Case Series: Microbacterium aurum Bacteremia in Immunosuppressed Patients—An Emerging Threat

ABSTRACT.

Blood stream infection with Microbacterium species in humans is rare and frequently linked to the presence of immunosuppressed conditions such as patients on chemotherapy or corticosteroids. Presence of indwelling catheters is also a potential risk factor for M. aurum infection. No case report has been documented in the literature regarding the pathogenic potential of M. aurum in causing bacteremia. This is the first case series reporting bacteremia by M. aurum describing the risk factors and sensitivity pattern of this pathogen. In this case series, we have described bacteremia caused by M. aurum. The risk factors and sensitivity pattern of this pathogen have also been evaluated. Here, we describe the clinical course and presentation of three patients whose blood culture showed growth of M. aurum. Indwelling venous catheter for hemodialysis or for chemotherapy for the treatment of acute lymphoblastic leukemia was found to be a risk factor in two patients. Rheumatoid arthritis was the underlying condition in the second patient and was started on immunosuppressants. Blood samples were collected during the febrile period. The blood culture samples of all these patients had pure isolates of M. aurum, identified by matrix-assisted laser desorption ionization–time of flight mass spectrometry. All three patients were managed according to the sensitivity reports and were discharged in stable condition.

INTRODUCTION

Microbacterium species are dry, yellow-colored, Gram-positive coryneform rods that are obligate aerobes. They belong to the family Microbacteriaceae^1–4 and are isolated from a variety of environmental sources, including soil, water, sewage, hospital humidifiers, and hospital air.⁵ Of the 30 Microbacterium species known to cause human infection, the most common are Microbacterium paraoxydans, Microbacterium oxydans, and Microbacterium foliorum.^1,2,6 The less common species isolated from human samples were Microbacterium resistens, Microbacterium aurum, Microbacterium hinotii, Microbacterium hydrocarbonoxydans, Microbacterium testaceum, and Microbacterium trichothecenolyticum.⁶

Infections with Microbacterium species in humans are rare⁷ and frequently linked to the presence of immunosuppressed conditions such as patients on chemotherapy or the presence of indwelling catheters.^2,8,9 Only sporadic infections caused by Microbacterium species have been documented in the literature; these include infections in patients with central venous catheter–associated sepsis, surgical site infection, and endophthalmitis. Blood has been retrieved as the primary source of Microbacterium species from clinical specimens.⁸

In this case series, we describe three patients whose blood cultures showed growth of M. aurum. Two of these patients had indwelling venous catheters (Case 1 and 3). Rheumatoid arthritis was the predisposing condition in the second patient (Case 2). To the best of our knowledge, no case report has been documented in literature regarding the pathogenic potential of M. aurum in causing bacteremia. This is the first case series reporting bacteremia by M. aurum describing the risk factors and sensitivity pattern of this pathogen.

Case 1.

A 70-year-old male was admitted to the nephrology department of our institution in November 2022 with uremic symptoms. He was nondiabetic; hypertensive for 10 years on medication, but with suboptimal control; had mild to moderate renal dysfunction since 2020; and was on conservative management. He was diagnosed with chronic kidney disease stage 5D and was on renal replacement therapy. Hemodialysis was planned and right non-tunneled internal jugular vein catheter (IJVC) was inserted. The patient developed fever after catheter insertion. Blood samples were collected from the IJVC line and peripheral line in BacTAlert bottles for culture and sensitivity to the microbiology department. They were incubated in the BacTAlert instrument. Both the blood culture bottles beeped positive on Day 2. Direct Gram stain was made from the positive bottle (Figure 1). Direct Gram stain showed Gram-positive rods of 4 to 5 µm. Culture was done on blood agar and MacConkey agar and incubated for 24 hours aerobically. After 24 hours, yellow pigmented, dry wrinkled, rough colonies of 2 to 3 mm were isolated on blood agar (Figure 2). Gram stain made from the culture plate growth showed Gram-positive rods of 3 to 4 µm (Figure 3). The isolate was identified as M. aurum by matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-ToF MS). Antibiotic sensitivity was performed by Kirby–Bauer disk diffusion. An inoculum of 0.5 McFarland was prepared on normal saline and lawn culture was done on Mueller–Hinton agar. The antibiotics tests were ceftazidime (30 µg), ciprofloxacin (5 µg), clindamycin (2 µg), co-trimoxazole (1.25/23.75 µg), doxycycline (30 µg), erythromycin (15 µg), gentamicin (10 µg), meropenem (10 µg), linezolid (30 µg), and vancomycin (30 µg). The isolate was sensitive to ceftazidime, doxycycline, co-trimoxazole, gentamicin, linezolid, and vancomycin. Repeat blood cultures were collected before starting antibiotics and sent to the laboratory for confirmation. The non-tunneled IJVC was removed, and the patient was started on co-trimoxazole. He responded well to the treatment. He was discharged with stable vitals and was called for hemodialysis twice weekly.

