Genetic analysis of methicillin-resistant Staphylococcus aureus from Singapore hospitals

Antibacterial resistance and their genetic location in MRSA isolated in Kuwait hospitals, 1994-2004

Background

Methicillin-resistant Staphylococcus aureus (MRSA) continues to be a major cause of serious infections in hospitals and in the community worldwide. In this study, MRSA isolated from patients in Kuwait hospitals were analyzed for resistance trends and the genetic location of their resistance determinants.

Methods

Between April 1994 and December 2004, 5644 MRSA isolates obtained from different clinical samples were studied for resistance to antibacterial agents according to guidelines from the National Committee for Clinical Laboratory Standards and the British Society for Antimicrobial Chemotherapy. The genetic location of their resistance determinants was determined by curing and transfer experiments.

Results

They were resistant to aminoglycosides, erythromycin, tetracycline, trimethoprim, fusidic acid, ciprofloxacin, chloramphenicol, rifampicin, mupirocin, cadmium acetate, mercuric chloride, propamidine isethionate and ethidium bromide but susceptible to vancomycin, teicoplanin and linezolid. The proportion of the isolates resistant to erythromycin, ciprofloxacin and fusidic acid increased during the study period. In contrast, the proportion of isolates resistant to gentamicin, tetracycline, chloramphenicol and trimethoprim declined. High-level mupirocin resistance increased rapidly from 1996 to 1999 and then declined. They contained plasmids of 1.9, 2.8, 3.0, 4.4, 27 and 38 kilobases. Genetic studies revealed that they carried plasmid-borne resistance to high-level mupirocin resistance (38 kb), chloramphenicol (2.8 – 4.4 kb), erythromycin (2.8–3.0 kb) and cadmium acetate, mercuric chloride, propamidine isethionate and ethidium bromide (27 kb) and chromosomal location for methicillin, the aminoglycosides, tetracycline, fusidic acid, ciprofloxacin and trimethoprim resistance. Thus, the 27 kb plasmids had resistance phenotypes similar to plasmids reported in MRSA isolates in South East Asia.

Conclusion

The prevalence of resistance to erythromycin, ciprofloxacin, high-level mupirocin and fusidic acid increased whereas the proportion of isolates resistant to gentamicin, tetracycline, chloramphenicol and trimethoprim declined during the study period. They contained 27-kb plasmids encoding resistance to cadmium acetate, mercuric chloride, propamidine isethionate and ethidium bromide similar to plasmids isolated in MRSA from South East Asia. Molecular typing of these isolates will clarify their relationship to MRSA from South East Asia.

Resistance to gentamicin and related aminoglycosides in Staphylococcus aureus isolated in Brazil

Isolates of Staphylococcus aureus obtained from a Brazilian university hospital were characterized in relation to resistance to gentamicin and related aminoglycosides. Thirty-six isolates were susceptible to methicillin (MSSA) and 14 were resistant (MRSA). All isolates were sensitive to nucleic acid-binding compounds. All MRSA isolates and one MSSA isolate were demonstrated to be resistant to gentamicin and were coincidentally resistant to amikacin, kanamycin, neomycin and tobramycin. Among the gentamicin sensitive MSSA isolates, five isolates were found to be resistant only to kanamycin/neomycin. The resistance to gentamicin (and related aminoglycosides: kanamycin and tobramycin) must be due to AAC(6')-APH(2") activity. As these isolates also showed resistance to neomycin, they must carry an additional genetic element, probably the one responsible for APH(3')III activity, which accounts for the high level of resistance to kanamycin and to amikacin. The resistance to kanamycin/neomycin in the gentamicin sensitive isolates could not be attributed to the AAD(4')(4") activity because of the tobramycin sensitivity, and so could be ascribed to the APH(3')III activity. Curing and transfer experiments, as well as electrophoresis procedures, indicate that gentamicin resistance in Staph. aureus strains here studied has, characteristically, chromosomal localization.

