Mutants, showing either constitutive (depressed) or non-inducible expression of chromosomally-mediated Type I beta-lactamase were obtained from clinical isolates of Enterobacter cloacae, Ent. aerogenes, Citrobacter freundii, Providencia stuartii, Morganella morganii, Serratia marcescens and Pseudomonas aeruginosa. The wild-type and mutant strains were compared for susceptibility to a range of beta-lactam antibiotics. Derepression of beta-lactamase synthesis generally, but not always, resulted in a marked reduction in susceptibility to the agents tested, including the '3rd generation' cephalosporins. In many cases, the observed resistance would preclude, or severely compromise, the therapeutic efficacy of the drugs. In this context, depressed mutants of Enterobacter spp., Citro. freundii and Ps. aeruginosa could be of primary concern although those of Ser. marcescens, Prov. stuartii and Morg. morganii often exhibited equally high resistance levels to older beta-lactams. Comparison of the susceptibilities of the non-inducible mutants with that of their inducible parents suggested variation in the beta-lactamase inductive potency of different compounds in different organisms. For example, cefoxitin was a powerful inducer in Ent. cloacae, Citro. freundii and one strain of Ps. aeruginosa; similarly cefazolin and cefuroxime were good beta-lactamase inducers in Ser. marcescens and Morg. morganii. Aminothiazolyl-oxime cephalosporins and ureido-penicillins were generally poor inducers. From such comparisons, the contribution of inducible Type I beta-lactamase to resistance phenotype could be ascertained.
Herpes simplex virus lesions recur in 8-30% of infants who receive a course of parenteral antiviral therapy for an initial infection. Long-term acyclovir is used by some clinicians to prevent recurrent Herpes simplex disease. We describe nine infants who were treated with doses of oral acyclovir which were chosen to achieve 2-h post-plasma concentrations of > or = 2 micrograms/ml. Eight infants had Herpes simplex encephalitis and one had multiple recurrences of dermal and ocular disease. The target plasma concentration was chosen in order to attain acyclovir cerebrospinal fluid distribution (< or = 50% plasma) for an estimated ID30 of Herpes simplex II strains of 0.1-0.5 microgram/ml. One of nine patients failed to achieve the target plasma acyclovir concentration. One of nine patients developed symptomatic recurrence of the central nervous system disease and none of the remaining eight patients experienced recognized dermal or neurologic recurrence of Herpes simplex disease. Renal and neurologic status were routinely monitored and no signs of acyclovir toxicity were observed. Plasma concentration of acyclovir > or = 2 micrograms/ml may be achieved with average oral doses of 1340 mg/m2/dose (1000-1740 mg/m2/dose) given at 12-h intervals.
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