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The use of continuously monitored blood culture systems has reduced the time taken to detect positive blood cultures. Once a positive blood culture is detected, presumptive identification of the organism relies on direct Gram stain of the inoculated blood culture broth. The appearance of Gram positive cocci in clusters on direct Gram stain is suggestive of staphylococcus species, but differentiation of Staphylococcus aureus from coagulase negative staphylococci can usually only be made 18-24 hours later, once the organism has been cultured on solid media. This distinction is important given the differences in virulence, and the relatively high frequency that coagulase negative staphylococci are isolated as contaminants.
"The main characteristics are the size of the bacterial cells, the number of cells in a typical cluster, and knowledge about whether the Gram stain was made from an anaerobic or aerobic blood culture bottle"
Over the past decade, technologists in our laboratory have noted that certain morphological characteristics can help distinguish S aureus from coagulase negative staphylococci in direct Gram stains from BacT/ALERT blood culture bottles (BioMérieux, Durham, North Carolina, USA). The main characteristics are the size of the bacterial cells, the number of cells in a typical cluster, and knowledge about whether the Gram stain was made from an anaerobic or aerobic blood culture bottle (table 1; fig 1). The purpose of our study was to evaluate these criteria in a blinded prospective fashion.
Table 1 Criteria used to distinguish Staphylococcus aureus from coagulase negative staphylococci in direct Gram stains from BacT/ALERT blood culture bottles
Bottle type | Organism | Cell size | Cluster characteristics |
Anaerobic | S aureus | Small (<1 [micro]m) | Irregularly clustered in large numbers (typically 200-300 cells) |
Coagulase-negative staphylococci | Large ([= or >, slanted]1 [micro]m) | Tetrads or small clusters up to 16 cells |
|
Aerobic | S aureus | Large ([= or >, slanted]1 [micro]m) | Very tight clusters (typically 8-32 cells). Individual cells cannot be distinguished |
Coagulase-negative staphylococci | Variable (typically <1 [micro]m) | Tetrads or small clusters up to 16 cells |
Gram stain smears from positive BacT/ALERT blood culture bottles showing typical appearances of Staphylococcus aureus and coagulase negative staphylococci, with key characteristics stated below in parentheses. (A) Anaerobic bottle, S aureus (small cells, large clusters); (B) anaerobic bottle, coagulase negative staphylococcus (large cells, tetrads (arrows), and small clusters); (C) aerobic bottle,