Introduction Airway bacterial infections are associated with exacerbations of COPD. The potential role of atypical bacteria as a trigger for exacerbations is not well understood. Atypical bacteria such as Chlamydophila pneumoniae (CP), Legionella pneumophila (LP) and Mycoplasma pneumoniae (MP) are difficult to culture as they are intracellular pathogens. LP can be detected by urinary antigen, and serology can be performed for MP, but these techniques give no indication as to the atypical bacterial load. Quantitative PCR (qPCR) offers an alternative approach to identification and quantification of bacteria in sputum.
Methods Multiplex qPCR was used to detect and quantify CP, LP and MP in 238 samples prospectively collected from 87 patients in the London COPD Cohort: mean (±SD) age 71.4 (±8.1); predicted FEV1 43.4% (±17.5%); male gender 47.9%; current smoker 49.2%. Baseline (n=104), exacerbation (n=95), and follow-up (n=39) samples were tested: Baseline was defined as at least 6-weeks without exacerbation, and exacerbation was defined as 2 consecutive days of two symptoms (Anthonisen criteria), at least one of which is a major symptom (dyspnoea; sputum purulence; sputum volume). Follow-up involved taking samples 2 or 5 weeks post-exacerbation onset. Using a qPCR developed by our clinical diagnostic service, the CP, MP and LP gene targets were RNA-polymerase β-chain; P1 adhesin protein; and MIP respectively. Routine microbiological analysis was also performed on these samples.
Results No samples were positive for the atypical organisms using culture. With qPCR analysis 6/238 samples (six separate patients) were positive for LP (2.5%), four at baseline and two at exacerbation/follow-up. One baseline sample was positive for MP (0.42%), and no samples were positive for CP. Atypical bacteria were present at 0.83% of exacerbations. Median (IQR) bacterial load was 4.3x104 cfu ml−1 (2.0x104–8.55x104) for LP PCR-positive samples; the MP-positive sample load was 2.64x107 cfu ml−1.
Conclusion Quantitative PCR was more sensitive and informative than standard microbiological culture for the detection of atypical bacteria. Atypical bacteria in sputum were detected at very few exacerbations of COPD; moreover, when they were detected by qPCR, the load was low, indicating little or no significance in the aetiology of these events.