Microbiology Case Study: An 81 Year Old Female with Persistent Fevers

Case History

The infectious disease service was consulted on an 81 year old female for persistent fevers. She initially presented a few weeks prior with cough & shortness of breath which was diagnosed as an acute chronic obstructive pulmonary disease (COPD) exacerbation for which she received levofloxacin and steroids. The patient continued to have a persistent cough and dysphagia after discharge. Her respiratory status and cough worsened and she was readmitted and intubated. Vancomycin, piperacillin/tazobactam and levofloxacin were started as well as fluconazole for suspected esophageal candidiasis. Her past medical history was significant for breast cancer, atrial fibrillation, and diabetes mellitus. Of note, patient was originally from Puerto Rico but moved to the United States 40 years ago and denied recent travel and any known tuberculosis exposures. She formerly worked in a deli packing cheeses.  A bronchoscopy was performed and a brochoalveolar lavage (BAL) specimen as well as blood and stool specimens were submitted for bacterial culture and ova and parasite exam.

Laboratory Identification

Image 1. Multiple larval forms in the stood specimen from an ova and parasite exam. (Iodine stain, 100X).
Image 2. High power of the larvae with a short buccal cavity (red arrow) and prominent genital primordium (blue arrow), (Iodine stain, 1000x).

The bronchoscopy revealed a bloody fluid admixed with clots which was clinically consistent with diffuse alveolar hemorrhage. The roundworms depicted above were identified in both the BAL and stool O&P exam. Based on the presence of the short buccal cavity and the prominent genital primordium and the absence of eggs, the identification of Strongyloides stercoralis was made. Given the large amount of larvae present in both the lungs and gastrointestinal tract, the patient was diagnosed with a strongyloidiasis hyperinfection.  

Discussion

Strongyloides stercoralis is classified as a nematode (roundworm) and is the cause of strongyloidiasis in humans. The helminth is found worldwide, especially in warm climates and underdeveloped countries, and is the cause of 30-100 million infections. Infection is due to fecal contamination of soil, where free-living forms are found, or water. Infective filariform larvae penetrate intact skin, particularly bare feet, resulting in infection. The free living cycle begins with the rhabditiform larvae passed through the stool develops into the infective filariform larvae or when the  rhabditiform larvae mature into free living adult male & female forms that mate and produce eggs which then hatch and become infective filariform larvae that can infect humans. The parasitic life cycle begins with the infective filariform larvae penetrates human skin. The worm is then either coughed up from the lungs and swallowed or migrates to the small intestine where eggs are laid and hatch.

Patients may present with gastrointestinal symptoms such as abdominal pain, bloating, and diarrhea, pulmonary symptoms like dry cough and throat irritation, or skin rashes along points of entry (feet, ankles). When the larvae are in the lung, Loeffler’s syndrome, characterized by pneumonia symptoms with coughing and wheezing, may develop due to an accumulation of eosinophils in response to the parasitic infection. In patients who are immunocompromised, the rhabditiform larvae can develop into the filariform larvae in the host and can directly penetrate the bowel mucosa or perianal skin resulting in autoinfection, dissemination throughout the body, and high parasite burden. Symptoms of hyperinfection include bloody diarrhea, bowel perforation, destruction of lung parenchyma with bloody sputum, meningitis, and septicemia. Hyperinfection most commonly occurs after steroid administration for asthma or COPD exacerbation, but can also be seen in those receiving chemotherapy or who have had organ transplants.  

