were genotyped by pulsed field gel electrophoresis. The ability to form biofilm was determined on a polystyrene surface. The resistance to complement was determined with a pooled human normal serum and TNFα release by infected macrophages was determined by ELISA. The 112 isolates from this study were associated with a 52% of mortality. All were resistance to β-lactams, fluoroquinolones, and trimethroprim-sulfamethoxal, 96 and 90% were resistant to meropenem and imipenem, respectively, but with high susceptibility to polymyxin B, colistin and tigecyclin. Isolates were classified in 11 different clones. Most isolates, 88% (99/112), were metallo-β-lactamases and carbapenemases producers, associated in 95% with the presence of bla_OXA-72 gene. Only 4/99 and 1/99 of the carbapenem-resistant isolates were related to efflux mechanism to meropenem or imipenem resistance, respectively. The loss of expression of 22, 29, and/or 33-36-kDa proteins was detected in 8/11 of the clinical isolates with resistance to carbapenem. More than 96% (108/112) of the isolates were high producers of biofilms on biotic surfaces. Finally, all isolates showed variable resistance to normal human serum activity and were high inductors of TNFα release by macrophages. In summary, these results suggest that multidrug-resistant A. baumannii can persist in the hospital environment through its ability to form biofilms. The high mortality observed was due to their ability to survive normal human serum activity and capability to induce potent inflammatory immune response making this nosocomial pathogen a serious threat to hospitalized patients.

LpxK Is Essential for Growth of Acinetobacter baumannii ATCC 19606: Relationship to Toxic Accumulation of Lipid A Pathway Intermediates

Acinetobacter baumannii is a Gram-negative pathogen for which new therapies are needed. The lipid A biosynthetic pathway has several potential enzyme targets for the development of anti-Gram-negative agents (e.g., LpxC). However, A. baumannii ATCC 19606 can grow in the absence of LpxC and, correspondingly, of lipid A. In contrast, we show that cellular depletion of LpxK, a kinase occurring later in the pathway, inhibits growth. Growth inhibition results from toxic accumulation of lipid A pathway intermediates, since chemical inhibition of LpxC or fatty acid biosynthesis rescues cell growth upon loss of LpxK. Overall, this suggests that targets such as LpxK can be essential for growth even in those Gram-negative bacteria that do not require lipid A biosynthesis per se. This strain provides an elegant tool to derive a better understanding of the steps in a pathway that is the focus of intense interest for the development of novel antibacterials.

Acinetobacter baumannii ATCC 19606 can grow without lipid A, the major component of lipooligosaccharide. However, we previously reported that depletion of LpxH (the fourth enzyme in the lipid A biosynthetic pathway) prevented growth of this strain due to toxic accumulation of lipid A pathway intermediates. Here, we explored whether a similar phenomenon occurred with depletion of LpxK, a kinase that phosphorylates disaccharide 1-monophosphate (DSMP) at the 4′ position to yield lipid IV_A. An A. baumannii ATCC 19606 derivative with LpxK expression under the control of an isopropyl β-d-1-thiogalactopyranoside (IPTG)-regulated expression system failed to grow without induction, indicating that LpxK is essential for growth. Light and electron microscopy of LpxK-depleted cells revealed morphological changes relating to the cell envelope, consistent with toxic accumulation of lipid A pathway intermediates disrupting cell membranes. Using liquid chromatography-mass spectrometry (LCMS), cellular accumulation of the detergent-like pathway intermediates DSMP and lipid X was shown. Toxic accumulation was further supported by restoration of growth upon chemical inhibition of LpxC (upstream of LpxK and the first committed step of lipid A biosynthesis) using CHIR-090. Inhibitors of fatty acid synthesis also abrogated the requirement for LpxK expression. Growth rescue with these inhibitors was possible on Mueller-Hinton agar but not on MacConkey agar. The latter contains outer membrane-impermeable bile salts, suggesting that despite growth restoration, the cell membrane permeability barrier was not restored. Therefore, LpxK is essential for growth of A. baumannii, since loss of LpxK causes accumulation of detergent-like pathway intermediates that inhibit cell growth.

IMPORTANCEAcinetobacter baumannii is a Gram-negative pathogen for which new therapies are needed. The lipid A biosynthetic pathway has several potential enzyme targets for the development of anti-Gram-negative agents (e.g., LpxC). However, A. baumannii ATCC 19606 can grow in the absence of LpxC and, correspondingly, of lipid A. In contrast, we show that cellular depletion of LpxK, a kinase occurring later in the pathway, inhibits growth. Growth inhibition results from toxic accumulation of lipid A pathway intermediates, since chemical inhibition of LpxC or fatty acid biosynthesis rescues cell growth upon loss of LpxK. Overall, this suggests that targets such as LpxK can be essential for growth even in those Gram-negative bacteria that do not require lipid A biosynthesis per se. This strain provides an elegant tool to derive a better understanding of the steps in a pathway that is the focus of intense interest for the development of novel antibacterials.

