Clinical and microbiological insights into Achromobacter denitrificans bacteremia: A case series from a tertiary care hospital

INTRODUCTION

Achromobacter denitrificans is an aerobic, motile, oxidase and catalase-positive, non-fermenting Gram-negative bacillus belonging to the family Alcaligenaceae. It is commonly found in soil, water, and moist hospital environments, and was historically considered a non-pathogenic environmental organism. However, emerging clinical evidence has increasingly highlighted its pathogenic potential, particularly among immunocompromised individuals and patients undergoing invasive procedures.^[1,2]

Recent reports have implicated A. denitrificans in a spectrum of healthcare-associated infections, including respiratory tract infections, catheter-related bloodstream infections (BSIs), and surgical site infections.^[3,4] A systematic review reported a mortality rate of 20.4% in Achromobacter infections, with significantly higher risk in patients with malignancies or neutropenia,^[5] underscoring the need for early recognition and targeted therapy.

This organism presents both diagnostic and therapeutic challenges due to its intrinsic resistance to aminoglycosides, cephalosporins, and several β-lactam antibiotics.^[6] Nevertheless, it often retains susceptibility to agents such as carbapenems, piperacillin-tazobactam, and trimethoprimsulfamethoxazole.^[7] Accurate identification using advanced tools such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is essential for prompt diagnosis and appropriate antimicrobial stewardship. In this case series, we describe five patients diagnosed with BSIs caused by A. denitrificans, each presenting in a distinct clinical context. These cases highlight the emerging clinical relevance of this organism and offer insights into its management across diverse patient populations.

CASE SERIES

Case 1

A 59-year-old female, a homemaker, previously diagnosed with carcinoma of the lower third of the esophagus, was admitted to the department of surgical oncology with complaints of persistent epistaxis, hemoptysis, and progressive dysphagia over the past 3 months. She had recently completed five cycles of chemotherapy with a carboplatin-paclitaxel regimen but demonstrated progressive clinical deterioration despite treatment. In view of worsening respiratory and gastrointestinal symptoms, she was scheduled for an elective esophagectomy.

In the immediate post-operative period, the patient developed high-grade fever. By post-operative day 4, she exhibited features of sepsis and altered sensorium, which rapidly progressed to multi-organ dysfunction syndrome. She was transferred to the intensive care unit (ICU), where she was intubated and placed on mechanical ventilation. Central venous and peripheral lines were inserted, and broad-spectrum antibiotics were initiated empirically. During her ICU course, she developed ventilator-associated pneumonia and continued to have melena suggestive of gastrointestinal bleeding.

Investigations

Relevant hematological and biochemical parameters have been summarized in Table 1. Blood, urine, and tracheal aspirate samples were submitted to the department of microbiology for culture and sensitivity testing. The aerobic blood culture bottle flagged positive in the automated blood culture system. Direct Gram stain revealed Gram-negative bacilli. Subculture on blood agar yielded small, grayish, non-hemolytic colonies, while MacConkey agar showed non-lactose fermenting colonies [Figure 1]. Gram staining confirmed the presence of Gram-negative bacilli. The isolate was positive for catalase and oxidase tests. Further identification using MALDI-TOF MS confirmed the organism as A. denitrificans. Antimicrobial susceptibility was interpreted using Clinical and Laboratory Standards Institute M45 guidelines for Achromobacter spp., which provide species-specific minimum inhibitory concentrations and disk diffusion breakpoints [Table 2]. Tracheal aspirate culture yielded heavy growth of Candida tropicalis (>10⁵ CFU/mL), which was sensitive to fluconazole, voriconazole, and itraconazole.

Table 1. Comparative hematological and biochemical profiles of five clinical cases.

