Comparison of culture media for detecting methicillin resistance in Staphylococcus aureus and coagulase negative staphylococci.

Abstract

SUMMARY The detection of methicillin resistance was examined in 51 strains of Staphylococcus aureus and 135 strains of coagulase negative staphylococci using Isosensitest, Diagnostic Sensitivity Test (DST), Mueller-Hinton (MH), Columbia, and Sensitest agars. MH agar with 5% added sodium chloride incubated at 35°C was the most effective in detecting resistance in S aureus, and Columbia agar with 5% added sodium chloride incubated at 35°C was most effective for coagulase negative staphylococci. For clinical purposes, a provisional report of sensitivity for S aureus could be issued after 18 hours; with coagulase negative staphylococci, only resistant strains could be reported at this time. For definitive results cultures must be examined after 40 hours of incubation.Resistance to methicillin in Staphylococcus aureus is usually heterogeneous and due to the presence of a component which sometimes only manifests resistance if grown on osmotically supportive media' or if cultured at 30'C2 for up to 48 hours.3 Snell et al4 recommended that incubation at 30°C or adding 5% sodium chloride to the medium, or both, be used for methicillin testing. Despite the heterogeneous nature of the resistance in S aureus to methicillin it has been shown to be clinically relevant in that it impairs the response to treatment with methicillin.5 Resistance to methicillin in coagulase negative staphylococci has also been shown to be due to heterogeneous populations and to be enhanced by growth on hypertonic medium or incubation at 30°C or for 48 hours.6In addition to these factors, the detection of methicillin resistance can be affected by the basal test medium used. Hindler and Inderleid7 showed that when MH, an undefined medium, was used from five different manufacturers there were differences in the detection of resistance of S aureus to methicillin.Brown and Kothari8 determined the minimum inhibitory concentration (MIC) of strains of S aureus using MH, DST, Sensitest, and Wellcotest agars, and all gave similar results when incubated at 30°C for 18 hours. In view of the increasing clinical importance of

Rationale for Eliminating Staphylococcus Breakpoints for -Lactam Agents Other Than Penicillin, Oxacillin or Cefoxitin, and Ceftaroline

“…All 31 strains had no zones of inhibition around methicillin (10 µg) and cephalexin (30 µg) disks, and 26 demonstrated reduced zones of inhibition for cephalothin (30 µg) and cephaloridine (30 µg) on trypticase soy agar containing 5% sodium chloride. When DD was performed on Mueller-Hinton agar, the same 26 strains demonstrated zones of 25–30 mm, which would have been interpreted as susceptible using former CLSI breakpoints, around the cephalothin and cephaloridine disks, confirming the ability of sodium chloride to improve the detection of β-lactam resistance [72] as well as the heterogeneous expression of resistance in these strains. MRS was recovered from blood culture after initiation of cephalosporin therapy in 21 of these patients, 20 of whom remained culture positive after day 3 of cephalosporin therapy.…”

Detecting methicillin‐resistant Staphylococcus epidermidis– disc diffusion, broth breakpoint or polymerase chain reaction?

“…Such a low-level resistance has not yet been described in S. epidermidis. Isolates with this mechanism of resistance would not be demonstrated using a mecA-specific PCR and probably not using the broth tube breakpoint test where the breakpoint is relatively high, 16 mg/l methicillin. However, they may have zones smaller than those of the susceptible isolates in a disc diffusion test.…”

“…Using another method, agar dilution, other workers have shown that the choice of medium is important when testing S. uureus (11) and coagulase-negative staphylococci ( 1 6). In these tests an increased concentration of NaCl in the medium enhances growth of the resistant subpopulations, but 48 h incubation is necessary for detection of all resistant strains of S. epidermidis ( 10,16). In the disc diffusion test the influence of the medium is on the size of the inhibition zone and breakpoints can be chosen where resistant and susceptible strains are best separated.…”

Comparison of test systems for recognition of methicillin resistance inStaphylococcus aureus

“…This has increased interest in optimal methods for MRSA recognition. Several papers on the subject have been published recently (5)(6)(7)(8)(9)(10). These studies make clear that there are different views on the criterion to use in defining methicillin resistance, i.e.…”