Meropenem selection induced overproduction of the intrinsic carbapenemase as well as phenotype divergence in Acinetobacter baumannii

Acinetobacter baumannii 37662 is a carbapenem-susceptible isolate with bla_OXA-51-like as the sole carbapenemase gene. Following selection with meropenem (MEM) at a subinhibitory concentration, two morphologically different mutants, designated 37662RM1 and 37662RM2, were obtained and characterised. Compared with the parent strain, resistant mutant 37662RM1 grew at a slower rate and had impaired capsule synthesis, whereas 37662RM2 grew fast and abolished capsule synthesis. In addition, the latter resistant mutant also lost pathogenicity but showed significantly enhanced biofilm formation. Transposition of the insertion sequence ISAba1 and formation of ISAba1-bla_OXA-51-like was responsible for the upregulated expression of bla_OXA-51-like. The bla_OXA-51-like gene of A. baumannii 37662 is a close variant of bla_OXA-138 and has been designated bla_OXA-508. Overproduction of OXA-508 conferred major carbapenem resistance to these two mutants. Overall, these results indicate that a subinhibitory concentration of MEM can induce phenotype divergence together with carbapenem resistance in A. baumannii.

Identification of unique cardiolipin and monolysocardiolipin species in Acinetobacter baumannii

Acidic glycerophospholipids play an important role in determining the resistance of Gram-negative bacteria to stress conditions and antibiotics. Acinetobacter baumannii, an opportunistic human pathogen which is responsible for an increasing number of nosocomial infections, exhibits broad antibiotic resistances. Here lipids of A. baumannii have been analyzed by combined MALDI-TOF/MS and TLC analyses; in addition GC-MS analyses of fatty acid methyl esters released by methanolysis of membrane phospholipids have been performed. The main glycerophospholipids are phosphatidylethanolamine, phosphatidylglycerol, acyl-phosphatidylglycerol and cardiolipin together with monolysocardiolipin, a lysophospholipid only rarely detected in bacterial membranes. The major acyl chains in the phospholipids are C16:0 and C18:1, plus minor amounts of short chain fatty acids. The structures of the cardiolipin and monolysocardiolipin have been elucidated by post source decay mass spectrometry analysis. A large variety of cardiolipin and monolysocardiolipin species were found in A. baumannii. Similar lysocardiolipin levels were found in the two clinical strains A. baumannii ATCC19606^T and AYE whereas in the nonpathogenic strain Acinetobacter baylyi ADP1 lysocardiolipin levels were highly reduced.

First Whole-Genome Sequences of Two Multidrug-Resistant Acinetobacter baumannii Strains Isolated from a Moroccan Hospital Floor

This report describes the whole-genome shotgun sequences of two multidrug-resistant Acinetobacter baumannii strains, ABE8_07 and ABE12_M, isolated from a Moroccan hospital floor. These two genome sequences will initiate the study and characterization of the Acinetobacter baumannii genome in Morocco.

Characterization of a highly virulent and antimicrobial-resistant Acinetobacter baumannii strain isolated from diseased chicks in China

Poultry husbandry is a very important aspect of the agricultural economy in China. However, chicks are often susceptible to infectious disease microorganisms, such as bacteria, viruses and parasites, causing large economic losses in recent years. In the present study, we isolated an Acinetobacter baumannii strain, CCGGD201101, from diseased chicks in the Jilin Province of China. Regression analyses of virulence and LD50 tests conducted using healthy chicks confirmed that A. baumannii CCGGD201101, with an LD50 of 1.81 (±0.11) × 10(4) CFU, was more virulent than A. baumannii ATCC17978, with an LD50 of 1.73 (±0.13) × 10(7) CFU. Moreover, TEM examination showed that the pili of A. baumannii CCGGD201101 were different from those of ATCC17978. Antibiotic sensitivity analyses showed that A. baumannii CCGGD201101 was sensitive to rifampicin but resistant to most other antibiotics. These results imply that A. baumannii strain CCGGD201101 had both virulence enhancement and antibiotic resistance characteristics, which are beneficial for A. baumannii survival under adverse conditions and enhance fitness and invasiveness in the host. A. baumannii CCGGD20101, with its high virulence and antimicrobial resistance, may be one of the pathogens causing death of diseased chicks.

