Growth comparisons of streptomycin-sensitive and streptomycin-resistant Micrococcus pyogenes var. aureus.
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Three methods employed to distinguish staphylococci from micrococci were compared, using clinical and environmental strains. When these methods are used, misinterpretation of results, as well as erratic results, may occur, and suggestions for eliminating these problems are provided. The most sensitive test that combines ease of use and speed in obtaining results for distinguishing the two genera is the lysostaphin susceptibility test. Two other tests, facultatively anaerobic growth in semisolid thioglycolate agar and fermentation of dextrose, may also be used to distinguish these two genera, but results are often slow in developing, are subject to technical difficulties, and may lead to incorrect assignment of certain species of staphylococci and micrococci to their proper genera.
Taxo A bacitracin disks (BBL Microbiology Systems, Cockeysville, Md.) were evaluated as a test for differentiating staphylococci from micrococci in clinical laboratories. All staphylococci tested were resistant to the disks, with no zone of growth inhibition. Zones of growth inhibition ranging from 10.5 to 25.0 mm in diameter were observed with all micrococci except two isolates which failed to grow on the test medium. These findings were confirmed by minimal inhibitory concentrations to bacitracin. Staphylococci had minimal inhibitory concentrations significantly higher than those of micrococci, with no overlap in the ranges of values.
Bacitracin susceptibility was evaluated as a laboratory method to differentiate staphylococci from micrococci. A total of 317 staphylococcal isolates and 108 micrococcal isolates were each tested for susceptibility to bacitracin by a disk-diffusion method using disks of three different potencies (0.04, 2.0, and 10.0 U) and a broth dilution method to obtain MICs. When a growth inhibition zone diameter breakpoint of greater than 10 mm was used to establish susceptibility with a 0.04-U disk, all micrococci were bacitracin susceptible and 94.6% of the staphylococci were resistant. Testing with disks of higher potency did not improve the specificity of the disk-diffusion method.
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