Maximizing identification bacteria in equine synovial and other body fluid samples

Isolation of bacteria from synovial and other body fluid samples is often not successful. Reportedly, isolation rates may be as low as 33% when conventional methods are used for culturing synovial fluid from horses clinically suspected with a synovial infection. Successful isolation of microorganisms from synovial fluid is however essential, not only to confirm the infectious aetiology but also to allow antimicrobial susceptibility testing.

To our knowledge, enrichment in blood culture medium is the preferred method for maximizing identification of bacteria in equine synovial fluid and other body fluid samples. Compared to plate culture, enrichment in liquid media offers the advantages of a larger volume of inoculum together with the dilution of growth inhibitors. Furthermore, commercial blood culture media contain resins and lytic agents to inactivate inhibitors and release phagocytosed organisms, respectively. When automated incubators are used, the enrichment step is kept as short as possible since culture bottles are automatically evaluated several times per hour for growth.

In our clinic, for already ten years, all presumably infected synovial and other body fluid samples are processed by blood culture medium enrichment in small volume pediatric bottles (BACTEC Peds Plus/F bottle, Becton Dickinson) that are incubated in an automated system with fluorogenic detection of organism growth (BACTEC FX40 series, Becton Dickinson). Isolation rates of synovial fluid culture of horses with a clinical diagnosis of synovial infection are as high as 80%. The time to detection (TTD), defined as the time from inoculation of the sample in the enrichment bottle until detection of microorganism growth by the automated incubator, is less than 24 h for the majority of samples and even <12 h for about 40% of the isolates. In a study of our group, it was found that the isolation rate and TTD after BACTEC enrichment is not affected by the administration of antimicrobials prior to sampling. Furthermore, BACTEC culture bottles serve as the ideal transport medium for synovial and other body fluid samples and if delivered to the laboratory within 12 h and stored at room temperature, there is no effect on the recovery rate. [...]