Different drugs for bad bugs: antivirulence strategies in the age of antibiotic resistance

The rapid evolution and dissemination of antibiotic resistance among bacterial pathogens are outpacing the development of new antibiotics, but antivirulence agents provide an alternative. These agents can circumvent antibiotic resistance by disarming pathogens of virulence factors that facilitate human disease while leaving bacterial growth pathways - the target of traditional antibiotics - intact. Either as stand-alone medications or together with antibiotics, these drugs are intended to treat bacterial infections in a largely pathogen-specific manner. Notably, development of antivirulence drugs requires an in-depth understanding of the roles that diverse virulence factors have in disease processes. In this Review, we outline the theory behind antivirulence strategies and provide examples of bacterial features that can be targeted by antivirulence approaches. Furthermore, we discuss the recent successes and failures of this paradigm, and new developments that are in the pipeline.

Biology of Acinetobacter baumannii: Pathogenesis, Antibiotic Resistance Mechanisms, and Prospective Treatment Options

Acinetobacter baumannii is undoubtedly one of the most successful pathogens responsible for hospital-acquired nosocomial infections in the modern healthcare system. Due to the prevalence of infections and outbreaks caused by multi-drug resistant A. baumannii, few antibiotics are effective for treating infections caused by this pathogen. To overcome this problem, knowledge of the pathogenesis and antibiotic resistance mechanisms of A. baumannii is important. In this review, we summarize current studies on the virulence factors that contribute to A. baumannii pathogenesis, including porins, capsular polysaccharides, lipopolysaccharides, phospholipases, outer membrane vesicles, metal acquisition systems, and protein secretion systems. Mechanisms of antibiotic resistance of this organism, including acquirement of β-lactamases, up-regulation of multidrug efflux pumps, modification of aminoglycosides, permeability defects, and alteration of target sites, are also discussed. Lastly, novel prospective treatment options for infections caused by multi-drug resistant A. baumannii are summarized.

Genetic Regulation of Virulence and Antibiotic Resistance in Acinetobacter baumannii

Multidrug resistant microorganisms are forecast to become the single biggest challenge to medical care in the 21st century. Over the last decades, members of the genus Acinetobacter have emerged as bacterial opportunistic pathogens, in particular as challenging nosocomial pathogens because of the rapid evolution of antimicrobial resistances. Although we lack fundamental biological insight into virulence mechanisms, an increasing number of researchers are working to identify virulence factors and to study antibiotic resistance. Here, we review current knowledge regarding the regulation of virulence genes and antibiotic resistance in Acinetobacter baumannii. A survey of the two-component systems AdeRS, BaeSR, GacSA and PmrAB explains how each contributes to antibiotic resistance and virulence gene expression, while BfmRS regulates cell envelope structures important for pathogen persistence. A. baumannii uses the transcription factors Fur and Zur to sense iron or zinc depletion and upregulate genes for metal scavenging as a critical survival tool in an animal host. Quorum sensing, nucleoid-associated proteins, and non-classical transcription factors such as AtfA and small regulatory RNAs are discussed in the context of virulence and antibiotic resistance.

Genetic Dissection of the Type VI Secretion System in Acinetobacter and Identification of a Novel Peptidoglycan Hydrolase, TagX, Required for Its Biogenesis

The type VI secretion system (T6SS) is a widespread secretory apparatus produced by Gram-negative bacteria that has emerged as a potent mediator of antibacterial activity during interbacterial interactions. Most Acinetobacter species produce a genetically conserved T6SS, although the expression and functionality of this system vary among different strains. Some pathogenic Acinetobacter baumannii strains activate this secretion system via the spontaneous loss of a plasmid carrying T6SS repressors. In this work, we compared the expression of T6SS-related genes via transcriptome sequencing and differential proteomics in cells with and without the plasmid. This approach, together with the mutational analysis of the T6SS clusters, led to the determination of the genetic components required to elaborate a functional T6SS in the nosocomial pathogen A. baumannii and the nonpathogen A. baylyi By constructing a comprehensive combination of mutants with changes in the T6SS-associated vgrG genes, we delineated their relative contributions to T6SS function. We further determined the importance of two effectors, including an effector-immunity pair, for antibacterial activity. Our genetic analysis led to the identification of an essential membrane-associated structural component named TagX, which we have characterized as a peptidoglycan hydrolase possessing l,d-endopeptidase activity. TagX shows homology to known bacteriophage l,d-endopeptidases and is conserved in the T6SS clusters of several bacterial species. We propose that TagX is the first identified enzyme that fulfills the important role of enabling the transit of T6SS machinery across the peptidoglycan layer of the T6SS-producing bacterium.