Investigation	Case 1	Case 2	Case 3	Case 4	Case 5	Units	Reference interval
Hemoglobin	8.6	11.3	10	10.8	8.4	gm/dl	Female: 12–15; Male: 13–17
Total leucocytes count	15,820	5230	19,350	330	15,000	cells/cumm	4,000–11,000
Neutrophils	85	62.9	96.9	–	82	%	40–70
Lymphocytes	12	29.8	2.7	–	13	%	20–40
Eosinophils	1	0.8	0	0	0	%	1–6
Monocytes	2	6.4	0	0	1	%	0–2
Basophils	0	0.1	0.4	0	0	%	0–1
Platelet count	0.36	2.81	6.05	0.17	1.37	Lakh cells/cumm	1.5–4.5
Total RBC counta	3.02	3.67	3.08	3.39	3.57	Million cells/µl	3.5–5.5
MPVb	11.43	10.2	7.7	7.4	8.70	fl	7.0–10.4
MCVc	90.2	93.1	95.7	110.9	75.6	fl	76–96
MCHd	26.42	30.8	32.3	31.9	23.5	pg	27–32
MCHCe	28.64	33.1	33.8	28.7	31.1	%	31–35
RDWf	15.63	15.6	15.4	18.9	17.1	%	11.5–14.5
HCTg	27.07	34.1	29.5	–	27.0	%	36–46
PCTh	12.67	4.86	13.43	1.78	–	ng/ml	<0.5
Serum urea	78.6	28.2	67.1	6.4	40.8	mg/dl	12.9–42.9
Serum creatinine	1.95	0.71	0.81	0.5	0.51	mg/dl	0.6–1.4
Serum sodium	130.6	133.9	132.7	133	133	mmol/l	135–145
Serum potassium	3.32	4.12	3.17	3.6	3.63	mmol/l	3.5–5.3
Serum ionic calcium	4.16	4.04	3.44	4.1	4.17	mg/dl	4.5–5.5

^aRBC: Red blood cell, ^bMPV: Mean platelet volume, ^cMCV: Mean corpuscular volume, ^dMCH: Mean corpuscular hemoglobin, ^eMCHC: Mean corpuscular hemoglobin concentration, ^f RDW: Red cell distribution width, ^gHCT: Hematocrit, ^hPCT: Procalcitonin

Table 2. Overview of findings from the case series on Achromobacter denitrificans bloodstream infections.

Case No.	Age/Sex	Underlying condition	Co- morbidities	Clinical Department/setting	Risk Factors	Specimen Source	Antibiotics (Sensitive)	Clinical outcome
1	59 y/F	Esophageal carcinoma (post- oesophagectomy)	None	Surgical Oncology→ICUa	Recent surgery, Chemotherapy, ICU stay	Blood	Meropenem, Imipenem, Piperacillin– Tazobactam, Ceftazidime	Died
2	44 y/F	Duodenal carcinoma	Carcinoma ovary	Surgical Gastroenterology	Chemotherapy, Major abdominal malignancy	Blood	Cotrimoxazole, Meropenem, Imipenem, Ceftazidime, Piperacillin- tazobactam,	Recovered
3	60 y/M	Left-sided cicatrisation collapse	COPDb, TBccontact	Respiratory Medicine→ICUa	Chronic lung disease, ICU admission, Septic shock	Blood	Cotrimoxazole, Piperacillin– Tazobactam	Died
4	33 y/M	Acute myeloid leukemia (FLT3+d)	None	Hematology	Febrile neutropenia, Chemotherapy, Immunosuppression	Blood	Ceftazidime, Piperacillin– Tazobactam, Meropenem, Imipenem, Cotrimoxazole	Recovered
5	11 y/M	Extrahepatic portal vein obstruction, portal hypertension	None	Pediatric Surgical Gastroenterology	Recent surgery (PSRSe), Postoperative thrombosis	Blood	Meropenem, Imipenem	Recovered

aICU: Intensive care unit, ^bCOPD: Chronic obstructive pulmonary disease ^cTB: Tuberculosis, ^dFLT3: FMS-like Tyrosine kinase 3 (mutation), ^ePSRS: Proximal splenorenal shunt

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Figure 1:

Colony morphology of Achromobacter denitrificans on blood agar and MacConkey agar. (a) Small, grayish, non-hemolytic colonies on blood agar; (b) pale, non-lactose fermenting colonies on MacConkey agar after 24 h of aerobic incubation at 37°C.

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Treatment

Empirical therapy with intravenous minocycline (100 mg twice daily) and meropenem (1 g every 8 h) was initiated. Following confirmation of A. denitrificans in blood cultures and based on susceptibility results, the antibiotic regimen was modified to include piperacillin-tazobactam (4.5 g every 8 h). Antifungal therapy was not initiated, as C. tropicalis was deemed a colonizer in the absence of clinical or radiological evidence of invasive fungal infection. Supportive management was continued in accordance with ICU protocols.

Outcome and follow-up

Despite aggressive antimicrobial therapy and comprehensive critical care support, the patient’s condition deteriorated. She developed worsening respiratory failure and ultimately suffered a cardiac arrest. After a 20-day ICU stay, she succumbed to septic shock and multi-organ failure.

