Mushin and Kerr investigated whether intestinal gram-negative bacilli possess coagulase enzymes capable of clotting plasma. They tested 52 Salmonella, 34 Shigella, 4 alkalescens-dispar, 53 Proteus, 140 paracolon, and 14 coliform strains using citrated and heparinized plasma, along with various citrate-containing media. While most Salmonella, paracolon, coliform, and alkalescens-dispar strains clotted citrated plasma within 24 hours, all Shigella and Proteus strains failed to do so. Crucially, clotting was not due to coagulase enzyme activity but rather resulted from bacterial metabolism of citrate, which released free calcium ions. This was demonstrated by the complete absence of clotting in heparinized or oxalated plasma where citrate was not the anticoagulant. Citrate utilization patterns varied among strains depending on the medium composition and plasma type used. The authors conclude that false-positive coagulase reactions commonly reported for gram-negative bacilli actually reflect citrate metabolism rather than true coagulase activity, and recommend using non-citrate anticoagulants for accurate coagulase testing.

Key findings

• Clotting of citrated plasma by gram-negative bacilli results from citrate metabolism and calcium ion liberation, not true coagulase enzyme activity
• All Shigella and Proteus species tested failed to clot citrated plasma and could not utilize citrate in any medium tested
• Clotting reactions depended on bacterial strain, plasma type, and citrate concentration; reactions were absent with heparinized or oxalated plasma
• Salmonella strains showed consistent citrate utilization across different media, while paracolon and coliform strains showed higher positive reactions in plasma-containing media
• Non-citrate anticoagulants should be used for coagulase testing to avoid false positive results from gram-negative bacilli