Figure 1.

Direct gram-stained smear showing Gram positive rods of 4 to 5 µm with parallel sides and rounded ends.

Figure 2.

Colonies of Microbacterium aurum on blood agar. After 24 hours, yellow pigmented, dry wrinkled, rough colonies of 2 to 3 mm were isolated on blood agar.

Figure 3.

Gram-stained smear showing Gram positive rods of 3 to 4 µm.

Case 2.

A 40-year-old female was admitted to the immunology department of our institution. She had a history of inflammatory polyarthritis in the small and large joints and was diagnosed with rheumatoid arthritis with extensive deep vein thrombosis (DVT) in the left lower limb extending up to infrarenal inferior vena cava. She was started on methotrexate + folate + hydroxychloroquine. She presented with fever and left lower limb swelling with acute onset, developing over 1 hour. The swelling was associated with pain and worsened with walking and weight bearing. The swelling progressively increased in size. Lower limb Doppler ultrasound showed extensive DVT up to the left common iliac vein. She was started on enoxaparin and warfarin. Antiphospholipid antibodies (APLA) were negative. Two sets of blood samples were collected in BacTAlert bottles and sent to the microbiology laboratory for culture and sensitivity. The blood culture bottle was positive at 26 hours. Direct Gram stain from positive bottles showed Gram positive bacilli of 4 to 5 µm, with parallel sides and rounded ends. Culture was done on blood and MacConkey agars and incubated for 24 hours aerobically. After 24 hours, yellow pigmented, dry wrinkled, rough colonies of 2 to 3 mm were isolated on blood agar. Gram stain made from the culture plate growth showed Gram positive rods of 3 to 4 µm. The isolate was identified as M. aurum by MALDI-ToF MS. Antibiotic sensitivity was performed with the Kirby–Bauer disk diffusion method. An inoculum of 0.5 McFarland was prepared on normal saline and lawn culture was done on Mueller–Hinton agar. The antibiotics testes were ceftazidime (30 µg), ciprofloxacin (5 µg), clindamycin (2 µg), co-trimoxazole (1.25/23.75 µg), doxycycline (30 µg), erythromycin (15 µg), gentamicin (10 µg), meropenem (10 µg), linezolid (30 µg), and vancomycin (30 µg). The isolate was sensitive to ceftazidime, ciprofloxacin, clindamycin, doxycycline, gentamicin, meropenem, linezolid, and vancomycin. Repeat blood cultures were collected before starting antibiotics and sent to the laboratory for confirmation. The patient was started on ceftazidime and ciprofloxacin. The patient responded well to the treatment. She was discharged in stable condition with treatment advice for rheumatoid arthritis.

Case 3.

A 33-year-old male was diagnosed with acute lymphoblastic leukemia and was on induction chemotherapy. He was brought to the oncology department of our institution with complaint of fever when perfused with port-a-cath catheter system. Blood sample was collected from the catheter and peripheral vein and sent to the microbiology laboratory for culture and sensitivity. The blood culture bottle was positive on day 2. Direct Gram stain from positive bottles showed Gram positive rods of 4 to 5 µm. Culture was done on blood and MacConkey agars and incubated for 24 hours aerobically. After 24 hours, yellow pigmented, dry wrinkled, rough colonies of 2 to 3 mm were isolated on blood agar. Gram stain made from the culture plate growth showed Gram positive rods of 3 to 4 µm. The isolate was identified as M. aurum by MALDI-ToF MS. Antibiotic sensitivity was performed by Kirby–Bauer disk diffusion method. An inoculum of 0.5 McFarland was prepared on normal saline and lawn culture was done on Mueller–Hinton agar. The antibiotics testes were ceftazidime (30 µg), ciprofloxacin (5 µg), clindamycin (2 µg), co-trimoxazole (1.25/23.75 µg), doxycycline (30 µg), erythromycin (15 µg), gentamicin (10 µg), meropenem (10 µg), linezolid (30 µg), and vancomycin (30 µg). The isolate was sensitive to ceftazidime, ciprofloxacin, co-trimoxazole, doxycycline, linezolid, and vancomycin. Repeat blood cultures were collected before starting antibiotics and sent to the laboratory for confirmation. Repeat cultures were positive for M. aurum infection. The catheter was removed, and the patient was started on ceftazidime and ciprofloxacin. The patient responded well to the treatment. She was discharged in stable condition.

