Biomedical subjects
Publications and source records attributed to W B Grubb.
The Royal Free Hospital in London has experienced the spread of a phage 85 methicillin-resistant Staphylococcus aureus (MRSA). An analysis of a sample has demonstrated that the plasmids carried by these strains, and the type and location of their drug-resistance determinants, are similar to those of MRSA strains which have been causing problems in some Australian hospitals.
The isolates from an outbreak of methicillin-resistant Staphylococcus aureus (MRSA) at Royal Perth Hospital (RPH) during a 3 month period in 1982 have been compared genetically with MRSAs isolated at the same hospital during 1969-1973. The 1982 isolates are genetically similar to isolates from eastern Australia and appear to have been introduced by a patient transferred from a hospital in another Australian state. Methicillin-resistant Staph. aureus isolated during 1969-1973 were genetically distinguishable from the 1982 isolates but were similar to strains reported from elsewhere in the world during these years. This indicates that the MRSA strains currently prevalent in Australia are either a new type or are related to previous MRSAs but have undergone considerable genetic change. These results give further support to suggestions that the strains of MRSA currently being isolated in Australian hospitals have special properties which have facilitated their spread.
Gentamicin-resistance plasmids in methicillin-resistant Staphylococcus aureus isolated from four Australian hospitals have been studied. All the plasmids conferred resistance to gentamicin, tobramycin, kanamycin and all but one conferred resistance to quarternary ammonium compounds. Plasmids which only carried these resistance determinants were ca 15.3 megadaltons and were only found in isolates from one hospital. The most common plasmid was ca 18.0 megadaltons and in addition encoded trimethoprim resistance. Two plasmids, one of ca 19.6 megadaltons and one of ca 28.5 megadaltons were found to carry the penicillinase determinant. However only the larger of these encoded heavy metal ion resistance and was sensitive to quaternary ammonium compounds. EcoR1 analysis indicated that all but the ca 28.5 megadalton plasmid were closely related. The EcoR1 analysis of the ca 28.5 megadalton plasmid indicated that it could have resulted from recombination between a gentamicin resistance determinant and a penicillinase plasmid.
Plasmid pWG115 isolated from a methicillin-resistant Staphylococcus aureus encodes resistance to cationic surface-active agents and trimethoprim. It has a molecular weight of ca 14.6 megadaltons and can be transferred to other strains of staphylococci in mixed-culture transfer with propamidine isethionate as a selective agent. Gentamicin resistance in Australian methicillin-resistant Staph. aureus isolates can be either chromosomal or plasmid-borne. The most common gentamicin resistance plasmid is 18.0 megadaltons and also encodes resistance to trimethoprim and cationic surface-active agents. This suggested that pWG115 was related to gentamicin resistance plasmids and that it may provide a target for the postulated gentamicin resistance transposon. This paper demonstrates that the chromosomal gentamicin resistance determinant from WG523 can transpose into pWG115 to generate an 18.0 megadalton plasmid, phenotypically indistinguishable from the naturally occurring gentamicin resistance plasmids such as pWG53. EcoR1 restriction enzyme analysis demonstrated that gentamicin resistance can transpose into at least two sites on pWG115. One of these sites generates EcoR1 restriction fragments identical to pWG53. The 5.2 kilobase pair (3.4 megadalton) element involved confers low-level resistance to gentamicin, cross resistance to tobramycin and kanamycin, and has been designated Tn3851.
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The drug-resistance determinants in methicillin-resistant Staphylococcus aureus (MRSA) from three different hospitals in Eastern Australia have been examined. With one exception, all the isolates had chromosomal determinants for penicillinase and resistance to cadmium (Cd), mercury (Hg), phenyl mercuric acetate, methicillin, tetracycline, erythromycin, lincomycin and low level streptomycin. The strain which was the exception differed in that it did not have chromosomal resistance to Cd and lincomycin. In addition, the strains often contained plasmids which belonged to one of three categories: a small cryptic plasmid of either c. 1.4 Mdal, c. 1.7 Mdal or c. 1.9 Mdal; a chloramphenicol resistance plasmid of c. 2.8 Mdal; and a gentamicin resistance plasmid within the range of c. 15.3 to c. 28.5 Mdal. The predominant gentamicin-resistant plasmid in isolates from two hospitals had a molecular weight of c. 18 Mdal, whereas the isolates from the third hospital had a plasmid of molecular weight c. 15.3 Mdal. The only other gentamicin resistance plasmids detected were associated with penicillinase determinants. In one isolate, this corresponded to a plasmid of c. 19.6 Mdal and in the other to a plasmid of c. 28.5 Mdal. These results indicate that MRSAs which are prevalent in Eastern Australian hospitals are substantially different in the location of their drug resistance determinants to earlier strains reported in the literature.
Gentamicin resistance has been studied in methicillin-resistant Staphylococcus aureus (MRSA) strains, from Royal Melbourne Hospital (RMH) and Sydney. Gentamicin resistance was transferred in mixed cultures to a plasmid free strain, and the determinants were examined. The Sydney strain had high level resistance to gentamicin, tobramycin, kanamycin and neomycin which was carried on a c.34 megadalton plasmid. The gentamicin resistant RMH isolates all had a determinant which conferred low level resistance to gentamicin, tobramycin and kanamycin and appeared to be chromosomal in one isolate, on a plasmid of c.28.5 megadaltons in another and on a plasmid of c.18 megadaltons in the other isolates. It is suggested that a gentamicin resistance transposon is being transferred in the MRSA at RMH.
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Methicillin-sensitive variants have been isolated from ageing broth cultures of the methicillin-resistant strain of Staphylococcus aureus NCTC 10442 and also from numerous other strains. There were wide variations and fluctuations in the incidence of sensitive variants from cultures incubated under uniform conditions.
Cold-sensitive (Cs) variants were obtained from ageing broth cultures of the Oxford strain of Staphylococcus aureus (NCTC 6571) and other penicillin-sensitive strains of this species. Growth of all strains was severely retarded on certain media at 30 degrees C but was stimulated by the addition of penicillins and cephalosporins. Revertants were derived which behaved as the parent cells did with respect to growth temperature requirements and response to these antibiotics.
Sodium chloride and sucrose were shown to elevate the maximum temperature at which both coagulase positive and coagulase negative staphylococci would grow on the surface of nutrient agar. All of 200 isolates tested showed this effect.
The methicillin resistant strain of Staphylococcus aureus NCTC 10442 when aged in a broth culture at 42 degrees C yielded variants showing responses of sensitivity, dependence and indifference to the antibiotic. These responses and that of resistance in the parent cells were displayed in agar dilution and agar diffusion experiments at 30 degrees C. At 42 degrees C, all four organisms were sensitive to methicillin.
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