Resistance of
Staphylococci to Antimicrobial Drugs
Hospital strains of S. aureus are usually resistant to a variety of
different antibiotics. A few strains are resistant to all clinically
useful antibiotics except vancomycin, and vancomycin-resistant strains
are increasingly-reported. The term MRSA refers to
Methicillin resistant Staphylococcus aureus. Methicillin
resistance is widespread and most methicillin-resistant strains are
also multiply resistant. A plasmid associated with vancomycin
resistance has been detected in Enterococcus faecalis which can
be transferred to S. aureus in the laboratory, and it is
speculated that this transfer may occur naturally (e.g. in the
gastrointestinal tract). In addition, S. aureus exhibits
resistance to antiseptics and disinfectants, such as quaternary
ammonium compounds, which may aid its survival in the hospital
environment.
Staphylococcal disease has been a
perennial problem in the hospital environment since the beginning of
the antibiotic era. During the 1950's and early 1960's, staphylococcal
infection was synonymous with nosocomial infection. Gram-negative
bacilli (e.g. E. coli and Pseudomonas aeruginosa) have
replaced the staphylococci as the most frequent causes of nosocomial
infections, although the staphylococci have remained a problem,
especially in surgical wounds.. S aureus responded to the
introduction of antibiotics by the usual bacterial means to develop
drug resistance: (1) mutation in chromosomal genes followed by
selection of resistant strains and (2) acquisition of resistance genes
as extrachromosomal plasmids, transducing particles, transposons, or
other types of DNA inserts. S. aureus expresses its resistance
to drugs and antibiotics through a variety of mechanisms.
Beginning with the use of the
penicillin in the 1940's, drug resistance has developed in the
staphylococci within a very short time after introduction of an
antibiotic into clinical use. Some strains are now resistant to most
conventional antibiotics, and there is concern that new antibiotics
have not been forthcoming. New strategies in the pharmaceutical
industry to find antimicrobial drugs involve identifying potential
molecular targets in cells (such the active sites of enzymes involved
in cell division), then developing inhibitors of the specific target
molecule. Hopefully, this approach will turn up new antimicrobial
agents for the battle against staphylococcal infections. In fact, in
the past two years alternatives to vancomycin have been approved with
the increase in VRSA (vancomycin resistant S. aureus) isolates.
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