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Detection and recovery of sublethally-injured enterotoxigenic Staphylococcus aureus
M K Sandel1, Y-F G Wu, J L McKillip
1School of Biological Sciences, Louisiana Tech University, Ruston, LA 71272, USA. mckillip@latech.edu
Aims:
To determine whether sublethally-injured (acid- or heat-shocked) Staphylococcus aureus cells are recoverable using selective agar overlays.
Methods And Results:
Brain Heart Infusion (BHI) Agar overlaid with either Baird-Parker Agar (BPA) or Gram-Positive Agar (GPA) was compared in the ability to resuscitate heat- and acid-shocked enterotoxigenic Staph. aureus. BHI/BPA overlays allowed for greater recovery of both heat- and acid-shocked cells than BHI/GPA, although the former was not selective and allowed growth of bacteria other than Staph. aureus. No significant difference existed in percent recovery of heat- and acid-shocked cells between the two overlay approaches. Significant differences were noted in counts on BHI/GPA plates and straight selective GPA/GPA plates, however. Viability of heat- and acid-shocked Staph. aureus was also examined using fluorescence microscopy, the relative counts of which correlated well to the calculated percent recovery on selective agar overlays.
Conclusions:
This work has shown that an improved agar overlay technique increases the sensitivity of the standard plate count while enumerating sublethally-injured enterotoxigenic Staph. aureus compared with direct plating onto selective media.
Significance And Impact Of The Study:
These data emphasize the need to develop practical and cost-effective methods that reliably detect and enumerate sublethally-injured pathogens such as Staph. aureus.