頭孢氨苄 Cephalexin

作用機轉

A first generation cephalosporin, cephalexin exhibits activ­ity against the bacteria usually covered by this class. Cephalosporins are bactericidal against susceptible bacteria and act by inhibiting mucopeptide synthesis in the cell wall resulting in a defec­tive barrier and an osmotically unstable spheroplast. The exact mechanism for this effect has not been definitively determined, but beta-lactam antibiotics have been shown to bind to several en­zymes (carboxypeptidases, transpeptidases, endopeptidases) within the bacterial cytoplasmic mem­brane that are involved with cell wall synthesis. The different affinities that various beta-lactam an­tibiotics have for these enzymes (also known as penicillin-binding proteins; PBPs) help explain the differences in spectrums of activity of these drugs that are not explained by the influence of beta-lactamases. Like other beta-lactam antibiotics, cephalosporins are generally considered to be more effective against actively growing bacteria. While there may be differences in MIC’s for individual first generation cephalosporins, their spectrums of activity are quite similar. They possess generally ex­cellent coverage against most gram-positive pathogens and variable to poor coverage against most gramnegative pathogens. These drugs are very active _in vitro_ against groups _A_ beta-hemolytic and B Streptococci, non-enterococcal group D Streptococci ( _S. bovis_ ), _Staphylococcus intermedius_ and _aureas_, _Proteus mirabilis_ and some strains of _E. coli_, _Klebsiella_ spp., Actinobacillus, Pasturella, _Haemophilus_ _equigenitalis_, Shigella and Salmonella. With the exception of _Bacteroides_ _fragilis_, most anaerobes are very susceptible to the first generation agents. Most species of Corynebacteria are susceptible, but _C. equi_ (Rhodococcus) is usually resistant. Strains of _Staphylococcus epidermidis_ are usually sensitive to the parenterally administered 1st generation drugs, but may have variable suscept