Evaluation of a selective media for the detection of gram-positive bacteria in leg ulcers and pressure wounds

Hard-to-heal ulcers are resource intensive due to the fact that they are difficult to treat and especially vulnerable to bacterial invasion. The bacterial culture contaminating these wounds often consist of several different bacterial organisms that originate from endogenous sources. Necrotic material in ischemic ulcers provide nutrition which support bacterial reproduction, increasing the risk of infection. Determining causative pathogen in infected ulcers proves to be difficult when culturing swab samples, however Staphylococcus aureus and hemolytic streptococci generally act as primary pathogens. The aim of the study was to investigate if the detection rate increased for S. aureus and hemolytic streptococci when culturing swab samples from ulcers on Columbia CNA; a media selective for gram-positive bacteria. In the experimental procedure the inhibitory action of CNA upon gram-negative bacterial growth was evaluated, using simulated ulcer samples (n=6) containing bacterial quality control strains in arbitrary concentrations. Additionally, patient samples (n=51) were cultured and screened for primary pathogens to investigate differences in the detection rate for CNA and the current culture media; Blood agar, Chocolate agar, Gentian violet blood agar and CLED agar. Results from simulated ulcer samples showed excellent inhibitory function regarding the antibiotic substances of the CNA agar. Culturing patient samples from lower leg- and pressure ulcers on CNA, provided indications of diverse circumstances yielding higher respectively lower detection rate concerning S. aureus and hemolytic streptococci. Samples containing mixed flora with gram-negative bacteria generated higher detection rate and samples containing S. aureus yielded a lower detection rate when culturing on CNA, compared with that of the routine method.