Genetic characterisation of resistance to metal ions in methicillin-resistant Staphylococcus aureus: elimination of resistance to cadmium, mercury and tetracycline with loss of methicillin resistance

Abstract

Susceptibility to six metal ions--cadmium (Cd), mercury (Hg), arsenate (Asa), arsenite (Asi), antimony (Sb) and zinc (Zn)--was tested in 23 independent isolates of methicillin-resistant Staphylococcus aureus (MRSA) obtained from Guy's Hospital (GH) during 1984-1986, which included 10 isolates of the UK epidemic EMRSA-1 strain. Strains were also tested for resistance to antibiotics and the nucleic-acid-binding compounds propamidine isethionate and ethidium bromide. A further 19 methicillin-resistant isolates, including 10 EMRSA-1 were obtained from other sources. Ten methicillin-sensitive, antibiotic sensitive isolates were from Guy's Hospital. Resistance to Hg was associated with methicillin resistance in 19 of the 20 EMRSA-1 isolates, all of which were resistant to Cd. Resistance to Cd and Hg was found in 13 out of 22 other MRSA isolates. Hg resistance was not present in the methicillin-sensitive isolates which were often (13 out of 19) moderately resistant to Cd. Multiple resistance to metal ions, including resistance to Hg, Asa, Asi and Sb, was uncommon. Resistance to Cd (MIC greater than 32 mg/L or 8-16 mg/L) was associated with increased resistance to Zn. In 11 of the consecutive MRSA isolates from Guy's Hospital seven distinct strains were recognised by phage type. Methicillin resistance in these strains varied from 16 to 1024 mg/L at 30 degrees C with a 2-8-fold lower minimum inhibitory concentration at 37 degrees C indicating some degree of heterogeneity. Representatives of the EMRSA-1 strain had the higher levels of resistance. Loss of methicillin resistance occurred in 0.2-5.0% of colonies tested after storage at room temperature in 10 of these isolates.(ABSTRACT TRUNCATED AT 250 WORDS)