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Mode of Action of Penicillins In Vivo and In Vitro in Bacillus megaterium

A new system in which the in vivo and in vitro formation of cross-links in the peptidoglycan of Bacillus megaterium can be compared directly has been developed. The method for the determination of the in vivo cross-linking consists of lysozyme digestion of acetylated [14C]diaminopimelic acid-labeled cells and Bio-Gel P-6 gel filtration of the digest. The elution profile indicates the cell wall synthesized in vivo consists of highly cross-linked fractions (44%), bisdisaccharide peptide(s) (38%), and disaccharide peptide(s) (18%). The in vitro system showed a high synthetic activity of cross-linked peptidoglycan. The synthesis was inhibited completely by 83.3 μg of ristocetin or vancomycin per ml or 10−4 M p-chloromercuribenzoate and inhibited only partially by penicillins. The polymerization was stimulated by high concentrations of sucrose, glycerol, amino acids, or dimethyl sulfoxide. The formation of cross-links was inhibited 50% at 0.3 μg of dicloxacillin per ml and 90% at 0.5 μg or more. It was also stimulated by high concentrations of sucrose, glycerol, or dimethyl sulfoxide. Effective concentrations of dicloxacillin on the growth, viability, and morphology of B. megaterium were determined. Sharp inhibition of cross-linking occurred in vivo and in vitro at these effective concentrations, whereas the incorporation of [14C]-diaminopimelate into bacterial cells was not affected at all. Cell-bound dicloxacillin reduced severely the degree of cross-linking in the cell wall synthesized after transfer to a dicloxacillin-free medium. Cell wall synthesized in the presence of dicloxacillin showed a higher rate of turnover than did the normal cell wall. Moreover, disaccharide peptide(s) was degraded faster than was bisdisaccharide peptide(s) in dicloxacillin-treated cells. From these observations, the primary target of penicillin action in B. megaterium is discussed in relation to the inhibition of cross-linking, penicillin-binding components, and cell lysis.