The Challenge of Identifying Burkholderia cepacia Complex (BCC)
The Burkholderia cepacia complex (BCC) represents a group of at least nine closely related bacterial species, or genomovars, that pose a significant clinical challenge, particularly for patients with cystic fibrosis (CF) and other immunocompromised individuals. Its presence in CF patients is associated with a poor clinical prognosis, often leading to a severe decline in pulmonary function known as “cepacia syndrome.” Accurately identifying BCC isolates is complicated by their metabolic plasticity, genetic heterogeneity, and phenotypic overlap with other non-fermenting Gram-negative bacilli, such as *Burkholderia gladioli* and *Stenotrophomonas maltophilia*. Therefore, while molecular methods like PCR and Multi-Locus Sequence Typing (MLST) are the gold standard for definitive species identification, conventional biochemical tests remain a critical, accessible first step for genus-level identification in clinical and resource-limited laboratory settings.
Initial Screening and Selective Culture
The laboratory identification process for BCC begins with basic microbiological screening. *Burkholderia cepacia* complex organisms are aerobic, motile, non-spore-forming, Gram-negative rods. A positive oxidase test is characteristic for the genus *Burkholderia*, including *B. cepacia* and *B. gladioli*, although this result should be interpreted in the context of other tests. When cultured on general media, BCC often produces non-lactose fermenting colonies on MacConkey agar. On nutrient agar, colonies may be smooth, circular, and occasionally develop a yellowish or greenish pigment.
A key diagnostic feature exploited by preliminary screening is the organism’s high intrinsic resistance to multiple antibiotics. BCC isolates are typically resistant to amoxicillin-clavulanate and gentamicin. Crucially, they are also one of the most common Gram-negative bacilli isolated in the clinical setting that exhibits resistance to polymyxin B. This resistance pattern is utilized in the formulation of selective and differential media. Two of the most common selective media for BCC isolation are *Burkholderia cepacia* Selective Agar (BCSA) and Oxidation-Fermentation Polymyxin B Bacitracin Lactose (OFPBL) agar. Growth on these media, combined with the characteristic non-lactose fermenting phenotype, strongly suggests the presence of a *Burkholderia* species and warrants further biochemical testing.
Conventional Biochemical Assays for Differentiation
A battery of conventional biochemical tests is necessary to differentiate *B. cepacia* from other non-fermenters and, to a limited extent, from other *Burkholderia* species. These tests assess the organism’s enzymatic profile and its ability to metabolize specific substrates:
**Carbohydrate Metabolism (Oxidation-Fermentation):** *B. cepacia* is generally a glucose oxidizer, meaning it breaks down glucose in the presence of oxygen, a characteristic tested using Hugh and Leifson oxidation-fermentation (O/F) medium. Analysis of other sugars often reveals variable results. For instance, the oxidation or fermentation of lactose and sucrose can vary within the complex, though positive reactions for xylose are frequently observed among *B. cepacia* strains.
**Decarboxylase Tests:** These tests determine the presence of enzymes (lysine decarboxylase, ornithine decarboxylase, and arginine dihydrolase) that cleave the carboxyl group from specific amino acids. *B. cepacia* typically gives a negative result for Arginine Dihydrolase, while the result for Ornithine Decarboxylase is frequently reported as variable. The Lysine Decarboxylase result is also performed as part of a conventional panel but does not offer clear differentiation on its own.
**Other Enzymatic and Metabolic Tests:** The **ONPG (O-nitrophenyl-β-galactopyranoside)** test detects the enzyme β-galactosidase. *B. cepacia* is generally **positive** for this test. A **Positive** reaction is also typically seen for both the **Catalase** and **Citrate** utilization tests. The **Gelatin Hydrolysis** test is variable. The **Urea** test (urease activity) is also included in many panels, but its diagnostic value is generally limited within the BCC.
Automated Systems and Phenotypic Variability
Commercial automated identification systems, such as Vitek 2 and MicroScan, provide rapid biochemical profiles. The **API 20NE** system has historically been cited as one of the best systems for the phenotypic identification of *B. cepacia* strains. Despite the use of these advanced panels, a significant limitation remains: conventional biochemicals alone often have a low concordance rate for species-level identification within the complex, sometimes falling as low as 28.2% compared to molecular methods like VITEK MS. This poor species-level accuracy is a direct consequence of the metabolic plasticity of BCC. Isolates may exhibit varied enzymatic profiles, or “genomovars” may show phenotypic characteristics that overlap, leading to potential misidentification with closely related, but distinct, species like *B. pseudomallei* or *B. gladioli*.
For example, a misidentification of *B. pseudomallei* as *B. cepacia*, or vice versa, by automated biochemical systems has been repeatedly reported, which carries severe clinical risk due to differences in virulence and required treatment. Specific enzymatic differences, such as the activity of NAGA (beta-N-acetyl-galactosaminidase) and BNAG (beta-N-acetyl-glucosaminidase), have been explored to improve the accuracy of Vitek 2 results, but the inherent variability within the BCC strains often confounds reliable differentiation.
Conclusion: The Dual Approach to BCC Identification
The biochemical testing of *Burkholderia cepacia* complex is a crucial and essential component of the clinical microbiology workflow, but it is rarely sufficient on its own. While the simple battery of conventional biochemicals (Gram-stain negative, oxidase positive, non-lactose fermenting, polymyxin B resistant, and positive for ONPG/Citrate) is highly efficient for reliably identifying the genus *Burkholderia*, it fails to consistently and accurately resolve the species within the *B. cepacia* complex. Therefore, the recommended diagnostic strategy involves a dual approach: initial isolation and genus-level identification using selective media and the API 20NE biochemical system, followed by molecular confirmation. Final, definitive species identification requires genotypic methods, such as PCR-Restriction Fragment Length Polymorphism (RFLP) analysis or MLST, which accurately classify the genomovar and ensure the correct clinical management for the patient.