Using polymerase chain reaction for evaluation of positive results of two automated blood culture systems which remain negative by smear and subcultures

Abstract number: 1134_01_325

Karahan Z.C., Mumcuoglu I., Tamer D., Guriz H., Balaban N., Aysev D., Akar N.

Objectives:  

Automated blood culture systems have improved the detection rate of blood-borne pathogens. False positivity of these systems is between 1–10%. In this study we aimed to evaluate whether smear and subculture-negative positivities observed in two automated blood culture systems (BacTAlert (Biomérieux) and Bactec (Becton Dickinson)) were due to real microbial growth or false positive signalling.

Methods:  

Microbial DNA was extracted from 90 aerobic BacTAlert and 65 aerobic Bactec blood culture bottles which gave positive signals but smear and sub-cultures were sterile. Each bottle belonged to a different patient. Eubacterial 16SrDNA and fungal ITS region directed PCR were performed to determine the presence of microorganisms in these samples. Positive results were subjected to automated sequence analysis and nucleotide sequences were matched from the GeneBank.

Results:  

None of the bottles gave positive results by fungal PCR. Only 10 BacTAlert bottles gave positive results by eubacterial PCR. These bottles gave positive signals between 2nd and 5th days of incubation (mean 3.5 days). Sequence analysis results were as follows: Pasteurella multocida (1), S.epidermidis (2), S.hominis (1), Micrococcus spp (1), S.pneumoniae (1), Corynebacterium spp (2), Brachibacterium spp (1), Arthrobacter/Rothia spp. (1).

Conclusions:  

P. multocida was determined from a patient who had undergone surgery and S. pneumoniae was from a patient who was admitted to emergency department. In other patients there were underlying diseases such as hematological malignancies or solid tumors. Although most of the determined bacteria were the inhabitants of the skin flora, characteristics of the host make them medically important for diagnosis. As these bacteria are easy to grow on laboratory media, their inability to grow on subculture may be due to their low number, or the antibiotic usage of the patient. Incubating these bottles for longer periods may increase the number of microorganisms and subcultures may become positive. In patients with underlying diseases, molecular methods can be used for quick identification of the microorganisms.