Bacterial resistance to antibiotics:modified target sites

Alteration in the target sites of antibiotics is a common mechanism of resistance. Examples of clinical strains showing resistance can be found for every class of antibiotic, regardless of the mechanism of action. Target site changes often result from spontaneous mutation of a bacterial gene on the chromosome and selection in the presence of the antibiotic. Examples include mutations in RNA polymerase and DNA gyrase, resulting in resistance to the rifamycins and quinolones, respectively. In other cases, acquisition of resistance may involve transfer of resistance genes from other organisms by some form of genetic exchange (conjugation, transduction, or transformation). Examples of these mechanisms include acquisition of the mecA genes encoding methicillin resistance in Staphylococcus aureus and the various van genes in enterococci encoding resistance to glycopeptides. © 2005 Elsevier B.V. All rights reserved.

Molecular analysis of methicillin-resistant Staphylococcus aureus reveals an absence of plasmid DNA in multidrug-resistant isolates

The number, diversity and restriction enzyme fragmentation patterns of plasmids harboured by 44 multidrug-resistant hospital-acquired methicillin-resistant Staphylococcus aureus (MR-HA-MRSA) isolates, two multidrug-resistant community-acquired MRSA (MR-CA-MRSA), 50 hospital-acquired MRSA (HA-MRSA) isolates (from the University Hospital Birmingham, NHS Trust, UK) and 34 community-acquired MRSA (CA-MRSA) isolates (from general practitioners in Birmingham, UK) were compared. In addition, pulsed-field gel electrophoresis (PFGE) type following SmaI chromosomal digest and SCCmec element type assignment were ascertained for each isolate. All MR-HA-MRSA and MR-CA-MRSA isolates possessed the type II SCCmec, harboured no plasmid DNA and belonged to one of five PFGE types. Forty-three out of 50 HA-MRSA isolates and all 34 CA-MRSA isolates possessed the type IV SCCmec and all but 10 of the type IV HA-MRSA isolates and nine CA-MRSA isolates carried one or two plasmids. The 19 non-multidrug-resistant isolates (NMR) that did not harbour plasmids were only resistant to methicillin whereas all the NMR isolates harbouring at least one plasmid were resistant to at least one additional antibiotic. We conclude that although plasmid carriage plays an important role in antibiotic resistance, especially in NMR-HA-MRSA and CA-MRSA, the multidrug resistance phenotype from HA-MRSA is not associated with increased plasmid carriage and indeed is characterised by an absence of plasmid DNA. © 2005 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.

The need for continued monitoring of antibiotic resistance patterns in clinical isolates of Staphylococcus aureus from London and Malta

Antibiotic resistance is an increasing problem in isolates of Staphylococcus aureus (S. aureus) worldwide. In 2001 The National Health Service in the UK introduced a mandatory bacteraemia surveillance scheme for the reporting of S. aureus and methicillin-resistant S. aureus (MRSA). This surveillance initiative reports on the percentage of isolates that are methicillin resistant. However, resistance to other antibiotics is not currently reported and therefore the scale of emerging resistance is currently unclear in the UK. In this study, multiple antibiotic resistance (MAR) profiles against fourteen antimicrobial drugs were investigated for 705 isolates of S. aureus collected from two European study sites in the UK (London) and Malta.