In the laboratory, the diagnosis of S. stercoralis is most often made by an ova and parasite exam of the stool, duodenal fluid, sputum or BAL specimens (Image 1). Most commonly the rhabditiform larvae are present and are identified by the presence of a short buccal cavity and prominent genital primordium (Image 2). These two features are helpful in distinguishing S. stercoralis from hookworms (Ancylostoma spp. and Necator americanus) which have a longer buccal cavity and indistinct genital primordium. The eggs of these two nematodes are also very similar, although typically S. stercoralis eggs hatch before they are passed in stool specimens. S. stercoralis can also be visualized on H&E histology sections in the crypts of intestinal biopsies where the adult female measures up to 2.2 mm in length. Finally, serologic testing can be helpful when there is a high suspicion of disease in the face of multiple negative stool exams, but cannot distinguish between a current or past infection. Most patients do not remember a specific exposure and prevention includes wearing gloves and shoes when handling or walking on soil that may contain contaminated fecal material. Treatment options for an acute or chronic S. stercoralis include a short course of ivermectin or albendazole. In the case of disseminated infection, ivermectin should be given until stool and sputum exams are negative for 2 weeks. In the case of our patient, she was started on ivermectin, but succumbed to the disease due to extensive pulmonary hemorrhage.   

-Jaswinder Kaur, MD, is a fourth year Anatomic and Clinical Pathology resident at the University of Mississippi Medical Center. 

-Lisa Stempak, MD, is an Assistant Professor of Pathology at the University of Mississippi Medical Center in Jackson, MS. She is certified by the American Board of Pathology in Anatomic and Clinical Pathology as well as Medical Microbiology. She is the Director of Clinical Pathology as well as the Microbiology and Serology Laboratories. Her interests include infectious disease histology, process and quality improvement, and resident education.

Microbiology Case Study: A 54 Year Old Male with Right Upper Quadrant Pain

Case History

A 54 year old male with a past medical history of Type II diabetes mellitus and obesity was admitted for a few days history of severe pain over right upper quadrant accompanied by fevers, chills, nausea, vomiting and diarrhea. Physical exam revealed a palpable gallbladder. Ultrasound imaging showed a distended gallbladder with a thickened, edematous and hyperemic wall that was interpreted as acalculus cholecystitis. The patient underwent percutaneous drainage of the gallbladder with plans to undergo a cholecystectomy once the acute phase of his illness stopped. The gallbladder fluid was sent to microbiology for analysis.

Laboratory Findings

Anaerobic plates obtained from organisms growing in thioglycollate broth grew low, convex opaque white colonies. The organisms did not produce the classic double zone of beta hemolysis (Image 1).  Gram stain of the culture showed gram positive bacilli that were “boxcar” shaped (Image 2). Aerobic plates had no growth. The organisms were catalase negative and non-motile. MALDI-TOF identified the organism as Clostridium perfringens.

Image 1. Growth on CNA plate in anaerobic environment
Image 2. Gram stain from the anaerobic culture shows boxcar-shaped gram positive bacilli.

The patient was also placed on piperacillin-tazobactam while in the hospital.  His condition improved and he was discharged home with a seven day course of cefpodoxime and metronidazole with a general surgery follow up appointment.

Discussion

Clostridium perfringens is a gram positive bacilli with blunt ends (boxcar shaped). These obligate anaerobes are spore formers, however these are rarely seen. When seen, they produce subterminal spores. These organisms cause of crepitant myonecrosis (gas gangrene), gangrenous cholecystitis, septicemia, and food poisoning.  They are present in large numbers as normal microbiota in the gastro-intestinal tract of humans and animals, the female genital tract and oral mucosa. Typically, infections are caused by endogenous strains gaining access to normal sterile sites due to a predisposing factor that compromise normal anatomy: surgery, trauma, or altered host defense mechanisms (diabetes, burns, immunosuppression, and aspiration).

Penicillin is recommended in most infections, however resistance has been reported. Optimal management of intra-abdominal infection is to achieve appropriate source control and drainage is important.   

References

  1. Tille P. Bailey & Scott’s Diagnostic Microbiology. Fourteenth Edition. Elsevier; 2017.
  2. Murray P. Medical Microbiology. Seventh Edition. Elsevier; 2013.

-Angela Theiss, MD is a 3rd 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.

Microbiology Case Study: A 19 Year Old Woman Passes a Worm in Her Stool

Case History

A 19 year old woman with no significant past medical history presented to an outside clinic with psychological distress after passing a worm per rectum. She had no other complaints or symptoms.