Antimicrobial drug resistance in Staphylococcus aureus isolated from cattle in Brazil

Isolates of Staphylococcus aureus obtained from apparently healthy cattle in the State of Paraiba, Brazil were characterized in relation to resistance to 21 antimicrobial agents. Among the 46 isolates obtained, resistance to penicillin was most frequent, followed by resistance to cadmium, streptomycin, arsenate, tetracycline, mercury, erythromycin and kanamycin/neomycin. All isolates were susceptible to fusidic acid, ethidium bromide, cetrimide, chloramphenicol, benzalkonium chloride, doxycycline, gentamicin, methicillin, minocycline, novobiocin, rifamycin, tylosin and vancomycin. Only six isolates were susceptible to all the drugs tested. With respect to the antibiotics, multi-resistant isolates were uncommon. These results are probably a consequence of the peculiarities of local drug usage pressures. In relation to metal ions, resistance to mercury was rare while resistance to arsenate was relatively frequent, which contrasts with the situation for human Staph. aureus strains. After treatment with ethidium bromide, elimination of resistance to penicillin, tetracycline, streptomycin, erythromycin and cadmium was observed, which was consistent with the genetic determinants being plasmid-borne.

Typing of methicillin-resistant Staphylococcus aureus with an M13 repeat probe

A bacteriophage M13 tandem repeat has been used to probe EcoRI digested genomic DNA of methicillin-resistant Staphylococcus aureus (MRSA). The patterns generated were found to be useful in typing MRSA and generally confirmed the relationships that had previously been recognized in other studies based on antimicrobial resistance and plasmid profiles. The epidemic MRSA of London hospitals (EMRSA) and the majority of the epidemic MRSA of eastern Australian hospitals (EA MRSA) gave the same pattern. However, two isolates previously classified as EA MRSA gave a different pattern and a third another pattern. One isolate from Dublin, two isolates from Nuneaton and two isolates from Singapore gave the same pattern as the two EA MRSA. With the exception of the early or classic MRSA all the other isolates examined gave their own distinctive patterns. With one exception the classic MRSA belonged to a separate group. The exception was of particular interest because it gave the same pattern as the majority of the EA MRSA. This suggests that there may be an evolutionary relationship between some of the classic MRSA and the EMRSA of London and the EA MRSA of Australia.

Comparative sensitivity to antibiotics and biocides of methicillin-resistant Staphylococcus aureus strains isolated from Saudi Arabia and Great Britain

Methicillin-resistant Staphylococcus aureus (MRSA) isolated in Saudi Arabia and Great Britain were examined for susceptibility to antibiotics and biocides. The strains differed in their sensitivity patterns. None of the Saudi strains showed resistance to propamidine isethionate, but most of the British gentamicin methicillin-resistant Staph. aureus (GMRSA) strains were highly resistant to this compound and to some other nucleic acid-binding (NAB) compounds. Both groups showed a low level of resistance towards quaternary ammonium compounds (QACs), but resistance to these compounds was not associated with resistance to gentamicin in the Saudi strains. The aminoglycoside-resistant determinants were non-conjugative in these strains. Natural MRSA strains were good recipients for pWG613, but transferred this plasmid in reciprocal crosses at significantly lower rates.

Hong Kong strains of methicillin-resistant and methicillin-sensitive Staphylococcus aureus have similar virulence

While most authorities agree that methicillin-resistant Staphylococcus aureus (MRSA) are as pathogenic as methicillin-sensitive strains (MSSA), some believe that MRSA are relatively avirulent opportunists, and that their importance has been exaggerated. We present evidence that Hong Kong strains of MRSA and MSSA are equally pathogenic: they have similar virulence in animal models; they are isolated in similar proportions from both deep and superficial clinical sites including blood; in patients with hospital-acquired bacteraemias mortality rates are similar when adjusted for clinical factors; and in both animals and patients with systemic MRSA infection, mortality rates are significantly reduced by vancomycin therapy. Efforts to control the spread of MRSA are justified, and in invasive sepsis early treatment with vancomycin may be life-saving.