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Resistance mechanism relatedness was studied in 18 clinical, European vanA vancomycin-resistant enterococci. Molecular analysis revealed 10 Tn1546-like elements, suggesting two evolutionary lineages. Lineage I dominated the European mainland, and lineage II dominated the United Kingdom and Israel. Geographic clustering reflected different types of meat consumption between countries, since each lineage is associated with colonization of different animals.

Three epidemiologically unrelated clusters of Haemophilus influenzae resistant to ampicillin, chloramphenicol, and tetracycline were studied. The biotypes and cell-envelope protein patterns were determined for 17 nonencapsulated strains, 6 from Dundee and 11 from Cheltenham, and for 6 type b encapsulated strains from Guildford. After mobilization by conjugation, large 32- to 36-MDa plasmids were purified from all the strains. The restriction fragment patterns of the plasmids were determined by ethidium bromide staining of digested purified plasmid or by Southern hybridization of digested total cellular DNA of the parent strains, probed with purified plasmid. Evidence is presented for a chromosomal location of the plasmids in the parent strains, the spread in nature of a plasmid between distinguishable strains of H. influenzae, the person-to-person spread of a strain within a cluster, and a high degree of sequence homology between distinguishable plasmids, implying their close relatedness.

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

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