DISCUSSION

A. denitrificans, a non-fermenting Gram-negative bacillus once relegated to the background as a benign environmental organism, is rapidly emerging as a clinically significant opportunistic pathogen.^[8] Conventionally, overshadowed by more notorious non-fermenters such as Pseudomonas aeruginosa or Acinetobacter baumannii, A. denitrificans is now being recognized in serious infections, particularly BSIs, in immunocompromised and post-operative patients.^[9-12]

Our case series underscores this shift. Five clinically diverse patients, including those with advanced malignancies, post-surgical states, hematologic neoplasms, chronic infections, and pediatric portal hypertension, developed A. denitrificans bacteremia.^[13] Notably, all had recent hospitalizations, invasive procedures, or immunosuppressive conditions, consistent with risk factors highlighted in global studies.^[14-16] The presence of central venous catheters, post-chemotherapy immunosuppression, and ICU exposure likely facilitated bloodstream translocation in these vulnerable hosts.^[17]

Despite their increasing frequency, Achromobacter infections are often underreported or misidentified. Phenotypic overlap with other non-fermenters complicates diagnosis, often leading to inappropriate empirical therapy.^[18] Our experience affirms the value of MALDI-TOF MS, which enabled the rapid identification of A. denitrificans in all five cases. However, even MALDI-TOF faces limitations in species-level resolution unless supported by robust reference databases.^[19,20] This technological gap is not merely academic, but it can delay accurate diagnosis and targeted treatment. Conventional phenotypic methods are often inadequate for reliable species-level identification within the genus Achromobacter due to overlapping biochemical traits with other non-fermenters. However, certain distinguishing features can help laboratories without MALDI-TOF MS achieve at least genus-level identification. Achromobacter spp. are typically oxidase-positive, catalase-positive, motile, and non-lactose-fermenting bacilli. In contrast, Stenotrophomonas maltophilia is oxidase-negative and non-motile, while P. aeruginosa can be differentiated by its characteristic pigment production (pyocyanin, pyoverdine), grape-like odor, and growth at 42°C. Burkholderia spp. may share oxidase positivity but can be separated by specific biochemical reactions such as lysine decarboxylase activity and differing antimicrobial resistance profiles. Thus, while MALDI-TOF MS remains the most reliable tool for species-level confirmation, these phenotypic cues remain useful in resource-limited settings to differentiate Achromobacter from other common non-fermenting Gram-negative bacilli.

The treatment of Achromobacter infections remains challenging due to the organism’s intrinsic resistance to aminoglycosides, cephalosporins, and certain β-lactams. In our series, isolates consistently demonstrate susceptibility to piperacillin-tazobactam, meropenem, imipenem, and trimethoprim-sulfamethoxazole.^[21,22] However, resistance patterns are evolving, and reliance on historical susceptibility profiles is no longer sufficient. Our observation underscores the need for region-specific antibiograms to guide empirical therapy.^[23]

While three patients recovered with appropriate therapy, two patients, both with significant comorbidities, succumbed to septic shock and multi-organ dysfunction. These findings mirror previous studies indicating a high mortality risk in neutropenic and oncology patients.^[17,24] Notably, the pediatric patient with EHPVO and the AML patient in remission responded favorably, highlighting the potential for good outcomes with early, targeted therapy.

This case series is among the few worldwide and Indian literature documenting A. denitrificans BSIs across oncology, hematology, gastroenterology, and pulmonology units [Table 3]. It reinforces the organism’s opportunistic nature and its ability to exploit host vulnerability and healthcare-associated exposures. Clinicians must remain vigilant for this pathogen, especially in settings involving central lines, ventilators, or cytotoxic therapies.^[25] Infection control measures must keep pace with this evolving threat. Hospital water systems, catheters, and respiratory equipment are all potential reservoirs.^[26-27] Surveillance strategies and strict aseptic protocols are critical to curbing nosocomial transmission.

CONCLUSIONS

A. denitrificans is an emerging multi-DR pathogen capable of causing serious BSIs, particularly in immunocompromised and hospitalized patients. This case series demonstrates its clinical relevance across oncology, critical care, hematology, and pediatric surgery, with infections often linked to recent surgery, malignancy, immunosuppression, or ICU admission. Accurate identification using MALDI-TOF MS and timely initiation of targeted antibiotics such as piperacillintazobactam, meropenem, or trimethoprim-sulfamethoxazole were key to favorable outcomes in most cases. However, delayed diagnosis or severe comorbidities contributed to poor prognosis in some patients. Clinicians should maintain a high index of suspicion for A. denitrificans in septic patients with risk factors, especially when non-fermenters are isolated. Ongoing surveillance, improved diagnostic capacity, and antimicrobial stewardship are essential to limit the clinical impact of this underrecognized but significant pathogen.