Acinetobacter Molecular Biology

The genus Acinetobacter is a group of Gram-negative, non-motile and non-fermentative bacteria belonging to the family Moraxellaceae. They are important soil organisms where they contribute to the mineralisation of, for example, aromatic compounds. Acinetobacter are able to survive on various surfaces (both moist and dry) in the hospital environment, thereby being an important source of infection in debilitated patients. These bacteria are innately resistant to many classes of antibiotics. In addition, Acinetobacter is uniquely suited to exploitation for biotechnological purposes.

Taxonomy
The genus Acinetobacter comprises 17 validly named and 14 unnamed (genomic) species. Some unrelated (genomic) species have common designations, while some other species seem to be congruent but have different names. The knowledge of the biology or ecology of acinetobacters at species level is limited. This is due to the fact that identification of acinetobacters at species level is difficult. A phenotypic species identification system has been described and a variety of genotypic methods has been explored and applied to investigate the diversity or phylogeny in the genus. These methods include high resolution fingerprinting with AFLP, PCR-RFLP with digestion of PCR amplified sequences, and analysis of various DNA sequences. Of these, AFLP analysis and amplified 16SrDNA ribosomal DNA restriction analysis have been validated with large numbers of strains of all described species. Nucleotide sequence based methods are expected to be the standard for identification in the near future.

Clinical Significance
Several species persist in hospital environments and cause severe, life-threatening infections in compromised patients. The spectrum of antibiotic resistances of these organisms together with their survival capabilities make them a threat to hospitals as documented by recurring outbreaks both in highly developed countries and elsewhere. An important factor for their pathogenic potential is probably an efficient means of horizontal gene transfer even though such a mechanism has so far only been observed and analyzed in Acinetobacter baylyi, a species that lives in the soil and has never been associated with infections.

Phage Therapy
A phage directed against Acinetobacter showed a remarkable lytic activity both in vitro and in vivo: as few as 100 pfu of phage protected mice against Acinetobacter. Further information on phage therapy ...

Biotechnology
Many of the characteristics of Acinetobacter ecology, taxonomy, physiology and genetics point to the possibility of exploiting its unique features for future applications. Acinetobacter strains are often ubiquitous, exhibit metabolic versatility, are robust and some provide convenient systems for modern molecular genetic manipulation and subsequent product engineering. These characteristics are being exploited in various biotechnological applications including biodegradation and bioremediation, novel lipid and peptide production, enzyme engineering, biosurfactant and biopolymer production and engineering of novel derivatives of these products. It is anticipated that progress in these fields will broaden the range of applications of Acinetobacter for modern biotechnology.

Catabolism of Aromatic Compounds
Acinetobacter strains isolated from the environment are capable of the degradation of a wide range of aromatic compounds. However the predominant route for the final stages of assimilation to central metabolites is through catechol or protocatechuate (3,4-dihydroxybenzoate) and the beta-ketoadipate pathway, and the diversity within the genus lies in the channelling of growth substrates, most of which are natural products of plant origin, into this pathway.

Molecular Basis of Antibiotic Resistance in Acinetobacter spp.
Members of the genus Acinetobacter have the ability to develop resistance to new antibiotics extremely rapidly. Most multiresistant isolates of Acinetobacter spp. belong to the Acinetobacter baumannii complex, and many clinical isolates of A. baumannii are now resistant to all conventional antimicrobial agents, including carbapenems. Molecular studies have characterised most of the responsible genes and mechanisms of resistance to antibiotics found within the genus. Multidrug resistance typically results from the accumulation of multiple mutations and/or the acquisition of resistance genes from other bacterial genera, with the latter occurring by a variety of mechanisms, including the transfer of plasmids, transposons and integrons carrying clusters of genes encoding resistance to several unrelated families of antibiotics simultaneously. Whole-genome sequence analysis has identified the presence of resistance islands, apparently built through the successive insertion of broad host-range mobile genetic elements into an insertion hotspot on the A. baumannii chromosome. This ability to 'switch' its genetic structure may explain the unmatched speed at which A. baumannii captures resistance markers when under antibacterial selection pressure. Overall, the emergence of resistance among clinical isolates of A. baumannii appears to be a combined effect of gene acquisition, following lateral gene transfer, and clonal spread of multiresistant clones.

For further information see the newly published book Acinetobacter Molecular Biology edited by Ulrike Gerischer