Effect of zinc concentration in Mueller-Hinton agar on susceptibility of Pseudomonas aeruginosa to imipenem.

The susceptibility of Pseudomonas aeruginosa to imipenem has been shown to vary according to zinc concentration in the media. MICs of imipenem for 68 unique clinical isolates of P. aeruginosa were determined in media supplemented with zinc at concentrations between 0.5 and 6.0 micrograms/ml. In agar containing up to 3 micrograms of zinc/ml, 75 to 82% of the strains were susceptible to imipenem at an MIC of < or = 4 micrograms/ml. In agar supplemented to contain 6 micrograms of zinc/ml, however, only 40% of the strains were susceptible to imipenem. Manufacturers should ensure that the concentration of zinc in commercial media is below 3 micrograms/ml to avoid false classification of isolates as resistant to imipenem.

Effect of time and temperature on inactivation of aminoglycosides by ampicillin at neonatal dosages.

The administration of gentamicin at least 1 hour before administration of ampicillin in neonates has been advocated because of in vitro inactivation of aminoglycosides by beta-lactam antibiotics. This method would cause a delay in ampicillin dosing in the treatment of serious bacterial infections and unnecessarily complicate nursing procedures. We studied the effect of varying concentrations of ampicillin (50, 100, 200, and 400 micrograms/ml) on aminoglycosidic antibiotics in vitro with the use of stock solutions diluted in pooled sera obtained from cord blood and incubated samples at 25 degrees C, 37 degrees C, and 40 degrees C. We found inactivation of aminoglycosides to be dependent on time, temperature, and ampicillin concentration, but the degree of inactivation was small and does not support temporal separation of parenteral administration of ampicillin and aminoglycosides to neonates.

Imipenem stability in a predried susceptibility panel.

We performed a 15-month study using 11 clinical strains and 1 control strain (ATCC 27853) of Pseudomonas aeruginosa to determine whether changes in the manufacturing process of Sensititre predried panels result in a reliable test of susceptibility to imipenem. MIC and breakpoint susceptibility results remained stable during the manufacturer's recommended shelf life of 18 months and compared well with standard agar disk diffusion and broth macrodilution results. Imipenem concentrations measured by high-pressure liquid chromatography were acceptable through 15 months but declined in the breakpoint panels by approximately 50% at 18 months. Between 9 months and panel expiration, 13 of 141 (9%) of the MIC panel packages had moisture entry, as indicated by pink desiccants, with a resultant loss of imipenem activity of 32 to 100%. It appears that the new manufacturing process produces MIC panels that are reliable for imipenem susceptibility testing until the labeled expiration date, provided that packages containing pink desiccants are not used.

Two cases of endocarditis due to Lactobacillus species: antimicrobial susceptibility, review, and discussion of therapy.

Endocarditis caused by lactobacilli may lead to death or to relapse of infection, despite antimicrobial treatment. We report two cases of lactobacillus endocarditis in individuals with native bicuspid aortic valves who survived without relapse and review the 39 other cases reported in the literature. In only 15 previously reported cases have patients been cured with medical therapy alone. One of our patients, who was infected with Lactobacillus acidophilus, was cured by medical therapy alone, and our other patient, who was infected with Lactobacillus casei subspecies rhamnosus, required surgical replacement of his aortic valve. L. acidophilus was tolerant and L. casei subspecies rhamnosus was resistant to many antibiotics tested. Combinations of penicillin or daptomycin and gentamicin were synergistic by time-kill assay. Synergistic therapy with a penicillin and an aminoglycoside was effective clinically and appears to provide optimal medical treatment on the basis of microbiological data.