Image 1. Eggs are spherical, 31 to 43 μm in diameter, with a thick yellow-brown (bile-stained) shell having radial striations and contain a hexacanth oncosphere (Image courtesy of: Fred Patterson, Parasitologist).
Image 2. Adult proglottids are longer (up to 20 mm) than they are wide (up to 7 mm) and have a genital pore (*) at the lateral margin.
Image 3. Formalin-fixed paraffin-embedded H&E stained section of a proglottid’s lateral uterine branches on each side of a central uterine stem (20x).

Discussion

Adult cestodes (tapeworms) have long, ribbon-like bodies made up of proglottids (egg-producing segments) that develop at the posterior of a scolex (specialized structure for attachment to the small intestine of a host). Taenia has 32 species, 2 of which are medically important for causing taeniasis: Taenia saginata and Taenia solium (beef and pork tapeworm infection, respectively). These parasites are distributed worldwide, with T. saginata being more common than T. solium.

The lifecycle of Taenia involves adult, egg, and larval stages. Adults release gravid proglottids and eggs that are passed in feces. The eggs reach pasture land via soil or water and are ingested by an intermediate host. For T. saginata, the intermediate host is a herbivore (cow), as eggs of T. saginata do not infect humans, and for T. solium, the intermediate host is a pig, human, or other animal. Ingested eggs hatch and release the hexacanth oncosphere (6-hooked embryo) that can penetrate tissues. Over 2 to 3 months, infective cysticerci (0.5 to 2.0 mm in diameter larvae) develop in muscles. Of note, only T. solium can cause cysticercosis (extra-intestinal larval forms within human tissues, ie. the human becomes the intermediate host) and this can be life-threatening if cysticerci invade the brain. When humans consume raw or undercooked beef/pork meat that is infected, cysticerci will attach to the small intestinal mucosa and, over 3 to 5 months, mature into the adult form. The adult T. saginata can reach 4 to 12 meters in length and the adult T. solium can reach 1.5 to 8 meters in length. Adult tapeworms can live within intestines for over 25 years while gravid proglottids and eggs are passed in stool.

Infections are usually asymptomatic or cause mild indigestion, anorexia, and abdominal discomfort. The eggs can be identified by ova and parasite examination (Figure 1) or a cellulose tape preparation of perianal skin. The oncosphere must be visualized to avoid misidentifying a pollen grain. However, eggs of Taenia species and Echinococcus species are indistinguishable. Diagnosis is also made by recovering gravid proglottids from the anal opening or passed in feces (Figures 2 and 3). Distinguishing the two species can be done by examining gravid proglottids for the number of lateral uterine branches present on one side of a central uterine stem. T. saginata have 15 to 30 lateral uterine branches while T. solium have 7 to 13 branches. Both species have a small anterior scolex (measuring 1 to 2 mm in diameter for T. saginata and 1 mm in diameter for T. solium) with 4 suckers. Definitive identification is possible since T. solium’s scolex has a rostellum (crown) with 2 rows of hooks whereas T. saginata’s scolex bears no rostellum or hooks. Treatment is a single dose of praziquantel and successful treatment is defined as passing zero proglottids over 4 consecutive months.

-Adina Bodolan, MD is a 3rd 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.

Microbiology Case Study: A 55 Year Old Woman with Cough and Headache

Clinical History

A 55 year old female presented to the gastroenterology clinic with a chief complaint of cough and headache. She reported no recent fevers, abdominal pain, or diarrhea. On further questioning she revealed she was originally from the Philippines and had a past history of a parasitic infection that was treated twice with praziquantel. She did not remember the name of the parasite but was concerned for a recurrent infection. A stool specimen for ova & parasite exam and basic laboratory work, including an IgE level, were collected and the patient was scheduled for a screening colonoscopy. Findings from the colonoscopy revealed no gross evidence of neoplastic or infectious disease; however, random rectal biopsies were obtained. 