DISCUSSION

Human infections by Microbacterium species that have been reported in previous literature include peritonitis by M. resistens,¹ central venous catheter infection by M. paraoxydans,^2,10 and peritonitis by M.paraoxydan.^5,11 Most of these patients had underlying immunocompromised conditions, such as treatment with chemotherapy or corticosteroids.^7,10 Long-term presence of venous catheters^2,7,12 or dialysis catheters^1,5 were also a significant risk factor for the bacteremia. The diagnosis of catheter-related bloodstream infection was confirmed according to the Infectious Disease Society of America (IDSA) guidelines for intravascular catheter–related infection.¹³ The IDSA guideline criteria include collection of blood before starting antibiotics and collection of paired samples, which includes a set of catheter site blood and peripheral blood. were met in Case 1 and 3.

This diverse group of bacteria comprises many genera that are rarely identified at the genus level in clinical microbiology laboratories and are usually considered as contaminants when isolated from clinical samples. Thus, it may be difficult to identify clusters or random episodes of infection brought on by these bacteria.⁸ Identifications of these species is also a challenge because there are limitations in the differentiating power of routine biochemicals.² This has led to an insufficient understanding of the epidemiology, appropriate course of treatment, and prognosis of the bacterial infection.^2,8

Detection of 16S rRNA is considered as the gold standard for the identification of organisms such as Microbacterium, where biochemicals reactions might be inconclusive. Further, MALDI-ToF MS has been shown to identify Microbacterium species accurately.² The relevance of appropriate identification of Microbacterium species is clear. Any coryneform bacteria isolated from an immunocompromised patient’s sterile body location and thought to be a possible pathogen should be identified at least to the genus level.

It has been documented that MALDI-ToF MS has higher specificity in identifying pigmented Gram-positive rods like Microbacterium species, Leifsonia species, Exiguobacterium species, and Cellulosimicrobium species, among others. For Microbacterium species, accurate species-level identification by MALDI-ToF MS was documented for M. aurum, M. testaceans, and M. oxydans.¹⁴ This emphasizes the reliability of MALDI-ToF MS in identification of Microbacterium species. Also, previous literature has established that conventional biochemicals are only capable of identifying Microbacterium species at the genus level.¹⁴ Further confirmation by 16S rRNA sequencing is a future area of research in this field.

The three patients in our study had significant risk factors with bacteremia due to M. aurum (Table 1). Patients 1 and 3 in our study had indwelling catheters, which may be the predisposing factor for the bacteremia by M. aurum. The second patient was on immunosuppressive drugs for rheumatoid arthritis. The presence of Gram-positive rods in blood samples should be further investigated and identified rather than reporting them as contaminants. The treatment of infections caused by Microbacterium species is significantly hampered by the high rate of recurrence and the lack of data on antibiotic susceptibility testing.^15,16 Most studies have conducted sensitivity testing by broth microdilution,⁶ whereas some have also used disk diffusion for comparison of the results.¹³ The isolates showed sensitivity to ceftazidime, ciprofloxacin, co-trimoxazole, doxycycline, linezolid, and vancomycin. Research indicates that most Microbacterium isolates are sensitive to linezolid, meropenem, vancomycin, and doxycycline.^2,6 The best results were seen when patients were treated with a combination of erythromycin and co-trimoxazole.⁵ Catheter removal or consideration of other contributing risk factor should be part or the treatment plan, along with initiation of appropriate antibiotics for treatment of M. aurum bacteremia.

Table 1.

Cases of Microbacterium aurum isolated from the blood cultures of patients admitted at a tertiary care referral center in North India

Case	Age/Sex	Diagnosis	Risk Factor	Day of Fever Onset	CRP Values (mg/dL)	Procalcitonin Values (ng/mL)	Type of Sample	Pathogen Isolated	Condition at Discharge
1	70-year-old/Male	Chronic kidney disease stage 5D on renal replacement therapy	Right IJVC line for hemodialysis	Day 2 of catheterization	43.2	109.2	Two blood cultures, central and peripheral	M. aurum	Stable
2	40-year-old/Female	RA with DVT in left lower limb extending up to infrarenal inferior vena cava	Immunosuppressants for RA	Day 5 of start of immunosuppressants	28.4	121.4	Two blood cultures, both from peripheral line	M. aurum	Stable
3	33-year-old/Male	Acute lymphoblastic leukemia (induction chemotherapy)	Presence of Port-a-cath catheter system	Day 2 of insertion of catheter	87.2	94.0	Two blood cultures (central and peripheral)	M. aurum	Stable

CRP = C-reactive protein; DVT = deep vein thrombosis; IJVC = internal jugular vein catheter; M. aurum = Microbacterium aurum; RA = rheumatoid arthritis.

CONCLUSION

Microbacterium aurum bacteremia has not received much attention in the literature. The organism has the potential to cause infection in the presence of risk factors such as indwelling catheters and immunosuppressants. Proper knowledge about this bacterium, based on direct microscopy of the sample, is important to avoid underreporting of it as a contaminant. Timely diagnosis of this pathogen can help reduce morbidity among immunosuppressed patients.