Analysis of the contaminant spectrum in the MB/BacT liquid culture system following C(18)-carboxypropylbetaine specimen processing.

Abstract

A detailed study comparing specimen processing methods for the isolation of mycobacteria was recently reported [1]. In the follow-up study presented here, 1,201 specimens (95% respiratory) were collected and split approximately equally such that one-half of each specimen was processed with 3% sodium dodecyl sulfate-1% sodium hydroxide (SDS-NaOH) [2] and the other half was processed with C18-carboxypropylbetaine (CB-18) followed by lytic enzyme decontamination [3, 4]. All processed specimens were analyzed by culture using the MB/BacT liquid culture system containing the antibiotic supplement MAS (bioM rieux, France). The SDS-NaOHprocessed specimens were also inoculated onto L wenstein-Jensen slants, while the CB-18-processed specimens were also inoculated onto 7H11-selective plates (50 g/ml carbenicillin, 200 U/ml polymyxin B, 10 g/ml amphotericin B, and 20 g/ml trimethoprim lactate). CB-18/lytic enzyme processing increased overall culture sensitivity by approximately 14.1% (P<0.05); however, the contamination rates in the MB/BacT system following SDS-NaOH and CB-18 processing were 0.8% and 8.7%, respectively, and on solid media the contamination rates were 2.6% and 4.3%, respectively [1]. All specimens that had been processed by the CB-18/ lytic enzyme method, and that were either flagged as positive in MB/BacT cultures or were observed with growth on the 7H11-selective plates, were subjected to acid-fast staining. Those culture-positive specimens that were negative for the presence of acid-fast bacteria, or that suggested the presence of contaminating microorganisms, were further analyzed to determine the nature of the contaminant. Portions of these cultures were first subcultured onto blood agar plates. Any microorganisms that grew were then subjected to Gram staining. Contaminant speciation was performed using conventional methods [5]. A total of 105 MB/BacT cultures were considered contaminated following processing with CB-18. These 105 cultures produced 116 contaminants (Table 1): 62.1%