Laboratory Identification

Image 1. Rectal biopsy showing unremarkable colonic mucosa with many calcified structures deep in the epithelium (H&E, 40x).
Image 2. Rectal granuloma surrounding a calcified structure (H&E, 400x).
Image 3. Intact parasitic egg with a small, inconspicuous spine measuring approximately 90 um in greatest dimension (H&E, 1000x oil immersion).

Discussion

Schistosoma japonicum is a trematode that can infect humans through direct penetration of the skin by the cercariae when wading or swimming in infected waters in the Far East, such as China, the Philippines, Indonesia, and Thailand. Infection can initially present as swimmer’s itch and then develop into Katyama syndrome, which includes fever, eosinophilia, muscle aches, lymphadenopathy, abdominal pain, and diarrhea.

S. japonicum migrates through tissues and the adult male & female forms take up residence in the mesenteric veins that drain the small intestine. The female lays eggs which travel to the lumen of the intestines and can be shed in the stool. The host immune response to the eggs is the major cause of clinical disease which presents as inflammation & ulceration in the intestines, portal fibrosis in the liver & splenomegaly, and more rarely, lesions in the central nervous system. As with all trematodes, snails serve as the intermediate host. 

In the microbiology laboratory, diagnosis is usually made by identification of the eggs in stool specimens. The eggs of S. japonicum are ovoid in shape with a transparent shell and a small, inconspicuous spine. The eggs typically measure between 70-100 um in greatest dimension. These eggs can commonly be visualized in rectal biopsies as well. It is important to get an accurate measurement of the size of the egg and multiple sections to be able to detect the location and morphology of the spine. The eggs of S. japonicum must be distinguished from those of S. mekongi which is similar in appearance; however, the latter is found along the Mekong River in Southeast Asia and is smaller in size (50-70 um in greatest dimension). Serology is also a viable diagnostic test in those that have traveled to endemic regions, but sensitivity and specificity of the assays vary depending on how the antigen is prepared and the Schistosoma species of interest.

Treatment of choices for those infected with S. japonicum is praziquantel divided into three doses over the course of one day and it should be administered at least 6 to 8 weeks after the last exposure to contaminated freshwater. Since our patient admitted to a recent visit to the Philippines with potential exposure to infected waters, she received another course of praziquantel therapy.

-Anas Berneih, MD, is a fourth year Anatomic and Clinical Pathology chief resident at the University of Mississippi Medical Center. 

-Lisa Stempak, MD, is an Assistant Professor of Pathology at the University of Mississippi Medical Center in Jackson, MS. She is certified by the American Board of Pathology in Anatomic and Clinical Pathology as well as Medical Microbiology. She is the Director of Clinical Pathology as well as the Microbiology and Serology Laboratories. Her interests include infectious disease histology, process and quality improvement, and resident education.

Microbiology Case Study: A 65 Year Old Female with Altered Mental Status

Case History

A 65 year old female with a past medical history significant for hypertension, type 2 diabetes, and asplenia, presented to a regional hospital with shortness of breath and altered mental status after the patient’s spouse found them obtunded. Emergency room workup was significant for hypoxia, acidosis, leukocytosis, and acute kidney injury with CT chest imaging showing multifocal right lung infiltrates concerning for aspiration pneumonia. The patient was started on broad spectrum antibiotics but subsequently developed increasing respiratory distress requiring intubation. The patient was transferred to our institution for further management.

Laboratory Findings

Additional testing revealed negative RSV, influenza PCR, and S. pneumoniae and Legionella urinary antigen tests. A sputum culture grew E. coli susceptible to several antibiotics and the antibiotic therapy was narrowed to doxycycline. The patient improved clinically and was transferred back to the regional hospital. Two days after transfer, one blood culture bottle which was collected upon admission to our institution (about 5 days prior), flagged positive for microbial growth. Gram stain revealed budding yeast forms, which were subsequent identified as Candida glabrata.