IMPORTANCE

Acinetobacter baumannii is one of the most troublesome and least investigated multidrug-resistant bacterial pathogens. We have previously shown that A. baumannii employs a T6SS to eliminate competing bacteria. Here we provide a comprehensive analysis of the components of the T6SS of Acinetobacter, and our results provide genetic and functional insights into the Acinetobacter T6SS. Through this analysis, we identified a novel peptidoglycan hydrolase, TagX, that is required for biogenesis of the T6SS apparatus. This is the first peptidoglycanase specialized in T6SS function identified in any species. We propose that this enzyme is required for the spatially and temporally regulated digestion of peptidoglycan to allow assembly of the T6SS machinery.

Evaluation of the ability of Acinetobacter baumannii to form biofilms on six different biomedical relevant surfaces

The human opportunistic pathogen, Acinetobacter baumannii, has the propensity to form biofilms and frequently cause medical device-related infections in hospitals. However, the physio-chemical properties of medical surfaces, in addition to bacterial surface properties, will affect colonization and biofilm development. The objective of this study was to compare the ability of A. baumannii to form biofilms on six different materials common to the hospital environment: glass, porcelain, stainless steel, rubber, polycarbonate plastic and polypropylene plastic. Biofilms were developed on material coupons in a CDC biofilm reactor. Biofilms were visualized and quantified using fluorescent staining and imaged using confocal laser scanning microscopy (CLSM) and by direct viable cell counts. Image analysis of CLSM stacks indicated that the mean biomass values for biofilms grown on glass, rubber, porcelain, polypropylene, stainless steel and polycarbonate were 0·04, 0·26, 0·62, 1·00, 2·08 and 2·70 μm(3) /μm(2) respectively. Polycarbonate developed statistically more biofilm mass than glass, rubber, porcelain and polypropylene. Viable cell counts data were in agreement with the CLSM-derived data. In conclusion, polycarbonate was the most accommodating surface for A. baumannii ATCC 17978 to form biofilms while glass was least favourable. Alternatives to polycarbonate for use in medical and dental devices may need to be considered.

SIGNIFICANCE AND IMPACT OF THE STUDY

In the hospital environment, Acinetobacter baumannii is one of the most persistent and difficult to control opportunistic pathogens. The persistence of A. baumannii is due, in part, to its ability to colonize surfaces and form biofilms. This study demonstrates that A. baumannii can form biofilms on a variety of different surfaces and develops substantial biofilms on polycarbonate - a thermoplastic material that is often used in the construction of medical devices. The findings highlight the need to further study the in vitro compatibility of medical materials that could be colonized by A. baumannii and allow it to persist in hospital settings.

Structural and Functional Investigation of FdhC from Acinetobacter nosocomialis: A Sugar N-Acyltransferase Belonging to the GNAT Superfamily

Enzymes belonging to the GNAT superfamily are widely distributed in nature where they play key roles in the transfer of acyl groups from acyl-CoAs to primary amine acceptors. The amine acceptors run the gamut from histones to aminoglycoside antibiotics to small molecules such as serotonin. Whereas those family members that function on histones have been extensively studied, the GNAT enzymes that employ nucleotide-linked sugars as their substrates have not been well characterized. Indeed, though the structures of two of these "amino sugar" GNAT enzymes have been determined within the past 10 years, details concerning their active site architectures have been limited because of a lack of bound nucleotide-linked sugar substrates. Here we describe a combined structural and biochemical analysis of FdhC from Acinetobacter nosocomialis O2. On the basis of bioinformatics, it was postulated that FdhC catalyzes the transfer of a 3-hydroxybutanoyl group from 3-hydroxylbutanoyl-CoA to dTDP-3-amino-3,6-dideoxy-d-galactose, to yield an unusual sugar that is ultimately incorporated into the surface polysaccharides of the bacterium. We present data confirming this activity. In addition, the structures of two ternary complexes of FdhC, in the presence of CoA and either 3-hydroxybutanoylamino-3,6-dideoxy-d-galactose or 3-hydroxybutanoylamino-3,6-dideoxy-d-glucose, were solved by X-ray crystallographic analyses to high resolution. Kinetic parameters were determined, and activity assays demonstrated that FdhC can also utilize acetyl-CoA, 3-methylcrotonyl-CoA, or hexanoyl-CoA as acyl donors, albeit at reduced rates. Site-directed mutagenesis experiments were conducted to probe the catalytic mechanism of FdhC. Taken together, the data presented herein provide significantly new molecular insight into those GNAT superfamily members that function on nucleotide-linked amino sugars.