Class-D OXA-type β-lactamases are bacterial enzymes commonly known to hydrolyse the isoxazolylpenicillin oxacillin much faster than classical penicillins, such as benzylpenicillin. Once overexpressed, the OXAAb enzymes, which are intrinsic to Acinetobacter baumannii, can confer resistance to carbapenem antibiotics.
Reversible colony switch between opaque and translucent forms is an interesting morphological feature that has recently been recognised in Acinetobacter baumannii. Such switching is associated with variations in the bacterial cell morphology, surface motility, biofilm formation, antibiotic resistance, and virulence.
Studies on the intra and extra hospital epidemiology of Acinetobacter baumannii in Lebanon have demonstrated that The majority of clinical isolates are multi-drug resistant, including high rates of carbapenem resistance.
Horizontal Gene Transfer and Natural Transformation
Horizontal gene transfer (HGT) plays a key role in the evolution of Acinetobacter baumannii toward multidrug resistance and sucessful adaptation to new environments. Natural transformation, a process where bacterial cells actively take up free DNA from the extracellular environment and recombine it into their chromosome or reconvert it into extra-chromosomal genetic elements, is one of the several possible pathways that mediate HGT in A. baumannii.
Tn6019 is the common backbone of all the AbaR0- and AbaR3-type resistance islands in Acinetobacter baumannii. It is a 16,332-bp class III transposon bounded by 26-bp inverted repeats and carries a set of five transposition genes (tniC, tniA, tniB, tniD, and tniE). Tn6019 also carries genes coding for arsenate/arsenite resistance, monooxygenase, thioredoxin-disulfide reductase, universal stress protein, and sulphate permease. The last open reading frame (orf) has no predicted function and is usually annotated as orf4.
One of the major virulence determinants of Acinetobacter baumannii is the presence of a capsule that forms a barrier around the bacterial cell wall, providing protection from environmental pressures including desiccation, disinfection, and host immune responses.