Image 1. Gram stain from blood culture showing budding yeast forms.
Image 2. Wet mount from a growth plate showing yeast without pseudohyphae
Image 3. Potato flake agar showing smooth creamy white colonies.

Discussion

Candida glabrata is a yeast which can cause infections of the bloodstream as well as urogenital tract and infections of the mucosal surfaces such as the oral cavity. Patients at increased risk for infection include immunocompromised patients, especially patients on immunosuppressant drugs, chemotherapeutic agents which cause neutropenia, and antibiotics. There has been an increasing incidence of systemic and mucosal yeast infection, thought to be due to the increasing use of immunosuppressing therapies as well as broad spectrum antibiotics. Candida species have been reported in some centers as the fourth most common cause of bloodstream infections and after Candida albicans, Candida glabrata has been reported as the second most frequent Candida species to cause nosocomial yeast infections. Candida glabrata is a significant pathogen as it can be resistant to azole antifungals, amphotericin, and echinocandins. There are several mechanisms of resistance to azoles which Candida glabrata utilizes including upregulation of genes CgCDR1 and CgCDR2 which encode drug efflux pumps. In addition, Candida glabrata also can have mutations in gene CgERG11, the usual enzymatic target of the azole antifungal drugs.

Candida glabrata appears as a budding yeast on gram stain with smooth white to cream colonies on agar plates. Microscopically, Candida glabrata appears as an oval yeast with single terminal budding. Distinguishing morphologic features of Candida glabrata include not forming pseudohypae at 37 degrees and not producing germ tubes in in vitro assays. Candida glabrata will also utilize the carbohydrate trehalose.

In some clinical cases, it is difficult to determine whether a Candida species recovered from culture represent an actual pathogen or a normal colonizer which was carried into the culture during collection. Candida species are normal commensal organisms of mucosal surfaces as well as skin, making it important to correlate clinical findings which microbial culture results. In this case, the patient had been stabilized and demonstrated great clinical improvement a few days prior to the positive blood culture result. Thus, this positive blood culture for Candida glabrata most likely represents a case where normal skin flora was inadvertently cultured.

-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.

The More You Know: Brucella and Laboratory Safety

Did you know that despite zoonotic transmission of Brucella spp. within United States being rare, Brucella is one of the most common laboratory transmissible infections?

This is why is important to be aware that on February of 2019 the Center of Disease Control and Prevention (CDC) emitted a food safety alert regarding potential exposures to Brucella RB51 in 19 states (https://www.cdc.gov/brucellosis/exposure/drug-resistant-brucellosis-linked-raw-milk.html).

The diagnosis of brucellosis can be a challenge if it is not suspected due to many reasons. Brucella spp. can be easily aerosolized and the infective dose to cause disease is very low. Clinically, the symptoms can present from 5 days to 6 months after exposure. The acute symptoms are flu-like and non-specific. Although brucellosis is rarely a deadly disease, it can cause chronic infections with multiple complications including arthritis, epididymitis/orchitis, hepatosplenomegaly, endocarditis, and CNS disease.  Despite completing the recommended treatment course of antibiotics, 5-10% of patients with brucellosis have relapse of disease and need additional antibiotic therapy.

In the laboratory, identification of Brucella is also a challenge: they appear as gram negative coccobacilli on Gram stain.  On solid agar they form small (1-2 mm in diameter) smooth and glistening colonies after 24 to 72 h of aerobic incubation. Biochemically they are oxidase, catalase, and urease positive. But there are some caveats: Brucella can stain gram variable or even gram positive (1).  Also, Brucella is one of the few gram negative bacteria that grows well on Colistin-Nalidixic Acid (CNA) agar, so it can be easily mistaken for a gram positive organism. Because of its slow-growing nature, performing biochemical tests to rule Brucella too early can yield false-negative results due to the paucity of organism tested.

The most common reasons for accidental Brucella exposure in the laboratory are lack of experience working with the organism and work performed on an unsuspected Brucella isolate on an open bench.  Not only is the person working with the isolate is exposed, but all those whom are within 5 feet due to aerosolized particles.  The whole laboratory section can be potentially infected depending of the aerosolizing event (https://www.cdc.gov/brucellosis/laboratories/risk-level.html).

