Acinetobacter baumannii IMPORTANT pathogen? Not to be missed by Dr.T.V.Rao MD

Acinetobacter baumannii IMPORTANT pathogen? Not to be missed by Dr.T.V. Rao MD – Today when we are reporting on bacteriology table, talk and discuss about a newly emerging pathogen. It has become famous with Iraq war. A.baumannii is referred to as 'Iraqibacter' due to its seemingly sudden emergence in military treatment facilities during the Iraq War. I come across atleast once in 1o days from ICU and MICU units. Acinetobacter baumannii is a typically short, almost round, rod-shaped (coccobacillus) Gram-negative bacterium. It can be an opportunistic pathogen in humans, affecting people with compromised immune systems, and is becoming increasingly important as a hospital-derived (nosocomial) infection. Given the predilection of A. baumannii to colonize and infect critically ill patients, who often have a poor prognosis irrespective of secondary infective complications, it has been challenging to determine the true clinical impact of this pathogen The genus Acinetobacter, as currently defined, comprises gram-negative, strictly aerobic, nonterminating, no fastidious, nonmotile, catalase-positive, oxidase-negative bacteria. Based on more recent taxonomic data, it was proposed that members of the genus Acinetobacter should be classified in the new family Moraxellaceae within the order We grow it on blood agar and Mac conkey agar as nonlactose fermenting, gram negative bacteria on Gram staining, These organisms form smooth, sometimes mucoid, greyish white colonies; colonies of the A. calcoaceticus-A. baumannii complex resemble those of Enterobacteriaceae, with a colony diameter of 1.5 to 3 mm after overnight culture, while most of the other Acinetobacter species produce smaller and more translucent colonies. However we are confused with Moraxella from many respiratory specimens however if we do oxidase test it is easier, We Clinical microbiologists typically differentiate members of the Acinetobacter genus from other Moraxellaceae by performing an oxidase test, as Acinetobacter spp. are the only members of the Moraxella to lack cytochrome oxidases. If we are aware of many articles published on ESKAPE pathogens, A. baumannii has also been identified as an ESKAPE pathogen (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species; a group of pathogens with a high rate of antibiotic resistance that are responsible for the majority of nosocomial infections. Authentic studies indicate, A. baumannii growth is suspected to be favoured in hospital settings due to the constant use of antibiotics by patients in the hospital. In a study of European intensive care units in 2009, A. baumannii was found to be responsible for 19.1% of ventilator-associated pneumonia (VAP) cases. We working with minimal infrastructure find to speciate from many others resembling Acinetobacter, Species identification with manual and semi-automated commercial identification systems that are currently used in diagnostic microbiology, such as the API 20NE, Vitek 2, Phoenix, and Micro Scan Walkaway systems, remains problematic. This can be explained in part by their limited database content but also because the substrates used for bacterial species identification have not been tailored specifically to identify Acinetobacter. When we are working with limited facilities there is more scope for wisdom and clinical situation to identify Acinetobacter where the patients are placed in the hospital for longer times, and use of multiple new generation antibiotics, as we see observe drug resistant isolates from many intensive care units.

Because most infections are now resistant to multiple drugs, it is necessary to determine what susceptibilities the particular strain has for treatment to be successful. Important to realise, there were many discrepancies between results obtained by broth micro dilution and those obtained by disk diffusion. Very major errors (susceptible according to disk diffusion but resistant according to broth micro dilution) occurred with ampicillin-sulbactam, piperacillin, piperacillin-tazobactam, ticarcillin-clavulanate, ceftazidime, and cefepime. In the absence of human or animal model data, it is impossible to determine which testing method is more clinically relevant traditionally, CLSI recommends that MICs for antibiotics versus Acinetobacter spp. be determined in broth, using cation-adjusted Mueller-Hinton broth, or on agar, using Mueller-Hinton agar. Disk diffusion should also be performed using Mueller-Hinton agar. As newer options Acinitobacter infections were treated with imipenem or meropenem, but a steady rise in carbapenem-resistant A. baumannii has been noted. Consequently, treatment methods often fall back on polymixins, particularly colistin. Colistin is considered a drug of last resort because it often causes kidney damage among other side effects. Prevention methods in hospitals focus on increased hand-washing and more diligent sterilization procedure. A. baumannii strains resistant to all known antibiotics have now been reported, signifying a sentinel event that should be acted on promptly by the international health care community. Antibiotic selection for empirical therapy is challenging and must rely on recent institutional-level susceptibility data. Time to effective therapy clearly impacts patient outcomes, and this may include patients with A. baumannii infection. Thus far, carbapenems have been thought of as the agents of choice for serious A. baumannii infections. However, although these drugs are still active against the vast majority of A. baumannii strains worldwide, the clinical utility of this class of antimicrobial is increasingly being jeopardized by the emergence of both enzymatic and membrane-based mechanisms of resistance, as described above, often working in concert. A concerning void of new therapeutic options exists for A. baumannii infections. Of the recently licensed antimicrobials, tigecycline, a 9-t-butylglycylamido semisynthetic derivative of minocycline, has provided some hope, but clinical data are still limited.

Note - In routine reporting, many times we over diagnose when we observe an NLF with coccobacillary appearance and Oxidase negative bacteria, as Acinetobacter baumannii. I wish the Microbiologists to Refer Diagnostic Microbiology by Bailey and Scot as matters have changed beyond our imaginations. (Ref - Acinetobacter baumannii: Emergence of a Successful Pathogen Anton Y. Peleg et al Clinical Microbiology Reviews)

Dr.T.V.Rao MD Professor of Microbiology Freelance Writer