Selective culture of coagulase-positive staphylococci

Polymyxin susceptibility in staphylococci differentiating coagulase-positive and coagulase-negative strains

47 staphylococcal reference strains representing 13 species were tested for polymyxin sensitivity using tablet and disc diffusion methods. Corresponding MIC and IC50 values were determined with a plate dilution assay. Coagulase-positive strains were found to be more resistant towards polymyxin, and could thereby be separated from coagulase-negative strains. Furthermore, 100 recently isolated staphylococci from clinical specimens were tested, and again the coagulase-positive strains could be identified by their smaller tablet inhibition zones. We conclude that the polymyxin test is an aid in the fast differentiation of staphylococci in laboratories performing primary sensitivity tests.

Comparison of animal sera for suitability in coagulase testing

The sera of several animals were examined for suitability in coagulase testing. The assay for coagulase-reacting factor (CRF) activities of the whole sera indicated the following relative concentrations of CRF: human > pig > rabbit > horse > bovine, chicken, and lamb. Human, pig, and rabbit sera had adequate amounts of CRF for coagulase testing. The plasmin activities of the different sera, arranged from the strongest to the weakest, were as follows: rabbit > human > lamb > horse > bovine, chicken, and pig. Fibrinolysis was observed when rabbit, human, lamb, or horse sera were incorporated into coagulase test agars. Pig serum was superior to the other sera for use in the plate test for coagulase since it had adequate amounts of CRF and the plasminogen-plasmin system was not activated by staphylokinase or staphylococcal Müller factor. Heparinized pig plasma was more suitable than citrated pig plasma since citrate interfered with the growth of Staphylococcus aureus, and the use of heparinized plasma prevented false-positive coagulase reactions due to citrate utilization.

Nasal carriage of Staphylococcus aureus in a general surgical unit

Nasal carriage of staphylococci was found in 50 per cent of in-patients in a general surgical unit and in 26 per cent of out-patients. While 32 per cent of in-patients carried resistant organisms in their noses a high percentage of out-patients (17 per cent) also were carriers of resistant organisms. Eighty per cent of staff carried staphylococci in their noses, 67 per cent being resistant organisms. Although the ‘hospital staphvlococcus’ was found in 5 per cent of patients and 15 per cent of attending staff, only 1 per cent of in-patients and none of the out-patients or staff carried multiple-resistant organisms. Age does appear to have some bearing on the carriage of bacteria in the anterior nares.

In contrast to other studies, this work failed to reveal any relationship between wound infection and nasal carriage of staphylococci. These findings suggess that general dissemination of bacteria in the hospital environment may be of greater importance than autoinfection as a cause of wound sepsis.

Selection of Staphylococcus aureus in cultures from air samples

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