If you work in a sentinel microbiology laboratory and think an isolate may be Brucella, you are advised to perform rule out biochemical tests (oxidase, catalase, and urea) in a biosafety hood using proper PPE and proper disposal techniques for items contaminated with the organism (glass slides, loops, etc).  Keep manipulation of the organism to a bare minimum and do not set up testing such as MALDI-TOF MS, API, or any additional biochemicals until Brucella can be ruled out.  Brucellosis is a mandatory reportable disease, and sentinel laboratories are required to send isolates that are suspicious for Brucella to their local public health departments instead of trying to pursue identification themselves.

What about MALDI-TOF, you may be thinking? Glad you asked! There are methods to inactivate highly infectious pathogens such as Brucella before utilization of MALDI-TOF, but this is not recommended for sentinel laboratories.  Definitive identification is best left to public health and other specialty laboratories with enhanced facilities for working with highly infectious pathogens. In addition, some MALDI-TOF MS databases do not contain spectra for Brucella spp., making it likely that testing will give you no identification or an organism misidentification. 

There are several tests to diagnose brucellosis and serology is a common method.  The CDC requirements for definitive diagnosis are (1) identification from culture or (2) Brucella antibody titers demonstrating a four-fold or greater rise in two weeks or more.  Presumptive diagnosis is defined as (1) Brucella detection by PCR, (2) Brucella total antibody titers of ≥ 1:160 by standard tube agglutination test (SAT), or (3) Brucella microagglutination test (BMAT) in one or more serum specimens.

Now to the present food safety alert: the causative agent is Brucella strain RB51 (RB51), a live-attenuated vaccine for cattle. RB51 can be shed in the milk and people can contract brucellosis after ingestion of raw or unpasteurized dairy products. The presence of RB51 infection is not detected by serological tests and the RB51 stain is resistant to rifampin. This is problematic in two ways: (1) for people exposed to RB51, serology cannot be relied upon for diagnosis and (2) doxycycline plus rifampin for six weeks is one of the most common drug regimens for treatment of brucellosis, but due to rifampin resistance this regimen is not optimal therapy against the RB51 strain of Brucella. 

References

  1. West, Tsubasa, Rhonda A. Warren, and Siu-Kei Chow. “Photo Quiz: A 66-Year-Old Man with Bloodstream Infection and Back Pain.” (2019): e00381-18.
  2. National Center for Emerging, Zoonotic and Infectious Disease. Centers for Disease Control and Prevention. Brucellosis Reference Guide: Exposures, Testing, and Prevention. February 2017. https://www.cdc.gov/brucellosis/pdf/brucellosi-reference-guide.pdf
  3. Food safety alert: Exposures to Drug-Resistant Brucellosis Linked to Raw Milk, February 8, 2019.  https://www.cdc.gov/brucellosis/exposure/drug-resistant-brucellosis-linked-raw-milk.html
  4. Third Case of Rifampin/Penicillin-Resistant Strain of RB51 Brucella from Consuming Raw Milk https://emergency.cdc.gov/han/HAN00417.asp
  5. Solera J. Update on brucellosis: therapeutic challenges. Int J Antimicrob Agents. 2010;36:18–20.

-Dennise E. Otero Espinal, MD is a Medical Microbiology Fellow at the University of Chicago (NorthShore).

-Erin McElvania, PhD, D(ABMM), is the Director of Clinical Microbiology NorthShore University Health System in Evanston, Illinois. Follow Dr. McElvania on twitter @E-McElvania. 

Microbiology Case Study: A 58 Year Old Female with Shortness of Breath, Fever, and Chills

Case History

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 Findings

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.

Image 1. Gram negative coccobacilli characteristic of Acinetobacter on Gram stain.
Image 2. Acinetobacter morphology on MacConkey agar.

Discussion

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.