The inventors have constructed a polyclonal antibody (pAb) for the specific detection of the multi-drug resistant (MDR) bacterial pathogen, Acinetobacter baumannii (A. baumannii), producing the antibody entitled, 'AB-pAb'. The AB-pAb was raised against a recombinant (His-tagged) 22 kDa outer membrane protein (OMP22), an antigenic protein which is conserved across the species. The gene encoding OMP22 was amplified from the clinical A. baumannii isolate, AR_0056, which belongs to the international clonal lineage II, a lineage associated with outbreaks worldwide. The AB-pAb is capable of recognizing purified, denatured, OMP22 by Western blot, in addition to the native protein in whole cells of A. baumannii in vitro. The pAb was optimized for diagnostic use by, firstly, removing antibodies within the heterogeneous pAb pool which were cross-reactive to other, clinically-relevant Gram-negative bacteria (GNB). This eliminates the issue of cross-reactivity often associated with polyclonal antibodies, which can limit their use as diagnostic tools. Moreover, testing was performed under conditions which mimic those of the blood and urine, further enhancing the novel AB-pAb's ability to recognize target bacteria in patient samples. When tested against a panel of clinical isolates by indirect-ELISA, for the recognition of A. baumannii from other clinically relevant GNB, the optimized AB-pAb had a sensitivity of 85.5% (95 % confidence interval: 76.11% to 92.3%) and a specificity of 99.5% (95 % confidence interval: 99.53% to 99.99%) at a cutoff, signal-to-noise ratio (SNR) of 0.1275. To our knowledge, no commercial anti-A. baumannii pAbs are currently available which target OMP22, specifically optimized for diagnostic purposes.