different species of coagulase-negative staphylococci should be validated before use. However, in general commercially available methods do perform well in the identification of S. epidermidis, the most commonly encountered coagulase-negative staphylococcal species.
Table 16-4 shows how the catalase-positive, grampositive cocci can be differentiated. Because they may show a pseudocatalase reaction, that is, they may appear to be catalase-positive, Aerococcus and Enterococcus are included in Table 16-4; Rothia (formerly Stomatococcus) is included for the same reason. Once an organism has been characterized as a gram-positive, catalase-positive, coccoid bacterium, complete identification may involve a series of tests, including (1) atmospheric requirements, (2) resistance to 0.04 U of bacitracin (Taxo A disk) and furazolidone, and (3) possession of cytochrome G as determined by the microdase (modified oxidase) test. However; in the busy setting of many dinical laboratories, microbiologists proceed immediately to a coagulase test based on recognition of a staphylococcal-like colony and a positive catalase test.
Microdase disks are available commercially (Remel, Inc., Lenexa, Kan). A visible amount of growth from an 18- to 24-hour-old culture is smeared on the disk; Micrococcus spp. turn blue within 2 minutes (see Figure 13-25).
Both for bacitracin and for furazolidone resistance, disk tests are used (Figure 16-2). A 0.04-U bacitracin-impregnated disk and a 100-pg furazolidone-impregnated disk, both available from Becton Dickinson and Company, are placed on the surface of a 5% sheep blood agar plate that has been previously streaked in
three directions with a cotton-tipped swab that has been dipped in a bacterial suspension prepared to match the turbidity of the 0.5 McFarland standard (i.e., the same as is used in preparing inocula for disk diffusion susceptibility tests as described in Chapter 12).
Micrococcus spp. and related genera are (1) not lysed with lysostapbin, (2) resistant to the antibiotic furazolidone, (3) susceptible to 0.04 U of bacitracin, and (4) microdasepositive; they usually will only grow aerobically. In contrast, staphylococci are (1) lysed with lysostaphin, (2)
• 2 hr PYR broth hydrolysis*
S. saprophytics subsp. saprophytics $. cohnii subsp. cohnii S. cohnii subsp. urealyticum S. sduri subsp. sciurf S. homlnis subsp. novobiosepticus Table 16-6
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