Shorter Incubation Times for Detecting Multi-drug Resistant Bacteria in Patient Samples: Defining Early Imaging Time Points Using Growth Kinetics and Total Laboratory Automation

BACKGROUND: The transition from manual processing of patient samples to automated workflows in medical microbiology is challenging. Although automation enables microbiologists to evaluate all samples following the same incubation period, the essential incubation times have yet to be determined. We defined essential incubation times for detecting methicillin-resistant Staphylococcus aureus (MRSA), multi-drug resistant gram-negative bacteria (MDRGN), and vancomycin-resistant enterococci (VRE). METHODS: We monitored the growth kinetics of MRSA, MDRGN, and VRE between two and 48 hours on chromogenic media to establish the time points of first growth, single colony appearance, and typical morphology for 102, 104, 106, and 108 colony forming units/mL. Subsequently, we imaged plates inoculated with 778 patient samples after 20, 24, and 36 hours. RESULTS: The first growth, single colony appearance, and typical morphology time points were inoculum-dependent. First growth appeared after 6–18 hours, 4–18 hours, and 8–48 hours for MRSA, MDRGN, and VRE, respectively, and single colonies appeared at 12–18 hours, 6–20 hours, and 12–48 hours, respectively. Typical morphology was visible at 14–22 hours and 12–48 hours for MRSA and VRE, but was not determined for MDRGN. By examining patient samples, ≥98% of MRSA and MDRGN were visible 20 hours after the start of incubation. Following 24 hours of incubation, only 79.5% of VRE were clearly visible on the respective plates. CONCLUSIONS: An incubation time of 20 hours is sufficient for detecting MRSA and MDRGN. VRE growth is much slower and requires additional imaging after 36 hours.

Evaluation of the Impact of Automated Specimen Inoculation, Using Previ Isola, on the Quality of and Technical Time for Stool Cultures

BACKGROUND: This study was designed as a quasi-experiment to evaluate automatic inoculation of fecal specimens, using the automated specimen inoculator Previ Isola (bioMerieux, France). METHODS: We evaluated the quality of cultures, recovery rates of enteropathogenic bacteria (Salmonella, Shigella, Campylobacter, and Yersinia species), and cost-effectiveness in terms of technical time. The Previ Isola recovery rates for the two-year period from August 2009 to July 2011 were compared with historical manual inoculation data of the previous two years (August 2007 to July 2009). The regional (Baden-Wurttemberg) and nationwide (Germany) trends of recovery rates for this four-year period were referred. RESULTS: A total of 5,884 fecal specimens were collected over the study period. Most positive cultures were for Salmonella, followed by Campylobacter. Compared with the historical data, the numbers of Campylobacter-positive specimens for a year between August and July were increased significantly, from 19 in 2007-2008 and 10 in 2008-2009 to 32 in 2009-2010 (P=0.002) and 32 in 2010-2011 (P=0.003), respectively. During the study period, the official data for our region and nationwide did not show this increase in the recovery rate of Campylobacter. For Salmonella, Shigella, and Yersinia, no significant changes were observed. Compared with manual inoculation, the mean hands-on time with Previ Isola inoculation was significantly shortened, from 37:30 min to 8:42 min per 15 fecal specimens. CONCLUSIONS: Inoculation by Previ Isola improves the quality of routine culture of fecal specimens, with better sensitivity for Campylobacter and less hands-on time.

Erratum: Evaluation of the Impact of Automated Specimen Inoculation, Using Previ Isola, on the Quality of and Technical Time for Stool Cultures

The title of the author was misprinted as Marlies Trampe, M.D., so it should be corrected to Marlies Trampe.