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Antibiotic susceptibilities and plasmid profiles of nosocomial methicillin-resistant Staphylococcus aureus: a
Abstract:
Methicillin-resistant Staphylococcus aureus strains isolated at a single Melbourne Hospital between 1969 and 1981 were examined for susceptibility to a range of antimicrobial agents and for the presence of plasmid DNA. Isolates obtained during 1969 possessed a plasmid of mol. wt 20 X 10(6), encoding heavy metal resistance and penicillinase production, and a plasmid of mol. wt 2.8 X 10(6), mediating tetracycline resistance. In the majority of isolates obtained after 1973, these functions were chromosomally encoded. Before 1980, both high- and low-level chromosomally-encoded gentamicin resistances were encountered, whereas isolates from 1980 and 1981 displayed low-level gentamicin resistance only; the latter phenotype was most commonly mediated by a plasmid of mol. wt 18 X 10(6) that also encoded resistance to tobramycin and kanamycin. Chloramphenicol resistance in strains isolated throughout the period was mediated by one of three plasmids, each of mol. wt c. 3 X 10(6).
Insights
Antimicrobial resistance in Methicillin-resistant Staphylococcus aureus (MRSA) evolved over time in Melbourne. Plasmid-mediated resistance shifted to chromosomal, with changes in gentamicin resistance patterns observed.
Area of Science:
- Microbiology
- Molecular Biology
- Infectious Diseases
Background:
- Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant public health threat.
- Understanding the evolution of antimicrobial resistance mechanisms in MRSA is crucial for effective treatment strategies.
- Hospital environments are key settings for the emergence and spread of resistant bacterial strains.
Purpose of the Study:
- To investigate the changes in antimicrobial susceptibility and plasmid DNA in MRSA strains over a 12-year period (1969-1981).
- To identify the genetic basis (plasmid vs. chromosomal) of resistance to various antimicrobial agents.
- To track the emergence and characteristics of gentamicin resistance in MRSA isolates.
Main Methods:
- Isolation and characterization of MRSA strains from a Melbourne hospital between 1969 and 1981.
- Antimicrobial susceptibility testing against a panel of agents.
- Plasmid DNA analysis using molecular weight determination.
Main Results:
- Early MRSA isolates (1969) carried plasmids encoding heavy metal, penicillinase, and tetracycline resistance.
- Post-1973, resistance functions predominantly shifted to chromosomal encoding.
- Gentamicin resistance evolved from high- and low-level chromosomal forms to primarily low-level, often plasmid-mediated (18x10^6 MW) by 1980-1981, also conferring tobramycin and kanamycin resistance.
- Chloramphenicol resistance was consistently plasmid-mediated (approx. 3x10^6 MW) throughout the study period.
Conclusions:
- MRSA resistance mechanisms in this Melbourne hospital demonstrated a significant temporal shift from plasmid to chromosomal encoding for several key resistance traits.
- The evolution of gentamicin resistance patterns highlights the dynamic nature of antimicrobial resistance development in hospital-acquired MRSA.
- Continued surveillance of resistance mechanisms and genetic elements in MRSA is essential for guiding clinical practice and infection control.