A 58 year old female presented to the emergency department with a chief complaint of shortness of breath, fevers and chills since the previous day. Her past medical history is significant for chronic obstructive pulmonary disease, hypertension, and borderline personality disorder. Vitals signs were significant for an oxygen saturation of 88%. Physical examination of the patient was difficult as the patient became increasingly agitated, however, the patient appeared in no acute distress with moist mucous membranes, anterior lung fields were clear to auscultation, there were no cardiac murmurs, and examination of their skin revealed no rashes or lesions.
Laboratory tests were significant for a lactate level of 2.5 with a white blood cell count and complete metabolic panel within normal limits. Chest x-ray did not show evidence of consolidation or interstitial infiltrates. Urinalysis was within normal limits. One set of blood cultures was also drawn during this initial encounter. The patient became increasingly agitated after initial examination and was discharged with some laboratory tests pending. After incubating for 20 hours, the aerobic blood culture bottle flagged positive for bacterial growth, with gram stain demonstrating a gram negative coccobacillus and a rapid Verigene identification of Acinetobacter. The patient came back to the emergency department the next day with stable vital signs and unremarkable complete blood count and chest x-ray. The patient was started on meropenem which was switched to ciprofloxacin two days later, after bacterial antibiotic susceptibility results showed susceptibility to carbapenems, amikacin, amp/sulbactam, ceftazidime, ciprofloxacin, gentamin and tobramycin.
Acinetobacter is a genus of gram negative bacteria, with some genospecies identified as human pathogens including species in the A. calcoaceticus-A. baumannii complex (ACB) which are difficult to differentiate by phenotypic characteristics. Species in the ACB include genospecies 1 (A. calcoaceticus), genospecies 2 (A. baumannii), genospecies 3, and genospecies 13TU.
In the laboratory, Acinetobacter appear as non-pigmented mucoid, domed colonies with a smooth surface on growth media. Acinetobacter are non-motile, aerobic, catalase positive, oxidase negative, indole negative bacteria. Acinetobacter are also non-glucose fermenters and do not utilize lactose.
Out of the ACB genospecies, A. baumannii is considered the most significant pathogen, causing 80% of nosocomial infection. A. baumannii is an environmental bacteria which inhabits soil and water. In hospital settings, A. baumannii can survive on environmental surfaces for extended periods of time and is resistant to desiccation and cleaning solutions. The most common settings in which A. baumannii infections occur are within intensive care units where there are immunocompromised patients utilizing medical devices such as ventilators or catheters which are surfaces A. baumannii frequently colonizes. Not surprisingly, sites where these medical devices preside are the most common sites of infection for A. baumannii including the respiratory tract (hospital acquired pneumonia), bloodstream infections, and wound infections. Interestingly, A. baumannii wound infection have also been seen at a high prevalence in wartime and disaster victims. A. baumannii has been recovered in 63% of wounds from soldiers in Iraq and Afghanistan and 20% of wounds from victims after a tsunami in 2004.
Importantly, A. baumannii can be resistant to several classes of antibiotics including fluroquinolones (DNA topoisomerase mutations), aminoglycosides (transposons), beta lactams (AMP C beta lactamase), and carbapenems (OXA carbapenemase), making infections with multidrug resistant organisms challenging to treat. In this case, the microbe had an OXA carbapenemase but was susceptible to carbapenems. In addition, this patient’s relatively benign presentation and normal laboratory results raise the question of whether this bacteria was causing a bloodstream infection or was simply a skin colonizer which grew after being inoculated into the blood culture media. Acinetobacter, in addition to colonizing hospital equipment and surfaces is a common colonizer of the skin as well as respiratory tract of patients on respiratory ventilators. Thus, Acinetobacter can be inadvertently cultured in blood and sputum samples, making correlation of the patient’s clinical symptoms and signs with culture results very important.
-Liam Donnelly, MD is a 1st year anatomic and clinical pathology resident at the University of Vermont Medical Center.
-Christi Wojewoda, MD, is the Director of Clinical Microbiology at the University of Vermont Medical Center and an Associate Professor at the University of Vermont.