Nocardia brasiliensis in diabetic foot abscess: A rare case report

ABSTRACT

Nocardia species are saprophytic bacteria, present in soil and water, usually associated with pulmonary infection and occasionally with abscesses in other body sites in immunocompromised patients and immunocompetent individuals. Microbiologically, they are aerobic, thin, branched, filamentous, gram-positive bacteria. Here, we report a case of diabetic foot abscess in an immunocompetent, adult male patient, due to Nocardia brasiliensis that could be diagnosed in a timely manner due to proper microbiological work-up and speciated using MALDI-TOF-VITEK-MS.

Introduction

Nocardia are opportunistic pathogens that are ubiquitously present and are usually associated with pulmonary infection/occasional abscesses in other body sites in immunocompromised patients. However, it can also infect immunocompetent individuals with or without comorbidities, such as diabetes or alcoholism.[1,2] Microbiologically, they are aerobic, thin, branched, filamentous, gram-positive bacteria and acid-fast in modified Ziehl-Neelsen (ZN) staining using decolorizer, 1% sulfuric acid; also called “aerobic actinomycetes” due to their resemblance to actinomycetes in gram stain.[3] Even in the absence of clinical suspicion of nocardiosis, proper microbiological work-up of a patient’s specimen can lead to correct diagnosis and initiation of appropriate antimicrobial treatment which can be life-saving for the patient.

Case History

In November 2022, a 50-year-old male patient reported to the outpatient department of our hospital with complaints of fever, pain, and swelling over the left foot for the last 15 days. He had been diagnosed with diabetes mellitus over the past two years. The patient recalled minor trauma to the same foot approximately 20 days prior. On examination, his foot was tender, edematous, and erythematous, but without any discharging sinuses. Routine laboratory workup revealed increased blood leukocyte counts, elevated C-reactive protein levels, and high blood glucose levels. Hb_a1c levels also exceeded normal limits. Radiography revealed no bone abnormalities. Incision and drainage were performed under aseptic conditions; the aspirated pus specimen was collected in a sterile container and sent to the microbiology laboratory for investigations.

Direct microscopy

Thin, branched, filamentous, beaded, gram-positive bacilli were observed in gram-stained primary smear with abundant pus cells [Figure 1]. However, acid-fast bacilli were not seen on the modified ZN stain (primary smear). Based on microscopy, Actinomyces/Nocardia was suspected.

Figure 1.

Branched, thin, filamentous, gram-positive bacilli on gram stain

Culture

The specimen was inoculated on 5% sheep blood agar and MacConkey agar, and incubated at 37°C in ambient air. After 72h of aerobic incubation, chalky-white, dry, wrinkled, non-emulsifiable colonies with an earthy smell were isolated on blood agar. Culture smears were prepared and, stained with gram and modified ZN stain. The modified ZN smear showed acid-fast, beaded, branched bacilli suggestive of Nocardia. Colonies were positive for catalase and urea hydrolysis. An LJ slant was also inoculated which showed growth after 72h of aerobic incubation at 37°C.

Antimicrobial susceptibility testing (AST)

AST was performed by disk diffusion method on blood agar in which cotrimoxazole was sensitive.

MALDI-TOF-MS

Identification and speciation were confirmed using bioMeriux MALDI-TOF-VITEK-MS, which identified the pathogen as Nocardia brasiliensis (99.9% confidence).

Patient treatment and follow-up

The patient was given cotrimoxazole based on the culture report, as Nocardia species are usually sensitive to sulphonamides and cotrimoxazole. Later, AST by disk diffusion method also showed that the isolate was sensitive to cotrimoxazole. It was administered orally at a dose of 10 mg/kg/day for eight weeks along with surgical aspiration of abscess. Follow-up was done weekly. The fever subsided three days after initiation of antibiotic treatment. The abscess healed by the end of four weeks. Treatment was continued for eight weeks.

Discussion

Nocardia species invade the human body through the respiratory tract, skin, or digestive tract, resulting in the development of nocardiosis. Lung is the most common site of nocardiosis, accounting for 70–80% of cases.[1] The case reported here is of a patient with diabetes mellitus who presented with a foot abscess. Diabetic foot abscesses due to Nocardia brasiliensis are not commonly reported in India. Localized, primary cutaneous/subcutaneous nocardiosis is a rare entity, typically affecting immunocompetent individuals with or without comorbidities like diabetes.[2]

The clinical diagnosis of nocardiosis is challenging owing to non-specific findings. Proper microscopic examination of the smear is important for early diagnosis or suspicion of nocardiosis, which is evident from this case which otherwise would have gone undiagnosed/misdiagnosed. Additionally, Nocardia grows slowly, usually taking 72h or more to form small colonies; therefore, prolonged incubation is required for culture to avoid misdiagnosis. In some cases, it may take up to 14-21 days of incubation.[4] In this case, prolonged incubation (>48h) of the inoculated culture plates was done because of initial suspicion based on microscopy. Nocardia species are beaded, weakly acid-fast on ZN staining, and gram-positive, filamentous bacteria. The diagnosis of nocardiosis is established when these organisms are isolated from sputum, broncho-alveolar lavage, pleural fluid, blood, pus (as in this case), subcutaneous tissues, or other samples from the infection site.[5]

Conventional methods cannot identify Nocardia species. However, mass spectrometry and other molecular technologies viz. PCR and gene sequencing can correctly identify

Nocardia

Nocardia species commonly causing human infections include N. asteroides, N. brasiliensis, and N. caviae.[6] A recent report has shown that up to 91% of Nocardia species can be accurately identified using the MALDI-TOF-VITEK-MS system.[7] N. brasiliensis is mostly associated with primary, localized skin/soft tissue infections in immunocompetent patients, as evident from other case reports, whereas cutaneous nocardiosis from N. asteroides is usually secondary to hematogenous dissemination.[2,3,8]

The Clinical and Laboratory Standards Institute (CLSI) recommends performing broth microdilution for AST of Nocardia. Other methods include the determination of minimum inhibitory concentration (MIC) using E-tests or VITEK-2 automated system. However, none of these methods were available, so antibiotic disk diffusion was performed on sheep blood agar. Cotrimoxazole, linezolid, and amikacin are active against >95% of Nocardia isolates. Cotrimoxazole is the drug of choice owing to its good oral bioavailability and high tissue concentration. It is also active against all Nocardia species. Therefore, cotrimoxazole monotherapy can be given to patients with skin or localized subcutaneous abscesses even when AST is unknown. However, in life-threatening pneumonia or central nervous system involvement, a parenteral, multi-drug regimen is preferred.[4] Thus, oral cotrimoxazole monotherapy was justified in this case.

Limitation

MIC could not be determined for Nocardia isolate due to the unavailability of tests.

Conclusion

Nocardiosis is a relatively uncommon infection that can infect any part of the body and presents with non-specific clinical manifestations. Clinicians should consider nocardiosis as a differential diagnosis for localized or multiple abscesses. Isolation of Nocardia from any clinical site should be considered significant and appropriately correlated. Even a provisional diagnosis of Nocardia infection, based on a gram-stain/acid-fast staining report from a clinical microbiology laboratory can help in early initiation of appropriate antibiotic therapy and prevent dissemination of the disease.

List of abbreviations

MALDI-TOF-MS = Matrix Assisted Laser Desorption Ionisation-Time of Flight-Mass Spectrometry

PCR = Polymerase Chain Reaction

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.